Temporal variation of stomatal sensitivity to VPD in western riparian trees

Jessica S. Guo, College of Agriculture and Life Sciences, University of Arizona, Tucson, AZ, Susan Bush, Biology, University of Utah, Salt Lake City, UT and Kevin R. Hultine, Research, Conservation and Collections, Desert Botanical Garden, Phoenix, AZ

Background/Question/Methods
Increasing atmospheric vapor pressure deficit (VPD) and decreasing soil moisture are known to imperil plant hydraulic function, but the role of VPD in stomatal regulation can be difficult to isolate. Western riparian trees are an ideal system in which to investigate stomatal regulation because they are subject to a wide range of ambient VPD yet lack soil moisture limitations. Recent evidence also suggests that plant hydraulic strategies, known to vary along a continuum across taxonomic and functional groups, can also fluctuate in response to changing environmental conditions. Here, we synthesize sap-flux-scaled canopy conductance data to elucidate the extent to which stomatal sensitivity to VPD changes over the course of a growing season. Using sap-flux data from nine riparian species located at four sites spanning an elevation gradient in northern Utah, USA, we fit a time-varying empirical model of stomatal conductance to VPD in a hierarchical Bayesian framework. This approach yielded seasonal estimates of stomatal sensitivity (sensu Oren) wherein values < 0.6 are considered anisohydric (less sensitive to VPD) and values ≥ 0.6 are considered isohydric (more sensitive to VPD).
Results/Conclusions
Seven species were found to have reference conductance (Gref, conductance at VPD = 1 kPa) that varied with growing degree days (gdd), an index of heat accumulation. Interestingly, the two non-native species at the lowest-elevation site, Tamarix ramosissima and Elaeagnus angustifolia, exhibited significantly positive correlation between Gref and gdd, whereas five species exhibited significantly negative correlations. Thus, as stomatal sensitivity is inversely proportional to Gref, our results showed that the non-native species became less sensitive to VPD during the growing season, while five of the seven native species became more sensitive to VPD during the growing season, indicating significant seasonal variation in whole-plant hydraulic conductance strategies. Perhaps surprisingly, sensitivity to VPD varied greatly among riparian trees with year-round access to groundwater. The lowest-elevation species experienced the highest VPD conditions and yet were the least sensitive to VPD (anisohydric), a combination that results in greater transpiration fluxes. Uniquely, non-native riparian trees become less sensitive to VPD over the growing season, which may have profound implications for the community composition and water budget of critical riparian habitat in a warmer and drier climate.