Abstract
Atmospheric CO2 (c a) rise changes the physiology and possibly growth of tropical trees, but these effects are likely modified by climate. Such c a × climate interactions importantly drive CO2 fertilization effects of tropical forests predicted by global vegetation models, but have not been tested empirically. Here we use tree‐ring analyses to quantify how c a rise has shifted the sensitivity of tree stem growth to annual fluctuations in rainfall and temperature. We hypothesized that c a rise reduces drought sensitivity and increases temperature sensitivity of growth, by reducing transpiration and increasing leaf temperature. These responses were expected for cooler sites. At warmer sites, c a rise may cause leaf temperatures to frequently exceed the optimum for photosynthesis, and thus induce increased drought sensitivity and stronger negative effects of temperature. We tested these hypotheses using measurements of 5,318 annual rings from 129 trees of the widely distributed (sub‐)tropical tree species, Toona ciliata. We studied growth responses during 1950–2014, a period during which c a rose by 28%. Tree‐ring data were obtained from two cooler (mean annual temperature: 20.5–20.7°C) and two warmer (23.5–24.8°C) sites. We tested c a × climate interactions, using mixed‐effect models of ring‐width measurements. Our statistical models revealed several significant and robust c a × climate interactions. At cooler sites (and seasons), c a × climate interactions showed good agreement with hypothesized growth responses of reduced drought sensitivity and increased temperature sensitivity. At warmer sites, drought sensitivity increased with increasing c a, as predicted, and hot years caused stronger growth reduction at high c a. Overall, c a rise has significantly modified sensitivity of Toona stem growth to climatic variation, but these changes depended on mean climate. Our study suggests that effects of c a rise on tropical tree growth may be more complex and less stimulatory than commonly assumed and require a better representation in global vegetation models.
Highlights
Tropical forests account for a third of global gross and net primary productivity, store 25% of the carbon in terrestrial ecosystems (Beer et al, 2010; Bonan, 2008) and drive fluctuations of the land carbon sink (Friedlingstein et al, 2019)
A key challenge in model predictions of tropical forest responses to these changes is the uncertainty of the magnitude of effects of elevated CO2 levels, commonly referred to as ‘CO2 fertilization effects’ (Cernusak et al, 2013; Fatichi, Pappas, Zscheischler, & Leuzinger, 2019; Körner, 2009; Lewis, Edwards, & Galbraith, 2015; Settele et al, 2014; Terrer et al, 2019)
Published climate–growth relations at monthly scales for our study sites and species show that climate effects occur both in wet season and transitional months (Heinrich et al, 2008, 2009; Rahman, Islam, & Bräuning, 2018; Vlam et al, 2014), in accordance with meta-analyses of climate– growth relations in tropical trees (Rozendaal & Zuidema, 2011)
Summary
Tropical forests account for a third of global gross and net primary productivity, store 25% of the carbon in terrestrial ecosystems (Beer et al, 2010; Bonan, 2008) and drive fluctuations of the land carbon sink (Friedlingstein et al, 2019). In climate– growth analyses of tree-ring chronologies, this would lead to reduced sensitivity of ring width to rainfall, causing the positive slope of the growth–rainfall relation to become flatter at high ca This is represented by a negative ca × P interaction. Hypothesis 2b At cooler sites, ca rise could enhance the positive effect of temperature on photosynthesis (Lloyd & Farquhar, 2008) and growth (Voelker et al, 2017), as ca-induced additional leaf warming increases photosynthesis This is represented by a positive ca × T interaction. | 4031 climate, providing support for Hypotheses 1a and 2b at cooler sites (2b: during the cooler dry seasons), and Hypotheses 1b and 2a at warmer sites
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