OVERSTORY-IMPOSED HETEROGENEITY IN SOLAR-RADIATION AND SOIL-MOISTURE IN A SEMIARID WOODLAND

Degradation of semiarid ecosystems is a major environmental problem wo rldwide, characterized by a reduction in the ratio of herbaceous to wo ody plant biomass. These ecosystems can be described as a set of canop y patches comprising woody plants and the intercanopy patches that sep arate them, yielding an overstory with intermediate closure. Field mea surements of microclimate at the scale of canopy patches, particularly for near-ground solar radiation and soil moisture, are largely lackin g from both nondegraded and degraded ecosystems. We tested for relatio nships among spatial patterns of the overstory, near-ground solar radi ation, and soil moisture in a semiarid pinon-juniper woodland in north ern New Mexico that had a highly heterogeneous overstory (approximate to 50% canopy cover) and was not degraded with respect to ground cover and erosion rates. We used measurements taken every 1 m along a 102-m transect-solar radiation indices were estimated monthly and annually using hemispherical photographs, and soil moisture was measured over 4 yr using time-domain reflectometry (TDR)- and analyzed the data using general least squares linear models that accounted for spatial autoco rrelation and temporal heteroscedasticity. Time-averages of solar radi ation and of soil moisture both were spatially autocorrelated at scale s of up to 4 m (P < 0.05), corresponding approximately to the average lengths of both canopy and intercanopy patches and to the scale of spa tial autocorrelation in the canopy/intercanopy pattern of the overstor y (3 m; P < 0.05). For near-ground solar radiation, we found expected spatial variation between patches (canopy < intercanopy; P < 0.0001) a nd within patches for centers vs. edges (canopy center < canopy edge a nd intercanopy center > intercanopy edge; P < 0.0001) and for north vs . south edges (canopy north edge < canopy south edge and intercanopy s outh edge < intercanopy north edge; P < 0.0001). For soil moisture, ca nopy locations were significantly drier than intercanopy locations (P < 0.0001), and edge locations were significantly wetter than center lo cations both overall and within both patch types (P < 0.0001). Spatial heterogeneity in soil mositure was attributed primarily to canopy int erception and drip on the basis of large differences a in snow cover b etween canopy and intercanopy locations. Spatial autocorrelation in th e residuals for soil moisture of up to 7 m was attributed to transpira tion by woody plants at scales corresponding to belowground root distr ibutions. The spatial heterogeneities in near-ground solar radiation a nd soil moisture are of sufficient magnitude to affect biotic processe s of woody and herbaceous plants, such as growth and seedling establis hment. Because land degradation problems in semiarid shrublands and wo odlands appear to result from differential impacts to intercanopy vs. canopy patches, our results can be used to help design effective mitig ation and remediation strategies. More generally, our results demonstr ate how the physical presence of woody canopies reinforces spatial het erogeneity in microclimate and, because our site has intermediate clos ure of the overstory, bridge the gap along a grassland-forest continuu m between related studies in relatively open savannas and in forests w ith nearly closed canopies.