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

Abstract

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.

A Study on the Vulnerability of the Gross Primary Production of Rubber Plantations to Regional Short-Term Flash Drought over Hainan Island

“…Ta+234.5 × f w (16) (18) where E s and E a are the saturated vapor pressure and actual vapor pressure (kPa), respectively; E s is a function of T a adjusted by P, the atmospheric pressure (hPa); atm is standard atmosphere (1013.25 hPa); and H is the altitude (m).…”

“…In addition, research on the causes of drought in the rubber ecosystem [17] and the mechanisms of drought resistance [9] are considered in depth, and systematic progress has been made. However, there are few findings on the vulnerability of rubber ecosystems to drought [18,19]. The term vulnerability refers to the frangible nature of a system threatened by different disasters [20] with different intensities and duration.…”

Drought Sensitivity and Vulnerability of Rubber Plantation GPP—Insights from Flux Site-Based Simulation

“…Consequently, models have been essential tools for understanding and forecasting the responses of tropical forests to climate change [21,22]. Completed studies have focused on carbon fluxes, water fluxes, biomass, and the productivity of rubber forests, such as using the soil vegetation atmosphere transfer (SVAT) model to simulate CO 2 and H 2 O fluxes in the canopy of rubber trees and the process-based Land Use Change Impact Assessment tool (LUCIA) to simulate the biomass and rubber yield of rubber plantations at the scale of individual trees, plots, and landscapes [23,24]. The community Land Model Version 5 (CLM5) has also been used to simulate rubber and has achieved excellent results in terms of rubber yield and carbon flux simulation, and has also been shown to capture the seasonal fluctuations of rubber production well.…”

“…Compared to tropical natural forests, tropical plantation forests have relatively homogeneous ecological environments and stand structures, making them advantageous entry points for studying the carbon cycle in tropical forests. Moreover, research has shown that as representative species of tropical plantation forests, rubber plantation forests have a carbon sequestration capacity per unit area that exceeds that of natural forests [24]. Hainan Island (18 • 10 ′ ~20 • 10 ′ N and 108 • 37 ′ ~111 • 03 ′ E) covers an area of about 35,400 km 2 and is the largest tropical island in China [30].…”

Measuring Photosynthesis of Entire Tree Crowns and Pulse Label Trees in Large Closed Chamber with 13CO2 in the Field: Design and Testing

“…However, such models are seldom validated by actual measurements of whole tree photosynthesis, while crown photosynthesis is influenced by the crown architecture (leaf angle and position, self-shading), the presence of several leaf cohorts, and neighborhood effects of adjacent crowns [3][4][5]. However, while gas exchanges between the forest canopy and the atmosphere can be measured at the ecosystem level using the eddy-covariance method [6,7], such fluxes include both the tree component and that of the understory and soil. In addition, the fluxes measured by eddy-covariance integrate large spatial areas of several tens of hectares, which makes it practically impossible to compare genotypes or either horticultural or silvicultural practices.…”