Soil nutrient manipulations alter tree growth but not water use efficiency in temperate forests

Jacob Malcomb, Todd Scanlon and Howard Epstein, Department of Environmental Sciences, University of Virginia, Charlottesville, VA, Matthew A. Vadeboncoeur, Earth Systems Research Center, University of New Hampshire, Durham, NH, Daniel L. Druckenbrod, Geological, Environmental, & Marine Sciences, Rider University, Lawrenceville, NJ, Mary Beth Adams, Northern Research Station, USDA Forest Service, Morgantown, WV, Matthew Lanning, Department of Earth Sciences, Indiana University–Purdue University Indianapolis (IUPUI), Indianapolis, IN, Lixin Wang, Earth Sciences, IUPUI, Indianapolis, IN

Background/Question/Methods
Forest water use efficiency (WUE), broadly defined as the ratio of carbon uptake via photosynthesis to water lost via transpiration, has increased in recent decades, a trend primarily attributed to increases in atmospheric CO2 concentrations. However, the magnitude of this trend varies among species, locations, and climate regimes, suggesting alternate controls on WUE. In the eastern US, acid deposition has been a major biogeochemical driver for decades, but it remains unclear how deposition and associated changes in soil nutrients influence WUE. Here, we examined how whole-watershed nutrient manipulations designed to alter soil acid/base status have influenced tree growth and WUE at three sites in the eastern United States: Hubbard Brook Experimental Forest (New Hampshire), where a one-time, whole-watershed calcium amendment was applied in 1999, and the Bear Brook Watersheds (Maine) and Fernow Experimental Forest (West Virginia), where watersheds received annual nitrogen and sulfur treatments beginning in 1989. Growth and intrinsic water use efficiency (WUEi) were derived from tree ring widths and stable C isotope ratios (δ13C) for four codominant species in treated and control watersheds at each site. Catchment water balance evapotranspiration was assessed at each site to constrain interpretation of changes in WUEi trends.
Results/Conclusions
At Hubbard Brook, tree growth was significantly greater in three of four examined species in the calcium-treated watershed, compared to the control watershed. Tree growth was significantly reduced in the N and S-treated watershed in three of four examined species at Fernow and two of four species at Bear Brook. WUEi of sugar maple was greater in the calcium-treated watershed at Hubbard Brook, but there was otherwise no evidence that nutrient manipulations influenced WUEi. Overall, nutrient manipulations altered tree growth to a greater extent than WUEi, suggesting that changes in tree growth were accompanied by proportional changes in transpiration. When aggregated across all measured trees in each region (West Virginia and New England), WUEi increased at the New England sites (0.63 mmol CO2 mol H2O-1 yr-1) between 1980-2015, whereas there was no temporal trend at Fernow. Divergent regional trends may reflect differences in background acid deposition loads and underlying soil properties. Together, results suggest that experimental nutrient manipulations have altered tree growth, but not WUEi, and that CO2-driven increases in WUE are not ubiquitous across eastern US forests.