Physiological stress and interspecies competition determine tree species distributions across a peatland to forest gradient in southeast Alaska

Background/Question/Methods

Abiotic and biotic processes shape the distribution and abundance of species. Biotic interactions likely determine the upward limit of plant distributions, while physiological tolerance of abiotic conditions likely determines the lower limit. Explaining how some species grow across environmental gradients while others cannot is central to describing species’ distributions and identifying niches. The topography of southeast Alaska forms a hydrologic gradient of ecosystem types, ranging from emergent wetlands to upland forests. Some species (Pinus contorta ssp. contorta and Picea sitchensis) are restricted to specific ecosystem types, while others (Tsuga heterophylla) grow across the entire gradient. This information is crucial to regional management and essential for understanding species’ response to predicted climatic conditions. To identify the factors and processes most important in determining species’ distributions, field sites were established across this gradient, and data was collected on species composition and biomass, water table position, nutrient dynamics, and light availability. A Bayesian modeling framework was then used to identify the factors driving the abundance and distribution of forest tree species across a hierarchy of spatial scales. Canopy cover and biomass of each tree species were included as parameters to assess the role of competition in determining distributions.

Results/Conclusions

Drivers varied among species and across the hierarchy of spatial scales. Species presence or absence at any given location was determined by the ability to tolerate stressful peatland conditions or to compete for resources under more favorable conditions. At the smallest sampling extent, the number of days where the water table was within the plant-rooting zone and interspecies competition most strongly influenced species’ presence or absence. At the largest sampling extent, the ecosystem type, results suggest that topographic position and presence of competing tree species are most important in determining the distribution of tree species across this landscape. These results support the notion that size and extent of sampling frame strongly influence study conclusions. Alteration of model parameters to account for conditions predicted under climate change showed a shift in tree species dominance and distribution across this gradient. These results support the use of this hierarchical modeling framework for evaluating the influence of abiotic and biotic factors on species distributions across different environments and scales. This model also proves to be a valuable method for assessing the impact of climate change on distributions and for predicting shifts in canopy dominance, tools invaluable for management and conservation of forest ecosystems.