Download Post disturbance vegetation succession and resilience in

Study plan for PhD project
Post disturbance vegetation succession and resilience in forest
ecosystems
Main supervisor: Associate professor Ulf Grandin, Dept. of Aquatic Sciences and Assessment, SLU
Co-supervisors: Associate professor David Angeler and associate professor Stefan Löfgren both at the
Dept. of Aquatic Sciences and Assessment, SLU, and doctor Thomas Dirnböck at Environment Agency
Austria.
Description of the planned doctoral project
Aims and goals
The project aims at a deeper understanding of plant ecological processes in Swedish and European
forest ecosystems, by in-depth analyses of long term data mainly from the international monitoring
program ICP-Integrated Monitoring. Main emphasis will be on understanding resistance and
resilience in unmanaged and semi-natural forests to impacts by drivers and pressures at different
scales from site specific local disturbances to large scale effects from eutrophication, acidification
and climate change.
The goals are to find out in what way forest plant biodiversity and composition are affected by
disturbances at various scales in space and time, and to what degree intrinsic resistance and
resilience may counteract changes in the ecosystem. The obtained knowledge could feed into
management to foster adaptation of forests to disturbances and maintain ecosystem regimes that
are desirable for societies in terms of ecosystem service provisioning.
Background and theoretical context
Forest ecosystems have evolved in the presence of large and small scale natural disturbances, e.g.
fire, wind and insect outbreaks. However, during the last century all ecosystems have faced other
kinds of increasing diffuse anthropogenic disturbances, mainly acidification, eutrophication and rapid
climate change. The forest ecosystems are since long also subjected to forestry, which has profound
effects on the structure and species composition, but managed forests are not the main focus in this
project. For unmanaged semi-natural forests, the diffuse anthropogenic disturbances may lead to
changes in species diversity and composition. However, ecological changes in semi-natural forests
may take time as many forest plants are sessile and often long lived, causing a resilience debt
(Johnstone et al. 2016) that may not manifest until after further disturbances. Nevertheless, changes
in forest conditions caused by the major anthropogenic disturbances may cause a loss of biodiversity
and a homogenisation of the species composition (Rooney et al. 2004). Ultimately, when disturbance
thresholds are passed ecosystems can shift into alternative regimes. These regimes are often
undesired because they provide less ecosystem services relative to the previous regime (Pope et al.
2014).
The data we will use offers a unique time series of detailed geochemical and vegetation data, which
we will utilise to investigate spatiotemporal patterns in forest vegetation composition and structure.
This will allow us to test hypotheses about how biodiversity relates to resistance (absorption of
disturbances without structural or functional change) and resilience (either recovery after or shift to
an alternative regime after disturbances) under pressures from both local and large scale
disturbances.
Scientific problems and relevance
Many studies have shown that increased forest resilience have a dampening effect on perturbations
caused by e.g. fire or insect outbreaks (Hood et al. 2016). Although there are multiple evidences that
decreased diversity decreases forest resilience (e.g. Thompson et al. 2009), there are few studies that
have been able to relate eutrophication to decreased forest biodiversity mainly because forestry is a
confounding factor in large scale data from national forest inventories (but see Dirnböck et al. 2014
and Simkin et al. 2016). By utilising the ICP-Integrated Monitoring network and related monitoring
data, we will be able to investigate effects by the major pressures (nitrogen and sulphur deposition
and climate change) on the composition and structure in forest vegetation, in the absence of direct
anthropogenic influence. This in turn will increase our knowledge about how adaptive management
(see Folke et al. 2004) of the forest ecosystem should be practiced to optimise diversity and
resilience while allowing sustainable forestry.
Specific research questions and hypotheses are presented under the preliminary descriptions of the
different papers, below.
Methods
The PhD student will primarily work with data from the ICP-Integrated Monitoring network (ICP-IM).
The ICP-IM has an extensive database with biological and chemical data from detailed studies of
unmanaged semi-natural forested catchments all over Europe. The project will also make use of the
ICP Forests data base to get even more long term data on vegetation, tree layer and chemistry. A
third data source may be selected sites in the LTER Europe network. However, data from ICP Forests
and to some degree also from LTER Europe are from managed forests which need to be addressed in
the analyses.
