Herbaceous production lost to tree encroachment in United States rangelands

Researchers quantify forage yield gap caused by woody encroachment

The ecological impacts of woody plants expanding onto U.S. rangelands are well documented. When trees move in, grassland wildlife move out, herbaceous plants are displaced, water supplies and soil moisture decline, forage production (grasses and forbs) for livestock and wildlife drops, and the risk of severe wildfires increases. These impacts affect western sagebrush-steppe rangelands and the Great Plains grasslands alike, where much of the landscape is privately owned and used for ranching.

New research from University of Montana researchers affiliated with Working Lands for Wildlife (WLFW) quantifies the economic impacts of lost herbaceous production throughout the American west. This research was supported by the USDA’s Natural Resources Conservation Service’s (NRCS) Conservation Effects Assessment Project – Wildlife Component, the Arkansas Game and Fish Commission, and the Bureau of Land Management.

Led by Scott Morford, the team tapped into the power of the Rangelands Analysis Platform to track changes in vegetation cover and biomass production across western rangelands from 1990 to 2019. The researchers analyzed and mapped these data to understand how herbaceous production (grass and forb biomass) changed when trees moved in and converted grasslands to woodlands and forests. In 2023, the team updated their analysis to include forage production losses through 2022.

The team then quantified the “yield gap” caused by tree encroachment for each county in 17 western states. Yield gap refers to the difference between the actual herbaceous production (after trees had moved in) and potential herbaceous production (in the absence of trees). The team used the gap in grassland yield to estimate the economic value of lost forage in terms of tonnage and dollars.

Across all western U.S. rangelands, tree expansion now creates an annual yield gap of roughly 5-6%. These large year-over-year losses are similar in magnitude to impacts on commodity crops caused by extreme drought but have a longer-lasting impact. The team estimated the total value of lost production due to expanding trees at $6.17 billion from 1990 to 2022. In 2022 alone, the yield gap exceeded $200 million in lost forage.

The team also found that tree cover increased in more than 25% of western rangelands during this 30-year period. Grasslands saw an 85% increase in tree cover, with roughly 8% of tree-free grasslands transitioning to woodlands, in turn elevating the vulnerability of adjacent tree-free rangelands to future degradation.

Working Lands for Wildlife is directly addressing woody expansion across both landscapes through its biome-specific frameworks for conservation action. These plans call for proactive management of woody species through a Defend the Core, Grow the Core, Mitigate Impacts strategy.

This conservation strategy flips the old, expensive, and ineffective way of thinking about this threat. Instead of focusing time and limited dollars on treating areas that have already transitioned to a woodland state, this strategy focuses on preventing treeless cores from being taken over by trees. Once core areas are protected, managers can expand intact cores by stopping further tree expansion. In areas where rangelands have fully transitioned to woodlands, conservation partners can enact efforts to mitigate impacts and help communities and wildlife adapt.

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Herbaceous production lost to tree encroachment in United States rangelands

Abstract: Rangelands of the United States provide ecosystem services that benefit society and rural economies. Native tree encroachment is often overlooked as a primary threat to rangelands due to the slow pace of tree cover expansion and the positive public perception of trees. Still, tree encroachment fragments these landscapes and reduces herbaceous production, thereby threatening habitat quality for grassland wildlife and the economic sustainability of animal agriculture.

Recent innovations in satellite remote sensing permit the tracking of tree encroachment and the corresponding impact on herbaceous production. We analyzed tree cover change and herbaceous production across the western United States from 1990 to 2019.

We show that tree encroachment is widespread in U.S. rangelands; absolute tree cover has increased by 50% (77,323 km2) over 30 years, with more than 25% (684,852 km2) of U.S. rangelands experiencing tree cover expansion. Since 1990, 302 ± 30 Tg of herbaceous biomass has been lost. Accounting for variability in livestock biomass utilization and forage value reveals that this lost production is valued at between $4.1 and $5.6 billion U.S. dollars.

Synthesis and applications: The magnitude of impact of tree encroachment on rangeland loss is similar to row-crop conversion, another well-known and primary mechanism of rangeland loss in the U.S. Prioritizing conservation efforts to prevent tree encroachment can bolster ecosystem and economic sustainability, particularly among privately-owned lands threatened by land-use conversion.

Citation: Morford, S. L., Allred, B. W., Twidwell, D., Jones, M. O., Maestas, J. D., Roberts, C. P., & Naugle, D. E. (2022). Herbaceous production lost to tree encroachment in United States rangelands. Journal of Applied Ecology, 00, 1– 12.

Permanent URL: https://doi.org/10.1111/1365-2664.14288

Acknowledgements: This work was made possible by the USDA-Natural Resources Conservation Service’s (NRCS) Conservation Effects Assessment Project – Wildlife Component, the NRCS’s Working Lands for Wildlife, the Arkansas Game & Fish Commission through cooperative agreement 1434-04HQRU1567, and the Bureau of Land Management. NVIDIA provided hardware used in this analysis through their Academic Hardware Grants program.