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D E PA RT M E N T O F G E O S C I E N C E S A N D N AT U R A L R E S O U RC E M A N A G E M E N T university of copenhagen Sebastián Kepfer Rojas Vegetation dynamics and community assembly in post-agricultural heathland D E PA RT M E N T O F G E O S C I E N C E S A N D N AT U R A L R E S O U RC E M A N A G E M E N T university of copenhagen Sebastián Kepfer Rojas Vegetation dynamics and community assembly in post-agricultural heathland Title Vegetation dynamics and community assembly in post-agricultural heathland Author Sebastián Kepfer Rojas Citation Rojas, S.K. (2014): Vegetation dynamics and community assembly in post-agricultural heathland. IGN PhD Thesis December 2014. Department of Geosciences and Natural Resource Management, University of Copenhagen, Frederiksberg. 46 pp Publisher Department of Geosciences and Natural Resource Management University of Copenhagen Rolighedsvej 23 DK-1958 Frederiksberg C +45 353 31500 [email protected] www.ign.ku.dk Responsible under the press law Gertrud Jørgensen ISBN 978-87-7903-671-0 (paper) 978-87-7903-672-7 (internet) Lay-out Inger Grønkjær Ulrich Printed by Novagraf A/S Number printed 30 Order Single issues are available from Department of Geosciences and Natural Resource Management Also published at www.ign.ku.dk. PREFACE A bit over 3 years ago I embarked at my PhD at the Department of Geosciences and Nature Resource Management at the University of Copenhagen. This PhD evolved in a different direction than was originally planned. When I first started, Nørholm heathland was going to be only part of my dissertation. After participating in fieldwork and some months of working with the historical dataset, the ecological relevance of this wonderful area became clear. The more I familiarized with the history of Nørholm, the more interesting ecological questions and patterns arose which ultimately led me to dedicate my PhD study solely to this site. Coming from Guatemala, most of my experience in ecology and conservation is in tropical ecosystems. This has, in many ways shaped the way I perceive and think about nature. Observing the overwhelming diversity of species that coexist in relatively small areas is difficult not to wonder how the few requirements organisms have can lead to these immensely complex systems. This has undoubtedly contributed to my interest for alternative or perhaps, complementary explanations for the coexistence of species and the structure of ecological communities. During this PhD I had the opportunity to explore some of these alternatives on a contrastingly different system: heathlands. The “simplicity” of this ecosystem and the close connection to human activities challenged my preconceived views on nature and nature conservation. Somewhat idealistically or maybe just naively, I believe that conservation of nature should aim at removing or at least reducing the influence of anthropogenic impacts on ecosystems and not necessarily lead to a predefined state but rather a state shaped by ecological and evolutionary processes. This raises the question on whether conservation efforts should be allocated at managing areas to maintain a predefined state or should we leave natural processes take over? To answer this question, evidence and knowledge on the consequences of these options need to be gathered. This PhD presents a study of the development of a plant community which has been without direct human intervention for over a century. Hopefully, this will contribute to answering this question. September, 2014 Sebastián Kepfer Rojas 3 ACKNOWLEDGEMENTS I would like to start by expressing my gratitude to my supervisors Inger Kappel Schmidt and Vivian Kvist Johannsen for allowing me the opportunity to follow my own research interests and ideas; for allowing to play with this wonderful data and “travel in time”; and for supporting me not only academically but also in tough times, thank you for your understanding. The same gratitude extends to Annemarie Bastrup-Birk, for guidance and support at initial periods of my PhD. I would like to thank the people at the Section of Forest, Nature and Biomass, especially Torben Riis-Nielsen for sharing his expertise in heathland ecology and botany; Thomas Nord-Larsen, for advising and checking up on me; Barbro Haar and Oksana Korsgaard for helping me with all the administrative issues and simply making work-life simpler; and all my fellow PhD /friends: Lucia, Johannes S., Ludovika, Yoppi (it was great sharing an office with you), Johannes R., Jonas, Mette, Nanna, Davide, Carmen, Andy, Petros, Teresa, Michael, Noah, Rikke, Geshere, Claudia, Seid, Marie, Pia, Mads, Heng, Søren and Anders, thanks for the good times. Johannes R., thanks for commentaries on my thesis and for the academic, political, social, cultural and what not discussions. Mette, thanks for helping with the Danish summary. During this PhD I also had the enriching opportunity to work along with researchers from external institutes. Special thanks to Kris Verheyen who acted almost as an external supervisor, your guidance and advice was instrumental for this PhD. Similarly, thanks to Christian Damgaard for welcoming my visit and introducing me to the world of structural equation modeling. I am also grateful to Francesco de Bello, Bill Shipley, Hanna Tuomisto, Nicholas Gotelli, Jacob Weiner and Anne Magurran for their willingness to provide advice and for taking the time to answer my questions; your help got me back in track. This work could not have been possible without the visionary work of I.K. Rosenørn-Teilmann, and C.H. Bornebush from “Forstlige Forsøgsvæsen”, who initiated the conservation action and the long-term survey at Nørholm Heath more than 100 years ago. In the same lines, former owners and the current owners, Martha and Karl Nielsen are thanked for welcoming the research. Further, I thank Allan O. Nielsen, Xhevat Haliti, Jane Kongstad, Geshere Abdisa, Annelies Van der Craats and Karen Sverrild and many others for contributing in collecting this wonderful data and Preben Frederiksen for help with the soil analysis. 4 On a more personal level I thank my family in Guatemala (Rodolfo, Eugenia, Walter and José), infinitas gracias por todo. Min vidunderlige svigerfamilie: Lars, Gitte og Søren. Tak, tusind tak, I har virkelig gjort det bedre for os. And last but never least, Pia, thank you for your support, love and for always being concerned about my happiness. Nikolas and Matias, you were born the day before the second interview for the PhD position, so it is fair to say that you have accompanied me along the whole way. Although I have learned a lot about myself during this PhD, nothing compares to what I have learned from being your Papa, los amo. 5 TABLE OF CONTENTS PREFACE ..................................................................................................................................................................................3 ACKNOWLEDGEMENTS .......................................................................................................................................................... 4 TABLE OF CONTENTS ............................................................................................................................................................. 6 SUMMARIES ............................................................................................................................................................................. 7 LIST OF PAPERS .................................................................................................................................................................... 12 GENERAL INTRODUCTION.................................................................................................................................................... 13 PAPER 1. DISTANCE TO SEED SOURCES AND LAND USE HISTORY AFFECT FOREST DEVELOPMENT OVER A LONG TERM HEATHLAND TO FOREST SUCCESSION ................................................................... ..33 PAPER 2- HINTS FOR ALTERNATIVE STABLE STATES FROM LONG-TERM VEGETATION DYNAMICS IN AN UNMANAGED HEATHLAND ..................................................................................................................... 53 PAPER 3. INDIRECT EFFECTS OF LAND USE LEGACIES DETERMINE TREE COLONIZATION PATTERNS IN ABANDONED HEATHLAND ........................................................................................................................... 71 PAPER 4. INTERACTIVE EFFECTS OF LAND-USE HISTORY, TREE ENCROACHMENT AND DISTANCE TO EDGE ON SPECIES RICHNESS OF GROUND VEGETATION IN A SUCCESSIONAL HEATHLAND .............. 92 SUMMARY AND SYNTHESIS ............................................................................................................................................... 