Influence of seasonal temperature on tree-ring δ13C in different-aged temperate pine forests

Tree growth rings from three specimens in two different aged (14- and 77-year old) white pine plantation forests were analyzed for stable carbon isotope ratios to identify both short- and long-term variations in physiological response to changing environmental conditions. Three isotopic (δ13Ccorr) time series records were constructed from whole wood samples extracted from paths parallel to the growth rings in each forest. These δ13Ccorr records were corrected for the long-term anthropogenically induced CO2 and compared to historical climate (temperature, precipitation) data from 1935 to 2016. High resolution inter-annual variations in trees in each stand displayed similar intra-annual cycles in δ13Ccorr, demonstrating the seasonal physiological response of these forests to environmental stressors. In both stands, growing season temperature acted as a significant control (p < 0.05) on δ13Ccorr values and hence carbon uptake through enhanced physiological discrimination against 13CO2. Wavelet transform methods applied to the carbon isotope composition extracted from tree-rings to investigate isotope-climate linkages revealed phase relationships linked to changes in stand structure and microclimate due to thinning treatment applied in the older stand in 1983. Cross wavelet transform (XWT) of mean annual temperature with latewood tree-ring isotope time series demonstrated a persistent 12- to 16-year phase-locked relationship, which suggests long-term temperature variations influenced stomatal conductance behavior in the post-thinning period (1983–2012). Overall, air temperature acted as a multi-year control on tree-ring carbon isotope compositions in these different-age forests and variations in δ13Ccorr time series provided a potential record of past temperature changes in the region.