Asian tropical forests assimilating carbon under dry conditions: water stress or light benefits?

Abstract

Tropical forests are characterized by vast biomass, complex structures, and mega-biodiversity. However, the adaptation processes of these forests to seasonal water availability are less understood, especially those located in the monsoonal and mountainous regions of tropical Southeast Asia. This study used four representative tropical forests spanning from 2° N–22° N in continental Southeast Asia to address dry-condition photosynthesis at the seasonal scale. We first provided novel and reliable estimations of ecosystem photosynthesis (gross primary production; GPP) seasonality at all four sites. As expected, both evergreen and deciduous seasonal forests exhibited higher GPPs during the rainy season than during the dry season. A bimodal pattern corresponding to solar radiation occurred in the GPP of the perhumid forest. The surface conductance (Gs) was consistently lower both in the dry season and during dry spells (DSPs) than during the wet season and non-dry spells. However, this did not prevent GPP from increasing alongside increasing irradiance in the perhumid forest, suggesting that other ecosystem physiological properties, e.g., the light-saturated photosynthetic rate, must have increased, thus surpassing the effect of Gs reduction. Thus, perhumid forests could be defined as light-demanding ecosystems with regards to their seasonal dynamics. Seasonal forests are water-stressed ecosystems in the dry season, as shown by the reductions in GPP, Gs, and related ecosystem physiological properties. At all four forest sites, we observed a lack of consistent adaptive strategy to fit the water seasonality due to the diversity in leaf phenology, soil nutrient availability, root depth, and other potential factors.

Strong Green‐Up of Tropical Asia During the 2015/16 El Niño

“…Additionally, we also note that at intra-annual time scale, tropical Asia did not demonstrate green-up in dry seasons like Amazon (Yang et al, 2023;Zhang et al, 2016), and the difference implied a different mechanism in the green-up in tropical Asia than in tropical America. Within the tropical Asia, the difference between continental and maritime tropical Asia (Zhang et al, 2016), and the difference between the west and east part of maritime tropical Asia (Wang et al, 2023) have been noted by other studies in terms of their sensitivities to climate.…”