As droughts increase in severity and duration following global climate trends, sustaining urban forests in dry regions requires a detailed understanding of their water use. However, heterogeneity, species diversity, and exposure to modified environments with secondary hydrologic and climate feedbacks make the biological processes of urban trees highly uncertain. The resulting water use patterns are highly location-specific, with large inter- and intra-specific variability. Transpiration of urban forests is determined by the responses of leaf stomata to multiple environmental factors. Current mechanistic representations of stomatal dynamics are complex and dependent on empirical parameters that are sometimes not clearly related to particular plant physiological or functional traits. Consequently, landscape planning and management relies on empirical coefficients and qualitative assessments of landscape water requirements. In search of a generalizable, physiology-based approach to assess urban tree transpiration, we applied the optimal stomatal control theory to sap flux datasets collected in situ in the Los Angeles Metropolitan area, CA and Salt Lake City, UT. We used the analytical solution of the optimality problem to represent sap flux responses of water-unlimited, irrigated trees to atmospheric vapor pressure deficit.
Results/Conclusions
The optimal stomatal control model explained the variability of sap flux responses of 192 water-unlimited urban trees to atmospheric vapor pressure deficit (VPD) with an accuracy similar to the commonly used empirical approach. The key parameter of the model – sap flux sensitivity to VPD – was species-specific and location-invariant. Thus, while this parameter varied among 20 studied species by an order of magnitude, the same species studied at different locations in the Los Angeles Metropolitan area (Eucalyptus grandis, Jacaranda mimosifolia, Pinus canariensis, Platanus racemosa, Sequoia sempervirens) and the same species studied in both Los Angeles and Salt Lake City (Gleditsia triacanthos and Platanus acerifolia) had similar sensitivity parameters. Moreover, our analysis allowed for establishing quantitative links between sap flux sensitivity to VPD and functional traits (vulnerability of branch xylem to cavitation and whole-tree water use efficiency) that were in agreement with theoretical predictions. Further work is needed to include more tree species and investigate how model coefficients change in the presence of additional environmental stressors, particularly soil water limitations. This approach is promising for developing a generalized, trait-based and relatively simple model for estimating transpiration of urban forests in disparate regions.