The regrowth of tropical secondary forests and plantations can not offset the carbon release caused by tropical deforestation, consequently determining net carbon losses on tropical lands. However, large uncertainties remain in relation to this assumption. Here, we used a biometric method to estimate the net dry matter production and net ecosystem production in a rubber forest, the most widespread plantation type in tropical Southeast Asia. According to biometric estimates made during the study, the ecosystem was a carbon sink (790 gC m-2 yr-1). Net ecosystem carbon fluxes were measured by the eddy covariance method. The carbon budget estimated using the FluxNet procedure (904 gC m-2 yr-1) was closer to the biometric estimates in comparison to a method based on data measured during neutral atmospheric conditions. Overall, when considering the whole life cycle, including deforestation of the prior-existing tropical forest, the hypothesis of plantations serving as large carbon sinks is not supported by our study.
The largest errors in assessing the terrestrial carbon balance are believed to stem from uncertain rates of tropical deforestation or rainforest conversion (
Currently, the micrometeorological-based eddy covariance method is widely used for the investigation of carbon balance in a variety of ecosystems (
An alternative to the above approach is the use an inventory-based biometric method, which has been used to estimate forest Net Ecosystem Productivity (NEP), although there are limits to the estimation of soil C sequestration. In the tropics, eddy covariance-based carbon budgets have been reported to be generally higher than biometric estimates both before and after
In the Xishuangbanna tropical rainforest, both long-term catchment water observations and direct eddy covariance estimates indicate that more water underwent evapotranspiration in the rubber plantations than in the rainforest (
In this investigation, we assessed the carbon budget of a 33-year-old rubber plantation in tropical China. Specifically, this study was designed to determine: (1) if adult tropical rubber plantations are carbon sinks or sources; (2) whether eddy covariance and biometric estimates converge in a monoculture forest; and (3) if rubber plantations are a potential carbon sink when compared to primary tropical rainforest.
The study site is located in the experimental area of the Xishuangbanna Tropical Botanical Garden (21° 55’ 30” N, 101° 15’ 59” E; size ~ 20 ha; elevation 570 m a.s.l.), Xishuangbanna, Yunnan Province, south-western China. In recent years, tropical rainforests have been largely replaced across the whole region by rubber plantations for economic reasons. The Xishuangbanna experimental area was established by the Chinese Academy of Sciences to study the impact of widespread rubber plantations on the local environment. The study site is included in a larger area, all planted with rubber trees.
The climate in this area is strongly seasonal with two air masses alternating over the year (
The 33-year-old rubber plantation studied has a mean canopy height of approximately 22 m. Trees were planted 2 m apart in rows with mixed spacing between the rows, from narrow (4 m) to wide (12 m),. The main rubber-tapping period is from May to November. Fertilization is performed in April and July of each year, and understory weeds and seedlings are excluded using herbicides or by hand weeding.