The first paper will investigate small scale processes at the ICP-IM site Aneboda, which have suffered
from a severe bark beetle outbreak (Löfgren et al. 2014). Aneboda belongs to the SITES network
under Asa, financed by the Swedish Research Council, which opens opportunities for comparisons
with managed forests and experiments that may not be conducted in the protected Aneboda
catchment.
In the following papers, the scope and scale will widen to include more sites from the international
ICP networks, to get more general results and conclusions.
The thesis will consist of at least four scientific papers. Here we describe preliminary plans and
hypotheses for three papers, and alternative plans for the topic of the fourth and a possible fifth
paper.
Paper I
Biological response after the storm Gudrun, at the Aneboda Integrated Monitoring site.
In this paper, we will primarily study effects on the field and ground layer vegetation from the
massive bark beetle infestation that occurred after the storm “Gudrun” in 2005 (Löfgren et al. 2014).
The study will be based on long term high resolution data on vegetation, edaphic factors,
meteorology, hydrology and chemistry. Could also include effects on the wood-fungi community
comparing affected and non-affected adjacent forest, discussing source populations, local and
regional species pools and dispersal in a fragmented landscape (however, there is currently no data
on fungi).
This monitoring site has undergone a rapid and drastic change from a closed canopy with a moss
dominated understorey to an open forest with an understorey dominated by ruderal vegetation. It is
hypothesised that the vegetation will develop in different directions in affected areas and
unaffected refuges, which could indicate a regime shift induced by the storm. We are especially
interested in looking at the fate of early successional species in affected parts and their replacement
by species with other traits, and to compare this with the vegetational patterns in the refuges.
As a reference, new field inventories may be carried out in the adjacent Asa experimental forest,
where we have stands of different types and managed forests at different age.
This study will also make use of the intense chemical monitoring and include the effects of released
nutrients on the plant succession, as explanatory variables in the analyses.
Paper II
Small scale site specific variation in response to storm and bark beetle attacks at ICP-Integrated
Monitoring sites in Europe.
In paper II we will be using bark beetle infestations as natural experiments studying variation in
resistance and resilience in the forest structure and composition at the site scale, over a wide
geographical gradient. Bark beetle infestations are expected to increase in a warmer climate (Bentz
et al. 2010), however there is still an ignorance regarding biogeochemical effects (Edburg et al. 2012).
Post disturbance successional development depends on several factors such as e.g. diaspore
availability, environmental harshness, competition and grazing pressure. A severe disturbance may
lead to multiple successional pathways resulting in different communities, depending on the
resistance and resilience of the initial community. In this study we will look at successional patterns
over a climate and depositional gradient across Europe, by using data from unmanaged forests in the
ICP-IM and LTER networks. The study will also look at effects on the biogeochemical cycles and
disentangle effects from bark beetle affected forests from effects in forests only affected by diffuse
anthropogenic disturbances. There is currently a need for deeper knowledge in how the
biogeochemical pathways change in bark beetle affects forests (Edburg et al. 2012). This new
knowledge will lead to better understanding of ecosystem services such as e.g. water and nutrient
retention are affected in bark beetle affected forests. We hypothesise that different forest types
within a catchment react differently on a massive bark beetle infestation, and that the succession
and the associated effect on the geochemical processes may differ along the studies gradient.
Paper III
Nutrient limitation or enrichment in European forests. Could increased CO2 levels lead to nutrient
deficiency for the forest understorey?
Since the ICP-IM monitoring started, we have seen significant decreases in sulphur deposition while
nitrogen deposition show varying trends. Data from the ICP-IM network were recently used in the
first scientific study ever showing negative effects on plant diversity from N deposition (Dirnböck et
al. 2014), and have been supported in later studies (Simkin 2016). However, recently Jonard et al.
