Research Papers
Hydrologic Vulnerability of Sagebrush Steppe Following Pinyon and Juniper Encroachment

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Abstract

Woodland encroachment on United States rangelands has altered the structure and function of shrub steppe ecosystems. The potential community structure is one where trees dominate, shrub and herbaceous species decline, and rock cover and bare soil area increase and become more interconnected. Research from the Desert Southwest United States has demonstrated areas under tree canopies effectively store water and soil resources, whereas areas between canopies (intercanopy) generate significantly more runoff and erosion. We investigated these relationships and the impacts of tree encroachment on runoff and erosion processes at two woodland sites in the Intermountain West, USA. Rainfall simulation and concentrated flow methodologies were employed to measure infiltration, runoff, and erosion from intercanopy and canopy areas at small-plot (0.5 m2) and large-plot (13 m2) scales. Soil water repellency and vegetative and ground cover factors that influence runoff and erosion were quantified. Runoff and erosion from rainsplash, sheet flow, and concentrated flow processes were significantly greater from intercanopy than canopy areas across small- and large-plot scales, and site-specific erodibility differences were observed. Runoff and erosion were primarily dictated by the type and quantity of ground cover. Litter offered protection from rainsplash effects, provided rainfall storage, mitigated soil water repellency impacts on infiltration, and contributed to aggregate stability. Runoff and erosion increased exponentially (r2  =  0.75 and 0.64) where bare soil and rock cover exceeded 50%. Sediment yield was strongly correlated (r2  =  0.87) with runoff and increased linearly where runoff exceeded 20 mm·h−1. Measured runoff and erosion rates suggest tree canopies represent areas of hydrologic stability, whereas intercanopy areas are vulnerable to runoff and erosion. Results indicate the overall hydrologic vulnerability of sagebrush steppe following woodland encroachment depends on the potential influence of tree dominance on bare intercanopy expanse and connectivity and the potential erodibility of intercanopy areas.

Resumen

La expansión del monte en pastizales de los Estados Unidos (EE.UU.) ha alterado la estructura y función de los ecosistemas de estepa arbustiva. La estructura potencial de la comunidad es aquella en la que los árboles son dominantes, las especies arbustivas y herbáceas declinan y la cobertura de rocas y suelo desnudo aumenta y se torna más interconectada. Investigaciones del Desierto del Sudoeste de los EE.UU. ha demostrado que las áreas debajo de los canopeos de los árboles efectivamente almacenan agua y recursos edáficos mientras que las áreas entre canopeos (el intercanopeo) generan significativamente más escorrentía superficial y erosión. Investigamos estas relaciones y los impactos de la invasión de árboles sobre los procesos de escurrimiento superficial y erosión en dos sitios de la región entre las Rocallosas y las Sierras Madre del Oeste de los EE.UU. Se emplearon metodologías de simulación de lluvias y flujo concentrado para medir infiltración, escurrimiento superficial, y erosión de áreas de intercanopeo y canopeo a escalas de parcelas pequeñas (0.5 m2) y grandes (13 m2). La escorrentía superficial y la erosión derivada del impacto de las gotas de lluvia y flujo laminar, y los procesos de flujo concentrado fueron significativamente mayores en áreas de intercanopeo que en áreas de canopeo tanto a escala de parcelas pequeñas como grandes, y se observaron diferencias sitio-específicas de propensión a la erosión. La escorrentía superficial y la erosión estuvieron determinadas principalmente por el tipo y la cantidad de cobertura basal. La broza ofreció protección de los efectos de las gotas de lluvia, proveyó almacenamiento de agua de lluvia, mitigó los impactos de la repelencia hídrica del suelo sobre la infiltración, y contribuyó a la estabilidad de los agregados. La escorrentía superficial y la erosión aumentaron de modo exponencial (r2  =  0.75 and 0.64) en lugares donde el suelo desnudo y la cobertura de roca fueron superiores al 50%. La cantidad de sedimentos producida estuvo fuertemente correlacionada (r2  =  0.87) con la escorrentía superficial y aumentó de modo lineal en lugares donde el escurrimiento superficial superó los 20 mm h−1. Las tasas de escurrimiento superficial y erosión medidas sugieren que los canopeos de los árboles representan áreas de estabilidad hidrológica mientras que las áreas de intercanopeo son vulnerables al escurrimiento superficial y la erosión. Los resultados indican que la vulnerabilidad hidrológica de la estepa de Artemisia luego de la expansión del monte depende de la influencia potencial de la dominancia de árboles sobre el área de intercanopeo y la conectividad y el potencial erosivo de dichas áreas.

Section snippets

INTRODUCTION

The 10-fold expansion in the density and distribution of pinyon (Pinus spp.) and juniper (Juniperus spp.) woodlands during the past 130 yr has altered the ecological structure and function of shrub steppe ecosystems in the western United States (Burkhardt and Tisdale 1969; Tausch et al. 1981; Tausch and Tueller 1990; Tausch and West 1995; Bunting et al. 1999; Bates et al. 2000; Miller et al. 2000; Roberts and Jones 2000; Miller and Tausch 2001; Miller et al. 2005; Tausch and Hood 2007; Miller et

METHODS

This study is part of the larger Sagebrush Steppe Treatment Evaluation Project (SageSTEP) aimed at investigating the ecological impacts of invasive species and woodland encroachment into sagebrush steppe ecosystems in the Great Basin and the effects of various sagebrush steppe restoration methods, including tree removal (McIver et al. 2010).

Vegetation, Water Repellency, and Soils

The observed preponderance of shrub skeletons in shrub-interspace zones at Marking Corral and Onaqui indicate coarsening of the vegetative structure with increasing tree dominance, and the lack of observed large tree stumps and woody debris imply the emergence of tree co-dominance was through tree encroachment into historic sagebrush steppe (Miller et al. 2005; Tausch and Hood 2007; Miller et al. 2008). Shrub-interspace zones represented approximately 75% of both study areas and were

DISCUSSION

The discretely different hydrologic behavior of intercanopy (shrub-interspace zones) and tree coppice areas in this study and others from pinyon and juniper woodlands (Roundy et al. 1978; Reid et al. 1999; Pierson et al. 2007a; Petersen and Stringham 2008) suggest the hydrologic impact of woodland encroachment on sagebrush steppe likely depends on its effect on the expanse of interspace and bare ground within the intercanopy. Tree coppices across spatial scales in this study represented

MANAGEMENT IMPLICATIONS

This study documents the high spatial variability in hydrologic function of tree and shrub-interspace zones after woodland encroachment into sagebrush steppe, and provides a basis for predicting amplified runoff and erosion as a function of intercanopy area. Sediment yield from the two woodland sites (25% tree cover) in this study was highly affected by runoff rates and surface protection afforded by litter. Relative runoff rates for a given rainfall intensity and initial soil moisture were

ACKNOWLEDGMENTS

The authors wish to thank Jaime Calderon and Matthew Frisby for assistance with data collection. We also thank the Desert Research Institute, Reno, Nevada, for assistance with processing the numerous soil samples.

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This is Contribution Number 15 of the Sagebrush Steppe Treatment Evaluation Project (SageSTEP), funded by the US Joint Fire Science Program.

Mention of a proprietary product does not constitute endorsement by USDA and does not imply its approval to the exclusion of the other products that may also be suitable.

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