119 6 SUMMARIES ENGLISH SUMMARY The structure and the assembly of plant communities are a result of historical anthropogenic activities and natural phenomena. Understanding and distinguishing between the underlying mechanisms is a challenging but integral part of ecological studies and conservation planning. Ecological theory proposes that biotic, abiotic and stochastic factors act as “ecological filters” to determine the assembly and structure of local communities. The functional strategies of the species in community play a central role as they interact with these filters to determine the presence and abundance of species in a community. This PhD study aims at understanding how biotic, abiotic and stochastic factors interact to structure a heathland community managed under different traditional land-use practices for centuries. Agriculture ceased in 1865 whereas low intensity grazing occurred until 1895, when traditional management was abandoned at the heathland. I explore the general hypothesis that the strength of biotic factors varies along abiotic gradients (i.e. soil fertility) and with the functional strategies of functional groups in the community (trees, herbs and dwarf shrubs). The data used in this work is part of one of the longest spontaneous successional studies of heathland vegetation, where tree colonization and understory vegetation patterns were measured in successive vegetation surveys initiated in 1921. This data was complemented with an intensive survey of the vegetation and ecological factors undertaken between 2009 and 2012. I found that even after a century after abandonment of agricultural practices, land-use legacies were still present in the soil and were important determinants of vegetation dynamics and community assembly. However, the effects of land-use legacies were mostly mediated by the understory vegetation and differed according to the functional groups. The distance to the edge, a proxy for the proximity to external seed sources, was arguably the most important factor affecting different components of the structure of the tree/shrub community, particularly at early stages. Similarly, the distance to the edge was also an important determinant of understory vegetation patterns, demonstrating the importance of dispersal in the development of the community. Furthermore, these results suggest a conceptual framework for the development of this heathland community. This framework proposes that the effect of the biotic interactions varies along abiotic gradients (e.g. soil fertility) and interacts with the functional strategies of species to determine the establishment of colonizing species, species’ performances and diversity patterns in the local community. This framework has broader implications for understanding the maintenance of 7 biodiversity, the csideoexistence of species and the stability of heathland communities, which can be helpful when designing conservation and management actions for this threatened habitat. “Plants are evidently in general, tolerably impartial as regards soil, if we except certain chemical and physical extremes (abundance of common salt, of lime, or of water), so long as they have not competitors" Eugenius Warming, Oecology of Plants (1909) 8 DANSK RESUMÉ Sammensætning og struktur af plantesamfund er et resultat af historiske, menneskeskabte aktiviteter og naturlige fænomener. At kunne forstå og skelne imellem de underliggende mekanismer er en udfordrende, men integreret del af økologiske studier og naturforvaltning. Den økologiske teori foreslår at biotiske, abiotiske og stokastiske faktorer fungerer som "økologiske filtre" til at bestemme sammensætning og struktur i plantesamfund. Arternes funktionelle strategier spiller en central rolle, da de interagerer med disse filtre og dermed bestemmer tilstedeværelsen og tætheden af arter i et samfund. Dette ph.d.-studie har til formål at forstå, hvordan biotiske, abiotiske og stokastiske faktorer interagerer og former vegetationen på en hede forvaltet under forskellige, traditionelle arealanvendelser i århundreder, før dyrkningen ophørte i 1865 og græsningen i 1895. Jeg udforsker den generelle hypotese, at styrken af biotiske faktorer varierer langs abiotiske gradienter (dvs. jordens næringsindhold) og med de funktionelle strategier i plantesamfundets funktionelle grupper (træer, urter og dværgbuske). De data, der anvendes i dette arbejde, er en af de længste spontane successionalstudier af lynghede, hvor kolonisering af træer og bundvegetationsmønstre er målt i en vegetationsundersøgelse indledt i 1921. Disse data blev suppleret med en intensiv undersøgelse af vegetation og økologiske faktorer foretaget imellem 2009 og 2012. Jeg fandt, at selv et århundrede efter opgivelse af landbrugsdrift, var der stadig forskelle i jorden relateret til tidligere arealanvendelse og at disse forskelle var afgørende for vegetationsdynamik og sammensætningen af plantesamfundet. Dog var effekten af tidligere arealanvendelse primært medieret af bundvegetationen og varierede i forhold til de funktionelle grupper. Afstanden til kanten af heden, en proxy for nærhed til eksterne frøkilder, var uden tvivl den vigtigste faktor, der påvirkede de forskellige dele af strukturen af træ/busksamfundet, især i de tidlige faser af successionen. Afstanden til kanten var ligeledes også en afgørende faktor for dynamikken i bundvegetationen, hvilket viser betydningen af spredning for udviklingen af samfundet. Disse resultater tyder desuden på en konceptuel ramme for udviklingen af dette dværgbuskhedesamfund. Denne ramme indikerer, at effekten af de biotiske interaktioner varierer langs abiotiske gradienter (f.eks. jordens frugtbarhed) og med funktionelle strategier af arter, og er bestemmende for etablering af koloniserende arter, arters ydeevne og mangfoldighed i plantesamfundet. Denne ramme har stor betydning for forståelsen af bevarelse af biodiversitet, sameksistens og stabilitet af dværgbuskheder. Det kan være nyttigt, når naturbeskyttelse og forvaltningsplaner skal designes for denne sårbare naturtype. 9 RESUMEN EN ESPAÑOL La estructura y ensamblaje de las comunidades vegetales son el resultado de actividades antropogénicas históricas y fenómenos naturales . Comprender y distinguir entre los mecanismos subyacentes es una parte difícil pero integral de los estudios ecológicos y planes de conservación. La teoría ecológica propone que factores bióticos , abióticos y estocásticos actúan como " filtros ecológicos" para determinar la estructura de ensamblaje y de las comunidades locales. Las estrategias funcionales de las especies en la comunidad juegan un papel central, ya que interactúan con estos filtros para determinar la presencia y abundancia de las especies en una comunidad. Este estudio de doctorado tiene como objetivo la comprensión de cómo los componentes bióticos, abióticos y estocásticos interactúan para estructurar una comunidad de brezal manejada bajo diferentes prácticas tradicionales de uso de la tierra por siglos antes de su abandono en 1865. Asimismo, se explora la hipótesis general de que la magnitud de los factores bióticos varía a lo largo de gradientes abióticos (e.g., la fertilidad del suelo) y con las estrategias funcionales de grupos funcionales en la comunidad (árboles , hierbas y arbustos) . Los datos utilizados en este trabajo forman parte de uno de los más largos estudios de sucesión espontánea de vegetación de brezales, donde la colonización de árboles y los patrones de vegetación del suelo fueron registrados en un estudio iniciado en 1921. Estos datos se complementaron con un estudio intensivo de la vegetación y factores ecológicos llevado a cabo entre 2009 y 2012. Se encontró que incluso después de un siglo del abandono de las prácticas agrícolas, los legados de la agricultura todavía estaban presentes en el suelo y fueron importantes en la determinación de la dinámica de la vegetación y el ensamblaje de la comunidad. Sin embargo, los efectos del legado del uso de la tierra fueron mediados principalmente por la vegetación del suelo y difieren según los grupos funcionales. La distancia al borde del brezal, un indicador de la proximidad a las fuentes externas de semillas, fue posiblemente el factor más importante afectando a los diferentes componentes de la estructura de la comunidad de árboles, sobre todo en las etapas iniciales. Del mismo modo , la distancia hasta el borde fue un factor determinante de los patrones de vegetación del suelo, demostrando la importancia de la dispersión en el desarrollo de la comunidad . Además, estos resultados sugieren un marco conceptual para el desarrollo de ésta comunidad. Este marco propone que el efecto de las interacciones bióticas varía a lo largo de gradientes abióticos (por ejemplo, de fertilidad del suelo) e interactúa con las estrategias funcionales de las 10 especies para determinar el establecimiento de especies colonizadoras, el rendimiento de las especies y los patrones de diversidad en la comunidad local. Este marco tiene implicaciones más amplias para la comprensión de la conservación de la biodiversidad, la convivencia y la estabilidad de las comunidades de brezales que pueden ser útiles al momento de diseñar acciones de conservación y gestión de éste tipo de hábitat. 