Unthinned slow-growing ponderosa pine (Pinus ponderosa) trees contain muted isotopic signals in tree rings as compared to thinned trees

Key message The muted wood isotopic signal in slow-growing trees of unthinned stands indicates lower responsiveness to changing environmental conditions compared to fast-growing trees in thinned stands. Abstract To examine the physiological processes associated with higher growth rates after thinning, we analyzed the oxygen isotopic values in wood (δ¹⁸O_w) of 12 ponderosa pine (Pinus ponderosa) trees from control, moderately, and heavily thinned stands and compared them with wood-based estimates of carbon isotope discrimination (∆¹³C), basal area increment (BAI), and gas exchange. We found that (heavy) thinning led to shifts and increased inter-annual variability of both stable carbon and oxygen isotope ratios relative to the control throughout the first post-thinning decade. Results of a sensitivity analysis suggested that both an increase in stomatal conductance (g_s) and differences in source water among treatments are equally probable causes of the δ¹⁸O_w shift in heavily thinned stands. We modeled inter-annual changes in δ¹⁸O_w of trees from all treatments using environmental and physiological data and found that the significant increase in δ¹⁸O_w inter-annual variance was related to greater δ¹⁸O_wresponsiveness to changing environmental conditions for trees in thinned stands when compared to control stands. Based on model results, the more muted climatic response of wood isotopes in slow-growing control trees is likely to be the consequence of reduced carbon sink strength causing a higher degree of mixing of previously stored and fresh assimilates when compared to faster-growing trees in thinned stands. Alternatively, the muted response of δ¹⁸O_w to climatic variation of trees in the control stand may result from little variation in the control stand in physiological processes (photosynthesis, transpiration) that are known to affect δ¹⁸O_w.