DIFFERENTIAL USE OF SPATIALLY HETEROGENEOUS SOIL-MOISTURE BY 2 SEMIARID WOODY SPECIES - PINUS-EDULIS AND JUNIPERUS-MONOSPERMA

1 Soil moisture in semiarid woodlands varies both vertically with dept h and horizontally between canopy patches beneath woody plants and the intercanopy patches that separate them, such that shallow soil layers in intercanopy locations are wettest, yet few studies have considered both dimensions of spatial variability in testing for acquisition of resources by plants. 2 Three hypotheses were tested relative to the us e of shallow water in intercanopy locations by two coexisting semiarid -woodland tree species, Pinus edulis (a pinon) and Juniperus monosperm a (a juniper): (i) both P. edulis and J. monosperma can use shallow wa ter from intercanopy locations;. (ii) J. monosperma is able to obtain more shallow water from intercanopy locations than P. edulis, and (iii ) the spatial arrangement of the trees influences the amount of water they obtain. Soil moisture and plant water potential (i.e. plant water stress) were measured before and after the addition of water to shall ow depths (0-30 cm) of intercanopy locations for trees of both species in two spatial arrangements: isolated and paired with a contiguous tr ee of the other species. 3 Both species responded to the addition of s hallow water in intercanopy locations, as measured by plant water pote ntial. The response of J. monosperma was significantly greater than th at of P. edulis, as measured by depletion of shallow soil moisture in intercanopy locations and by change in plant water potential per unit change in soil water potential (the difference was not detectable on t he basis of plant water potential alone); in addition, the amount of d epletion was correlated with basal area of J. monosperma but not of P. edulis. The responses were not influenced by spatial arrangement (iso lated vs. paired with a contiguous tree of the other species). 4 The r esults of this study are consistent with differences in the relative a bundances of the two species across locations, suggesting that species differences in ability to use shallow water in intercanopy locations is important in structuring semiarid woodlands. Further, the results s uggest that current theoretical concepts for semiarid ecosystems, whic h ignore either vertical or horizontal variability in soil moisture, m ay be inadequate for predicting changes in the ratio of woody to herba ceous plant biomass, particularly for plant communities with co-domina nt woody species that differ in ability to acquire spatially heterogen eous resources.