11 12 GENERAL INTRODUCTION RAPIDLY CHANGING ECOSYSTEMS ON A HUMAN-DOMINATED PLANET We live in a human dominated planet with a total population of approximately 7 billion. Meeting the growing demands of humans has led to more rapid and extensive changes in ecosystems over the last 50 years than in any comparable period of time in human history (Millenium Ecosystems Assessment, 2005). These staggering demands affects natural ecosystems and biodiversity through different interrelated processes (Ellis et al. 2013). Among them, land-use changes, plays a primary role (Foley et al. 2005; Foley et al. 2011). Land-use change is considered the major threat to terrestrial and marine ecosystems globally (Vitousek et al. 1997; Sala et al. 2000) and is strongly linked to economic development (Flinn & Marks, 2007; Foster et al., 2003; Verheyen et al., 2003). Following the pace of industrialization; intensification and technification of agricultural practices led to high levels of deforestation in North America and Europe until the first half of the 20th century (Flinn & Vellend 2005). Hereafter, following a change to a service economy coupled with human exodus to urban areas, an inversion of the direction of change occurred in some areas (Verheyen et al. 1999; Flinn & Marks 2007; Améztegui et al. 2010). Extensive former agricultural areas were abandoned, giving place to reestablishment of forests. In Denmark the general trend has been similar, although the magnitude of change has been quite dramatic. At the onset of the 19th century, forest cover was reduced to a mere 3-4 % (Plum 1989). Since then, forest cover has slowly increased to the current level of ca. 14 % (Nord Larsen et al. 2013). Although afforestation has been mainly driven by economic incentives, international agreements on conservation of biodiversity (e.g. Nagoya protocol), have set ambitious goals to halt and reverse the negative trends. A strategic part of these plans in Europe consists of conservation and restoration of semi-natural and forest habitat areas considered to be important for biodiversity (Habitat Directive; Natura2000) and, in the case of Denmark, of further increases in the area covered by forest (Skov og Natustyrelsen 2002; Petersen et al. 2012). Due to the intensive and extensive modifications to landscapes and ecosystems by former agriculture, expansion of current nature reserves or establishment of new ones requires reclamation of agricultural areas with different agricultural history. An implication is that nature conservation has to be considered within an agricultural context and that the effects of environmental legacies of previous agricultural practices have to be understood (Perrings et al. 2006). A necessary first step then is to determine the impact of land-use legacies on the different components of ecosystems and ecological communities. 13 In a step towards this direction, this thesis makes use of a long-term monitoring dataset complemented with an intensive survey of current vegetation and ecological properties to examine the effect of land-use change on the development of the vegetation at Nørhom heathland (RiisNielsen et al. 2005).The long-term data series enables the study of fundamental ecological processes and provides a solid basis for understanding the current patterns in the vegetation. Central to this work is to understand the impact and determine the importance of land-use legacies on the structure, dynamics and assembly of the vegetation community. In this section, I first describe the ecological foundations and the general conceptual framework that motivated and support this work. Second, I present the objectives. Third, I briefly introduce the study system, highlighting the features that make it a model system and unique site to study vegetation dynamics. Fourth, I summarize the methods used, and finally, I give a short overview on the papers included in this thesis. A GENERAL FRAMEWORK FOR DEALING WITH THE COMPLEXITY OF ECOLOGICAL COMMUNITIES A longstanding and fundamental goal in ecology is to understand the processes that determine the structure of communities. This has proven to be a difficult task. Attempts to find a general law of community organization have focused in concepts including: keystone predation (Paine 1966), niche theory (McArthur and Levins 1967), energy flow (Odum, 1969), regeneration dynamics (Grubb 1977), resource competition (Tilman 1982), neutrality (Hubbell 2001) and metabolic theory (Brown 2004). Although these concepts have greatly advanced ecological theory, some limitations have emerged when these hypotheses have been put to empirical test (Roughgarden 1996, Lawton 1999). The lack of a general theory of community organization led to strong criticism of community ecology as a scientific discipline (Lawton, 1999). An important reason for the failure of developing a general theory is that community structure is highly contingent (Lawton 1999; Simberloff 2004). At local scales, the interactions between organisms with each other and with their environment are so complex and site dependent, that even when ecological studies can decipher them, the results are not likely to be transferable to other sites, even when environmental and historical conditions are similar (Lawton 1999). Ecologists advocating in favor of community ecological studies argue though that lack of generalization should not render abandonment of ecological studies at local scales; but that it does require a new method of study (Keddy 1992; Simberloff 2004; Roughgarden 2009, McGill 2006). In this matter, Roughgarden (2009) writes: “the sciences rarely deliver exceptionless generalizations and scientific effort instead may be directed toward finding an 14 ‘‘invariant toolkit’’ of mechanisms that yield a variety of outcomes resulting from different interplays among the mechanisms in various situations”. In other words, when attempting to explain the factors that determine the local structures of communities, hypotheses on interrelated and multifactorial control have to be considered and contingencies have to be taken into account (Grace, 1999). Renewed interest in community assembly during the last 20 years, has directed much of its effort in reformulating a framework through which community structure can be studied (Belyea & Lancaster 1999; McGill et al. 2006; Weiher et al. 2011; HilleRisLambers et al. 2012). The idea behind this framework (Fig. 1.1) is that species from the regional pool need to pass a series of biotic, abiotic and dispersal filters to finally become members of the local communities (Zobel 1992; Belyea & Lancaster 1999). These filters act in a hierarchical multifaceted manner to determine the identity, number and abundance of the different species (Bello et al. 2013). More concretely, at the scale of local communities, abiotic factors (topography, water availability, soil fertility) select species from the species pool with the necessary physiological adaptations, reducing the species pool to those that can withstand the local conditions (Grime 2006). At finer scales, biotic interactions will vary along the local environmental gradients to FIGURE 1.1. Community assembly framework. Local communities are subsets of the regional species pool that pass through biotic and abiotic filters. Dispersal limitation and chance influence the ability of species to pass the filters. Once species establish in the local communities, feedbacks from species interactions and species´ effects on the environment modify the filters. Modified from HilleRisLambers et al. (2012). determine which species persist and become dominant. A necessary requirement is though that the species possess the required functional adaptations to colonize and establish at a local community (dispersal filters). The “filtering paradigm” is embedded within traditional ecological theory on community structure mechanisms. At the core of most of these theories are the differences in life history and functional traits of species and how they mediate the performance of species in different biotic and abiotic 15 milieus. Basic niche theory predicts a limit to the similarity between coexisting species (Abrams 1983). According to the “competitive exclusion principle” (Grime 1973), ecologically similar species (i.e. similar life history/functional traits) cannot coexist because the best competitor will always outcompete the others. However, a certain degree of similarity is also predicted because “environmental filters” select species with the necessary and thus similar adaptations (particularly at harsh environments). The balance between these opposing forces can vary according to environmental gradients and is largely determined by the functional and life-history traits of species. The importance of these mechanisms varies depending on local site conditions and on the presence of different functional groups in the community (McGill et al. 2006). For instance, in nutrient poor environments the success of perennial species is determined by their ability to conserve rather than capture mineral resources and species possessing traits for this type of strategy are favored (Grime 1979, Aerts 1999). However, if nutrients become available, species that can capture resources more effectively can have the advantage thus shifting the balance of competitive strength between these groups (Aerts and Heil, 1993). As mentioned above dispersal is another important factor structuring communities (Bullock 2002). However, a successful colonization depends not only on effective