A critical process that allows multiple, similar species to coexist in an ecological community is their ability to partition local habitat gradients. The mechanisms that underlie this separation at local scales may include niche differences associated with their biogeographic history, differences in ecological function associated with the degree of shared ancestry and trait-based performance differences, which may be related to spatial or temporal variation in habitat. In this study we measured traits related to water-use, growth and stress tolerance in mature trees and seedlings of three oak species (Quercus alba L., Quercus falcata Michx. and Quercus palustris Münchh). which co-occur in temperate forests across the eastern USA but tend to be found in contrasting hydrologic environments. The three species showed significant differences in their local distributions along a hydrologic gradient. We tested three possible mechanisms that influence their contrasting local environmental distributions and promote their long-term co-existence: (i) differences in their climatic distributions across a broad geographic range, (ii) differences in functional traits related to water use, drought tolerance and growth and (iii) contrasting responses to temporal variation in water availability. We identified key differences between the species in both their range-wide climatic distributions (especially aridity index and mean annual temperature) and physiological traits in mature trees and seedlings, including daily water loss, hydraulic conductance, stress responses, growth rate and biomass allocation. Taken together, these differences explain the habitat partitioning that allows three closely related species to co-occur locally.
Bibliographical noteFunding Information:
We thank the SERC for logistical support, William Brinley and Nathan Phillips for technical assistance in construction of the sapflow sensors and Geoffrey Parker for providing access to the 50-ha plot and for other support. We thank Marilyn Fogel for allowing us to use her former facilities the Geophysical Lab at the Carnegie Institution in Washington, DC for isotopic analyses, and Lauren Urgenson, George Raspberry (posthumously), Roxane Bowden, Kati Dawson Andrea Krystan, and Patrick Neale for technical and other assistance. The water table and soil moisture data were gathered as part of an NSF funded project to Sean McMahon (NSF Grant 1137366) and is curated by Rutuja Chitra-Tarak.
Funding for this project was provided by the Smithsonian Institution and the Oaks of the Americas Project (NSF DEB 1146380).
© 2019 The Author(s). All rights reserved.
- drought stress
- niche partitioning