Neighborhood association affects nocturnal and daytime transpiration fluxes of a common desert shrub, Larrea tridentata

Background/Question/Methods

Larrea tridentata (creosote bush) is an evergreen shrub that is a dominate species throughout the arid, southwestern United States. Many studies have explored Larrea’s physiological behavior including transpiration dynamics, under varying moisture conditions, but few have evaluated the influence of neighboring plants on Larrea’s transpiration dynamics. In addition, recent studies indicate that a significant fraction of Larrea’s total water flux budget can be attributed to nocturnal transpiration, yet nothing is known about how neighborhood associations may affect Larrea’s nocturnal water loss. This study addresses two questions: 1) How do plant neighbor associations affect Larrea’s nocturnal and daytime transpiration dynamics? 2) How important is nocturnal transpiration relative to daytime transpiration for understanding Larrea’s seasonal water loss dynamics? Transpiration was measured across four neighborhood associations: 1) Larrea growing “alone” (e.g. more than 1.5m from neighboring species) and Larrea growing in close proximity (overlapping canopies) with 2) Prosopis velutina (mesquite), 3) Olneya tesota (ironwood), and 4) Ambrosia deltoidea (bursage). Transpiration and stomatal conductance were measured on twelve Larrea shrubs from December 2011 through December 2012 with a portable gas exchange system (LI-6400), and the data was analyzed in a Bayesian model framework to address the two research questions.

Results/Conclusions

Larrea’s daytime transpiration rates were significantly affected by Larrea neighbor associations during the spring, summer, and fall. Larrea growing near small trees, (i.e. Prosopis and Olneya had the highest transpiration rates throughout all three seasons. During summer and spring, the mean transpiration of Larrea growing in association with Prosopis was approximately 40% higher than lone Larrea. Regardless of the neighborhood association, the highest daytime transpiration rates occurred in the winter. During the winter, lone and Larrea paired with Prosopis both had the highest transpiration rate (~3.08 mmol/m²/s), and Larrea paired with Ambrosia had transpiration 20% lower than lone Larrea. The highest nocturnal transpiration rates occurred in winter, and Larrea paired with Olenya had the highest rates, roughly 25% of their daytime rates. Nocturnal transpiration varied significantly between pairs during the spring and summer seasons. In both the spring and summer, Lone Larrea had the highest transpiration, ~30% of the daytime rate. The results indicate that plant neighborhood associations appear to play a significant role in both daytime and nocturnal plant water fluxes. The results also demonstrate that nocturnal transpiration can comprise a considerable portion of plant water budgets with nocturnal fluxes representing approximately 19-30% of daytime rates.