dispersal. Local site conditions and the availability of open space can determine establishment of colonizers (Ehrlén & Eriksson 2000; Myers & Harms 2011). Similarly to species performances, colonization is determined by the interplay between local environmental conditions, biotic interactions and life history traits of colonizers. For example, on a nutrient availability gradient, productivity is usually higher at high nutrient levels. The more vigorous, established vegetation can then limit colonization of other species due an increase in its competitive strength (Jurena & Archer 2003; Chauchard et al. 2007). However, the degree to which establishment of colonizers is limited depends on the life history traits of colonizing species (Ehrlén & Eriksson 2000). Colonization/competition trade-offs are typically invoked as being responsible for the differences in colonization success (Tilman 1993; Westoby et al. 2002). There is a trade-off between the ability to colonize and the ability to compete so that the best competitor cannot displace all other species in the area because it is a poor colonizer. New openings in the vegetation (as caused by disturbances) allow poor competitors to colonize and persist even in the presence of superior competitors (Tilman 1982). Clearly, the ideas of the filtering framework are very appealing when studying local community structure as they provide a flexible framework to identify the mechanisms that operate in different situations and on when they might be the dominant structuring force (rather than pursuing to find a single unifying mechanism). When combined with information on the life history strategies of 16 functional groups a more mechanistic and rigorous theory on community structure can arise (Mcgill 2006). By considering how the influence of structuring forces varies across environmental gradients and how they interact with different functional strategies, it is possible to evaluate the large range of factors and mechanisms that determine the structure and assembly of communities. TEMPORAL DYNAMICS Up to this point, community assembly has been presented as the outcome of the structuring forces acting upon communities. In the majority of ecological studies, communities are typically studied at fixed point in time, i.e. when the effects of assembly processes have exerted its influence in the community; without considering historical processes. This provides a narrow view, as in many cases the influence of multiple interacting factors can obscure underlying mechanisms (Chase & Myers 2011). Long-term vegetation studies consisting of repeated measurements of the vegetation offer a better opportunity to disentangle the effect of interacting mechanisms, to compare the relative magnitude and importance of their effects, and to determine when and how they change over time (Bakker et al. 1996; Rees et al. 2001). Studies of long-term vegetation dynamics have traditionally focused on the concept of vegetation succession (sensu Cowles 1899), i.e. the orderly replacement of species composition along a temporal scale (after a disturbance in secondary succession). Similarly, community assembly refers to the temporal changes in species composition over time, however, community development is determined by random colonization of species and by the difference in the likelihood of establishment and persistence of colonizers (Young et al. 2001). There is a subtle but important difference between the two concepts: Community assembly allows for a more stochastic view of vegetation change, where historical factors, random colonization and species interactions determine the outcome and often lead to multiple stable states (Chase 2003). The debate on whether communities develop in a deterministic or stochastic manner dates back to the early work of plant ecologists like Clements (1916, 1938) and Gleason (1926, 1927). According to Clements, succession proceeded through a series of states towards a final stable state (“climax”) in a more or less predictable way, even when the starting vegetation state was different. Gleason´s, and later Egler´s (1954) views, differed in that community development was determined by stochastic dispersal events and that the order of arrival of species could determine different stable endpoints (multiple stable states). Species arriving first could become dominant and then inhibit colonization by others species, either because first arrivers pre-empt space and resources 17 or through plant-soil feedbacks. An important point is that, in order to demonstrate the occurrence of multiple stable states, it is necessary that the initial environmental conditions are identical and that all species have access to the locality (Connell and Sousa 1983), which is rarely the case in field studies. Current views on the importance of stochastic vs deterministic processes affecting the structure of communities have moved away from considering them as mutually exclusive phenomena and recognize that both processes may act simultaneously (Gravel et al. 2006; Chase & Myers 2011). Focus has now moved to discerning the strength of these processes along environmental gradients, history of assemblage space and time (Chase & Myers 2011). With respect to the latter, Stokes and Archer (2010) proposed a modification to the filter paradigm to incorporate a change in the importance of structuring forces, from stochastic in early stages after disturbance, to deterministic at later phases of development (Weiher and Keddy 1999; McGill et al. 2006). The rationale for that can be traced back to the seminal work of Gleason (1926), who observed that immediately after disturbances, colonization is not strongly constrained by abiotic and biotic factors and thus dispersal limitation is the prominent structuring force. As vegetation develops and the abiotic conditions are modified, environmental selection and biotic interactions become the driving force. Despite the limitations that come along with long-term monitoring programs, the long-term dataset used in this thesis provides an opportunity to examine whether different structuring forces operate at different points in time and to draw insights on possible underlying mechanisms and infer on their relative importance. HEATHLANDS Outside the natural occurring areas, the vast majority of the north-west European lowland heathlands (as the one in this study) are closely linked to anthropogenic activities (Gimingham 1979, Gimingham and Schmidt 1983). It is currently accepted that the origin of this type of heathland can be traced back to the Neolithic period (ca. 2500 B.C.) when nomadic farmers cleared shrubs and woodlands to establish cultivation fields (Aerts and Heil 1993). Up until the turn of the 20th century, heathlands dominated the landscape of many north European countries and were part of the traditional agricultural system. With the advent of modern agricultural techniques and artificial fertilization, the extension of heathlands has decreased severely and many of the remaining heathlands occur in isolated and fragmented areas (Piessens et al. 2005). 18 Heathland communities are often dominated by schlerophyllous vegetation from the Ericaceae family (ericoid dwarf shrubs). Dominant species typically vary from Calluna vulgaris and Empetrum nigrum at dry heathlands to Erica tetralix in high moisture sites (Aerts and Heil 1993). Under natural conditions grasses like Deschampsia flexuosa and Molinia caerulea are also present in heathland communities. These groups (dwarf shrubs and grasses) present two diametrically contrasting functional strategies. While, dwarf shrubs possess traits characteristic of a resource conservation strategy (e.g. slow growth rates, low tissue turnover and thick leaves), grasses are adapted to a rapid resource acquisition (e.g. faster growth rates, high tissue turnover and thin leaves). These contrasting strategies are not only key for the coexistence between these groups, but they also determine the outcome of their interactions (Aerts & Berendse 1988; Aerts & Peijl 1993). Hence, any factor that affects the availability of nutrients (e.g. atmospheric nitrogen deposition, fertilization from agriculture) can disrupt this coexistence mechanism (Aerts and Bobbink 1998; Mitchell et al. 2000). Traditional agricultural practices in heathlands involved natural fertilization and tillage resulting in soils with reduced organic matter and increased nitrogen and phosphorus (Webb 1998; von Oheimb et al. 2008). Increased nutrient availability can then affect the stability of heathlands and trigger the transition to a grassland or woodland (Bakker & Berendse 1999). Post-agricultural heathlands present a model setting to study vegetation dynamics and community assembly, for many reasons: 1. The harsh environmental conditions present a pervasive abiotic filter, selecting for species with the necessary adaptations; 2. Land-use legacies and tree colonization can alter the levels of nutrient availability and light levels, modifying the abiotic filter; 3. The coexistence and balance of biotic interactions (biotic filter) of common functional groups with distinctive and almost opposing functional strategies is altered by the modification to abiotic conditions; and 4. Because of few species and contrasting functional groups, heathlands are relatively “simple” communities to study ecological processes. Moreover, because of the vulnerability of heathlands to current land use practices, most of the remaining areas are situated in nature reserves and depend on some type of management. Therefore, understanding the mechanisms involved in structuring heathland communities is necessary as a basis for conservation, management and restoration of these communities. 