(2015) have shown that European forests on the contrary undergo an increasing nutrient limitation
driven by increased tree growth as an effect of increased CO2 levels. If this true, we hypothesise that
increasing tree growth not only lowers nutrient availability for understorey plants but also causes
increased shading from a denser canopy, which together withholds major responses in
understorey plants. We will use existing data from the ICP and LTER networks on leaf and soil
nutrient content, at relevant sites in Europe and compare the results with trends in understorey
vegetation. If necessary, we may also take new samples from a selected sites to analyse nutrient
status (possibly utilising the Transnational Access program in the Horizon 2020 project eLTER). By
utilising longitudinal long term vegetation data in combination with recent and past data on nutrient
status, we may explain why few studies have been able to show effects in the understorey from the
increased nitrogen deposition over the past half a century.
Paper IV
Exact focus of paper IV is be decided
We plan that paper IV should deal with large scale patterns in forest vegetation, utilising data from
different long term monitoring programs, such as LTER, ICP-Forests and Integrated Monitoring. Paper
IV could either test results in papers I-III on a larger scale, or have new aims. Depending on the
obtained results in papers I-III, we suggest one of the following themes for the fourth paper:
a) Forest ecosystem integrity and disturbances at different scales.
A conceptual and general study on forest biodiversity issues related to natural and diffuse
anthropogenic disturbances and climate change. This may include regime shifts and thresholds in
species composition and structure. A local disturbance may cause cascade effects in the trophic
structure and hierarchy by giving rise to increased diversity in different guilds through an
increased structural diversity (sensu Noss 1990). Increased structural and species diversity are
generally seen as stabilising factors supporting the ecosystem function and residence. However,
when the disturbance is in the form of a slow but directed change in climate, or elevated
deposition of nutrient enriching substances, the effect of the disturbance may take other
pathways leading to decreased ecosystem stability and function. The objectives of this paper
would be to investigate and discuss differences between large and small scale disturbances, on
the effects on forest ecosystem integrity. This study would utilise data from suiatable ICP-IM,
ICP Forests and LTER Europe monitoring sites.
b) Effect of local and large scale disturbances on forest carbon flux
Element budgets, primarily carbon, at sites within the ICP-IM and Forests network. This study
would not be a direct continuation of papers I-III, but anyway closely related and based on the
same data sets. Old growth forests generally act as carbon sinks (Luyssaert et al. 2008). However,
more recent studies show uncertainty in the estimates of the carbon flux in the boreal forest
ecosystem (Bradshaw & Warkentin 2015). The detailed and repeated monitoring of permanent
forest plots in the ICP and LTER networks, will allow us to quantify turnover rate of trees, in the
form of seedling recruitment to tree death and decay rates, and relate this to the carbon cycling.
Some of the monitoring sites have in addition to the general stress factors in the form of
eutrophication and global warming also been affected by local and more severe disturbances in
the form of insect attacks and storm felling. One objective of this study would be to disentangle
effects of local disturbances from general stress factors, and to assess how this affects the
carbon storage in the tree biomass. This could lead to new and important knowledge whether
forests under more extreme climate (drought, fire, flooding and insect attacks) act as carbon
sinks or sources.
Possible paper V, depending on the development of analysis techniques of LIDAR data
From forest post-management succession to internal dynamics
In 2011 the Swedish ICP-IM sites were scanned by high resolution LIDAR. Since then all four sites
have shown increased tree mortality. This tree die-back has been expected as the sites are in a late
post-management successional state, on the transition towards a system characterised by internal
gap dynamics rather than a slow but still directed forest succession. A new LIDAR scanning in
combination with field inventories would give excellent opportunities to investigate small scale forest
dynamics and explore factors controlling forest resistance and the tipping point process from
succession to dynamics. Objectives and hypotheses for this study needs to be developed further,
after we know what kind of information we can get from the LIDAR data.
National and international cooperation
The supervisor team are persons with wide connections in international monitoring and research
networks. The PhD student will be introduced to relevant networks and contacts, whenever
appropriate.
The whole project will be based on forest monitoring data from harmonised international monitoring
programs. We plan that the student will attend the ICP-Integrated Monitoring Task Force meetings,
to present results. Moreover, the International LTER network has a special Task Force on effects of N
deposition, in which this project would fit very well. This Task Force is very active and has courses
and workshops, which we plan to attend.
