Increasing atmospheric carbon dioxide (CO2
) concentrations and global climate change are of growing concern to humankind. The world’s forests are a critical component of the terrestrial ecosystem and play a significant role in regulating the global carbon (C) cycle by serving as C sinks, thereby potentially mitigating future impacts of climate change [1
]. Tropical forests represent about half of the global forest area and are believed to be the largest C reservoir of terrestrial biota [2
]. They hold approximately 470 Pg C in live biomass, debris, and soil organic matter, which is approximately 55% of the total C stored in the world’s forests [3
]. Large areas of naturally-regenerated tropical forests have been lost due to land use changes, which resulted in the emission of around 1.4 Pg C year−1
to the atmosphere during 1990–2010 (~15% of global anthropogenic CO2
emissions for that time period) [4
]. In order to combat global warming and reduce CO2
emissions from land-use transition, especially due to deforestation and forest degradation, accurate and reliable estimates of C sequestration in various forest ecosystems are needed [5
Rubber tree (Hevea brasiliensis
Muell. Arg.) is a cash crop that provides a variety of raw materials to industrial products. During the past decades, state-owned farms and particularly smallholder farmers have gained unprecedented wealth in southern China and elsewhere due to the rising global rubber prices and the sustainability of government subsidies, e.g., the latex return reached approximately 36,000 CNY ha−1
or 13,000 CNY per person in Xishuangbanna (Yunnan Province) in 2010 [6
]. Accordingly, large areas of forests have been converted into rubber plantations in the tropics, resulting in major alterations in ecosystem C dynamics due to deforestation and soil erosion [7
]. Rubber plantations presently cover an area of 10 million ha in southeast Asia, out of which 1 million ha is in tropical China. The rubber-based income from these plantations may be decreased, however, due to the risk of falling latex prices, although it can be increased by the sell of stumpage when rubber trees have reached their latex-tapping lifespan [6
]. To evaluate the trade-offs between ecological and economic functions, rubber transitions have been the subject of considerable study [5
]). Rubber plantation C stocks in biomass and soil have been studied [11
], but information on the C sequestration including its latex C is still lacking, particularly in tropical China. Given the expansion area and the potential expansion rates in the future, a deeper understanding of C sequestration in rubber plantation ecosystems is needed on the part of policymakers, managers, researchers and other groups in the scientific community.
C sequestration in forest ecosystems is affected by stand age, topography, climate, soil type, tree species, and management regime [13
]. It has been known that the C stored in tree biomass increases with stand age, but the trend may be different for the mineral soil C storage [11
]. Several authors have revealed enhanced biomass C in rubber plantations with stand age, ranging from 1.4 to 6.7 Mg ha−1
], but these results excluded an important C component that is stored in latex. Meanwhile, whether net primary productivity (NPP) is related to stand age is still not clear, because Kotowska et al. [10
], for example, observed no obvious correlation between total biomass C and NPP when including latex yield. On the other hand, soil C pool might be independent of rubber age sequence. For example, in one study, soil organic C (SOC) stock under rubber plantation increased 1.13 Mg ha−1
over 11 years [20
], whereas in another study, a forest-to-rubber transition led to SOC stock loss by an average of 37.4 Mg ha−1
in 120 cm depth over 46 years [9
Therefore, the objectives of this study were to: (i) quantify above- and belowground biomass C as well as latex C in five different-aged (7-, 13-, 19-, 25- and 47-year-old) rubber plantations; and (ii) relate SOC storage with increasing stand age. We hypothesized that (i) rubber biomass C and its NPP including latex yield has a close quadratic relationship with stand age; and (ii) SOC stock will not be affected by stand age.
2. Materials and Methods
2.1. Study Site and Plot Establishment
The study site is located in Mengla County of Xishuangbanna, Yunnan Province, southwest China (21°09′–22°23′ N, 101°05′–101°50′ E), which borders Myanmar and Laos at the source of the Mekong River. The region has a typical tropical monsoon climate with a rainy season (May–October) and a dry season (November–April). The mean annual rainfall is 1550 mm, out of which 83% occurs in the rainy season. Mean annual evapotranspiration is 1200 mm. Average relative air humidity ranges from 76% to 89%. Mean annual air temperature is 21.5 °C. Temperatures often exceed 38 °C during March and April when the relative humidity is below 40%. The average annual number of sunshine hours is 1800 h with 170 foggy days a year.
