Millions of hectares in the American Southwest are occupied by Gambel’s oak (Quercus gambelii Nutt.). Native to North America, Gambel’s oak is a deciduous hardwood species existing as a shrub or subdominant tree with the capability of producing large thickets (e.g. Harper et al 1985). Following fire, rapid Gambel’s oak re-growth can form dense thickets suppressing herbaceous and coniferous species resulting in reduced biodiversity which can lead to a Gambel’s oak climax with no conifer regeneration (e.g. Huston 1994). In semi-arid ecosystems of the southwestern United States ephemeral and intermittent streams make up over 80 percent of all streams (NHD; USGS, 2006). It may be that intermittent and ephemeral streams and fire influence the spatial distribution of Gambel’s oak and abundance. The spatial pattern of plant species is often used to determine the presence of competition or interference between species. The objectives of this study are to quantify the presence of shallow groundwater adjacent to ephemeral and intermittent streams, determine the effects of shallow groundwater on Gambel’s oak water use efficiency, distribution and abundance, and determine the existence of competition between Gambel’s oak and ponderosa pine. Three study sites on the Cibola National Forest, New Mexico were used. Within each of the study sites shallow groundwater near intermittent and ephemeral drainages were monitored via water wells. Associated vegetation was measured with modified Whitaker plots and transects. Weather variables were measured via weather stations. Gambel’s oak stomatal conductance, transpiration, carbon isotope fractionation, and water use efficiency were measured. Nearest neighbor analysis of Gambel’s oak and ponderosa pine was measured.
Results/Conclusions
In this study it appears that fire can significantly reduce Gambel’s oak cover and water use efficiency on a large scale, but on small scales it appears that fire may increase groundwater and photosynthetic rates without a rise in transpiration rate. Gambel’s oak can compete well in either upland or riparian areas suggested by the evenly distributed Gambel’s oak patches in the upland and stream locations. Gambel’s oak patch size and distribution was not different between stream and upland locations. This suggests that water is likely not the most limiting resource to Gambel’s oak. Furthermore, there was a relationship between Gambel’s oak and ponderosa pine near intermittent and ephemeral streams suggesting competition. This relationship suggests that water may not be limiting to Gambel’s oak but may be limiting to ponderosa pine.