Background: Understanding ecosystem-level evolutionary responses of dominant riparian trees to climate-induced changes in precipitation may provide insight into global feedbacks to the water cycle. Across a landscape, evapotranspiration is a key ecosystem service, recycling water to promote future precipitation. Plants play a crucial role in this process; transpiration can depend on plant photosynthetic rate, stomata, and rooting depth. These traits are susceptible to selection as they are often genetically-based and considerably variable across environmental gradients.
Question: Have differences in physical limitation (water and energy) driven population differentiation in functional plant traits?
Methods: The Budyko theoretical framework is a hybrid hydrological framework that allows consideration of both energy and mass controls on evapotranspiration rates. The curve provides a reference condition for water balance with an expectation of how precipitation is partitioned into streamflow and evapotranspiration with changing climate. As climate changes, shifts along the curve suggest maintenance of ecosystem function while shifts away from the curve suggest changing function. Operating within this framework, we determine the physical limitations of sites from which trees from 17 populations of Populus angustifolia were collected in the Western United States. All trait and soil measurements were made in a greenhouse.
Results/Conclusions
Results: We show functional trait differentiation among populations of trees along a gradient of physical limitation. Trees from more water-limited sites show significantly longer shoot length and internode length than trees from less water-limited sites. However, there was no difference in internode diameter between trees from the two catchment-states. Additionally, soil pH was higher in pots of trees from more water-limited sites, while soil C:N was higher in pots from sites that are less water-limited.
Conclusions: The functional response of a water catchment to global changes may be moderated by a plant’s role in evapotranspiration. Understanding the evolutionary responses of a dominant riparian tree to climate-induced changes in stream flow, precipitation, and temperature at a macroscale will be crucial to predicting how ecosystems will respond to landscapes largely in drought.