Mark C Vanderwel, Vassily S Lyutsarev, and Drew W Purves
4 June 2013
The geographic distributions of different forest types are expected to shift in the future under altered climatic conditions. At present, the nature, magnitude and timing of these shifts are uncertain because we lack a quantitative understanding of how forest distributions emerge from climate- and competition-related variation in underlying demographic processes. Forest dynamics result primarily from the manner in which the physical environment and competition for limiting resources affect tree growth, mortality and recruitment. We sought to uncover the relative importance of these processes in controlling the geographic limits of different forest types.
We parameterized a climate-dependent forest dynamics model with extensive observations of tree growth, mortality and recruitment from forest inventory data. We then implemented the resulting demographic models in simulations of joint population dynamics for seven plant functional types (PFTs) across the region. By removing various climate effects in a series of simulation experiments, we assessed the importance of climate-dependent demography and competition in limiting forest distributions.
Distributions that emerged from simulated population dynamics approximated the current distributions for all seven PFTs well and captured several known patterns of succession. Temperature-related increases in mortality determined the southern boundaries of three out of four boreal and northern temperate PFTs, whereas temperature-related decreases in recruitment controlled the northern limit of all three southern temperate PFTs. Changes in growth rates and competitor performance had only minor effects on the distribution limits of most PFTs.
Our results imply that dynamic global vegetation models, which are widely used to predict future vegetation distributions under climate change, should seek to more appropriately capture the observed climate sensitivity of mortality and recruitment. Understanding the mechanisms controlling forest distributions will enable better predictions of their future responses to climate change.
In Global Ecology and Biogeography
Rosie Fisher, Nate McDowell, Drew Purves, Paul Moorcroft, Stephen Sitch, Peter Cox, Chris Huntingford, Patrick Meir, and F. Ian Woodward. Assessing uncertainties in a second-generation dynamic vegetation model due to ecological scale limitations, New Phytologist, August 2010.
Emily R Lines, David A Coomes, and Drew Purves. Influences of Forest Structure, Climate and Species Composition on Tree Mortality across the Eastern US, PLoS-One, PLoS, October 2010.
John P. Caspersen, Mark C. Vanderwel, William G. Cole, and Drew W. Purves. How stand productivity results from size- and competition-dependent growth and mortality, PLoS ONE, December 2011.
Mark C Vanderwel, David A Coomes, and Drew W Purves. Quantifying variation in forest disturbance, and its effects on aboveground biomass dynamics, across the eastern United States, Global Change Biology, Wiley, January 2013.
Drew Purves. The demography of range boundaries vs range cores in Eastern US tree species, Proceedings of the Royal Society Series B, 25 February 2009.
Jeremy W Lichstein, Jonathan Dushoff, Kiona Ogle, Anping Chen, Drew W Purves, John Caspersen, and Stephen W Pacala. Unlocking the forest inventory data: relating individual-tree performance to unmeasured environmental factors, Ecological Applications, April 2010.
Drew W Purves and Stephen W Pacala. Predictive Models of Forest Dynamics, Science, 13 June 2008.