Amazon rainforest photosynthesis increases in response to atmospheric dryness

Abstract

Earth system models predict that increases in atmospheric and soil dryness will reduce photosynthesis in the Amazon rainforest, with large implications for the global carbon cycle. Using in situ observations, solar-induced fluorescence, and nonlinear machine learning techniques, we show that, in reality, this is not necessarily the case: In many of the wettest parts of this region, photosynthesis and biomass tend to increase with increased atmospheric dryness, despite the associated reductions in canopy conductance to CO2. These results can be largely explained by changes in canopy properties, specifically, new leaves flushed during the dry season have higher photosynthetic capacity than the leaves they replace, compensating for the negative stomatal response to increased dryness. As atmospheric dryness will increase with climate change, our study highlights the importance of reframing how we represent the response of ecosystem photosynthesis to atmospheric dryness in very wet regions, to accurately quantify the land carbon sink.

Principal component regression analysis demonstrates the collinearity-free effect of sub estimated climatic variables on tree growth in the central Amazon

“…These results show that the effect of VPD on photosynthesis depended on the level of atmospheric moisture. Green et al (2020) reported that ecosystem photosynthesis can increase at VPD values lower than 10 hPa, Lee et al (2013), on the other hand, estimated that ecosystem photosynthesis declined as VPD progressively increased from 3.5 hPa (wet season) to 32 hPa in the dry season, which is in agreement with the results found in our study.…”

“…This discrepancy can be ascribed to difference in environmental conditions during data collection. For instance, Green et al (2020) reported that ecosystem photosynthesis increases in the central Amazon when VPD increased from 0.1 to 10 hPa. In tropical rainforests, the optimum temperature for photosynthesis is about 29 °C (Liu, 2020), with decreasing photosynthetic rates at higher temperatures.…”

“…There are reports associating tree growth or ecosystem photosynthesis to variations in temporal rainfall variability in the Amazon region (Lee et al, 2013;Méndez, 2018;Yang et al, 2018) or VPD (Lee et al 2013;Green et al, 2020). Some studies that aim to assess the effect of rainfall seasonality on tree growth in the Amazon have led to different results.…”

“…Rainfall and SWC seem to be the major factors that affect tree growth in the Amazon region, but there is still under debate whether Amazonian trees grow faster in the wet season than in the dry season. Although in most studies tree growth or ecosystem photosynthesis seems to decrease in the dry season (Méndez, 2018;Wagner et al, 2014;Yang et al, 2018;Antezana-Vera & Marenco, 2020), the opposite effect has also been reported (Laurance et al, 2009;Green, et al, 2020). Whereas Laurance et al (2009) reported that tree growth was fastest during the dry period and positively correlated with maximum temperatures (T max ), Wagner et al (2014) found no significant effect of T max on tree growth.…”

“…Tree growth can be defined as the increase of biomass through time and it is often estimated by measuring the increment of stem diameter over time -a proxy of biomass gains at the ecosystem level (Wagner, Rossi, Stahl, Bonal, & Herault, 2012;Wagner et al, 2014;Dias & Marenco, 2016;Antezana-Vera & Marenco, 2020). As tree growth is greatly affected by factors that affect photosynthesis, sun-induced fluorescence -a proxy of ecosystem photosynthesis has been used to estimate the effect of environmental factors on total carbon gain of the ecosystem (Lee et al, 2013;Yang et al, 2018;Green et al, 2020). DOI 10.15517/rbt.v69i2.44489 Tree growth can be affected by intrinsic factors (e.g., genetic make-up) and environmental factors, such as nutrient availability, irradiance, temperature, rainfall, and soil water content (SWC).…”