Assessment of the net ecosystem production was carried out by the forest department using the most commonly employed biometric method (
where
For rubber plantations, rubber tapping (as it relates to carbon loss) should be included in the NEP estimation. Thus,
where
A method based on tree growth theory was proposed for estimating the biomass increment. Using the assumption that the biomass accumulation of a monoculture stand will be similar to that of individual tree growth, the logistic equation was used to fit the stand age
where
In our study area, we selected a stand where the age series ranged from 5 to 47 years. By using the investigation data of 30 rubber trees with different ages in the 10 stands (3 trees per stand) and diameter at breast height, a regression model of the biomass relating with DBH (diameter at breast height, 1.3 m) was established and applied to estimate the biomass of 30 rubber plantations with different stand ages (
We selected 4 plots (10×15m) in the rubber plantation and 3 sub-plots (4×5m) in each plot, which were set as control (CK) and root trenching (NR). In the NR plots, we used a PVC pipe ( 630 mm, H 500 mm) to carry out root trenching (PVC pipes can prevent new root regrowth). In the center of each sub-plot one PVC connector was inserted 3 cm underground, mating a PVC pipe ( 200 mm, H 200 mm), creating a respiration box for measuring soil respiration. Soil respiration was measured using a static chamber combined with an infrared gas analyzer (Li-820, Li-Cor Inc., Lincoln, NE, USA). Sampling was performed every morning between 09:00 and 11:00 local time from March 2008 to February 2009 (
An open-path eddy covariance system consisting of a three-dimensional sonic anemometer (CSAT3, Campbell Scientific Inc., Logan, UT, USA) and an open-path infrared gas analyzer (Li-7500, Li-Cor Inc., Lincoln, NE, USA) were mounted at a height of 38.0 m on a 55-meter iron meteorological observation tower (Fig. S1 in
The net ecosystem CO2 exchange (
where
The software used to perform the eddy flux data processing is recommended by FluxNet and maintained online by the Max Planck Institute (http://www.bgc-jena.mpg.de/~MDIwork/eddyproc/index.php). The used software implements the Reichstein
Micrometeorological theories and hypotheses are generally more accurate under neutral atmospheric conditions. Based on this consideration, we also used a method called “neutral atmospheric condition based eddy flux estimation” to process our carbon budget dataset. In this procedure, we selected the neutral atmospheric condition data (-1<ξ<1), which accounted for about 80% of all data, and filled the gap during non-neutral atmospheric conditions with environmental response curves derived from neutral atmospheric conditions (
where
Additional data quality assessments, such as ecosystem energy closure (Fig. S2 in
Logistic equations can be used to describe the relationship between stand age and carbon density as expected (
Rubber tree is an evergreen species in its native habitat, but in our study area trees become deciduous and shed their leaves after the coldest month (January). Aboveground litterfall production peaked in February (
Soil respiration was higher during the rainy season than during the dry season, while the heterotrophic to total respiration ratio was higher during the dry season than during the rainy season (
Based on the presented data and according to
To avoid the effects of calm nights and strong winds, the neutral atmospheric condition method has been used as an alternative approach to studying an ecosystem’s carbon balance (
According to traditional biometric methods, the 33-year-old rubber plantation in our study is a carbon sink (790 gC m-2 yr-1). This value is larger than that obtained from a young
The closest site to the rubber plantation at which comparable carbon flux measurements have been made is a primary tropical rainforest (approximately 10 km distant). The carbon sink strength (790 g C m-2 yr-1) of the rubber plantation is higher than that of the tropical rainforest (359 gC m-2 yr-1, based on the biometry method -
Belowground litter input plays an important role in the production estimation. According to previous studies, belowground litter production is reported to be even twice as large as aboveground litterfall (
According to the biometric estimates and the eddy-flux
We thank Xiao-Long Bai, Xue-Hai Fei, Jin-Xiang Xiong and Hong-Li Ji for their assistance in the field. This work was supported by Xishuangbanna Station for Tropical Rain Forest Ecosystem Studies (XSTRE), and by the National Science Foundation of China (41001063, 41271056, 41071071, 31200347), the Development Program in Basic Science of China (2010CB833501), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant no. XDA05070303) and the Chinese Academy of Sciences 135program (XTBG-T03).
The relationship between stand age and carbon density for the rubber plantation studied.
The monthly variations of aboveground litterfall production (
The monthly variations of soil respiration in the rubber plantation from March 2008 to February 2009. (Black columns): total soil respiration; (grey columns): heterogeneity respiration).
The carbon flux of the rubber plantation by different data processing procedures. (A): FluxNet procedure method; (B): Neutral atmospheric condition method; (hollow circles): ecosystem respiration (
Fig. S1 - The tower in the rubber plantation.
Fig. S2 - The relationship between net radiation (Rn) and the sum of latent heat (LE) and sensible heat (H).
Fig. S3 - The power spectrum of wind velocity components for air near forest floor in rubber plantation.
Fig. S4 - Light response of daytime flux and the temperature response of nighttime flux.
Fig. S5 - Cumulative frequency distribution of wind speed.