Hydrologic, abiotic and biotic interactions: plant density, windspeed, leaf size and groundwater all affect oak water use efficiency

Plant water use in drylands can be complex due to variation in hydrologic, abiotic and biotic factors, particularly near ephemeral or intermittent streams. Plant use of groundwater may be important but is usually uncertain. Disturbances like fire contribute to complex spatiotemporal heterogeneity. Improved understanding of how such hydrologic, abiotic, and biotic factors affect plant water use is needed not only to address management issues related to land use but also due to climate change, particularly for drylands projected to become warmer and drier such as much of the southwestern USA. Here we focus on the interactive roles of hydrologic, abiotic, and biotic factors in determining plant water use of Gambel oak in ephemeral and/or intermittent riparian areas of ponderosa pine forest in central New Mexico, USA along a fire disturbance gradient with varying oak densities. More specifically, the purpose of this research is to determine: (1) depth to and frequency of groundwater associated with intermittent and ephemeral streams, (2) isotopic indications of whether or not groundwater is used by plants, and (3) plant water use (conductance, transpiration, and water use efficiency) interactions with groundwater. Depth to groundwater, which could be as shallow as 1 m or less, increased with plant density but plant water use efficiency decreased. Photosynthesis:transpiration ratios maximizing water use efficiency for a given successional stage appeared to be also affected by windspeed and leaf size, highlighting interactive effects of hydrologic, abiotic and biotic affects—a finding that may be ecohydrologically relevant for other dryland riparian systems. Published 2010. This article is a US Government work and is in the public domain in the USA.