1. Research outline - Laboratory of Tree-Ring Research Download

1. Research outline
Project description:
Growth responses of perennial forbs to climate change
Introduction and state of research
There is much evidence for an ongoing, global change that has accelerated during the last
decade (Houghton, et al. 2001). Two of the most important factors affected are temperature
that faced a global increase of 0.6°C during the last century, and precipitation patterns that
are expected to show more heavy rainfalls and longer periods of drought (Frich, et al. 2002;
Fay, et al. 2003; Karl & Trenberth 2003; Bell, et al. 2004). While there are large regional
differences in the magnitude and direction of these changes (Forchhammer & Post 2000;
Frich, et al. 2002; Walther, et al. 2002; Karl & Trenberth 2003), effects of climate change will
be stronger in high latitudes and altitudes (Root, et al. 2003).
Despite this knowledge, it is complicated to attribute recent trends in herbaceous vegetation to climate change, since non-climatic influences dominate local, short-term biological
responses (Parmesan & Yohe 2003). However, many studies give evidence for recent biological and ecological responses, e.g. earlier phenological timing (Fitter & Fitter 2002; Parmesan & Yohe 2003; Root, et al. 2003), polwards and upslope shifting of species area
boundaries (Walther, et al. 2002; Parmesan & Yohe 2003; Callaghan, et al. 2004), and altering ecosystem processes such as changes in community composition and competition relationships (Harte & Shaw 1995; Holmgren, et al. 2001; Klein, et al. 2004; Wahren, et al.
2005), favoring southerly and lower-elevation species (Walther, et al. 2002; Callaghan, et al.
2004; Heegaard & Vandvik 2004), changes in productivity (Holmgren, et al. 2001; Fay, et al.
2003) or facilitated invasion of exotic species (Kriticos, et al. 2003; Grigulis, et al. 2005;
Neilson, et al. 2005).
Particularly for herbaceous plants with longer generation cycles, such as perennial
forbs, the success of a species in rapidly changing environments mainly depends on phenotypic plasticity and/or genetic variability in its life history traits (e.g. growth rate, reproductive
effort and longevity; Sardans, et al. 2006), because the speed of changes exceeds the capacity to genetically evolve (Jump & Penuelas 2005), especially in marginal habitats. At low
elevations in the temperate zones, increasing temperatures and lower, increasingly variable
soil water availability may shift life history traits towards more stress tolerance, i.e. phenotypic plasticity will express, for instance, in slower growth, lower annual reproductive effort,
greater longevity and functional adaptation to drought (Wookey, et al. 1995; Stanton, et al.
2000; Stinson 2004). In contrast, at high elevation in the temperate zones, abiotical constraints will relax, and growth conditions may generally improve, as long as biotical pressure
from lower, more productive species is not overruling (REF).
One problem when investigating phenotypic plasticity of perennial forbs is that shortand long-term responses to climate change may differ (Shaver et al. 2000, Hollister et al.
2005. Furthermore, species respond individualistically to environmental variables (Callaghan
et al. 2004). Studies based on lifetime data of individual plants may circumvent these difficul-
ties, the more as local and short-term influences on plant growth, such as disturbance or
herbivory, are filtered out (Parmesan & Yohe 2003). However, this way-out conflicts with
resources and the common time period of one to three growing seasons given for most studies. Also long-term data known from tree-ring series (e.g. Woodhouse 2003) cannot be
transferred to non-woody plants, because woody plants represent different systems that may
often show lagged or prolonged responses (Orwig & Abrams 1997).
A possible approach to these issues is ‘herb-chronology’, the analysis of annual rings
in the root xylem of perennial forbs (e.g. Dietz & Ullmann 1997; von Arx & Dietz 2006). In
this method, annual radial root increments (hereafter referred to as ‘ring width’) are used as
proxies for annual vegetative growth. Given the relatively long lifespan of some perennial
forb species of up to 50 years (Schweingruber & Poschlod 2005; von Arx, et al. 2006), lifetime series of ring width patterns (hereafter referred to as ‘growth trajectories) contain valuable information on past growth of individual plants that can be compared between different
habitats and in the relative context of each individual plant. Moreover, diameter of vessels in
annual rings may be closely linked to moisture availability in the respective years (von
Wilpert 1990; von Arx & Dietz 2005; cf. Table 1), and therefore allows to investigate impacts
of changed precipitation regimes on plant growth. Herb-chronology is widely applicable, because about two thirds of all perennial forbs that grow in the northern seasonal zone and
possess a persistent main root produce at least fairly clearly developed annual rings (e.g.
