Is soil wetter beneath trees or in the spaces between them?

Details

Degradation of semiarid ecosystems is a major environmental problem worldwide and is characterized by a decline in the ratio of herbaceous to woody plant biomass. These ecosystems can be viewed as woody canopy patches separated by intercanopy patches, producing an overstory with intermediate closure. Field measurements of patch-scale microclimate, especially near-ground solar radiation and soil moisture, are largely lacking in both degraded and nondegraded ecosystems.

The researchers tested relationships among overstory patterns, near-ground solar radiation, and soil moisture in a semiarid piñon–juniper woodland in northern New Mexico. Its overstory was highly heterogeneous, with approximately 50% canopy cover, and the site was not degraded with respect to ground cover and erosion rates. Measurements were 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 years using time-domain reflectometry (TDR). The researchers analyzed the data using generalized least squares linear models that accounted for spatial autocorrelation and temporal heteroscedasticity.

Time-averaged solar radiation and soil moisture were both spatially autocorrelated at scales up to 4 m (P < 0.05), approximately matching the average lengths of canopy and intercanopy patches and the spatial autocorrelation scale of the overstory canopy/intercanopy pattern (3 m; P < 0.05). Near-ground solar radiation was lower under canopies than between them (P < 0.0001). Within patches, it was lower at canopy centers than canopy edges but higher at intercanopy centers than intercanopy edges (P < 0.0001); it was also lower at north than south canopy edges and lower at south than north intercanopy edges (P < 0.0001). Soil was significantly drier beneath canopies than between them (P < 0.0001), while edges were significantly wetter than centers overall and within each patch type (P < 0.0001).

Based on large differences in snow cover between canopy and intercanopy locations, the researchers attributed soil-moisture heterogeneity primarily to interception and drip by canopies. They attributed spatial autocorrelation in soil-moisture model residuals at scales up to 7 m to woody-plant transpiration at scales corresponding to belowground root distributions. The differences in near-ground radiation and soil moisture were large enough to affect processes such as growth and seedling establishment in woody and herbaceous plants. Because degradation in semiarid shrublands and woodlands appears to result from different impacts on intercanopy and canopy patches, the researchers suggested their findings could help design mitigation and remediation strategies. More broadly, they concluded that the physical presence of woody canopies reinforces microclimate heterogeneity, and that this site's intermediate overstory closure helps bridge studies along the grassland–forest continuum between relatively open savannas and nearly closed-canopy forests.