Effect of woody-plant encroachment on livestock production in North and South America

Abstract

A large fraction of the world grasslands and savannas are undergoing a rapid shift from herbaceous to woody-plant dominance. This land-cover change is expected to lead to a loss in livestock production (LP), but the impacts of woody-plant encroachment on this crucial ecosystem service have not been assessed. We evaluate how tree cover (TC) has affected LP at large spatial scales in rangelands of contrasting social-economic characteristics in the United States and Argentina. Our models indicate that in areas of high productivity, a 1% increase in TC results in a reduction in LP ranging from 0.6 to 1.6 reproductive cows (Rc) per km(2). Mean LP in the United States is 27 Rc per km(2), so a 1% increase in TC results in a 2.5% decrease in mean LP. This effect is large considering that woody-plant cover has been described as increasing at 0.5% to 2% per y. On the contrary, in areas of low productivity, increased TC had a positive effect on LP. Our results also show that ecological factors account for a larger fraction of LP variability in Argentinean than in US rangelands. Differences in the relative importance of ecological versus nonecological drivers of LP in Argentina and the United States suggest that the valuation of ecosystem services between these two rangelands might be different. Current management strategies in Argentina are likely designed to maximize LP for various reasons we are unable to explore in this effort, whereas land managers in the United States may be optimizing multiple ecosystem services, including conservation or recreation, alongside LP.

Keywords: food production; global change; social–ecological systems.

Fig. 1.

Main environmental gradients (mean annual precipitation and mean annual temperature) in the US and Argentinean rangelands. Rangelands in this paper are defined as those areas encompassing the regular and regime mountain divisions of prairie, savanna, temperate and subtropical desert, and steppe, according to Bailey’s ecoregions (1). Within these areas, our work focused on those counties with mean annual precipitation values between 600 and 1,260 mm (Methods and Fig. 2). For both areas, national (bold lines) and county (thin lines) borders are shown. In the United States, state borders are also shown (bold lines).

Fig. 2.

LP, NPP, and TC for our study counties. Rangelands not included in the analyses (in gray) are those counties with an annual precipitation less than 600 mm or more than 1,260 mm (light gray) or those counties with less than 1,000 km² in rangelands or less than 25% of their total area in rangelands (dark gray; Methods).

Fig. 3.

Response models of LP to NPP and TC in the US and Argentinean rangelands. The equations for the response models are shown in Table 1. The red area indicates the NPP range where the impact of TC on LP is negative, whereas the green area indicates the positive effect.