19 AIMS OF THE STUDY This thesis used long-term and current vegetation patterns to identify how biotic, abiotic, historical and stochastic factors interact to structure a heathland plant community undergoing natural development. Central to this thesis was to identify whether the importance of these structuring factors vary in time and with functional strategies of the species in the community. The specific aims of this thesis were to: Ͳ Evaluate the importance of stochastic vs deterministic factors on vegetation dynamics (Paper 1, 2 and 4) Ͳ Assess the importance of land-use legacies as determinants of community structure (Paper 1, 2, 3 and 4) Ͳ Determine the effects of biotic, abiotic and stochastic factors differ according to functional groups (Paper 2, 3 and 4). Ͳ Assess the relative importance of direct and indirect effects of abiotic, biotic and stochastic factors as determinants of diversity patterns (Paper 1 and 4) 20 MATERIAL AND METHODS NØRHOLM: A unique heathland Nørholm (NH) is the subject of this thesis and has formerly been the subject of a series of ecological studies. Detailed topographic, climatic and geological descriptions of NH can be found in Hansen (1932), Opperman and Bornebush (1930), Riis-Nielsen et al. (2005). In addition, the papers in this thesis include descriptions of the site and specific methods relevant to each study. Instead of making an exhaustive description, in this section I will only outline the most relevant ecological and historical processes that have influenced the development of the vegetation and briefly summarize the methods used in this thesis. NH is a 350 ha heathland in the southwestern part of the Jutland peninsula in Denmark (Fig 1.2A). As most of the Atlantic European lowland heathlands, Nørholm is strongly linked to human activities (Webb 1998). Following the introduction of agriculture (~4000 BC), the area covered by forest in Denmark decreased giving place to heathland vegetation and agricultural fields (Nielsen et al. 2010). In the Jutland peninsula, heathlands were a main component of the landscape between 1000 BC- 1900 AD (Riis-Nielsen et al. 2005; Nielsen et al. 2010). During this period, NH was managed under traditional management practices, consisting of a so called “infield /outfield system. In this type of system, different areas in a farm were utilized at different levels of intensity. In NH, the infields were cultivated while the outfields were used for grazing. In winter periods, grazing animals were kept in stables were their manure was collected and applied in the infields as natural fertilizer, transferring nutrients to the agricultural fields (Christiansen 1978; Webb 1998). This type of system had a remarkable influence on the abiotic environment due to the mobilization of nutrients, alteration to the physical soil structure and removal of vegetation that accompanied ploughing in the infields. This system was maintained until abandonment of agricultural practices in 1865. Low intensity grazing continued until 1895 when NH was left without any human management. Nowadays, the effect of land-use legacies is still evident in the soil properties. For example, phosphorus concentrations are clearly higher in the former infields (Fig 1.2B). In 1913, the owner of NH initiated actions that led to the protection of the heathland with the objective of conserving it at its “natural condition” (Hansen 1932). At that time, livestock grazing was considered one of the main threats to heathland vegetation. To protect the heathland from grazing, a hedge row of mountain pine (Pinus mugo) was planted along the south and east borders of the heathland. This event had a tremendous impact on the development of the vegetation. The 21 pine dispersed and established widely into the heathland. In addition, scarce forest remnants enhanced colonization by other pioneer tree species. Tree colonization occurred in an exponential way, increasing from about 1200 individuals in 1920´s to ca. 900,000 at present time. Currently, forest covers ca. 30 % of the heathland (Fig 1.2C and Fig 1.3). FIGURE 1.2. Map of NH (A) showing the grid used for tree registration and the position of the vegetation plots for the 2009-2012 survey and permanent vegetation plots. The thick line separates the cultivated area to the West and the uncultivated to the East. The inlet shows the location of the study site in Denmark. Predicted total phosphorus content (B) obtained by regression kriging using topographical variables as predictors (R2 = 0.62). Vegetation height classification map (C) obtained from LiDAR images acquired in 2007. Although the land-use development and the landscape context of NH are typical of other sites in Denmark and Europe, there are important aspects that make NH a unique and important site for conservation and ecological research. First, the land-use history and the development of the vegetation are very well known. A series of aerial photographs, cartographic and cadaster records have been used to reconstruct the land-use history to a great detail. In addition, a monitoring program initiated in 1921 has systematically registered changes in the vegetation using permanent plots and tree colonization surveys. These surveys have been done in 10 occasions (11 for permanent plot) at irregular intervals spanning over 90 years. To our knowledge this is one of the longest spontaneous succession vegetation surveys. 22 Second, NH has not been managed for over 100 years. Long-term studies of spontaneous plant secondary succession are in general rare, and even more so for heathlands. As mentioned before, because of the conservation status of heathland in Europe and in Denmark, the majority of the areas are maintained under some type of management actions (e.g. burning, grazing, sod cutting, tree removal) aiming at controlling invasion by grasses and trees (Pywell et al. 2011). Third, in spite of the lack of management, typical heathland vegetation has been remarkably stable in NH. In other abandoned heathlands, tree and grass colonization is known to occur within few decades, and usually results on drastic alterations to the vegetation (Hester et al. 1991; Manning et al. 2004; Ascoli & Bovio 2010). A schematic description of the historical development of the vegetation is given in Fig. 1.3. FIGURE 1.3. Schematic development of vegetation at Nørholm heathland. Dashed lines are hypothesized vegetation trends obtained from vegetation studies in Denmark and in the area (Odgaard & Rasmussen 2000; Nielsen et al. 2010). Full lines correspond to measurements obtained from a long term vegetation study initiated in 1921 (see methods). ᬅ = introduction of agriculture; ᬆ = Cessation of agricultural practices. Notice that the x-axis is not scaled. Vegetation communities are dynamic. Quantifying and trying to predict how communities change has been a major task in ecological studies. The task is difficult because changes in vegetation communities occur at a pace that cannot be covered by typical ecological studies. To bypass this problem, ecologists have relied on chronosequences (i.e. space for time substitutions). Although, these studies have been insightful and pivotal for successional studies, they have also been 23 criticized mainly because in most cases, the assumptions of the method are not likely to hold for studies of plant communities and they are rarely tested (Johnson and Miyanishi 2008). On the contrary, and in spite of some limitations, repeated measurements of vegetation are a straightforward method to study long-term vegetation development over time (Bakker et al. 1996; Rees et al. 2001; Chazdon 2008). However, depending on the type of plant community, vegetation development can take several decades or even centuries and thus repeated measurement studies are rarely available. In NH, the Royal Veterinary and Agricultural University, initiated a survey in 1921 to study vegetation changes and forest succession (Riis-Nielsen et al. 2005). Changes in ground vegetation were recorded by means of 17 (later 20) permanent plots, each of 10 X 10 m. Between 1921 and 2014, 11 vegetation surveys were performed. To study tree colonization, individual trees were measured and registered using a 400 X 400 m grid of 33 quadrats covering the heathland entirely (Fig 1.2.A). Between 1921 and 2014, 10 tree surveys were performed (RiisNielsen et al. 2005). In this thesis, I first used the long-term tree surveys data (Paper 1, for methodological details) and permanent vegetation plots (Paper 2, for methodological details) coupled to the historical records to describe the long-term vegetation dynamics. This data was complemented with an intensive survey (2009-2012) of 140 plots (0.03 ha) used to register trees and shrubs and 12 subplots (0.1 m2) nested in each plot (n = 1680) to register the understory vegetation (Fig. 1.2A). In addition, soil samples, digital elevation models, full cover vegetation structure maps (LiDAR) were included in analysis focusing on current patterns of tree seedlings colonization (Paper 3) and diversity of understory vegetation (Paper 4); and to complement the long-term studies. 