Long-term trends in vegetation on Bureau of Land Management rangelands in the Western United States

Authors: KLEINHESSLINK, ANDREW - University Of Montana; KACHERGIS, EMILY - Bureau Of Land Management; McCord, Sarah; SHIRLY, JUSTIN - Bureau Of Land Management; HUPP, NICOLE - Bureau Of Land Management; CARLSON, JOHN - Bureau Of Land Management; MORFORD, SCOTT - University Of Montana; JONES, MATTHEW - University Of Montana; SMITH, JOSEPH - University Of Montana; ALLRED, BRADY - University Of Montana; NAUGLE, DAVID - University Of Montana

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2022
Publication Date: 12/15/2022
Citation: Kleinhesslink, A.R., Kachergis, E., McCord, S.E., Shirly, J., Hupp, N.R., Carlson, J.C., Morford, S.L., Jones, M.O., Smith, J.T., Allred, B.W., Naugle, D.E. 2022. Long-term trends in vegetation on Bureau of Land Management rangelands in the Western United States. Rangeland Ecology and Management. 87:1-12. https://doi.org/10.1016/j.rama.2022.11.004.
DOI: https://doi.org/10.1016/j.rama.2022.11.004

Interpretive Summary: New remote sensing tools enabled us to conduct the first ever assessment of trends in vegetation cover and production for all BLM rangeland allotments spanning ~1 million km2. Trends revealed striking increases in cover and production of annual forbs and grasses and widespread expansion of trees into rangelands. Emblematic of this rapid invasion was a doubling of allotments in which annual cover now exceeds that of perennials. Findings provide strong inference that the increases in annuals and corresponding declines in bare ground are linked, with annuals invading the interstitial bareground between larger perennials and shrubs (Fig. 7). Tree cover also rapidly expanded in many ecoregions, including the Northern Great Plains where it is expected to double by 2040 if current trends continue (Fig. 5). Remote sensing revealed the highly patchy nature of tree expansion into rangelands, and provided evidence that trees are primarily expanding outward from existing stands. The preponderance of variance in trends at the sub-allotment scale reinforces the idea that rangelands are defined by their heterogeneity in space and time. The advent of wall-to-wall RS data empowers managers with the capability to better target future management to hotspots of change.

Technical Abstract: The U.S. Bureau of Land Management (BLM) manages publicly owned rangelands that support vital open space for recreation, habitat for biodiversity and resources for livestock production. We used newly developed remote sensing tools to conduct the first ever assessment of trends in vegetation cover and production for all BLM rangeland allotments spanning ~1 million square kilometers. Trends revealed striking increases in cover and production of annual forbs and grasses, a troubling decline in perennial cover and widespread expansion of trees. Annuals increased in six ecoregions, and their cover and production now exceed that of perennials on over 1,600 BLM allotments, marking a fundamental shift in the ecology of these public lands. The most dramatic rise in annuals was in the Western Cold Desert of Nevada and parts of surrounding states where annual production has tripled. In contrast, we found declines in perennial cover in seven ecoregions and declines in bare ground in five. Trends in annuals were negatively correlated with trends in bare ground but not with trends in perennials, suggesting that annuals are filling in bare ground rather than displacing perennials. Tree cover has increased in five of eight ecoregions, confirming that woodland expansion is a widespread threat to rangelands across the West. Woodland expansion was most rapid in the vicinity of existing tree stands. Vegetation trends varied more within allotments than between allotments or between BLM field offices, a finding which highlights the value of wall-to-wall remote sensing data for understanding the full picture of vegetation changes on spatially heterogeneous rangelands. The use of next-generation remote sensing datasets ushers in new possibilities for rapid and spatially detailed monitoring of BLM rangelands. Future integration of remote sensing with plot-level field data will accelerate adaptive management and improve communication with diverse stakeholders.