Seasonal stomatal behavior of a common desert shrub and the influence of plant neighbors.

Stomata simultaneously regulate plant carbon gain and water loss, and patterns of stomatal conductance (g(s)) provide insight into water use strategies. In arid systems, g(s) varies seasonally based on factors such as water availability and temperature. Moreover, the presence and species identity of neighboring plants likely affects g(s) of the focal plant by altering available soil water and microclimate conditions. We investigated stomatal behavior in Larrea tridentata, a drought-tolerant, evergreen shrub occurring throughout the arid southwestern United States. We measured g(s) in Larrea over multiple seasons in the presence of neighbors representing different woody species. The data were analyzed in the context of a commonly used phenomenological model that relates g(s) to vapor pressure deficit (D) to understand spatial and temporal differences in stomatal behavior. We found that g(s) in Larrea was affected by neighborhood association, and these effects varied seasonally. The greatest effect of neighborhood association on g(s) occurred during the winter period, where Larrea growing alone (without neighbors) had higher g(s) compared to Larrea growing with neighbors. Larrea's stomatal sensitivity to D and reference conductance (i.e., g(s) at D = 1 kPa) also differed significantly among different neighbor associations. Random effects indicated reference g(s) varied over short time scales (daily), while stomatal sensitivity showed little daily or seasonal variation, but was notably affected by neighbor associations such that neighboring species, especially trees, reduced Larrea's sensitivity to D. Overall, seasonal dynamics and neighborhood conditions appear critical to understanding temporal and spatial variation in Larrea's physiological behavior.