24 THESIS OUTLINE GENERAL INTRODUCTION- Delimits the study and presents the theoretical background that motivated this work. PAPER 1- examines and compares the effects of land-use legacies and distance to seed sources on the long-term development (100+ years) of emergent properties of the tree/shrub community´s structure: species abundances, richness and composition. PAPER 2- describes the long-term development (100+ years) of the understory vegetation using permanent vegetation plots. Change in cover of species and functional groups are used to determine whether the trajectories in vegetation development are deterministically determined by agricultural legacies in the soil. The possibility of the presence of multiple stable states is raised. PAPER 3- compares the patterns of tree and shrub seedling establishment between areas with different land-use history and between species with different functional strategies to determine how the importance of biotic interactions varies along a nutrient availability gradient. PAPER 4- proposes a model of multivariate control of the diversity patterns of the understory vegetation. Using structural equation models, the interacting effects of land-use legacies and forest development on three functional groups are assessed. 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Hints for alternative stable states from long-term vegetation dynamics in an unmanaged heathland. (accepted for publication in Journal of Vegetation Science) 3. Kepfer-Rojas, S., Schmidt, I. K., Johannsen, V.K. & Verheyen, K. Indirect effects of land use legacies determine tree colonization patterns in abandoned heathland. (manuscript in preparation) 4. Kepfer-Rojas, S., Schmidt, I.K., Riis-Nielsen, T. & Damgaard, C. Interactive effects of landuse history, tree encroachment and distance to edge on species richness of ground vegetation in a successional heathland. (manuscript in preparation) The Papers are not included in this version of WKe thesis due to copyright. SUMMARY AND SYNTHESIS The overall aim of this thesis was to study how historical, biotic, abiotic and stochastic factors interact to structure plant communities and vegetation dynamics of a heathland undergoing spontaneous development. More specifically, this thesis integrated long-term and current information to evaluate the impact of land-use history and the proximity to seed sources on the development of the overstory and understory vegetation. Land-use legacies and distance to seed sources respectively played determinant roles affecting the probabilities of establishment and dispersal after abandonment of agricultural practices. Thus, differential effects of these factors on the structure of developing communities were identified. To increase our understanding on how they influenced the dynamics and assemblage of vegetation communities, this work focused on drawing inferences on the importance of these driving factors, on the mechanisms underlying them, on whether and how they changed over time and on how they interacted with life history and functional strategies of species. In this chapter I first summarize how land-use legacies and proximity to seed sources affected different properties of the structure of the community. Secondly, a synthetic framework for the development of the vegetation is proposed; and finally, some perspectives for management and future research at NH are given. LAND-USE LEGACIES: DIRECT AND INDIRECT EFFECTS An important first step to determine the impact of land-use legacies on the development of the vegetation is to determine what these legacies consist of, and whether they are still present (Flinn & Vellend 2005). Land-use legacies involve a series of processes that can potentially influence the development of plant communities. The most obvious effects are the disturbance of the vegetation and impacts on the soil properties (Flinn & Vellend 2005; Hermy & Verheyen 2007). Based on the latest survey, it was confirmed that land-use legacies were still evident in the soil even after more than one century since abandonment of agriculture. However, although nutrient levels and pH were higher in the formerly cultivated area of the heathland, the effects of traditional heathland agricultural practices are only moderate when compared to the more intense, modern agriculture (Von Oheimb et al. 2008; Pywell et al. 2011). Even at this relatively lower intensity, the impact of land-use was evident in many aspects of the development of the vegetation. Nevertheless, the effect varied in magnitude and was dependent on the functional groups. Arguably, the most obvious effect of land-use legacies was observed on the understory vegetation (Paper 4). Higher nutrient availability directly affected the current species richness patterns. However, this effect was only important for herbs (grasses and forbs), which responded directly to increased nutrients by achieving higher cover and species number in the formerly cultivated area. Importantly, species richness of herbs increased directly with increased availability of nutrients (i.e. independent of species cover), supporting the notion that a release from an environmental filtering increases the number of species that can coexist at a local community (Grime 2001, Craine 2009). This effect was particularly important for forb species which were almost exclusively found in areas with high nutrient availability, indicating that this particular functional group is limited by nutrient availability. On the other hand, land-use legacies did not affect the long-term patterns of species richness and composition of the tree and shrub communities (Paper 1). However, the rate at which species colonized was lower at the formerly cultivated areas (Paper 1). Coupled with the patterns of seedling colonization (Paper 3), it was demonstrated that increased nutrient availability increased the ability of the established vegetation to suppress colonization of tree species (an indirect effect of land-use legacies). Higher productivity of faster growing species Deschampsia flexuosa, was promoted by nutrient availability and consequently limited colonization of pioneer species with a high dispersal and resource acquisition strategy (Paper 3), highlighting that the importance of biotic interactions is a function of both environmental conditions and the relationship between functional strategies of established and colonizing species. As mentioned above, another important factor is the disturbance of the vegetation that follows abandonment of agricultural fields (although not exclusively related to land-use legacies). When the fields are abandoned, space for colonization becomes available. If the duration and intensity of agriculture have depleted the seed sources, colonization becomes a “race for space” where life history traits and dispersal limitation can be important determinants of the structure of the assembling community (Cramer, 2008). This mechanism likely explained the long-term development of the understory vegetation and the long-term increase in cover of grasses independent of nutrient availability in the permanent plots (Paper 2). To conclude, although effects of land-use legacies affected the development to the vegetation widely, most of the effects were indirectly mediated by the effects of nutrient availability on biotic interactions and priority effects. Importantly, even when the increase in nutrient availability was only moderate, sustained changes in the development of the vegetation can occur. DISTANCE TO SEED SOURCES AND DISPERSAL Although, grass and tree encroachment are occurring at NH, the patterns observed show some important deviations to what would be expected under an assumption of strict deterministic control by soil properties. The long-term development of the tree and shrub community was only indirectly determined by land-use legacies. Instead the distance to seed sources, a proxy for dispersal limitation, was the single most important factor determining the abundance and identity of the colonizing species (Paper 1). Similarly, the distance to the edge was an important determinant of the distribution, abundance and diversity of the understory vegetation (Paper 4). Furthermore, despite increased nutrient availability and lack of management, grass and tree encroachment have been limited to some areas and ericoid species have been able to persist, even in areas with higher nutrient availability (Paper 4). Likewise, the long-term changes in the composition of the understory vegetation could not be directly associated to soil properties, but was rather explained by the history of disturbance of the different plots which allowed colonization of grasses (Paper 2). Altogether, these results demonstrate that stochastic factors like dispersal and history of colonization are important factors structuring communities. SYNTHETIC FRAMEWORK FOR INTERACTING DRIVERS OF VEGETATION DYNAMICS AND COMMUNITY ASSEMBLY This thesis demonstrates the wide applicability and flexibility of the filtering paradigm of community assembly to understand changes in vegetation communities. Integrating the results from the longterm and the current vegetation analyses, a mechanistic conceptual framework of vegetation dynamics and community assembly is proposed (Fig 6.1 and 6.2). This framework illustrates how life history/functional traits (fig6.1A), species tolerances to abiotic conditions (fig6.1B) and species’ performances (that determine the outcome of biotic interactions), (Fig.1C) interacted over time to structure this community. FIGURE 6. 