Carbon and Water Cycling in Two Rubber Plantations and a Natural Forest in Mainland Southeast Asia.

Rubber plantations have rapidly replaced natural forests (NFs) in Mainland Southeast Asia, yet the relevant impacts on the terrestrial carbon cycle remain uncertain especially with an increase in drought frequency. Our study compared eddy‐covariance measurements of carbon and water fluxes from two rubber monoculture plantations (at a northern marginal site and a southern traditional plantation site) with a second‐growth NF between 2015 and 2018, and their responses to a prolonged drought during 2015/2016. The NF had higher light use efficiency, water use efficiency and gross primary productivity (GPP, 2.94 ± 0.41 kg C m−2 yr−1) than the northern rubber (NR) monoculture (2.45 ± 0.17 kg C m−2 yr−1), while lower ecosystem carbon use efficiency (eCUE) caused a lower net ecosystem productivity (NEP, 0.75 ± 0.25 kg C m−2 yr−1) compared to the plantation (1.19 ± 0.22 kg C m−2 yr−1). Drought decreased the NF eCUE by 23% with significant carbon uptake restrictions across multiple seasons, while the rubber GPP reduction was only substantial in the warm‐dry season with an overall 17% decline in eCUE. The NR site's GPP was mainly controlled by soil water content throughout the year. Higher light availability offset the negative effect of drier conditions on the rubber GPP, resulting in larger carbon uptake compared to the southern plantation (GPP, 2.12 ± 0.12 kg C m−2 yr−1; NEP, 1.07 ± 0.14 kg C m−2 yr−1). In contrast, the NF GPP was mainly restricted by vapor pressure deficit, especially during the drought. Plain Language Summary: Both rubber expansion from south to north and increased drought events lead to large uncertainties in forest carbon and water cycles in Mainland Southeast Asia. To better understand their potential impacts, our study compared the carbon and water fluxes, and how they respond to drought in a marginal rubber planting area with a traditional planting area, and with a second‐growth natural forest (NF) near the marginal plantation over 4 years using eddy‐covariance technique. We found the NF absorbed more carbon with a higher use efficiency of light and water, while it was largely used for ecosystem respiration resulting in decreased carbon uptake than the rubber plantation. The decreased carbon uptake during drought was significant across multiple seasons in the NF, while it was only substantial in the warm‐dry season in the rubber plantations. Compared to the traditional planting area, carbon uptake in the marginal site was higher and mainly driven by soil moisture. The negative effect of drier conditions was offset by higher light availability during the rainy season. In contrast, the NF carbon uptake was mainly controlled by atmospheric water demand, especially during the drought. Key Points: Rubber plantations had a lower annual gross primary productivity yet higher carbon sequestration than a natural forest (NF)Higher light availability offset the negative effect of drier conditions on the rubber gross primary productivity in a marginal areaDrought increased the importance of soil moisture and vapor pressure deficit on the rubber and NF carbon uptake, respectively [ABSTRACT FROM AUTHOR]