A Global Assessment of Long-Term Greening and Browning Trends in Pasture Lands Using the GIMMS LAI3g Dataset

Benjamin Cook,Stephanie Pau

doi:10.3390/rs5052492

Abstract

Pasture ecosystems may be particularly vulnerable to land degradation due to the high risk of human disturbance (e.g., overgrazing, burning, etc.), especially when compared with natural ecosystems (non-pasture, non-cultivated) where direct human impacts are minimal. Using maximum annual leaf area index (LAImax) as a proxy for standing biomass and peak annual aboveground productivity, we analyze greening and browning trends in pasture areas from 1982–2008. Inter-annual variability in pasture productivity is strongly controlled by precipitation (positive correlation) and, to a lesser extent, temperature (negative correlation). Linear temporal trends are significant in 23% of pasture cells, with the vast majority of these areas showing positive LAImax trends. Spatially extensive productivity declines are only found in a few regions, most notably central Asia, southwest North America, and southeast Australia. Statistically removing the influence of precipitation reduces LAImax trends by only 13%, suggesting that precipitation trends are only a minor contributor to long-term greening and browning of pasture lands. No significant global relationship was found between LAImax and pasture intensity, although the magnitude of trends did vary between cells classified as natural versus pasture. In the tropics and Southern Hemisphere, the median rate of greening in pasture cells is significantly higher than for cells dominated by natural vegetation. In the Northern Hemisphere extra-tropics, conversely, greening of natural areas is 2–4 times the magnitude of greening in pasture areas. This analysis presents one of the first global assessments of greening and browning trends in global pasture lands, including a comparison with vegetation trends in regions dominated by natural ecosystems. Our results suggest that degradation of pasture lands is not a globally widespread phenomenon and, consistent with much of the terrestrial biosphere, there have been widespread increases in pasture productivity over the last 30 years.

Highlights

Pasture, defined as land cover used to support domesticated grazing livestock, represents the most widespread anthropogenic land use today [1]
We focus our analyses around the following research questions: (1) How strong, widespread, and significant are greening and browning trends in global pasture lands? (2) What is the contribution of changes in climate to these trends? (3) How do trends in pasture areas compare with regions dominated by natural vegetation?
Median increases across pasture grid cells were on the order of 5%–10%, and the most spatially extensive greening was found in South America, Africa, and localized areas of Asia and Australia

Summary

Introduction

Pasture, defined as land cover used to support domesticated grazing livestock, represents the most widespread anthropogenic land use today [1]. [1,2], and pastures provide 38% of the global net primary productivity appropriated for meat consumption and production [4]. Given their importance for both subsistence and commercial agriculture, it is critical to understand the extent to which pastures are experiencing long-term increasing (greening) or decreasing (browning) productivity trends and whether these changes can be attributed to climatic, anthropogenic, or other factors. Global assessments using remotely sensed vegetation datasets have documented widespread greening and increased vegetation growth and productivity for many regions and ecosystems (e.g., [5,6,7,8]), including some pasture areas [9,10]. Degraded pasture lands have been documented locally in many regions, including Africa [20], Asia [24], North America [25], and South America [26]

Methods

Results

Discussion

Conclusion