Abstract
Pinyon (Pinus spp.) and juniper (Juniperus spp.) woodlands are an important vegetation type in the Great Basin, Colorado Plateau, and southwestern desert regions of the western US that is undergoing substantial changes associated with land management, altered disturbance regimes, and climate change. We synthesized literature on the ecohydrologic impacts of pinyon and juniper tree reductions across plot to watershed scales, short- and long-term periods, and regional climatic gradients. We found that the initial plot- to hillslope-scale ecohydrologic and erosion impacts of tree reduction on pinyon and juniper woodlands by fire, mechanical tree removal, or drought depend largely on: (1) the degree to which these perturbations alter vegetation and ground cover structure, (2) initial conditions, and (3) inherent site attributes. Fire commonly imparts an initial increased risk for hillslope runoff and erosion that degrades over time with vegetation and ground cover recovery whereas tree reductions by mechanical means pose fewer initial negative ecohydrologic impacts. Tree reduction by either approach can enhance understory vegetation and improve site-level ecohydrologic function over time, particularly on sites with an initially favorable cover of native herbaceous vegetation and a cool-season precipitation regime. Understory vegetation and ground cover enhancements appear to increase ecohydrologic resilience of some woodland communities to disturbances such as drought, fire, and insect infestations. In contrast, intensive land use, prolonged drought or repeated burning associated with invasions of fire-prone grasses can propagate long-term site degradation through persistent elevated runoff and erosion rates. Our synthesis suggests the annual precipitation requirement for increases in plot- to hillslope-scale soil water availability for herbaceous enhancement through tree removal likely ranges from 200–400 mm for sites in the Great Basin and northern Colorado Plateau (cool-season precipitation regimes), and, although suggested with great uncertainty, likely exceeds 400 mm for woodlands with rain-dominated precipitation regimes in the southwestern US. Overall, literature is inconclusive regarding tree reduction impacts on watershed-scale changes in groundwater and streamflow. To date, there is little evidence that drought-related changes to vegetation in pinyon and juniper woodlands substantially affect watershed-scale water availability and streamflow at the annual time scale. Our synthesis identifies key knowledge gaps to overcome in improving understanding of the ecohydrologic and erosion impacts of broadly occurring pinyon and juniper tree reductions in the western US.
Highlights
Pinyon (Pinus spp.) and juniper (Juniperus spp.) woodlands are an ecologically important vegetation type throughout the western US owing to their broad distribution, dynamic ecological impacts, and provision of an array of ecosystem services [1,2,3,4]
We address three specific research questions: (1) How do tree reductions affect plot- to hillslope-scale ecohydrologic and erosion processes in the short- and long-term across regional climate gradients? (2) Is there a specific climatic threshold for increases in water availability with tree reduction, and, if so, (3) Do increases in water availability with tree reduction translate to increases in groundwater and streamflow at the watershed scale? To address these questions, we review substantial literature on PJ woodlands, inclusive of our own research, regarding ecohydrologic and erosion impacts of tree reductions associated with management practices, natural disturbances, and changing climate
Wilcox [190] further noted that evapotranspiration is the dominant water-loss mechanism on southwestern US PJ woodlands, that streamflow from these woodlands is typically ephemeral, and that the seasonality of runoff for these landscapes is strongly related to the precipitation regime, with winter flows more common on snowy uplands and high summer flows occurring following intense summer thunderstorms
Summary
Pinyon (Pinus spp.) and juniper (Juniperus spp.) woodlands (hereafter PJ woodlands) are an ecologically important vegetation type throughout the western US owing to their broad distribution, dynamic ecological impacts, and provision of an array of ecosystem services [1,2,3,4]. PJ woodlands are commonly found at elevations ranging from several hundreds of meters to >2000 m above mean sea level throughout the Great Basin, Colorado Plateau, and the southwestern US, inclusive of the mountainous regions in the Sonoran and Chihuahuan Deserts in Arizona and New Mexico (Figure 1) [1,4,5,6,7]. These woodlands occupy approximately 190,000 km across this broad area, a 10-fold increase since the mid to late 1800s [1,2,8,9,10].
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