1. Relationship between functional traits, species tolerances to abiotic conditions and species performances along resources gradients. The position of the functional groups along resource use and establishment strategies (A), influence: 1) the tolerances to local abiotic conditions (shaded area) and the abundances along this gradient (B); and 2) the performance of species along a resource gradient (C). These factors work in a hierarchical way to determine species membership into local communities (fig. 6.2). At the top level, the presence of species is constrained by abiotic conditions (fig. 6.1B and fig 6.2). The nutrient poor and acidic soils in heathlands function as strong environmental filters, limiting species membership into the local communities to those species with the necessary functional adaptations. In this sense, the legacy of past agriculture has a direct effect, releasing the community from this abiotic filter and allowing more species to colonize. FIGURE 6.2. Synthetic filtering framework of community assembly at Nørholm heathland. Local community assembly is presented as a function of time (X-axis) and distance to external sources (Y-axis at the bottom of each panel). The local community assembles from species from the regional species pool (classified as functional groups) passing through abiotic and biotic filters. Only species adapted to local conditions can pass the abiotic filter. The effect of biotic interactions changes over time and depends on environmental conditions and functional group feedbacks. Access to the local community is determined by colonization/competition tradeoffs, mediated by environmental conditions. Curved arrows represent biotic interactions and the functional group effect on local conditions. Symbols: as in fig 6.1A. The effect of the release from the abiotic filter becomes evident first after abandonment of management, because management actively removes species that otherwise could have established. Over the course of community assembly the abiotic filter may change if the colonizing species modify the abiotic conditions. For instance, colonization by woody species can affect the soil properties through plant soil feedbacks and by limiting light availability (Mason et al. 2012). These novel conditions can then limit or facilitate further colonization by species depending on their tolerances to and performances at the modified nutrient and light regimes (Cramer 2008). This is exemplified by the decrease in the cover of dwarf shrub species and colonization of shade tolerant species, (e.g. Deschampsia flexuosa, paper 4) under the developed tree canopy at later phases of community assembly. At the next hierarchical level, biotic interactions limit species membership into the local community. This implies that even when species possess the necessary adaptations, they cannot establish in the local community because other species outcompete them or simply because all available space is already occupied by these species. In this sense, the occurrence and persistence of the species along environmental gradients depends on how species’ performances and the outcome of biotic interactions are determined by the local environmental conditions (fig 6.1C). Furthermore, the susceptibility of the colonizing species to the biotic filter depends on the life history strategies and functional trade-offs of the established and the colonizing species (Fig 6.1B). For instance, at the poor extreme of the nutrient availability gradient, functional groups with a resource conservation strategy (e.g. dwarfshrubs) can become dominant and further reduce access to the nutrient pool, limiting colonization or hindering the performance of fast growing species like grasses (Aerts & Peijl 1993; Aerts 1999). On the richer part of the nutrient gradient a combination of rapid colonization and fast growth by grasses can lead to priority effects, preventing colonization by pioneer trees (Paper 3), and/or suppressing dwarf shrubs (Paper 4). A requirement for the biotic and abiotic filters to operate is that colonizing species need to have access to the local community. Thus, the probability of different species colonizing the local community is incorporated as another element in this framework (represented as the distance from external sources in fig 6.1). In this sense, areas closer to seed sources are more likely to be colonized. In situations where colonization is not limited by abiotic and biotic factors, stochastic colonization can lead to a stochastic community assembly (Myers & Harms 2011). However when biotic interactions are strong the likelihood of a stochastic community assembly decreases. The stochastic component of the community assembly is expected to be stronger at initial stages and to decrease in importance as vegetation develops and deterministic processes like competitive exclusion and niche differentiation become more important (Stokes & Archer 2010), or because priority effects become stronger over time in the absence of disturbances (Chase 2003). Life history traits and trade-offs in functional strategies play a central role in the dynamics of the vegetation and the assembly of the community. First, life history traits determine whether species can pass the abiotic filter and access the local community. Second, resource use strategies can determine the performance of species along environmental or fertility gradients. Third, functional traits controlling competition/colonization trade-offs can determine whether species can successfully disperse and establish at local communities. Finally, because of the interaction of functional strategies with environmental gradients, different outcomes in community assembly can occur depending on the arrival and performance of species along environmental gradients. NO MANAGEMENT, BAD MANAGEMENT? Despite that grass and tree encroachment are occurring, NH differs from other areas in that the transition to grassland or woodland has occurred at a very slow pace. Even more than a century without management, open heathland is the dominant vegetation at a significant portion of the area. The temporal scope of this study allowed evaluating the long-term stability of the vegetation community facing recognized threats (increased nutrient availability and tree encroachment). Furthermore, the lack of management presented a unique opportunity to study the development of the vegetation under natural dynamics. Some of the findings in NH challenge the paradigmatic view of heathland conservation, i.e. that management actions are necessary for the maintenance of these communities (Mitchell et al. 2000; Pywell et al. 2011). A factor that was of mayor importance to the colonization of grasses and trees was the disturbance of the vegetation. Paradoxically, current management actions such as burning, grazing, sod cutting and mechanical removal of trees necessarily disturb the vegetation and can create establishment windows for colonizing species. Although these actions seem to be effective in some cases, they are required to be implemented periodically to maintain the vegetation at some desired state (Mitchel et al. 2000). This can be problematic not only because it is costly; but because it necessarily requires the continuous intervention and does not lead to a “self-sustainable” vegetation community. However, it is likely that the particular circumstances at this heathland are responsible for some of the observed patterns. For example, perhaps one of the most important factors explaining the slow colonization rates of trees is that seed source were scarce and at a considerable distance from the heathland when agriculture was abandoned. This caused a delay in the arrival of first colonizers, giving grasses, and to a lesser extent dwarf shrubs, time to establish and to modify the environment and hinder further colonization (Paper 2 and 4). Despite these contingencies, the conceptual framework developed here (fig 6.1 and 6.2) indicates that conservation or management designs should consider at least three factors: 1) the influence of functional strategies and colonization/competition trade-offs; 2) the importance of abiotic factors as determinants of plant interactions; and 3) the effect of the established vegetation, particularly in the absence of disturbances. For instance, because of competition/colonization tradeoffs, fast colonizers might have an initial