Spatial gradients in ecohydrologic properties within a pinyon‐juniper ecosystem

Abstract

The influence of woody vegetation and biological soil crusts on infiltration capacity is one of the several uncertainties associated with the ecohydrologic effects of woody plant encroachment into arid and semi-arid land systems. The objective of this study was to quantify the effects of Utah juniper (Juniperus osteosperma) and pinyon pine (Pinus edulis) on subcanopy and intercanopy ecohydrologic properties. We measured soil sorptivity, unsaturated hydraulic conductivity [K(h)], soil water content (SWC), and water repellency along radial line transects from under Juniperus and Pinus trees into the centre of the intercanopy space between trees. In the subcanopy, litter mounds, hydrophobic soils, and roots all appear to contribute to preferential flow to below-surface soils via wetted patches. For both Juniperus and Pinus, K(h) was significantly lower in the subcanopy than the intercanopy average; however, line transect measurements did not show distinct boundaries in K(h) between the two regions. K(h) increased by eight-fold across a gradient extending outward from near the edge of the canopy to approximately 2 times the canopy radius (CR). This suggests that the influence of these species on hydrologic properties extends significantly into the intercanopy region. Analysis of biological soil crust cover within the intercanopy showed that beyond the gradient zone, increasing structural development of biological soil crust was associated with increased K(h). Furthermore, these results indicate that the distance from the canopy and direction should be considered in the assessment and modelling of woody plant and biological soil crust influence on infiltration capacity.

Horizontal heterogeneity in the frequency of plant‐available water with woodland intercanopy–canopy vegetation patch type rivals that occuring vertically by soil depth

“…Because the effects of woody plants on water budget may be greater when foliage is of higher density, 513 evergreen and extending closer to the ground surface, we hypothesize that our results may be applicable to other piñon-juniper woodlands and potentially other savanna and woodland systems that share similar foliar characteristics (Breshears and Ludwig, 2009). Horizontal heterogeneity in soil water content at shallow depths has been documented in a variety of systems Rambal, 1988, 1993;Belsky et al1989a,b;Breshears et al, 1997;Guo et al, 2002;Kropfl et al, 2002;Madsen et al, 2008), with soil moisture being greater in canopy than in intercanopy patches at some times or places, and the converse occurring at other times or places. Our results here are noteworthy in that they translate this heterogeneity into plant-available water, quantify the large differences at the patch scale ( Figure 9) and provide probabilities of occurrence.…”

“…Several aspects of the water budget that do differ between canopy and intercanopy patches likely are contributing to the observed horizontal heterogeneity in plant-available water. Canopy patches generally receive reduced precipitation inputs due to foliar interception of precipitation (Skau, 1964;Collings, 1966;Owens et al, 2006), and this likely affects heterogeneity in soil water content Madsen et al, 2008), particularly in association with interception-related differences in snow cover . The large reduction in plant-available water from the upper to the lower soil layer of canopy patches (Figure 9) is likely due in part to interception effects.…”

“…Cornet et al, 1992). In addition, edges between canopy and intercanopy patches can be important transition zones that, in some cases, may have the most soil moisture due to canopy drip Madsen et al, 2008). Importantly, both the vertical and the horizontal aspects of soil moisture heterogeneity may be important in determining the distribution and abundance of plant functional types and their responses to land use change (e.g.…”

A review of concentrated flow erosion processes on rangelands: Fundamental understanding and knowledge gaps

“…Prolonged storage at the ground surface allows water to slowly infiltrate, even in the presence of water repellent soils (Leighton-Boyce, Doerr, Shakesby, & Walsh, 2007;Pierson et al, 2010;Pierson et al, 2013a;Pierson, Robichaud, Moffet, Spaeth, & Williams, 2008;Pierson, Williams, Kormos, & Al-Hamdan, 2014;Williams et al, 2014). Hydraulic conductivity and infiltration rates can be as much as 25-to 30-fold lower for water repellent versus wettable soils (DeBano, 1971;Madsen, Chandler & Belnap, 2008). The litter layer in vegetated areas buffers repellency effects on infiltration by trapping water input and allowing it to slowly infiltrate via macropores and breaks in the water repellent layer or slow wetting of the soil profile (Doerr, Shakesby & Walsh, 2000;Meeuwig, 1971;Pierson et al, 2008a;Williams et al, 2014a).…”

Spatial patterns of infiltration vary with disturbance in a shrub-encroached woodland

“…thin and smooth crusts) have been known to generate run-off due to their waterrepellency properties (Yair, 1990), some others could actually increase water retention by forming micro-catchments (Yair et al, 2011), analogous to litter dams. Increasing structural development of biological crust cover, generally characterized by thicker and darker-colored crusts, has been recorded to improve soil water-absorbing capacity (Madsen et al, 2008). Their composition, however, might be random, likely generating high infiltration variability at small scale.…”