COS 123-9
Population dynamics across tree elevational ranges: implications for climate change responses
Predicting how, and how quickly, species geographic ranges may respond to climate change is a current focus in ecology and conservation. Accurate predictions of range shifts will require an explicit consideration of how population growth rates depend on individual vital rates (i.e., growth, survival, and reproduction) that may vary across ranges, as range shifts are fundamentally the result of increasing population growth rates at leading range edges and decreasing population growth at trailing range edges. Thus, to assess which demographic processes (if any) will constrain or facilitate range shifts, we examined the population dynamics of a dominant conifer species (Abies amabilis) across its elevational range on Mt. Rainier, Washington (USA). Specifically, we assessed 1) how range position influences tree growth, survival, reproduction, and ultimately, population growth rates; 2) the sensitivity of population growth rates to individual vital rates; 3) how projected shifts in vital rates due to climate change may influence future range dynamics; and 4) how stage distributions at leading range edges (primarily young individuals) and trailing range edges (primarily old individuals) can influence population projections.
Results/Conclusions
We found that tree growth declined towards upper range edges, but survival and reproduction were not correlated with range position. Population growth rates were also not correlated with elevation (mean lambda 0.996). This consistency in population growth rates across elevation probably results from the low sensitivity of population dynamics to growth (elasticity range 0.003 – 0.02) and reproduction (elasticity range 0.003 – 0.01), compared to the high sensitivity to survival (elasticity range 0.08-0.43) . For example, the effect on population growth rates from a 1% change in survival would be roughly equivalent to a 30% change in growth or an 80% change in reproduction. Overall, these results suggest that the geographic range of this species will be slow to respond to climate change, unless climate change affects survival rates. Our models also indicate that population stage structure can modify the importance of different vital rates in controlling population growth rates under climate change. For example, shifts in growth could be particularly important for newly establishing populations at upper range edges, primarily composed of small individuals. Conversely, shifts in survival would have larger impacts on contracting populations at lower range edges as they are primarily composed of larger individuals.