Distribution of Terrestrial Ecosystems and Changes in Plant Community Composition

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1 Distribution of Terrestrial Ecosystems and Changes in Plant Community Composition Abstract Plant communities have been transformed by global changes such as land-use change and biological invasion in recent decades, and climate change will drive further transformation in the coming decades. Plant species can respond to changing climates by shifting their ranges to new areas in order to track optimal conditions and/or by adapting to these changes in situ. Future climates have the potential to alter species productivity, phenology and biotic interactions. In addition, species ranges are expected to shift towards higher latitudes, tracking favorable climate conditions as the planet warms. The same trend is expected along altitudinal gradients, where species are expected to move uphill. However, distributions of communities and ecosystems depend on their individual species responses to climate change, and are therefore more complicated to predict. Future plant community assemblages will differ from the ones that have existed historically, and these changes could themselves influence the rate of global climate change. Keywords Terrestrial ecosystem distribution; plant community composition; distributional shifts; niche envelope models; biotic interactions; niche evolution; adaptive potential Definition Climatic variables strongly influence the distributions and growth rates of plant species. As climates change, plant responses will alter the composition, distribution, and functioning of terrestrial ecosystems. Key Information Altered temperature, precipitation, and resource availability will lead to changes in ecosystem distributions and changes in the composition of plant communities. When large-scale changes in the environment occur, especially in the climate, plants and other organisms can respond in only a few ways. One possibility is that species tolerate or adapt to the new conditions. Alternatively, species' distributions can shift into new areas, such that populations continue to inhabit suitable climates. These range shifts contribute to a shift in the greater ecosystem s distribution and composition. Species unable to tolerate, adapt to, or shift their ranges in response to the new climate will go locally extinct. Climate change is only one of many factors of global change. Land use change, exotic species introductions, anthropogenic nitrogen deposition, and elevated CO (although closely tied to climate change) also affect terrestrial ecosystems. Land use change and climate change are expected to be the two most important factors driving changes in biodiversity over the course of this century; however the relative importance of each differs among biomes. Climate is expected to be most important in biomes with relatively less human disturbance, such as artic, alpine and boreal systems; whereas land use is already an important factor in areas closely tied to human activity, such as grassland, Mediterranean and tropical systems (Sala et al. 000). Therefore, while climate change is an increasingly important driver of global change, it should be considered within a broader context. 1

2 Species Responses to Global Change and the Re-structuring of Plant Communities Climate affects many aspects of plant growth and function. In response to a changing climate, species may modify phenology, thermal tolerance, and dispersal traits. Phenological responses to warming can prolong growing periods and enhance growth in many species. Warming commonly alters plant flowering time and advances bud break. Differences in phenological responses across species can alter inter-specific competitive ability by changing the timing of light, temperature, and water availability at the extremes of the growing season. Phenological changes can also alter plant competition for pollinators or cause life cycle asynchrony between plants and mutualist animals by affecting flowering time and plant distribution. Life cycle asynchrony can disrupt pollination cycles, potentially reducing plant reproductive output and population density. Rising temperature, altered precipitation, elevated atmospheric CO concentration, and increasing anthropogenic nitrogen deposition can affect species diversity patterns within communities, affect competitive relationships among species, and influence trophic interactions. Rising CO levels have been found to facilitate invasion of C grasslands by C 4 woody species, although warming is expected to favor warm season C grasses. Morgan et al. (011) found that 3 4 combined CO enrichment and warming favored higher productivity in C grasses, suggesting future grasslands may be 4 more resilient to C invasion. Furthermore, nitrogen deposition and other global environmental changes are thought to 3 have opened some ecosystems to invasion by non-native plant species (Hellmann et al. 008). These changes result in novel interactions of species that lack a common evolutionary history, making impacts on community composition difficult to predict. At the microevolutionary scale, the high level of local adaptation to climate and the great amount of genetic variation for traits defining species ecological niches suggests a high potential for species to adapt to new climatic conditions. For instance, several studies have reported significant genetic variation of functional traits related to phenology, growth or gas exchange and drought tolerance. However, niche evolutionary potential will require more than just genetic variation in defining traits. It will also strongly depend on constraints imposed by the species realized niche, such as interspecific competition, demographic dynamics, and trait plasticity (Lavergne et al. 009). Projected Species Range Shifts and Biome Distributions As climate change shifts species ranges, it will also alter the distribution of ecosystems and biomes. As the planet warms, species are expected to shift to higher latitudes, tracking favorable climate conditions. Species are largely influenced by their tolerance to temperature and precipitation extremes. For many species, low winter temperatures prevent expansion into higher latitudes and competition with better climatically-adapted species limits expansion into lower latitudes. A similar trend exists along altitudinal gradients. The commonly adopted altitude-for-latitude temperature model suggests that distributional shifts resulting from temperature changes will occur proportionally along altitudinal and latitudinal gradients. Because temperature decreases by 1 C over a ~167m increase in altitude or a ~145km increase in latitude, each 1 C increase in surface temperature should cause ranges to shift ~167m higher and ~145km poleward (Walther, 003). Communities previously relegated to lower elevations have already started moving uphill, replacing communities of higher elevations at their distributional margins. Over the past 50 years, mountain treelines have commonly shifted upwards by up to 130m (Jump et al. 009, Walther 003). This trend will reduce the extent of mountaintop communities, which are not adapted for warmer conditions. Given sufficient warming, many mountain communities may be outcompeted by communities from lower altitudes as mountaintops become more hospitable to more competitive species. Precipitation is another critical component influencing species distributions, and as such, altered precipitation regimes can determine which plant communities occur in a given region (Wu et al. 011). Expected changes in precipitation depend on the region being considered, and sometimes on the climate model, especially in the temperate zone. Differences among model projections add to the uncertainty in predictions of future ecosystem distributions, although general patterns can be observed across models. Inter- and intra-annual variability in precipitation are expected to become more extreme, exposing ecosystems to greater risk of prolonged drought and heat waves (Knapp et al. 008). This will have consequences for community composition, as described previously, but will also make otherwise suitable areas unfit for some ecosystems, thus modifying the poleward and uphill trend expected when only considering changes

