Landscapes Forest and Global Change - ESA - Escola Superior ...

E.S. Meier et al. 2010. Projections of shifts in species distributions
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abiotic factors such as climate, topography and soil may primarily constrain species ranges
under unfavourable growing conditions and here competition is generally low (Bertness and
Callaway 1994). Where abiotic conditions are more favourable, biotic interactions increase, and
competition may then, additionally to abiotic constraints, further determine species range limits
(MacArthur 1972). The constraining effect of interlinked abiotic and biotic processes may not
only be important when predicting current species distributions (Meier, 2010), but may be even
more important when estimating species range shifts due to changing environmental conditions.
Range shifts are mainly determined by the rate of plant establishment, growth and survival at a
new location and dispersal abilities (Higgins et al. 2003), and are hence strongly linked to
abiotic and biotic conditions. Moreover, because most modern landscapes are highly fragmented,
the density of individuals producing propagules is reduced and fewer and more distant sites for
those propagules to colonize are available. This may further slow down migration rates (Iverson
et al. 2004). Studying the interlinked effects of macroclimate, inter-specific competition and
landscape-fragmentation may help to better estimate colonisable suitable habitats of species.
2. Methodology
2.1 Variation partitioning approach
We examined the extent to which the variance in spatial patterns of species explained by species
distribution models (SDMs) can be partitioned among abiotic and biotic predictors, and how
these partitions depend on species characteristics. We fitted generalized linear models (GLMs)
for 11 common tree species in Switzerland using three different sets of predictor variables:
biotic, abiotic, and the combination of both sets. We estimated by variance partitioning the
proportion of the variance explained by biotic and abiotic predictors, jointly or independently.
We then analyzed the linkage of these partitions with species traits using non-parametric tests
(Mann–Whitney U-test and the Kruskal–Wallis test, depending on the number of classes
differentiated).
2.2 Co-occurrence patterns
Further, we analysed correlations between the relative abundance of European beech
(Fagus sylvatica) and three major competitor species (Picea abies, Pinus sylvestris and
Quercus robur) in environmental space, analyzing the variation in correlation along two major
environmental gradients, namely summer rainfall and annual degree-day sum. In a next step, we
projected these co-occurence patterns to geographic space. In a following spatial analysis, we
used generalized additive models (GAM) to predict the spatial patterns of species abundances,
and we evaluated from these models where and how much the simulated F. sylvatica
distribution varied under current and future climates if potential competitor species were in- or
excluded. For the analysis of co-occurrence we used ICP Forest level I data as well as climatic,
topographic and edaphic variables as predictors for modelling the spatial distribution of species
using SDMs.
2.3 Implementing migration rates in SDMs
In a final step, we calibrated SDMs using generalized linear models (GLMs) with ICP forest
level I data and climatic, topographic, edaphic and land-use variables to predict current and
future tree distributions, assuming either no or full migration when projecting to scenarios of
future climate. Additionally, we combined our SDMs with an explicit simulation of dynamic
tree migration rates (i.e., depending on interlinked effects of climate, inter-specific competition
and landscape connectivity). Dynamic migration rates were estimated from a process model
(TreeMig; Lischke et al. 2004) using an intense sensitivity analysis of migration rates along
gradients of climate, species competition and distance between suitable habitats in fragmented
landscapes. We combined the derived migration response with a GIS path cost analysis, to
estimate climatically suitable and colonisable habitats and the rate of expected migration of
Forest Landscapes and Global Change-New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.