Ecohydrological controls on soil moisture and hydraulic conductivity within a pinyon‐juniper woodland

Abstract

The impact of pinyon‐juniper woodland encroachment on rangeland ecosystems is often associated with a reduction of streamflow and recharge and an increase in soil erosion. The objective of this study is to investigate vegetational control on seasonal soil hydrologic properties along a 15‐m transect in pinyon‐juniper woodland with biocrust. We demonstrate that the juniper tree controls soil water content (SWC) patterns directly under the canopy via interception, and beyond the canopy via shading in a preferred orientation, opposite to the prevailing wind direction. The juniper also controls the SWC and unsaturated hydraulic conductivity measured close to water saturation (K(h)) under the canopy by the creation of soil water repellency due to needle drop. We use this information to refine the hydrologic functional unit (HFU) concept into three interacting hydrologic units: canopy patches, intercanopy patches, and a transitional unit formed by intercanopy patches in the rain shadow of the juniper tree. Spatial autoregressive state‐space models show the close relationship between K(h) close to soil water saturation and SWC at medium and low levels, integrating a number of influences on hydraulic conductivity.

Spatial gradients in ecohydrologic properties within a pinyon‐juniper ecosystem

“…(Seyfried, 1991;Pierson et al, 1994;Wilcox et al, 2003). Our finding of lower SWC in the subcanopy would seem to support previous findings of interception by Pinus and Juniperus trees (Owens et al, 2006;Lebron et al, 2007), although low soil moisture content is not a reliable indication of canopy interception in hydrophobic soils. On the other hand, measurements of lower K(h) in the subcanopy when compared to the intercanopy would appear to contradict findings from previous infiltration studies in semi-arid rangeland environments.…”

“…Soils are developed from sandstone parent material, to form the Rizono-Rock outcrop complex (loam, mixed (calcareous), mesic, Lithic Ustic Torriorthents) and are generally thin and poorly developed. Texture of the soils was found to be loamy sand with similar texture within both the subcanopy soil and intercanopy (Lebron et al, 2007). Soil organic matter was higher under vascular plant canopies, where rich organic O and slight A1 horizons develop, than the intercanopy zone.…”

“…Recent studies have shown woody plants can affect soil microclimate well into the intercanopy region through shading and interception, while affecting under ground resources through root uptake (Breshears, 2006;Lebron et al, 2007). In a transect study with a Pinus tree and a Juniperus tree as endpoints, Lebron et al (2007) attributed a gradient in soil moisture between the canopy and intercanopy HFUs to interception, and modified the existing HFU model to address meteorologically-driven directional gradients in ecohydrological processes. They observed a similar gradient in unsaturated hydraulic conductivity away from the Juniperus canopy into the intercanopy, but it was not clear whether the result was solely dependent on soil moisture.…”

“…For example, in an early study of rangeland ecohydrology, Seyfried and Wilcox (1995) hypothesized that improved infiltration capacity could extend beyond the canopy edge and thus have a significant role in patch to landscape scale infiltration processes (Ludwig et al, 2005). Recent studies have shown woody plants can affect soil microclimate well into the intercanopy region through shading and interception, while affecting under ground resources through root uptake (Breshears, 2006;Lebron et al, 2007). In a transect study with a Pinus tree and a Juniperus tree as endpoints, Lebron et al (2007) attributed a gradient in soil moisture between the canopy and intercanopy HFUs to interception, and modified the existing HFU model to address meteorologically-driven directional gradients in ecohydrological processes.…”

Fragmentation Effects on Soil Aggregate Stability in a Patchy Arid Grassland

“…Furthermore, 60-90% of burned tree plots generated runoff from the dry-and wet-run simulations whereas none and 50-75% of unburned tree plots produced dry-and wet-run runoff at Marking Corral and Onaqui, respectively. Our results support previous work showing soil water repellency is a natural phenomenon in tree canopy areas on unburned and burned piñon and juniper woodlands in the Great Basin (Lebron et al 2007;Madsen et al 2008;Pierson et al 2010;Robinson et al 2010;Madsen et al 2011Madsen et al , 2012Pierson et al 2013;Williams et al 2014). Further, we find the presence of soil water repellency and its effect on infiltration in tree canopy areas are not necessarily fire-created, but rather are exacerbated by fire removal of litter.…”