Dietz & Ullmann 1997; Dietz & Schweingruber 2002). Thus, herb-chronology is an elegant
handle on the study of lifetime phenotypic responses of perennial forbs to climate change,
particularly with regards to changes in temperature and moisture availability.
n = 10
R2 = 0.42
F = 13.5**
Vessel cross-sectional area (µm2)
R2 = 0.18
x: 6.0*
n = 10
Vessel cross-sectional area (µm2)
Altitude (m asl)
Fig. 1 Variation of vessel cross-sectional area (VCA) as obtained with ROXAS analysis (von Arx &
Dietz 2005). a) Silene vulgaris at a (dry) gravel and a (moist) forest site (Davos, Switzerland, 1700 m
asl), b) Penstemon venustus across an altitudinal gradient (Wallowa Mountains, Oregon, USA). The
90% quantile of each individuals VCA was used for analysis, because the diameter of the wide earlywood vessels was supposed to be more influenced by environment than the narrow latewood vessels.
The proposed project aims to evaluate the importance of phenotypic plasticity in growth and
functional root anatomy of perennial forbs to climate change. In addition, phenotypic plasticity in these traits will be used to detect and quantify individual responses to climate change at
the present time.
To this end, spatial changes of moisture across natural gradients are used as proxies
for the predicted temporal changes at a given site (“space-for-time” substitution; e.g. Dunne,
et al. 2004). To quantify differences in plasticity, lifetime growth and functional root anatomy
of several perennial forb species will be investigated using herb-chronological methodology.
In parallel, I will infer from manipulation of moisture as a key factor on its importance for
growth. Thus, accurate but short-term experimental data will be used to calibrate observational lifetime data of perennial forbs, an integrated approach that has been repeatedly
called for (cf. Shaver, et al. 2000; Beier 2004; Dunne, et al. 2004). Long-lived perennial forbs
growing in marginal habitats that have experienced relaxation from abiotical constraints will
be analysed to detect within-individual responses to climate change.
Questions and related hypotheses
Among co-occuring perennial forb species, are there differences in the phenotypic
plasticity of lifetime growth and functional root anatomy across a moisture gradient?
What is the importance of moisture for changes in growth patterns and patterns of
functional root anatomy? Is a particular lifetime growth pattern related to a particular
pattern of functional root anatomy?
 While general patterns across moisture gradients resembles among different perennial forb species, phenotypic plasticity differs between species, possible making
some species better adapted for future climate change than others.
 At the moist margin and in the centre of their distribution range, species will show
no consistent or linearly declining lifetime growth trajectories, indicating either favourable growth conditions or the confounding effects of different micro-habitats. At
the dry margin of their distribution range, species will show curvilinear lifetime
growth trajectories because of the overriding effect of the unfavourable macrohabitat conditions (cf. von Arx, et al. 2006; Fig. 2a).
Fig. 2 Hypothesized shape of growth trajectories during plant life (a) and the corresponding distributions of vessel diameters (b) across a moisture gradient. Grey-shaded areas in a) represent the establishment period with mainly vegetative growth.
 Depending on their position within the species distribution range, plants adjust their
distributions of vessel diameters in a way to maximize water transport efficiency and
minimize the risk of vessel cavitation (Fig. 1; Arnold & Mauseth 1999; Gorsuch, et
al. 2001; Hacke & Sperry 2001). At the dry margin of species range, the risk of cavitation due to drought is greater, and thus narrower, more secure vessels are more
abundant than in the centre and the wet margin of the distribution range (Fig. 2b).
 A distribution of vessel diameters that represents a ‘safety strategy’ corresponds to
a lifetime growth trajectory that shows constraining growth conditions.
In perennial forbs, are changes in growth patterns and patterns of functional root anatomy in response to climate change and shifting climate zones already observable?
 At their southern distribution boundary, old plants show responses to increased
drought stress in terms of strongly declining growth trajectories during the second
part of their life. At the same time, the vessel diameter will decrease to prevent vessel dysfunction due to cavitation. In contrast, at their northern distribution boundary,
growth trajectories of old plants decline only little or not at all due to the relaxing
trend of severing growth factors such as freezing or short vegetation period. Correspondingly, vessels will grow wider, because cavitation risk due to freezing is lower.
