Plant community structure along semiarid gradients is dependent on not only the spatial heterogeneity in soil water content, but also on physiological relationships, particularly those related to plant water use.

We have studied several aspects of water use by predominant semiarid woodland and forest species, including piñon (Pinus edulis), juniper (Juniperus monosperma), blue grama (Bouteloua gracilis), and ponderosa pine (Pinus ponderosa).

 

• Summary of results on physiology of plant water use

PJ Hillslope

Collectively, these studies highlight that where plants obtain their water from, in conjunction with how they use that water, is important in determining plant community composition along the grassland-forest continuum. Many studies of plant water use in semiarid environments have focused on differences between herbaceous and woody plants as a function of depth. An important finding of our studies is that some woody plants (e.g., juniper) can effectively utilize shallow intercanopy water, and thereby are likely competing with herbaceous plants. Spatial differences in how plants obtain water and spatial heterogeneity in soil water—both vertical heterogeneity with depth and horizontal heterogeneity between canopy and intercanopy patches—are both important in determining plant community structure and dynamics. These concepts can be integrated with current theories for vegetation dynamics in semiarid ecosystems.