Comparing drought response in two riparian species Platanus occidentalis L. (American sycamore) and Salix nigra Marshall (black willow) for use in riparian restoration

Background/Question/Methods: The restoration of riparian ecosystems in addition to protecting natural waterways has been proposed as a mechanism to buffer the effects of climate change. In the southeast Black Willow (Salix nigra Marshall) is used extensively in riparian restoration. Less information has been collected on American Sycamore (Platanus occidentalis L.) and its potential use for this purpose.  This research compares the ecophysiology of both species looking into the feasibility of using American Sycamore for riparian restoration. We assessed differences in water use (thermal dissipation probes; TDPs), CO₂ assimilation, and endomycorrhizal (AM) root colonization for trees of both species growing in the field alongside a small (< 1 m width) perennial stream on the campus of Kennesaw State University in North West Georgia. Tolerance to drought and inundation, as well as drought tolerance with and without mycorrhizae were assessed for both species under greenhouse conditions. For the field component, TDPs were used on three Platanus and four Salix trees and sap flow was recorded continuously over the summer of 2013. For CO₂ assimilation, A_max was determined from light curves generated using a LICOR 6400 photosynthesis system (n=6 Platanus, n=4 Salix). For AM root colonization, 160 root samples were collected from ten trees of each species. Roots were stained (trypan blue) and the presence of mycorrhizal structures was quantified using the root piece method. Greenhouse experiments were conducted using a randomized block design and subjecting 10 individuals of each species to control, inundation, or drought conditions; and drought with and without the addition of mycorrhizal spores. 

Results/Conclusions: Although both species showed strong correlations of water use with light and vapor pressure deficit (VPD); for VPD a higher correlation coefficient was found for Salix compared to Platanus (Salix r = 0.77; Platanus  r = 0.68; p < 0.001 for both). Also, Salix root samples exhibited higher levels of AM root colonization (Chi squared p < 0.05) compared to Platanus. Under field conditions A_max rates were similar for both species (~ 6 mmolm^-2s^-1), and preliminary results from greenhouse experiments showed no significant difference in A_max among treatments of drought and inundation, but maximum assimilation rates were ½ their field values for Platanus (3 mmolm^-2s^-1) probably due to nutrient deficiency. Our results indicate Platanus as a viable and strong species to use in restoration alongside the more frequently used Salix.