Controls on gross production by a semiarid forest growing near its warm and dry ecotonal limit

Climate change is expected to move the spatial patterns of temperature and water availability poleward and upslope, with concomitant shifts in vegetation distribution. Vegetation growing near its southern or low-elevation range limit may prove especially susceptible to mortality and displacement. We investigated the controls on Gross Primary Production (GPP) by an oak and pine stand located just above the lower forest limit in Southern California's San Jacinto Mountains. The local climate was montane Mediterranean, and the stand experienced extensive mortality in the early 1990s and from 2002 to 2004 coincident with extended droughts. The maintenance of high rates of CO2 uptake in winter, and access to water deep in the soil column in summer, allowed for a year-round growing season. The evergreens at the site remained photosynthetically active year-round despite frequent freezing nights. High rates of CO2 uptake were observed at air temperatures below 8°C, which is colder than has been reported for other ecosystems. Winter cold exerted a minor limitation on GPP, and winter warming would have a small effect on GPP. Vegetation withdrew water from the soil, saprolite, and fractured granitic bedrock to support transpiration and CO2 uptake during the dry summer, which further expanded the growing season. Access to a reliable supply of moisture deep in the soil and regolith appears critical for the survival of large trees at the site. These trees may prove vulnerable to climate change if increasing evaporation rates or interannual precipitation variability causes a more frequent or severe depletion of deep regolith moisture. © 2012 Elsevier B.V