Abstract
Tropical forests are a crucial component of the global carbon cycle and their response to future atmospheric changes may either accelerate or slow down the pace of climate change. The negative effects of warming on tropical forest trees may be offset by favourable effects of atmospheric CO2 (ca) rise on tree physiology and growth. However, the magnitude of this ‘CO2 fertilization effect’ is a major uncertainty in models. We aim to reduce this uncertainty in a retrospective study of tree responses to 33% ca rise during the past 100 years. In a large-scale tree-ring study, we evaluate whether ca rise has increased photosynthesis, reduced water use and stimulated diameter growth. We also assess how these responses are modified by rainfall, temperature and soil nutrient availability. We use Toona ciliata (Meliaceae) – a long-lived, widely distributed tree species that produces reliable growth rings – as a model system. We study >1000 trees at 15 sites (Asia, Australia) that span large climatic and edaphic gradients. We test for ca-induced photosynthesis stimulation (hypothesis H1) using Deuterium-isotopomers of tree-ring cellulose, a novel isotopic proxy for photorespiration. We test ca-induced reduction in water use (H2) by quantifying trends in hydraulic conductivity based on vessel size measurements (X-ray tomography). Next, we evaluate tree-growth responses to ca rise using trend and chronology analyses (H3). Finally, we implement empirical results in an existing tree-growth model to improve predictions of CO2 fertilization effects. Our study yields insights into tropical forest responses to climate change and has implications for earth system modelling.
Netherlands