Five adjacent rubber plantations with different ages were selected based on similar topography, management practices, and previous vegetation composition (dominated by Millettia
sp., Castanopsis indica
and Phoebe lanceolata
) in January 2009. The rotation period for rubber plantation is about 40 years in this region due to high tree mortality caused by pests and disease [21
]. We identified a chronosequence of rubber stands that were 7-, 13-, 19-, 25-, and 47-years of age. These different-aged stands were first tapped for latex when the trees were 7 years old. All five stands were located within an approximately 10-km radius of each other. Three sampling plots (25 × 40 m) were randomly established in each stand. Within each plot, diameter at breast height (1.3 m, DBH) and height for individual trees were measured using a diameter tape and a Haglöf Vertex clinometer, respectively. Topographical features of each plot, including elevation, slope and aspect, were recorded (Table 1
2.2. Field Sampling and Measurements
On the basis of the DBH and height measurements in the sampling plots, we harvested six standard trees from different diameter classes in each stand for biomass measurements (30 trees in total). The aboveground portions of the standard trees were divided into 5-cm sections for measurement. We measured the fresh weights of stems, branches and foliage in situ. The belowground portions were obtained by total excavation of the standard trees, extending radially out from the trunk and downwards to bedrock until no more roots were visible. We measured the fresh weights of root collars, stump roots, thick roots (diameter (D
) ≥ 2 cm), small roots (0.5 ≤ D
< 2 cm), and fine roots (D
< 0.5 cm) in situ. Samples (500 g) of each compartment in each standard tree were collected, and then oven-dried at 70 °C to a constant weight to calculate the ratio of fresh weight to dry biomass. We built regression models for the different compartments to evaluate tree biomass (n
= 30), and the models can be obtained in our previous work [22
Latex was collected three times a month from the onset of tapping in May 2009 and extended about 7 months. The latex production was recorded through weighing of the fresh material separately for all trees of each plot. The dry weight was measured after oven-drying representative subsamples of latex (six collecting bottles) at 70 °C to a constant weight. Meanwhile, monthly tapping times were also recorded. Then, a model (y = −0.002x2 + 0.089x + 0.584; P < 0.05) was fitted using annual yield (y, t ha−1 year−1) and stand age (x). The total production during the whole tapping phase was calculated by accumulating consecutive annual yield.
Litter, including leaves, twigs (D < 2 cm), flowers/fruits, and miscellaneous litter compartments, were collected every month between January 2009 and December 2009 using 10 litter traps (1 × 1 m) that were randomly distributed in each sampling plot. Litter samples were oven-dried at 70 °C to a constant weight to determine the ratio of fresh weight to dry biomass. Plant biomass in the shrub and herbaceous layers was negligible or practically missing due to regular removal by local farmers, except for their litter, and has thus not been investigated.
Three soil profiles in each plot were dug to a depth of 100 cm, and the samples were taken from six depths (0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm) using a soil corer (5 cm inner diameter). Soil samples from the same depth in the same plot were mixed in equal volume proportions and air-dried at room temperature. Bulk density for each soil depth was determined by collecting samples from a stainless steel cylinder (100 cm3) and oven drying the soil core at 105 °C for constant weight. Bulk density was calculated by dividing the mass of oven-dried soil by the volume of the core. Three soil samples were taken from every depth.
For analysis of C concentration, biomass samples were ground and passed through a 1 mm sieve. Mineral soil samples were sieved through a 0.149 mm sieve before chemical analysis. The C concentrations of tree, latex and soil samples were measured by a dry combustion method with a vario MAX CNS elemental analyzer (Elementar, Langenselbold, Germany). The tree, latex and litter C stocks (Mg ha−1) were calculated by multiplying C concentration by dry biomass (t ha−1). The tree NPP (t ha−1 year−1) was calculated by dividing total C stock by the corresponding stand age. The latex and litter NPP was estimated by total collection amount during a year. The SOC stock (Mg ha−1) was calculated by multiplying C concentration by bulk density and thickness of the soil layer with a correction for stone content.
2.3. Statistical Analysis
One-way ANOVA following Tukey’s Honest Significant Difference (HSD) test was used to test the differences in biomass production, and C stocks in different compartments affected by stand age. A quadratic polynomial regression was applied to check the relationship between C stocks in different compartments and stand age. The software SPSS 13.0 (SPSS Inc., Chicago, IL, USA) was employed for all statistical analyses at a significance level of 0.05.
In a chronosequence of rubber plantations in tropical China, the TBC and NPPtotal, whether including latex C or not, had a close quadratic relationship with stand age. Regardless of stand age, around 68% of the C was stored in aboveground biomass, and NPPlatex contributed to approximately 18% of C sequestration. However, SOC stock in the 100 cm depth was age-independent, and decreased with soil depth for each stand. The TEC across stands averaged 159.6, 174.4, 229.6, 238.1 and 291.9 Mg ha−1, respectively, of which more than 45% was stored in the soil. When considering a maximal life expectancy of rubber trees, the C accumulated in biomass would exceed soil over age sequence. Regression analysis showed that rubber plantations have a potential role in improving the regional C budget in the long term (achieving maximum at about 54 years), and thus they can be considered as alternative land use without affecting forest ecosystem C storage. We also suggest that further research on ecosystem C pool dependence on stand age in rubber agroforestry systems is needed to fully achieve a “win-win” balance between environmental and economic benefits in tropical China.