Long-term increases in intrinsic water-use efficiency do not lead to increased stem growth in a tropical
monsoon forest in western Thailand
CHARLES A . NOCK*, PATRICK J. BAKERw , WOLFGANG WANEKz, ALBRECHT L E I S , MICHAEL GRABNER} , SARAYUDH BUNYAVEJCHEWINk and PETER HIETZ*
*Institute of Botany, University of Natural Resources and Applied Life Sciences Vienna, 33 Gregor Mendel Strasse, A-1180 Vienna, Austria, wSchool of Biological Sciences, Monash University, Clayton, Vic. 3800, Australia, zDepartment of Chemical Ecology and Ecosystem Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, Institute of Water Resources Management, Hydrogeology and Geophysics, Joanneum Research, Elisabethstrasse 16/II, A-8010 Graz, Austria, } Institute of Wood Science and Technology, University of Natural Resources and Applied Life Sciences Vienna, Peter Jordan Strasse 82, A-1190 Vienna, Austria,
kNational Parks, Wildlife and Plant Conservation Department, 10900 Chatuchak, Bangkok, Thailand
Abstract
Rising atmospheric carbon dioxide [CO2] can accelerate tree growth by stimulating photosynthesis and increasing intrinsic water-use efficiency (iWUE). Little evidence exists, however, for the long-term growth and gas-exchange responses of mature trees in tropical forests to the combined effects of rising [CO2] and other global changes such as warming. Using tree rings and stable isotopes of carbon and oxygen, we investigated long-term trends in the iWUE and stem growth (basal area increment, BAI) of three canopy tree species in a tropical monsoon forest in western Thailand (Chukrasia tabularis, Melia azedarach, and Toona ciliata). To do this, we modelled the contribution of ontogenetic effects (tree diameter or age) and calendar year to variation in iWUE, oxygen isotopes, and BAI using mixed-effects models. Although iWUE increased significantly with both tree diameter and calendar year in all species, BAI at a given tree diameter was lower in more recent years. For one species, C. tabularis, differences in crown dominance significantly influence stable isotopes and growth. Tree ring D18O increased with calendar year in all species, suggesting that increasing iWUE may have been driven by relatively greater reductions in stomatal conductance – leading to enrichment in D18O – than increases in photosynthetic capacity. Plausible explanations for the observed declines in growth include water stress resulting from rising temperatures and El Nin?o events, increased respiration, changes in allocation, or more likely, a combination of these factors.
Keywords: carbon dioxide, carbon isotopes, intrinsic water-use efficiency, oxygen isotopes, stable isotopes, temperature, topical monsoon forest, tree growth, tree rings
Received 19 February 2010 and accepted 10 March 2010
Global Change Biology (2010), doi: 10.1111/j.1365-2486.2010.02222.x
