Forests cover 30% of the Earth’s terrestrial surface and have large influences on biogeochemical processes and provide a wide range of ecosystems services. However, throughout the world, an increasing number of tree species are reported to be declining in abundance due to large-scale tree mortality and/or reduced regeneration. Many factors have been implicated as drivers of tree decline, but stress caused by elevated temperatures and drought associated with anthropogenic climate change have emerged as important factors. Understanding the dynamics and spatial extent of tree decline is of great importance given the essential role that trees play in ecological systems.
Quaking aspen (Populus tremuloides) is ideally suited for studying the dynamics of tree decline: 1) it is the most widespread tree species in North America; 2) it inhabits a wide range of environments that are exceptionally variable in topography and climate; and 3) previous research has shown that its performance can be quite variable, exhibiting declines in some parts of its range and no changes in others. Using long-term data from a continent-wide plot network, we have evaluated mortality, growth and recruitment rates of aspen across its range to better understand the vulnerability of this species to global change.
Results/Conclusions
Our data summarize long-term patterns in mortality, growth and recruitment rates of aspen in forest inventory networks established by state, provincial and federal agencies in Canada and the U.S. Mortality rates of adult aspen increased substantially over the past 20-50 years, with patterns persisting in stands of different ages, densities, and compositions. In contrast, recruitment rates of adults decreased through time, with patterns varying with forest composition and age structure. To a lesser extent, growth rates of aspen also decreased through time, although they varied less with composition and stand structure.
Averaged over the past 20 years, demographic rates of aspen varied greatly among the five biomes that the tree inhabits in North America. For example, recruitment and growth rates were substantially lower in coniferous and boreal forests compared to three other biome types. Within all biomes, we explore the importance of recent climatic anomalies and topographic heterogeneity in predicting demographic rates of aspen.
In summary, our analyses of the long-term dynamics of aspen across its entire geographic range indicate that aspen populations are experiencing region-specific shifts in background demographic rates, with changes most pronounced in regions most affected by recent climate change (i.e., arid and high-latitude environments).