Analyses of δ13C and δ18O in tree rings of Callitris columellaris provide evidence of a change in stomatal control of photosynthesis in response to regional changes in climate

Abstract

We examined relationships between stable isotopes of carbon (δ13C) and oxygen (δ18O) in tree rings of Callitris columellaris F. Muell in the semi-arid Pilbara region of north-western Australia. To test the hypothesis that stomatal control of photosynthesis decreases during drier periods, we developed δ13C and δ18O chronologies spanning 1919-1999, and used a permutation regression approach to relate a 21 -year running correlation between δ13C and δ18O to rainfall and temperature at Marble Bar and our study site. The relationship between δ13C and δ18O switched from being always negative before 1955 to being consistently positive after 1976, suggesting an increase in stomatal control of photosynthesis in recent decades. Changes in the δ 13C-δ18O relationship reflected changes in rainfall, which has increased in the region by 30% since 1976. The correlation between δ13C and δ18O was positively related to the 21-year running mean of normalized rainfall anomalies at both the study site (P = 0.045, Adj. r2 = 0.47) and Marble Bar (P = 0.046, Adj. r 2 = 0.48). In addition, the δ13C- δ18O correlation was negatively related (P = 0.047, Adj. r 2 = 0.61) to temperatures at Marble Bar. Our interpretation of the role of changes in climate affecting the relationship between tree-ring δ13C and δ18O is supported by evidence from the isotope composition of foliage samples: foliar δ13C and δ18O were negatively correlated with log stomatal conductance (δ13C, r=-0.41; δ18O, r=-0.42), whereas the correlation between foliar δ13C and δ18O was positive (r = 0.63, P = 0.027) after the summer wet period. Our data indicate that stomatal control of photosynthesis in Callitris adjusts to region-wide changes in climate and that, in a warmer and drier world, trees might adapt by increasing non-stomatal control of photosynthesis. © 2008 Heron Publishing.