PARENT SESSION
Poster Session 5: Climate Dynamics
Tuesday, August 9, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Sensitivity of canopy transpiration to altered precipitation regimes in an upland oak forest.

Wullschleger, Stan¹, Hanson, Paul¹, ¹ Environmental Sciences Division, Oak Ridge, TN, USA

ABSTRACT- Climate-induced changes in regional precipitation could have important implications for the hydrologic balance of forest ecosystems. Few studies, however, have examined the response of temperate deciduous forests to increases or decreases in precipitation. Therefore, the Throughfall Displacement Experiment (TDE) was established in 1993 to investigate the sensitivity of an upland oak (Quercus spp.) forest to normal, above-normal (+33%), and below-normal (-33%) precipitation. Sap flux measurements on several co-occurring tree species were scaled using species-specific estimates of stand sapwood area to derive daily and seasonal rates of canopy transpiration from 2000 to 2003. With the exception of 2003, which was an extremely wet year, daily rates of transpiration in the dry plot declined as water potential in the upper 0.35-m soil profile approached -3.0 MPa. Extended periods of drought on the dry plot in 2000, 2001, and 2002 were sufficient to reduce rates of seasonal transpiration by 26 to 30% compared to the ambient plot. Although patterns of soil water availability across years and treatments were complex, there was a strong correlation between seasonal transpiration and the water stress integral, a cumulative index of drought severity and duration. A polynomial fitted to these data indicated that reductions in transpiration during the growing season on the order of 40% were possible given inter-annual and TDE-imposed reductions in soil water potential. Application of this equation to all years (1994 to 2003) of the project revealed considerable inter-annual and treatment-specific variation in canopy transpiration. Removal of throughfall on the dry plot during 1995, 1998, and 2002 resulted in a 23 to 32% reduction in seasonal transpiration compared to the ambient plot. While droughts in temperate 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. It will be important to explore interactions between precipitation frequency and intensity, and impacts on forest carbon and water cycles in more detail in future manipulative studies and model-based activities.

Key words: Global change, Sap flow, Drought, Precipitation