Population dynamics of epiphytic orchids in a metapopulation context

Background and Aims Populations of many epiphytes show a patchy distribution where clusters of plants growing on individual trees are spatially separated and may thus function as metapopulations. Seed dispersal is necessary to (re)colonize unoccupied habitats, and to transfer seeds from high- to low-competition patches. Increasing dispersal distances, however, reduces local fecundity and the probability that seeds will find a safe site outside the original patch. Thus, there is a conflict between seed survival and colonization. Methods Populations of three epiphytic orchids were monitored over three years in a Mexican humid montane forest and analysed with spatially averaged and with spatially explicit matrix metapopulation models. In the latter, population dynamics at the scale of the subpopulations (epiphytes on individual host trees) are based on detailed stage-structured observations of transition probabilities and trees are connected by a dispersal function. Key Results Population growth rates differed among trees and years. While ignoring these differences, and averaging the population matrices over trees, yields negative population growth, metapopulation models predict stable or growing populations because the trees that support growing subpopulations determine the growth of the metapopulation. Stochastic models which account for the differences among years differed only marginally from deterministic models. Population growth rates were significantly lower, and extinctions of local patches more frequent in models where higher dispersal results in reduced local fecundity compared with hypothetical models where this is not the case. The difference between the two models increased with increasing mean dispersal distance. Though recolonization events increased with dispersal distance, this could not compensate the losses due to reduced local fecundity. Conclusions For epiphytes, metapopulation models are useful to capture processes beyond the level of the single host tree, but local processes are equally important to understand epiphyte population dynamics

Experimental Evaluation of Seed Limitation in Alpine Snowbed Plants

Background: The distribution and abundance of plants is controlled by the availability of seeds and of sites suitable for establishment. The relative importance of these two constraints is still contentious and possibly varies among species and ecosystems. In alpine landscapes, the role of seed limitation has traditionally been neglected, and the role of abiotic gradients emphasized. Methodology/Principal Findings: We evaluated the importance of seed limitation for the incidence of four alpine snowbed species (Achillea atrata L., Achillea clusiana Tausch, Arabis caerulea L., Gnaphalium hoppeanum W. D. J. Koch) in local plant communities by comparing seedling emergence, seedling, juvenile and adult survival, juvenile and adult growth, flowering frequency as well as population growth rates lambda of experimental plants transplanted into snowbed patches which were either occupied or unoccupied by the focal species. In addition, we accounted for possible effects of competition or facilitation on these rates by including a measure of neighbourhood biomass into the analysis. We found that only A. caerulea had significantly lower seedling and adult survival as well as a lower population growth rate in unoccupied sites whereas the vital rates of the other three species did not differ among occupied and unoccupied sites. By contrast, all species were sensitive to competitive effects of the surrounding vegetation in terms of at least one of the studied rates. Conclusions/Significance: We conclude that seed and site limitation jointly determine the species composition of these snowbed plant communities and that constraining site factors include both abiotic conditions and biotic interactions. The traditional focus on abiotic gradients for explaining alpine plant distribution hence appears lopsided. The influence of seed limitation on the current distribution of these plants casts doubt on their ability to readily track shifting habitats under climate change unless seed production is considerably enhanced under a warmer climate

Fixed effect coefficients (FE, together with their standard errors, SE) of linear (juvenile and adult growth) or generalized linear (all other rates) mixed effects models relating the performance of experimental plants of four snowbed species of the northeastern Calcareous Alps (Austria) to their occurrence at experimental plots (occupancy) and to the biomass of the surrounding vegetation.

<p><i>t</i> is FE/SE and <i>p</i>-values are for one sided Student's <i>t</i>-tests of the hypotheses that occupancy has a positive and that biomass a negative effect on the respective rates.</p

Relative contribution of demographic processes to population growth rates λ of experimental snowbed plant populations of the northeastern Calcareous Alps (Austria).

<p>Values represent elasticities, which were standardized following <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0021537#pone.0021537-Franco1" target="_blank">[42]</a>.</p><p>The sum of the elasticity values equals one for each species.</p

CATS: A high‐performance software framework for simulating plant migration in changing environments

Abstract Considering local population dynamics and dispersal is crucial to project species' range adaptations in changing environments. Dynamic models including these processes are highly computer intensive, with consequent restrictions on spatial extent and/or resolution. We present CATS, an open‐source, extensible modelling framework for simulating spatially and temporarily explicit population dynamics of plants. It can be used in conjunction with species distribution models, or via direct parametrisation of vital rates and allows for fine‐grained control over the demographic and dispersal processes' models. The performance and flexibility of CATS is exemplified (i) by modelling the range shift of four plant species under three future climate scenarios across Europe at a spatial resolution of 100 m., and (ii) by exploring consequences of demographic compensation for range expansion on artificial landscapes. The presented software attempts to leverage the availability of computational resources and lower the barrier of entry for large‐extent, fine‐resolution simulations of plant range shifts in changing environments