These hypotheses are based on the fact that there is a spatial (and temporal) mismatch between a species’ present distribution range and its physiologically possible
distribution range. The same pattern
 In stable habitats with no or only slow succession, young plants (e.g. ≤ 5 years old)
grow wider annual rings and wider vessels than old plants (e.g.  25 years old) in
their corresponding developmental stage (i.e. during their first years of life). Such a
pattern can be interpreted as relaxation of abiotical growth constraints.
Significance of project
This project is likely to make significant contributions to assessing the impact of climate
change on the growth of perennial forbs. The following outcomes of this project may be particularly useful: (1) This proposal addresses the fundamental question of how growth of perennial forbs will be influenced by climatic change. (2) The integration of climate gradients
and experiments will yield reliable estimates of the presence, magnitude and plasticity of
lifetime growth responses to climate change. (3) Combining growth and anatomical data in
one study is a unique approach that may give additional, otherwise not accessible insight
into plant responses to different environments. (4) Comparison of response plasticity between different types of species, especially between native and invasive species, will give
valuable information on which species may profit more or suffer less from climate change;
this will hence provide first directions for possible management actions.
Study site. The natural climatic conditions of the USA are extraordinary well-suited to
study questions related to climate change for two main reasons: First, there are distinct
moisture and temperature gradients that constitute natural, long-term experiments. Second,
the USA are characterized by a great abundance of relatively long-lived perennial forb species that develop clearly demarcated annual rings and that grow across wide environmental
gradients (Dietz & Schweingruber 2002; Dietz & von Arx 2005; von Arx, et al. 2006).
Study species. For the present study, two sets of common native perennial forb species will be included, each of them from a phylogenetically wide range. The first set includes
moderately long-lived species from different families with life spans of mostly five to 15 years
(Table 1). In such species the width of the annual rings and root anatomy is expected to be
most plastic and respond most sensitively to environmental factors (von Arx, et al. 2006).
The second set includes only long-lived species with maximum life spans of not less than 25
years that grow in low density, high elevation habitats.
Herb-chronological root analysis. From all collected plants, the proximal parts of the
persistent main roots will be collected for further analysis. Thin root cross-sections are obtained using a sledge microtome and the woody structures (walls of xylem vessels and lignified walls of parenchyma cells) are stained reddish using HCl-Phloroglucinol. The stained
cross-sections are then photographed through the phototube of a dissecting microscope and
the resulting digital images will be used for further analysis. A root xylem analysis system
(ROXAS; cf. State of own research) will produce data on number and width of individual
annual rings, individual vessel diameters, vessel density and water conductance capacity
within every annual (von Arx & Dietz 2005).
Table 1 The different species sets and sample sizes used in the proposed project. Number of replicates is the number of individuals sampled/measured per species and site.
# Species
# Replicates/Sp.
Example species
61  10
Moisture gradients
32  15-20
Field experiment
Dalea purpurea (Fabaceae), Rudbeckia
occidentalis (Asteraceae), Agastache
urticifolia (Lamiaceae), Potentilla arguta
(Rosaceae), Lupinus sp. (Fabaceae),
Penstemon sp. (Scrophulariaceae), Potentilla pulcherrima
Altitudinal observation
Lupinus sp. (Fabaceae), Penstemon sp.
(Scrophulariaceae), …
Levels across moisture gradients. 2 Levels in moisture treatments
Work package 1: Patterns of growth and functional anatomy across moisture
gradients. (Question 1) Across several moisture gradients, sets of native perennial forb
species (Table 1, set A1) will be randomly sampled each at five to six intervals representing
distinct populations. For the selection of suitable moisture gradients, only precipitation during
the warm season from April to October will be considered. To reduce interfering competition
and disturbance effects, sampling will exclude high-density communities and heavily disturbed habitats. Sampling will be restricted to larger, established plants, since young plants
are not suitable for analysis of lifetime growth patterns (von Arx, et al. 2006). For each plant,
measures of above-ground performance, such as number of shoots and shoot height, will be
recorded in addition to herb-chronological data. Diameters of the populations of vessels will
be analysed using VC50, an index that gives the percentage of the population’s vessels that
remain water filled and conductive after water stress has caused the population to lose half
its conducting capacity (Mauseth & Stevenson 2004). To account for different site characteristics, factors such as land management and soil type will be considered as co-factors in
statistical analysis.
