Variations in the intrinsic water-use efficiency of Araucaria araucana trees under a present climate change scenario: tree age and site conditions effects

HADAD M. A.; ROIG FIDEL; ARCO MOLINA, JULIETA; GERHARD HELLE

Valparaiso

Conferencia; 11th International Conference on the Applications of Stable Isotope Techniques to Ecological Studies; 2018

Stable isotopes are of great interest to conduct ecophysiological and paleoenvironmental researches. The analyses of the tree-ring 13C/12C ratios (δ13C) allow to study the environmental changes effects on tree development and water use over time. Thus, the δ13C values allow to estimate the intrinsic water-use efficiency (iWUE) of trees. Moreover, the iWUE variations depend on local conditions, tree ontogeny or both. In this sense, the product of this study revealed the physiological responses of young and adult Araucaria araucana trees from two open xeric and two dense closed mesic forests to the atmospheric CO2 concentrations (Ca) increase and climate changes during the 20th century in northern Argentine Patagonia. In each site, 4 young trees (≤ 214 years) and 4 adult trees (≥ 264 years) were sampled. The α-cellulose δ13C contents of 5 tree-ring segments were analyzed. The results indicated that although an increase in the iWUE was shown by all trees during the last century in response to both changes in Ca and climate, tree responses strongly depend on ontogenetic, local and geographical characteristics. Trees from xeric sites presented greater iWUE and lower 13C discrimination (Δ13C) and intercellular CO2 concentration (Ci) values than the mesic ones. Moreover, iWUE and Δ13C values from mesic sites seemed to be mainly affected by temperature and light conditions, while trees from xeric condition appeared to be more affected by other factors, as precipitation. In general, adult trees presented significantly higher iWUE values than young trees, indicating that processes related to tree age and/or height induce different responses, mainly in dense and closed forests. These greater iWUE values can be attributed to hydraulic limitations reducing stomatal conductance in taller trees, to changes in irradiance and photosynthetic capacity depending on the tree position in the stand structure, and to more drought conditions in the upper part of the canopy experienced by taller trees. Finally, given the heterogeneous responses of trees to climate and given the endangered status of the A. araucana forests, further studies are needed to better understand how these forests may be affected in their physiology in the present and future climate change scenarios.