3 in temperature. Topography also determines the diversity of microclimates experienced by any ecosystem, and can thus influence distributional shifts. Furthermore, changes in species distributions may be significantly influenced by human habitat modification or a combination of both climate and human disturbances. For example, Hättenschwiler and Körner (1995) have suggested that the high level of nitrogen deposition and the cessation of cattle grazing in the Swiss Central Alps may have enhanced the replacement of Pinus sylvestris seedling populations by Pinus cembra, a species previously unreported in at that altitude. Modeling Impacts of Climate Change Projections of future ecosystem distributions can be based on species distribution models (SDM) of representative species of that ecosystem. SDMs consider the contemporary climate of regions associated with a given species and map future distributions of those species based on where similar climates are expected to exist in the future. Compiling the model projections for several species can suggest possible future distributions of their respective ecosystems. However, SDMs are static models and are limited by their assumptions. This limitation is based on whether the model considers a species fundamental niche (all possible locations where that species could potentially exist considering abiotic factors) or a species realized niche (only locations where that species exists in reality, which is determined by abiotic and biotic factors). While SDMs utilize the observed range, and therefore the realized niche, of the considered species in order to define that species niche, they often only consider the fundamental niche of a species when making projections. Thus, SDMs partially omit the importance of the biotic interactions that can define the realized niche, and therefore the distribution, of the species. For instance, temporary downhill species range shifts in the French mountain forests might be facilitated by competitive release from other alpine species at the range margins brought on by climate-induced disturbance (Lenoir et al. 010). Furthermore, most models do not necessarily include the dispersal patterns and evolutionary potential of species (see Elith & Leathwick 009) although some attempts have been made to include these parameters. Despite their limitations, species distribution models are a useful tool in predicting the impacts of climate change. A variety of abiotic and biotic factors will limit or alter the response of each species to changes in climate. These limitations will put some species at risk of being unable to establish in a region with a suitable climate. For instance, soil characteristics can hamper the otherwise successful establishment of plant species in new regions. Barriers to dispersal, such as habitat fragmentation or a lack of land, can also limit the ability of species to track suitable climates. A lack of natural area will be particularly detrimental to many species mobility in regions with high human populations, while natural barriers will be of greater concern in high latitudes and mountainous regions. In some cases the time required for a species to establish in a new community and spread to others may be too great compared to the rate of climate change. As a result, some species may decline or become extinct. The idiosyncratic responses of individual species to global change can lead to complex and unexpected responses at the community level. Consequently, predicting the composition of future plant communities is difficult. Species-level responses to climate change are likely to result in assemblages of species that have not historically interacted. These new assemblages, known as no-analog communities, could support novel species interactions. No-analog communities may function differently than contemporary communities, and consequently provide different ecological services. Thus, while models can provide an estimate of individual species responses, the communities of the future may be very different from the ones that have been observed historically. Changes in ecosystem distribution and plant community composition feed back to climate through several pathways. Forests store large amounts of carbon; consequently, alterations to their community composition, productivity, or distribution will influence global carbon dynamics and climate. In addition, ecosystems influence climate by influencing the fraction of radiation absorbed by the surface, the rate of evapotranspiration, which contributes to cloud creation and other atmospheric processes and the physical structure of the land surface. 3