Work package 2: Calibration of responses of growth and functional anatomy.
(Question 1 & 2) To assess the importance of moisture for phenotypic responses in growth
patterns and patterns of functional root anatomy, a common garden experiment will be performed. To this end, individuals of the species of work package 1 (Table 1, set A2) will be
carefully excavated in early autumn 2007 across natural moisture gradients. To avoid unwanted age-effects, plants of all ages will be included. Leaving roots intact, plants will be
planted into pots (width  height = 20  30 cm) with original soil. All pots will be placed in a
plot of the experimental garden facility provided by the University of Arizona, where they will
be randomly distributed. Plants will overwinter without further treatment, except for occasional watering and moderate weeding, if necessary. During a four-month period from May
to August 2008, I will impose a soil moisture treatment. Therefore, I will use mobile plastic
tents constructed of polyethylene supported by flexible tubing (cf. Wookey et al. 1993; Marion et al. 1997) as rain shelters. Compared to other solutions this device has the advantage
of low (about 10%) reduction in photosynthetic active radiation (PAR; Marion et al. 1997) and
its cost effectiveness. The tents will be constructed such as to allow good airflow (open
base) and prevent a considerable increase in air humidity. The tents will be removed from
the plots most of the time to minimize unwanted side effects. Plants will be subjected to ei-
ther of three treatments: 1. normal water supply (corresponding to an annual mean of ca.
800-1000 mm, ‘control’), 2. moderate water supply (corresponding to an annual mean of ca.
600-700 mm, ‘moderate’) and 3. scarce water supply (corresponding to an annual mean of
ca. 400-500 mm, ‘dry’). To simulate changes in precipitation patterns, periods between watering will be varied between treatments (every third, fourth and sixth day for ‘control’, ‘moderate’ and ‘dry’ treatment, respectively) while keeping quantities per watering constant.
Equally distributing the individuals from the different sites across the (original) natural gradients to these three treatments will help to distinguish between ecotypic and phenotypic variation (Stinson 2004). Plants will then be analysed with herb-chronological methodology. For
each plant, measures of above-ground performance, such as number of shoots and shoot
height, will be recorded in addition to herb-chronological data. Some words on how to calibrate with observational data!
It can be seen that the relation between soil water potential and tracheid diameter is
very close during periods of severe drought. (Wilpert 1991)
Tree rings are likely to record deficits more faithfully than surfeits, since climate control
of tree-ring growth works through the most limiting factor. (Hughes 2002)
Site chronologies from near moisture borders have a strong relationship to precipitation (Hughes 2002)
Work package 3: Presently existing responses to climate. (Question 2). The part
of this work package dealing with indications for already existing phenotypic responses when
comparing southern and northern marginal habitats will use and further analyse data available from the moisture gradient and species set A1 (cf. Table 1). For the second sub-project
that investigates already existing responses of individuals to climate change, careful selection of sample sites is crucial. Only sites where growth conditions have been stable as to
competition, disturbance or nutrient availability are suitable. Mountain slopes above tree line
sometimes meet these demands, because they are often unmanaged and succession of
vegetation may be slow or even absent. Moreover, high-altitude regions are expected to be
particularly strongly affected by climate change (Root, et al. 2003), therefore, possible phenotypic responses may be particularly strong, as well. Possible responses will be further
enhanced, because plants generally grow older at higher altitudes (Körner 2003; von Arx, et
al. 2006), thus individuals will be subjected to greater changes in climate during their lifetime.
At least five different, long-lived forb species (attaining life spans of approx. 25 years) will be
sampled at distinct locations (Table 1, set B). To avoid undesirable cohort effects, about 30
to 40 individuals of all age classes will be collected at each site and analysed with herbchronological methodology.
Expected publications
Based on prior experience and the kind of data to be generated, at least three publications in
international peer-reviewed journals should result from the proposed project. Possible titles
Phenotypic responses of lifetime growth and functional anatomy to climate change in
perennial forbs
How climate change affects growth of individual perennial forbs
Current state of own research
My main research interest has thus far been agroecology and plant ecology. In my diploma
thesis at the Geobotanical Institute ETH under the supervision of Prof. P. J. Edwards and Dr.
H. Dietz I used GIS to investigate spatial distribution patterns of plant diversity. We found
that the border structures in a typical farmland area support most of the species while making up for only minor parts of the area. Vegetation diversity (both alpha and beta diversity)
was negatively related to land-use intensity, thus stressing the importance of border structure for vegetation diversity in agroecosystems (von Arx, et al. 2002).
