Decreased forest water use efficiency under acid deposition

Alteration of soil calcium availability caused by acid deposition has been identified as a new control affecting forest water use at the watershed scale. Calcium fills a unique role as a secondary messenger ion in regulating stomatal aperture. Decreased calcium supply appears to cause prolonged stomatal opening and is consistent with the current understanding of plant physiology. However, the impacts of soil biogeochemistry on water use efficiency have yet to be evaluated. 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.