Vegetation response to precipitation across the aridity gradient of the southwestern United states

Abstract

Atmospheric water demand affects a variety of factors, including primary production and the terrestrial water balance. Precipitation gradients from arid to humid regions also impact the water balance and play a large role in vegetation dynamics. Focusing on a 23-year period (1989–2011), we examine precipitation during the growing season in conjunction with the Normalized Difference Vegetation Index (NDVI) series for 21 satellite scenes spanning across the southwestern United States. We classify the satellite scenes into three different groups, based on the United Nations Aridity Index (AI). Group 1 is categorized as relatively humid with AI≥0.65, group 2 is intermediate with 0.50 ≤ AI<0.65, and group 3 is relatively dry with AI<0.50. We target three types of vegetation covers: shrubland, pasture, and grassland. On a long-term basis, we find significant positive trends in the NDVI series for all types of vegetation in groups 1 and 2. The magnitude of the trend in NDVI decreases with the aridity level. However, neither the total precipitation nor the number of precipitation events (>3 mm and >13 mm) changed during this time. We also use cross-correlation analyses to establish the lagged behavior of the three types of vegetation in relation to precipitation amount and number of events. The vegetation response is similar between precipitation amount and number of precipitation events. However, in the arid region, we find distinct responses to precipitation depending on the vegetation type. The magnitude and significance of the vegetation response to precipitation patterns increase with environmental aridity. There is thus a meaningful disparity of vegetation behavior in time and space.