Seasonal and interannual relations between precipitation, surface soil moisture and vegetation dynamics in the North American monsoon region

The North American monsoon (NAM) region in northwestern Mexico is characterized by seasonal precipitation during the summer that leads to a major shift in ecosystem processes. Seasonal greening in the semiarid region is important due to its impact on land surface conditions and its potential feedback to atmospheric and hydrologic processes. In this study, we analyzed vegetation dynamics using remotely-sensed Enhanced Vegetation Index (EVI) images over the period 2004-2006 for the Río San Miguel and Río Sonora basins, which contain a regional network of precipitation and soil moisture observations. Results indicate that changes in vegetation greenness are dramatic for all ecosystems and are directly related to differences in hydrologic conditions. Vegetation responses depend strongly on the plant communities, with the highest greening occurring in mid-elevation Sinaloan thornscrub, which also exhibited the largest greenness-precipitation ratio (GPR), a measure of the plant capacity to convert precipitation into biomass. Analyses of the spatial and temporal persistence of EVI fields are used to distinguish the spatial organization of the vegetation response during the NAM. Correlation of vegetation greenness and accumulated monthly precipitation increased with the number of preceding months, while the correlation between greenness and surface soil moisture was equal to that of precipitation for the current month and lower than precipitation for longer lagged periods. Comparisons across ecosystems indicate that different plant water use strategies exist in response to hydrologic variations and are strongly controlled by elevation along semiarid mountain fronts.