Long-term trends in intrinsic water-use efficiency and growth of subtropical Pinus tabulaeformis Carr. and Pinus taiwanensis Hayata in central China

Abstract

This study analysed tree-ring stable carbon isotope composition (δ13C) of Pinus tabulaeformis Carr. and Pinus taiwanensis Hayata, from a subtropical forest located in central China for the last 130 years, to obtain the long-term trends of δ13C, carbon isotope discrimination by plants (Δ), leaf internal carbon dioxide (CO2) concentration (c i ) and intrinsic water-use efficiency (iWUE) in response to elevated atmospheric CO2 concentration (c a ) and climate change and explore how environmental changes affected long-term tree physiological responses and growth. Tree-ring cores were taken in Dabie Mountains, the border of Hubei, Henan and Anhui Provinces of China. δ13C was undertaken at every 3-year interval, and Δ, c i , iWUE and basal area increment (BAI) were determined. Regression analysis was used to quantify the trends in climate and the relationships of c i , iWUE and BAI with elevated c a and climate. Partial correlation analysis was used to distinguish the effects of c a and climate on c i and iWUE. δ13C of P. tabulaeformis and P. taiwanensis decreased in the past 130 years, but Δ had no obvious change over time for the two tree species. Both c i and iWUE increased significantly with the calendar year. BAI of P. tabulaeformis continuously increased during 1897–1993, but decreased slightly in the recent 20 years. However, BAI of P. taiwanensis did not present obvious change in the period 1882–2010. The c i and iWUE of P. tabulaeformis and P. taiwanensis also increased linearly with elevated c a in the past 130 years and with mean annual temperature during 1960–2007. Partial correlation analysis showed that elevated c a , not temperature, induced the changes in c a and iWUE. BAI of P. tabulaeformis since 1897 and that of P. taiwanensis during 1975–2010 responded quadratically to elevated c a . Warming-deduced drought in the study area in the recent 30 years resulted in increases in iWUE and decreases in BAI. This study showed that while iWUE increased in the past 130 years, tree growth of two subtropical tree species in central China responded to rising c a non-linearly. Negative effects of some factors on tree growth, such as climate change (particularly warming-induced drought), nutrient limitation and physiological long-term acclimation to elevated c a , have overridden the CO2 fertilization effects in the past 30 years.