During my PhD at the Geobotanical Institute ETH supervised by Prof. P. J. Edwards
and Dr. H. Dietz I established the methodological basis for the wider ecological use of herbchronological analysis.
In the main study I could experimentally verify the annual nature of growth rings in perennial forbs (von Arx & Dietz 2006). Therefore, plants from nine unrelated forb species were
grown from seed and subjected to competition and clipping treatments, and anatomical developments in the roots of the individuals were tracked during five growing seasons. A second objective was the application of herb-chronology in age- and growth-related questions in
plant ecology. In a correlational study we showed large-scale coherent climatic imprints on
the growth of herbaceous plants in lowland and mountain habitats in the USA and in alpine
habitats in Switzerland (Dietz & von Arx 2005) in response to the exceptionally warm summer in 1998. These results provided first evidence that growth of herbaceous plants responds rapidly and sensitively to climatic fluctuation. In an observational study we gained
post hoc insights into changes of the growth strategy of perennial forbs in response to the
changing environmental conditions along a 1000-m altitudinal gradient in the Wallowa Mountains (OR, USA). With increasing altitude, plants from the three included species grew older
but slower, and the lifetime growth trajectories changed from a linear decline towards a curvilinear relationship with highest growth increments in mid-life, reflecting a shift towards a
more conservative life history strategy (von Arx, et al. 2006). A third objective was the standardization and improvement of herb-chronological analysis. Therefore, I developed a root
xylem analysis system (ROXAS) that automatically analyzes digital images of root crosssection. ROXAS detects xylem vessels, identifies and measures annual rings, determines
plant age and provides additional anatomical parameters such as vessel size, vessel density
or percentage of area occupied by vessels. In an evaluation of ROXAS, anatomical parameters varied substantially between the five investigated perennial forb species, indicating anatomical adaptations to the constraints of the specific habitats (von Arx & Dietz 2005).
ROXAS is currently further developed during a nine-month post doc lasting until end of
April 2007 that is financed by the VELUX foundation, to improve its functionality and userfriendliness. At the beginning of the proposed project, an improved version of ROXAS will be
available and play a fundamental role in the analysis of root cross-sections (cf. Herbchronological root analysis).
Choice of hosting institute
The group of Prof. Malcolm K. Hughes at the Arizona Laboratory of Tree Ring Research
(LTRR; http://tree.ltrr.arizona.edu) is one of the leading groups in tree ring research with much
expertise in climate change research. As such, Prof. Hughes is a contributor of the IPCC
Third Assessment Report (TAR). The LTRR is equipped with all necessary materials for
herb-chronological analysis (except for …). LTRR includes 11 principal investigators and
their staff researchers (6), post-doctoral fellows (3) and graduate students (18). Two of the
postdoctoral fellows and 3 of the graduate students work directly with Professor Hughes.
One of these doctoral students that uses herb-chronology as a method in her thesis, I am
already in exchange. Professor Hughes is also associated with the School of Natural Resources (University of Arizona, Tucson), where Prof. Steve Archer’s Savanna/Woodland
Ecology Lab is based (http://ag.arizona.edu/research/archer/index.html ) Professor Archer and his
group will be an ideal complement to give valuable input, since he works on grass/woody
plant dynamics in changing environments, with a focus on dry habitats.
Joining the Arizona group thus offers an invaluable opportunity to draw upon extensive
knowledge, engage in fruitful cooperation and work in a highly stimulating research environment.
Training-research ratio
During the proposed work, I can fully use my experience in observational studies and in the
analysis of annual rings in the roots of perennial forbs (herb-chronology) while I will be invasive to a new field: climate change. The training aspect will be substantial particularly for
functional root anatomy and interpretation of growth responses in the light of climate change.
For the observational and experimental study, on the other hand, I feel already well
equipped. This would be the first time for me to work in a group that is mainly concerned
with climate change and I expect to engage in interesting and helpful discussions particularly
regarding interpretation of results. After the proposed project I would be able to investigate a
wide range of ecological questions with cutting-edge technique.
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Time line (18 months)
10 11 12
Observational sampling 1 (set A1)
Field experiment (set A2)
Observational sampling 2 (set B)
Root analysis
Data analysis and writing
Intesive phase
Less intensive phase
10 11 12