Another important arena for making international contacts and networking is the ALTER-Net summer
school, which attracts top lectures and PhD students from all major European institutes working with
long term environmental research and monitoring, including top level staff at the funding bodies in
EU.
National cooperation will in the beginning be through contacts within the research schools Ecologybasics and applications and Focus on soils and water at SLU. We also plan that results should be
disseminated at the annual meetings with the Swedish OIKOS society.
References
Bentz, B.J., Régnière, J., Fettig, C.J., et al. 2010. Climate Change and Bark Beetles of the Western
United States and Canada: Direct and Indirect Effects. BioScience 60: 602-613. DOI:
10.1525/bio.2010.60.8.6.
Bradshaw, C.J., Warkentin, I.G. 2015. Global estimates of boreal forest carbon stocks and flux. Global
and Planetary Change 128: 24-30.
Dirnböck, T., Grandin, U., Bernhardt-Römermann, M., et al. 2014. Forest floor vegetation response to
nitrogen deposition in Europe. Global Change Biology 20: 429-440. DOI: 10.1111/gcb.12440.
Edburg, S.L., Hicke, J.A., Brooks, P.D., Pendall, E.G., Ewers, B.E., Norton, U., Gochis, D., Gutmann, E.D.,
et al. 2012. Cascading impacts of bark beetle‐caused tree mortality on coupled biogeophysical and
biogeochemical processes. Frontiers in Ecology and the Environment 10: 416-424.
Folke, C., Carpenter, S., Walker, B., Scheffer, M., Elmqvist, T., Gunderson, L., Holling, C.S. 2004.
Regime shifts, resilience, and biodiversity in ecosystem management. Annual Review of Ecology,
Evolution, and Systematics 35: 557-581. DOI: 10.2307/annurev.ecolsys.35.021103.30000021.
Hood, S.M., Baker, S., Sala, A. 2016. Fortifying the forest: thinning and burning increase resistance to
a bark beetle outbreak and promote forest resilience. Ecological Applications 26: 1984-2000. DOI:
10.1002/eap.1363.
Johnstone, J.F., Allen, C.D., Franklin, J.F., et al. 2016. Changing disturbance regimes, ecological
memory, and forest resilience. Frontiers in Ecology and the Environment 14: 369-378. DOI:
10.1002/fee.1311.
Luyssaert, S., Schulze, E.-D., Börner, A., Knohl, A., Hessenmöller, D., Law, B.E., Ciais, P., Grace, J. 2008.
Old-growth forests as global carbon sinks. Nature 455: 213-215.
Löfgren, S., Grandin, U., Stendera, S. 2014. Long-term effects on nitrogen and benthic fauna of
extreme weather events: Examples from two Swedish headwater streams. AMBIO 43: 58-76. DOI:
10.1007/s13280-014-0562-3.
Noss, R.F. 1990. Indicators for monitoring biodiversity: a hierarchical approach. Conservation biology
4: 355-364.
Pope, K.L., Allen, C.R., Angeler, D.G. 2014. Fishing for resilience. Transactions of the American
Fisheries Society 143: 467-478.
Rooney, T.P., Wiegmann, S.M., Rogers, D.A., Waller, D.M. 2004. Biotic impoverishment and
homogenization in unfragmented forest understory communities. Conservation Biology 18: 787-798.
DOI: 10.1111/j.1523-1739.2004.00515.x.
Simkin, S.M., Allen, E.B., Bowman, W.D., Clark, C.M., Belnap, J., Brooks, M.L., Cade, B.S., Collins, S.L.,
et al. 2016. Conditional vulnerability of plant diversity to atmospheric nitrogen deposition across the
United States. Proceedings of the National Academy of Sciences 113: 4086-4091. DOI:
10.1073/pnas.1515241113.
Thompson, I., Mackey, B., McNulty, S., Mosseler, A. 2009. Forest resilience, biodiversity, and climate
change. Secretariat of the Convention on Biological Diversity, Report Technical Series no. 43,
Montreal, 1-67 pp.