Landscape dynamics of Great Basin pinyon-juniper woodlands: expansion, or regional decline?

Pinyon-juniper woodlands in various regions of the southwestern US have experienced pronounced diebacks that are associated with climate change. However, such massive declines have not yet been reported for similar woodlands of the Great Basin, where land management continues to emphasize large-scale tree removals to reverse historical woodland expansion. We report on recent trends in woodland dynamics for the Nevada Great Basin, integrating information from a remote sensing analysis using the Landsat archive (1984-2016) with a resampling study (2005-2015) of 98 field plots across 11 mountain ranges. The remote sensing study found that recent drought-related die-off, wildfire and management treatments have balanced expansion, resulting in little net change in tree cover. Across the 34,000-km² study area, 12% showed canopy cover decline, 13% showed canopy cover increase, and 75% had no significant change. Areas experiencing canopy cover loss were more common at lower elevations, on south-facing slopes, and on sites with greater climatic water deficits. The field study found substantial tree mortality, an order of magnitude greater than reported background levels, with stem mortality and canopy dieback more likely in hotter, drier sites and in localized patches of higher tree density. Thus, the recent trajectory of landscape change in Great Basin woodlands highlights the potential for large-scale decline, should regional drought trends continue. Woodland expansion into cooler, wetter sites at higher elevations and on north-facing slopes, combined with increased drought mortality at lower elevations and drier sites, likely reflects shifting tree species distributions in response to climate change. The dominant management paradigm for Great Basin pinyon-juniper woodlands - one of "landscape restoration from woodland expansion" - needs to be broadened to achieve resilience of the overall landscape mosaic to climate change, fire regime shifts, and other stressors.