The role of population demography in modeling plant community dynamics under future climate scenarios

Background/Question/Methods Understanding the potential location and composition of plant communities under future climate scenarios is the first step toward making informed predictions about patterns of biodiversity, ecosystem function, carbon stores, inter alia.  This information has immediate consequences for conservation strategy, carbon management, and  other socio-ecological policy.  Most area-based dynamic global vegetation models (DGVMs) do not explicitly include parameters for plant demographic processes such as reproduction, establishment and regeneration. These processes are likely sensitive to changing climate, so including them in plant populations models using future climate scenarios may influence outcomes. The objective of this study was to determine if including demographic processes in a DGVM affected the predicted plant responses to climate change.  General Circulation Models (GCMs) have predicted that snowfall in our study sites in the Sierra Nevada mountains (California, USA) will likely change.  Snow is the important hydrologic input in this system and plants are sensitive to changes in both snow depth and duration.  We observed reproduction and establishment rates of co-dominant pine species, Pinus contorta and Pine jeffreyi, as well as two dominant woody shrubs (Artemesia tridentata and Pursia tridentata).  We planted greenhouse-germinated Pinus seedlings on plots where snow was manipulated to be either above, equal to, or below ambient snow levels (snow levels modified by snow fences).  Using these experimentally derived demographic data, we created rules and parameters for a ‘regeneration niche’ in a prototype DGVM, Biomap. Regeneration was modeled as a seeding probability function resulting from reproductive carbon allocation of plants already within the area, as well as a small seeding probability of any other plant type that could regenerate in that area under the climate conditions summarized at the end of each model year.  We ran Biomap, both including and excluding the regeneration niche, to determine if regeneration changes predicted plant population outcomes under future climate scenarios. 

Results/Conclusions Generally, when Biomap included regeneration, it predicted greater woody expansion into areas with decreased snow and predicted a species shift by one pine species becoming dominant.  Pinus establishment was influenced by microclimate conditions, and was higher under shrubs.  Shrub microclimates were higher in moisture and nitrogen, and this effect was increased in the high snow plots.  There appears to be an indirect effect of snow on tree regeneration, mediated by snow’s effects on shrubs.  Models that include demographic processes, especially when calibrated with experimental data, may be capable of more accurate vegetation predictions under future climate scenarios.