Sensitivity of canopy transpiration to altered precipitation in an upland oak forest: evidence from a long-term field manipulation study

Climate-induced changes in regional precipitation could have important implications for the carbon, water, and nutrient cycles of forest ecosystems. However, few studies have examined the response of deciduous forests to increases or decreases in precipitation. Therefore, the throughfall displacement experiment (TDE) was established in 1993 near Oak Ridge, Tennessee to examine the sensitivity of an upland oak (Quercus spp.) forest to ambient, wet (+33%), and dry (-33%) precipitation regimes. Sap flux measurements on co-occurring tree species were scaled using species-specific estimates of stand sapwood area to derive daily and seasonal rates of canopy transpiration (E-C) from 2000 to 2003. With the exception of 2003, which was an extremely wet year, daily E-C in the dry plot, and occasionally during extended droughts in the ambient and wet plots, declined as water potential in the upper 0.35 m soil profile approached -3.0 MPa. Seasonal patterns of soil water potential and treatment-specific differences in E-C were dependent on precipitation frequency and intensity. Supplemental precipitation added to the wet plot increased seasonal E-C on average by 9% (range -1% to 19%), whereas extended periods of drought on the dry plot in 2000, 2001, and 2002 were sufficient to reduce seasonal E-C by 26-30% compared with the ambient plot. There was a strong correlation between seasonal E-C and the water stress integral, a cumulative index of drought severity and duration. A polynomial fitted to these data indicated that reductions in seasonal E-C on the order of 40% were possible given TDE-imposed reductions in soil water potential. Application of this equation to all years of the TDE (1994-2003) revealed considerable interannual and treatment-specific variation in canopy transpiration. In general, a 33% removal of throughfall on the dry plot during 1995, 1998, and 2002 resulted in a 23-32% reduction in seasonal E-C compared with the ambient plot. While droughts in deciduous forests are often limited in duration and tend to occur late in the growing season, soil water deficits of the magnitude observed in this study have the potential to impact local and regional forest water budgets.