4 Figure 1. Relationship between individual species responses, community changes and ecosystem modifications with climate change. Idiosyncratic responses of individual species will create no-analog communities. Feedbacks from terrestrial ecosystems further complicate predictions of future climate. References Elith J, and Leathwick JR (009) Species distribution models: ecological explanation and prediction across space and time. Annual Review of Ecology Evolution and Systematics 40: Hättenschwiler S, and Körner C (1995) Responses to recent climate warming of Pinus sylvestris and Pinus cembra within their montane transition zone in the Swiss Alps. Journal of Vegetation Science 6: Hellman JJ, Byers JE, Bierwagen BG, Dukes JS (008) Five potential consequences of climate change for invasive species. Conservation Biology (3): Knapp AK, Beier C, Briske DD, Classen AT, Luo Y, Reichstein M, Smith MD, Smith SD, Bell JE, Fay PA, Heisler JL, Leavitt SW, Sherry R, Smith B, and Weng E (008) Consequences of more extreme precipitation regimes for terrestrial ecosystems. Bioscience 58(9): Lavergne S, Mouquet N, Thuiller W, and Ronce O (010) Biodiversity and climate change: integrating evolutionary and ecological responses of species and communities. Annual Review of Ecology, Evolution, and Systematics 41: Jump AS, Mátyás C, and Peñuelas J (009) The altitude-for-latitude disparity in the range retractions of woody species. Trends in Ecology & Evolution 4: Lenoir J, Gegout JC, Guisan A, Vittoz P, Wohlgemuth T, Zimmermann NE, Dullinger S, Pauli H, Willner W, and Svenning JC (010) Going against the flow: potential mechanisms for unexpected downslope range shifts in a warming climate. Ecography 33: Morgan JA, LeCain DR, Blumenthal DM, Kimball BA, Carrillo Y, Williams DG, Heisler-White J, Dijkstra FA, and West M (011) C grasses prosper as carbon dioxide eliminates desiccation in warmed semi-arid grassland. Nature 476: Walther G (003) Plants in a warmer world. Perspectives in Plant Ecology, Evolution and Systematics 6: Wu A, Dijkstra P, Koch GW, Peñuelas J, and Hungate BA (011) Responses of terrestrial ecosystems to temperature and precipitation change: a meta-analysis of experimental manipulation. Global Change Biology 17:

5 Additional Recommended Reading Franklin J (009) Mapping Species Distributions: Spatial Inference and Prediction. Cambridge University Press, New York Loarie SR, Duffy PB, Hamilton H, Asner GP, Field CB, Ackerly DD (009) The velocity of climate change. Nature 46: Sala OE, Chapin FS III, Armesto JJ, Berlow E, Bloomfield J, Dirzo R, Huber-Sanwald, E, Huenneke LF, Jackson RB, Kinzig A, Leemans R, Lodge DM, Mooney HA, Oesterheld M, Poff NL, Sykes MT, Walker BH, Walker M, Wall DH (000) Global biodiversity scenarios for the year 100. Science 87: Schweiger O, Biesmeijer JC, Bommarco R, Hickler T, Hulme PE, Klotz S, Kühn I, Moora M, Nielsen A, Ohlemüller R, Petanidou T, Potts SG, Pyšek P, Stout JC, Sykes MT, Tscheulin T, Vilà M, Walther GR, Westphal C, Winter M, Zobel M, Settele J (010) Multiple stressors on biotic interactions: how climate change and alien species interact to affect pollination. Biological Reviews 85: Thuiller W, Albert C, Araújo MB, Berry PM, Cabeza M, Guisan A, Hickler T, Midgley GF, Paterson J, Schurr FM, Sykes MT, Zimmermann NE (008) Predicting global change impacts on plant species distributions: future challenges. Perspectives in Plant Ecology, Evolution and Systematics 9: Williams JW, Jackson ST (007) Novel climates, no-analog communities, and ecological surprises. Frontiers in Ecology and the Environment 5: Michael Schuster Lorena Torres Martinez Jeffrey S. Dukes Purdue University, West Lafayette, USA Department of Biological Sciences, Purdue University, West Lafayette, USA Department of Forestry and Natural Resources, Department of Biological Sciences, Purdue University, West Lafayette, USA DOI: URL: Part of: /_ Global Environmental Change Editor: PDF created on: Dr. Bill Freedman March, 8, 01 1:5 5