Vegetation controls on soil moisture distribution in the Valles Caldera, New Mexico, during the North American monsoon

Abstract

Soil moisture distributions are expected to be closely tied to ecosystem processes in water‐limited environments of the southwest United States. Nevertheless, few studies have addressed how soil moisture varies across grassland to forest transitions frequently observed in semiarid mountain settings. In this study, we quantify the vegetation controls on surface soil moisture by sampling a range of different ecosystems present in the Valles Caldera, New Mexico. Soil and atmospheric variables were measured during a 2‐week field campaign conducted in late July to early August 2005 during the North American monsoon. Field observations were supplemented by a network of continuous instruments used to assess conditions prior to and after the sampling campaign. Results reveal that soil moisture responds directly to summer precipitation events and is mediated by plant interception, which differs across the grassland–forest continuum. The nature of the spatial and temporal variations in soil moisture changes across the different sampled ecosystems: wetlands, riparian forests, grasslands, ponderosa, deciduous and mixed conifer forests. In particular, statistical analyses of soil moisture distributions indicate that distinct regimes (e.g. probability density functions) exist along the semiarid vegetation gradient, which may not be revealed through simple metrics such as the ecosystem average. Ecosystem differences are further elucidated through comparison of the spatial variations in each vegetation type, indicating higher variability in wetland and grassland sites. Copyright © 2008 John Wiley & Sons, Ltd.