Horizontal heterogeneity in the frequency of plant-available water with woodland intercanopy-canopy vegetation patch type rivals that occuring vertically by soil depth

Soil moisture integrates and drives ecohydrological processes in dryland ecosystems. However, despite the central importance of soil moisture, relevant field Studies have not holistically assessed key inter-related aspects of ecohydrological spatiotemporal variation: the threshold-like manner in which soil texture controls the frequency at which soil water is readily available for plants, assessment of horizontal heterogeneity associated with vegetation patches in addition to vertical heterogeneity associated with depth, seasonal variation associated with precipitation type (snow vs rain) and inter-annual variation spanning notably wet and dry periods. We measured soil water content by neutron probe in a semiarid pinon-juniper woodland (Pinus edulis and Juniperus monosperma) in northern New Mexico, USA, over 15 years and evaluated an ecohydrological metric-plant-available water, estimated as the percentage of time that soil water content was sufficiently wet to be generally available to plants. The frequency of plant-available water varied significantly across all variables assessed: precipitation amount (across years or seasons), precipitation type, vertically with soil depth and horizontally with vegetation patch type (canopy patches beneath trees, intercanopy patches between trees and edges between the two patch types). Notably, in many cases, horizontal heterogeneity in plant-available water associated with vegetation patch wits as substantial as vertical heterogeneity associated with depth, yet such horizontal heterogeneity is not included in most ecological or hydrological models. Our results highlight spatiotemporal variation in the frequency of plant-available water that is substantial, often overlooked, and may need to be explicitly considered for predicting dryland vegetation responses to land use and climate change. Copyright (c) 2009 John Wiley & Sons, Ltd.