Impact of woody-plant encroachment on animal production in U.S. grasslands
A large fraction of the world grasslands and savannas are undergoing a rapid shift from herbaceous to woody-plant dominance. Woody-plant encroachment (WPE) is expected to lead to a loss in pasture-based livestock production in these areas. As a response, brush management has been widely used to reduce the cover of encroaching shrubs during the last decades, although its cost-efficiency is debatable. The objective of our work was to assess the effects of WPE on animal stocking rate, which was used as a proxy of animal production, on U.S. grasslands by combining remote sensing data, biological data and modeling tools. Towards this goal, we first quantified WPE from remotely sensed data and obtained measures of stocking rate (i.e., cattle stocking rate) from USDA census data. We then modeled the impact of WPE on stocking rate under the general model SR = ƒ (ANPP, WPE), where stocking rate (SR) is function of aboveground net primary productivity (ANPP) and WPE. In order to account for the scale-dependency of the relationships, we developed independent models: one at the county scale and one at the farm scale.
Our results indicated a negative relationship between WPE and stocking rate in the U.S. grasslands, although the strength of the relationship varied with mean annual precipitation. The response of stocking rate to both ANPP and WPE was stronger in areas with precipitation values above 600 mm/yr. At the county level of analysis, a variance partition analysis showed that PPT was the main driver of SR, but that WPE had a notable impact (SR = 0.00195 + 0.30730*ANPP – 0.27739*WPE, R2=23.1%; pure effects= 13.14% and 3.5% respectively, joint effect=6.43%). Models at the two spatial scales of analysis showed similar patterns. Our results support the hypothesis that the negative impact of WPE on stocking rate occurs via reduction of palatable biomass rather than via a reduction in aboveground net primary productivity. Our study provides a spatially explicit quantification of the reductions of stocking rate due to WPE and can be used to assist in the cost analysis for brush management.