PS 23-72 - Using longitudinal surveys to assess the distribution and ecohydrological function of pinyon-juniper woodlands in the western USA

Abstract: Across the western USA, woodland communities comprised of Pinyon (Pinus spp.) and Juniper (Juniperus spp.) species (e.g., PJ Woodlands) are infilling within their existing ranges as well as expanding into adjoining shrubland and grassland ecosystems. Expansion of PJ Woodlands may in part be due to alterations to historical fire and climatic regimes with consequences of decreased herbaceous and forb cover and diversity with accelerated risk of soil surface erosion. Satellite-based remote sensing has been a valuable tool for assessing regional PJ Woodlands distribution and modern rates of change through time. However, remote sensing typically lacks the ability to assess entire plant communities (e.g., species-level herbaceous and forbs associating with PJ) with on-the-ground measurements of ecosystem hydrologic stability, both of which are important for conservation planning and land management efforts. Here, we offer plot-level data from multiple longitudinal assessment, inventory, and monitoring frameworks to estimate the current PJ Woodland community distribution and ecohydrologic status across the western USA. Our approach is to operationalize plot-level (~23,000; ~0.2 ha plots) longitudinal surveys from the US Department of Agriculture-Natural Resources Conservation Service National Resource Inventory (NRI), collected on private rangelands from 2004-2020, and US Department of the Interior-Bureau of Land Management Assessment, Inventory, and Monitoring (AIM) plots (~29,000; ~0.3 ha) collected on public rangelands from 2011-2020. Plant community and ecohydrologic surveys extend to the surrounding representative area (0.5-1 ha), while metrics like plant foliar cover are collected along 47.5 m (NRI) or 50 m (AIM) transects.

A total of 7,504 plots contained PJ Woodland species of interest with associated ecohydrologic surveys across 17 western US. A higher density of PJ species occurrence decreased overall species richness (Spearman ρ = -0.11, p < 0.001) and total perennial grass and forb cover (ρ = -0.21, p < 0.001). We found that 52% ± 1.4 (mean ± SE) and 76% ± 1.8 of survey areas were rated with at least a moderate departure from their historical reference condition for hydrologic function and biotic integrity, respectively, while only 42% ± 1.9 for soil/site stability. This preliminary assessment suggests that 1) PJ Woodland expansion and infilling is occurring across the western US with consequences of increased risk of losses to plant species richness and soil erosion, and 2) large scale ground-based surveys like NRI and AIM provide valuable detailed information on plant community dynamics and ecosystem health beyond currently and openly available with satellite-based remote sensing.