Loss of Homeostatic Gas Exchange in Eastern Hemlock in Response to Pollution and Rising CO2?

Abstract:

In eastern North American, multiple environmental effects, natural and anthropogenic, may impinge upon tree-ring based stable carbon isotope ratios when examined over long time periods. Investigation of relationships between a Vermont (USA) eastern hemlock δ¹³C (1849-2010) chronology and local and regional climate variables, as well as a regional sulfur dioxide time series revealed the decoupling of δ¹³C from significant climate drivers such as May-August maximum temperature (r=0.50, p<0.01) and, raise the possibility that this decoupling can be attributed to foliar and soil leaching of calcium due to acidic deposition since the 1960s. Further, investigation of derived photosynthetic isotope discrimination (Δ¹³C) time series showed an overall decreasing trend in Δ¹³C in response to rising atmospheric carbon dioxide (c_a), but with a slight rise in Δ¹³C in the last decade. Comparison of time series of leaf intercellular CO₂ concentration (c_i), c_i/c_a, and intrinsic water use efficiency (iWUE) showed homeostatic maintenance of c_i levels against c_a until 1965 and rising iWUE. Then, c_i increased proportional (1965-2000) and later at the same rate as c_a (2001-2010) and iWUE leveled off indicating a potential loss of sensitivity to increasing atmospheric carbon dioxide. This more recent passive response may be an indication of a loss of homeostatic maintenance of stomatal control and/or may be linked to changing climate in the region (e.g., wetter conditions).