Variation of Stem CO2 Efflux and Estimation of Its Contribution to the Ecosystem Respiration in an Even-Aged Pure Rubber Plantation of Hainan Island

The stem CO2 efflux (Es) plays an important role in the carbon balance in forest ecosystems. However, a majority of studies focus on ecosystem flux, and little is known about the contribution of stem respiration to ecosystem respiration (Reco) for rubber (Hevea brasiliensis) plantations. We used a portable CO2 analyzer to monitor the rate of Es in situ at different heights (1.5 m, 3.0 m and 4.5 m) in an even-aged rubber plantation from 2019 to 2020. Our results showed that Es exhibited a significant seasonal difference with a minimum value in April and a maximum in September. The mean annual rate of Es at 3.0 m in height (1.65 +/- 0.52 mu molm(-2)s(-1)) was slightly higher than Es at 4.5 m in height (1.56 +/- 0.59 mu molm(-2)s(-1)) and Es at 1.5 m in height (1.51 +/- 0.48 mu molm(-2)s(-1)). No obvious differences in vertical variations were found. An area-based method (Ea) and a volume-based method (Ev) were used to estimate stem respiration at stand levels. One-way ANOVA showed that Ea had no obvious differences in vertical variation (p = 0.62), and Ev indicated differences in vertical variation (p < 0.05). Therefore, the Ea chamber-based measurements at breast height were reasonable and practical extrapolation proxies of stem respiration in an even-aged rubber plantation. With the use of the area-based method, the stem carbon values released from a mature rubber forest were estimated to be 1.214 t Chm(-2)a(-1) in 2019 and 1.414 t Chm(-2)a(-1) in 2020. Ea/Reco and Ev/Reco showed seasonal changes, with a minimum value in April and a maximum value in December. The leaf area index (LAI) and soil volumetric moisture content (VWC) were the major impact factors of Ea/Reco in an even-aged pure rubber plantation.

Automated summary

Stem CO2 efflux is important for the carbon balance in forest ecosystems, but its contribution to ecosystem respiration for rubber plantations is not well understood. This study monitored the rate of stem CO2 efflux at different heights in a rubber plantation and found significant seasonal differences with a minimum value in April and a maximum in September. Both area-based and volume-based methods were used to estimate stem respiration, and the results showed reasonable extrapolation proxies. The study also found that leaf area index and soil volumetric moisture content were the major factors influencing the ratio of stem respiration to ecosystem respiration in the rubber plantation.