DECREASED FOREST WATER USE EFFICIENCY UNDER ACID DEPOSITION

Globally, about sixty-five percent of land precipitation is returned to the atmosphere by evapotranspiration, mainly through plants. Forests ecosystems contribute a significant portion of global transpiration, yet the mechanisms that regulate plant water use in forests are still not fully understood. Recent work has shown that the alteration of soil calcium availability caused by acid deposition has been identified as a new control affecting forest water use at the watershed scale. Physiologically, calcium fills a unique role as a secondary messenger ion in regulating stomatal aperture in plants. Decreased soil calcium supply appears to cause prolonged stomatal opening, consistent with the current understanding of plant physiology. We performed stable isotope (¹³C) analysis on tree cores collected from the Fernow Experimental Forest in both acidified and control catchments to address the impact of acid deposition on forest water use efficiency. Our results indicate that leaching of the soil calcium supply decreased water use efficiency in the acidified catchment in addition to increasing the net amount of water lost from the watershed (3.6 million gallons annually). Surprisingly, the control watershed did not experience any increase in water use efficiency over the study period, contrary to what was expected with the rise in atmospheric CO₂ and the historical curtailment of anthropogenic acid deposition. These results indicate that not only does current acid deposition impact plant water use, but past deposition regimes may have lasting effects currently unaccounted for.