advantage over dwarf shrub species in colonizing post agricultural sites. Thus, if the objective is to restore a dwarf shrub community in areas with high nutrient contents and depleted seed banks, it might be necessary to assist colonization (e.g. seed/seedling addition, combined with removal of fast colonizers) at early stages after disturbance until dwarf shrubs develop and can constrain other species (e.g. shading). On the other hand, in low fertility soils, dwarf shrubs are able to persist up to a point were low light availability strongly limits them. In this case, allowing more light through the canopy might be a more effective way than tree removal to maintain the cover of dwarf shrubs until self-thinning of the canopy occurs. To sum up, considering how different functional strategies respond to abiotic factors; how they determine the outcome of biotic interactions; and how these relationships vary along environmental gradients to influence the development of vegetation can help guiding restoration and conservation of this threatened ecosystem. FUTURE RESEARCH AT NØRHOLM HEATHLAND The long-term dataset collected at NH was the backbone of this thesis, evidencing the invaluable insights into fundamental ecological questions that can be gained with long-term ecological studies. Furthermore, the absence of direct human intervention during more than a 100 years following centuries of traditional agricultural practices, presented a unique opportunity to study how vegetation communities change under natural conditions. Despite the limitations that come along with this type of studies, it was shown that thorough documentation of changes in the vegetation and of relevant ecological factors combined with advanced analytical and statistical tools, offer the opportunity to study the development of communities under “real-life” conditions and to understand how historical factors affect this development. On the other hand, it is acknowledged that experimental studies would be necessary to confirm or test emerging hypotheses on causal relationships and mechanisms. This presents the conundrum of whether research should continue under a strict “no-human intervention policy” or whether experimental studies should be implemented at NH? The answer is not very simple but it is likely to be, at least partly dependent on the scale and intensity of the experiments. Small scale experiments that do not involve intensive disturbances to the vegetation, combined with observational studies can provide answers to some unresolved questions. For instance, stochastic disturbance events such as climatic anomalies, heather beetle attacks and herbivory undoubtedly influence the structure and assembly of communities. Regular monitoring programs could be valuable to assess the impact of these stochastic factors. However, this will require that 1) ad hoc studies can be implemented when these events occurs; 2) that the number of the permanent plots is increased; and 3) that monitoring becomes regular and at shorter time intervals (e.g. 5 years). The information obtained can then be complemented with small scale field experiments (e.g. disturbances, seed additions and fencing) along the main environmental gradients at NH to study for example, the relationship between disturbances, dispersal limitation and abiotic gradients. Another potential direction would be to conduct more detailed studies on tree colonization patterns. Although, this work identified the importance of the established vegetation, other factors are likely to be involved. To mention some: conspecific density dependence, the presence of mycorrhiza (Collier & Bidartondo 2009), allelopathic effects (Zackrisson & Nilsson 1992) and herbivory. To study these factors under natural conditions, a first step could be to establish large permanent plots (e.g. 20 ha) to map all individual stems and seedlings and conduct neighborhood effect studies. These results can then be complemented with small scale field studies and ex situ experiments. Understanding how these factors interplay with environmental gradients and functional strategies of colonizing species can be an exciting research direction. NH is a unique site to answer fundamental and applied ecological questions and to make comparative studies. Thus the continuation of monitoring programs and addition of new research projects will likely contribute fruitfully to our understanding of vegetation dynamics, community assembly, and many other interesting ecological processes. All in all, this work highlighted the importance and uniqueness of NH as an interesting area for ecological research and as a benchmark for monitoring. Hopefully, NH will continue to serve as a source of knowledge and inspiration for the years to come. REFERENCES Aerts, R. 1999. Interspecific competition in natural plant communities: mechanisms, trade-offs and plant-soil feedbacks. Journal of Experimental Botany 50: 29–37. Aerts, R., & Peijl, M.J. Van Der. 1993. A simple model to explain the dominance of low productive perennials in nutient-poor environments. Oikos 66: 144–147. Chase, J.M. 2003. Community assembly: when should history matter? Oecologia 136: 489–98. Collier, F. a., & Bidartondo, M.I. 2009. Waiting for fungi: the ectomycorrhizal invasion of lowland heathlands. Journal of Ecology 97: 950–963. Craine, J.M. 2009. Resource strategies of wild plants. Princeton University Press, Princeton, USA. Cramer, V.A., Hobbs, R.J., & Standish, R.J. 2008. What’s new about old fields? Land abandonment and ecosystem assembly. Trends in ecology & evolution 23: 104–12. Flinn, K.M., & Vellend, M. 2005. Recovery of forest plant communities in post-agricultural landscapes. Frontiers in Ecology and the Environment 3: 243–250. Grime, J.P. 2001. Plant strategies, vegetation processes, and ecosystem properties. John Wiley & Sons, Chichester, UK. Hermy M, Verheyen K. 2007. Legacies of the past in the presentǦday forest biodiversity: a review of past landǦuse effects on forest plant species composition and diversity. Ecological Research 22, 361Ǧ371. Mason, N.W.H., Richardson, S.J., Peltzer, D.A., de Bello, F., Wardle, D.A., & Allen, R.B. 2012. Changes in coexistence mechanisms along a long-term soil chronosequence revealed by functional trait diversity. Journal of Ecology 100: 678–689. Mitchell, R.J., Auld, M.H.D., Duc, M.G. Le, & Marrs, R.H. 2000. Ecosystem stability and resilienceௗ: a review of their relevance for the con- servation management of lowland heaths. 3: 142– 160. Myers, J. A., & Harms, K.E. 2011. Seed arrival and ecological filters interact to assemble highdiversity plant communities. Ecology 92: 676–86. Stokes, C.J., & Archer, S.R. 2010. Niche differentiation and neutral theory: an integrated perspective on shrub assemblages in a parkland savanna. Ecology 91: 1152–62. Pywell, R.F., Meek, W.R., Webb, N.R., Putwain, P.D., & Bullock, J.M. 2011. Long-term heathland restoration on former grassland: The results of a 17-year experiment. Biological Conservation 144: 1602–1609. Stokes, C.J., & Archer, S.R. 2010. Niche differentiation and neutral theory: an integrated perspective on shrub assemblages in a parkland savanna. Ecology 91: 1152–62. Von Oheimb, G., Härdtle, W., Naumann, P.S., Westphal, C., Assmann, T., & Meyer, H. 2008. Long-term effects of historical heathland farming on soil properties of forest ecosystems. Forest Ecology and Management 255: 1984–1993. Zackrisson, O., & Nilsson, M. C. 1992. Allelopathic effects by Empetrum hermaphroditum on seed germination of two boreal tree species. Canadian Journal of Forest Research-Revue Canadienne De Recherche Forestiere, 22, 1310–1319. 1 FORMER ISSUES Maj 2013 August 2013 August 2013 Landskabsbyens æstetik. En undersøgelse af fikmmediet som redskab til belysning af forstadens omgivelseskarakter Mads Farsø ISBN 978-87-7903-614-7 Smallholder tree farming systems for livelihood enchancement and carbon storage James Michael Roshetko ISBN 978-87-7903-629-1 Translating Harbourscape. Site-specific Design Approaches in Contemporary European Harbour Transformation Lisa Diedrich ISBN 978-87-7903-626-6 January 2014 Changing heathlands in a changing climate. Climate change effects on heathland plant communities Johannes Ransijn ISBN 978-87-7903-644-4 April 2014 Deriving harmonised forest information in Europe using remote sensing methods. Potentials and limitations for further applications Lucia Maria Seebach ISBN 978-87-7903-651-2 October 2014 Cemeteries: Organisation, management and innovation. Diffusion of maintenance specifications in Danish national church cemetery administrations Christian Philip Kjøller ISBN 978-87-7903-673-4 November 2014 Parks, People and Places. Place-based governance in urban green space maintenance Julie Frøik Molin ISBN 978-87-7903-675-8 December 2014 Vegetation dynamics and community assembly in post-agricultural heathland Sebastián Kepfer Rojas ISBN 978-87-7903-671-0 PhD Thesis December 2014 ISBN 978-87-7903-671-0 Sebastián Kepfer Rojas Vegetation dynamics and community assembly in post-agricultural heathland department of geosciences and natural resource management university of copenhagen rolighedsvej 23 dk-1958 Frederiksberg tlf +45 35 33 15 00 [email protected] www.ign.ku.dk