Acid deposition during the 20^th century depleted base cations in the northeast United States and around the globe, substantially altering terrestrial ecosystem function. To understand ecological response to depleted calcium (Ca) soil concentrations, watershed 1 (WS1) at the Hubbard Brook Experimental Forest was amended with enough Ca to bring soil Ca concentrations to pre-industrial levels.

Results/Conclusions
Following the amendment, the hydrology of WS1 changed; annual evapotranspiration (ET; calculated as the difference between precipitation and runoff) increased by 25%, 18%, and 19% respectively for the three years following treatment, followed by a return to normal ET rates ever since. Concurrently, stream water chemistry changed, showing high Ca retention following the wollastonite addition. We hypothesize that trees in WS1 responded to the Ca amendment through a few possible mechanisms that stimulated transpiration for the three years following treatment. Hydrologic change resulting from the Ca amendment suggests that historical and future changes to soil Ca concentrations may have concurrently altered or will alter water cycle dynamics.