Proximity to and identity of plant neighbors affect the water status of Larrea tridentata, a dominant desert shrub

Background/Question/Methods

Arid ecosystems are characterized by water limitation of biological processes. Water status of individual plants in the Sonoran Desert is controlled by precipitation through its affect on soil water dynamics, but competition and facilitation from neighboring plants may also affect soil water availability. Previous studies have shown that plant neighbor associations significantly affect plant rooting depth and transpiration. We studied water status of Larrea tridentata (creosote bush), a drought tolerant evergreen shrub common to the arid southwestern U.S., to address two questions: 1) How does species composition of and distance to neighboring plants affect Larrea’s water status? 2) Are there seasonal differences in the spatial variability of Larrea’s water status? Relative leaf water content samples were collected from ~40 Larrea shrubs per 11 sampling periods. Larrea shrubs were classified into four neighbor associations: 1) growing alone, or growing near 2) other Larrea, 3) tree species including Prosopis velutina (mesquite) or Olneya tesota (ironwood), or 4) Ambrosia deltoidea (bursage). Upon completion of pressure volume curves with a WP4 dew point potentiometer, we converted relative leaf water content to leaf water potential with a simple non-linear model. We analyzed leaf water potential data in a Bayesian framework to address our research questions. 

Results/Conclusions

Larrea’s relative leaf water content and leaf water potential significantly varied between wet and dry seasons as well as different neighbor associations. Moreover, approximately 17% of the variation in leaf water potential was explained by the distance to the nearest neighboring Prosopis, Olneya, Ambrosia, and Larrea. Leaf water potential was lowest during the dry season (mean = -6.4 MPa), and highest during the wet season (mean = -3.1 MPa). Furthermore, site level variation in leaf water potential was greatest during the dry season (CV = 1.08) and lowest during the wet season (CV = 0.82). During both dry and wet seasons, leaf water potential was highest in Larrea whose nearest neighbors were other Larrea (mean = -4.7 MPa [dry season] and -2.6 MPa [wet season]), whereas the neighbor association associated with Larrea’s lowest water potential differed between the dry season (Ambrosia, mean = -8.7 MPa) and wet season (Olneya or Prosopis, mean = 3.7 MPa). These results indicate that recent precipitation patterns as well as the distance to and species identity of the nearest neighbor significantly affect Larrea’s leaf water status, suggesting that these temporal and spatial factors are important to understanding plant water relations in this dominant desert shrub.