Improving understanding of carbon stock characteristics of Eucalyptus and Acacia trees in southern China through litter layer and woody debris

C concentration of the litter layer and the woody debris

The present findings demonstrate that the relative C concentration of both the litter layer and the woody debris in Eucalyptus and Acacia plantations was species-dependent, partially supporting our first hypothesis. In the present study, a fixed C concentration or mean value, rather than a measured C concentration, would have been inappropriate for calculating the C stock of litter layer and the woody debris for Eucalyptus plantations, leading to large estimated error of 12 and 9%, respectively. However, the assumed concentration would have been more accurate for Acacia plantations (5% estimated error in the C stock in both studied materials). The implication of this finding is that, in order to accurately estimate the size of the C stock, plantation type-specific measurements are necessary. It has been suggested that the C concentration in the woody debris on the floor of a humid evergreen broad-leaved plantation (49.0%) is very similar to that estimated for the canopy as a whole (50.4%)²⁴, consistent with our finding in Acacia plantations. A recent study has shown that forest structure, in particular trunk diameter, may influence the C concentration of woody debris²². It is likely that the high mortality rate suffered by Acacia trees gives other living trees more opportunity to absorb both sunlight and soil nutrient, thereby – at least to some extent – explaining the higher DBH values associated with their arbor layer (Table 1). The observed increase of DBH values implies a rise in the relative contribution of tree trunk wood to the total biomass, which is of relevance given that the trunks’ C concentration tends to be higher than that of other parts of the biomass. As a result, the C concentration of the woody debris in Acacia plantations was found to be higher than in the Eucalyptus plantations.

The C concentration in the litter layer and woody debris depends on a number of factors, including forest type, stand condition and local climate^{6,7,21,24,32,33,34}. The estimated C concentration of the litter layer in the two types of plantations examined in the present study was comparable with values obtained from the broad-leaved plantations of subtropical China³⁵, and local evergreen broad-leaved forests in the PRD region^7,24. Possibly because the two species stem from the tropical zone, the estimated C concentration of woody debris in the two plantations fell within the range recorded for tropical species²⁵. While the estimated C concentration of the woody debris of the Acacia plantations was somewhat higher than what has been reported for a tropical mixed Metrosideros polymorpha/Acacia plantations³³, but was comparable with the value obtained from a range of coniferous and broad-leaved forests in temperate climates^21,34, as well as from evergreen broad-leaved forests in the PRD region^7,36.

C stock of the litter layer

The C stock of the litter layer in the two plantations types, particularly the Acacia, was compared to estimates derived from the evergreen broad-leaved forests at a nationwide survey³⁷, and the broadleaved tree forests in subtropical and mid-subtropical zone^5,38, specific data see Table 7. The litter layer appears to serve an important role in C accumulation in plantations ecosystem, in particular for Acacia plantations. The size of the C stock contained in the litter layer was significantly higher in the Acacia than that in Eucalyptus plantations, especially in the leaf debris, which was the largest contributor to the litter layer C stock. Its relative contribution to the litter layer reached 72.6% in the Acacia plantations, in agreement with both global estimates (60–76%) and its contribution in tropical Eucalyptus forests (79%)³². In contrast, its contribution in the Eucalyptus plantations examined in the present study was only 54.3%. The quantity of leaf litter has been reported to be associated with the annual increment in the DBH³⁹. Here, particularly in the Acacia plantations, the DBH increased significantly with stand age, implying that the contribution of leaf litter to the C stock would increase accordingly.

Characteristics of the woody debris C stock

C stock of woody debris and its distribution in snags and logs

The measured C stock of the woody debris in the two forest types was significantly lower than that reported for both tropical montane wet forests and moist evergreen broad-leaved forests^33,35. A comparison among evergreen broad-leaved forests in the PRD region revealed that the C stock of woody debris in the Acacia plantations was similar to that in forests composed of middle-aged and mature trees reported by Sun and Guan⁸, but significantly lower than that measured in mature forests by Yang et al.² It is possible that the high growth rate of Acacia does not allow insufficient time for the woody debris to accumulate; the size of the C stock of woody debris in the mature Acacia plantations was significantly higher than in the mature Eucalyptus ones (6.68 vs 1.21 t ha⁻¹), levels comparable to those seen in other subtropical plantations^5,40 (Table 8).

Our study showed that forests type and stand age affect the amount of C stored in woody debris and its distribution in snags and logs. Snags represented the principal component of the woody debris, contributing 60.5% in the middle-aged Acacia plantations and 59.0% in the mature ones. These proportions differ markedly from the composition in tropical and subtropical broad-leaved forests, where logs account for 59–95% of the woody debris^2,5,8,40. It has been suggested that most of the woody debris production (especially snags) is generated as a results of competition between adjacent trees, the death of old trees and external factors, such as weather events, pest and disease damage, and logging^4,33. A large amount of snags present in Acacia plantations may be explained by the high mortality rate of standing trees, in line with conclusions drawn from similar studies¹⁷. With the exception of the mature Acacia plantations, the mortality rate of canopy in our study falls within the range of that for subtropical forests in the PRD region (1.7–12.3% per annual)⁴¹. The size of the C stock in the logs of the Acacia plantations was greater than that for Eucalyptus. It is likely that many of these logs were the result of wind-induced breakage; therefore, their frequency is expected to be higher in the Acacia than in the Eucalyptus plantations.

C stock in various diameter class of woody debris

Fallen wood of smaller diameter experiences a greater degree of contact with the ground, and therefore tends to decompose faster⁴². Woody debris material of Small diameter was more frequent in the Acacia plantations than in the Eucalyptus ones, verifying the hypothesis that forest type affects the C stock within the woody debris of a specific diameter class. The difference between the two plantations types is most likely due to the contrasting wind resistance, particularly for A. mangium and E. urophylla. The PRD region is the area in China where typhoons occur most frequently, and two strong typhoons were recorded here in 2010 alone. Consequently, the proportion of the C stock within woody debris of small diameter (2.5–10 cm) in the Acacia plantations was larger than the range of 40–75% reported in secondary subtropical broad-leaved forests of comparable age³⁶, as well as that in forests within the United States (28%)⁴³. The C stock proportion in both the mature Eucalyptus and Acacia plantations (17 and 16%, respectively) was markedly higher than the ~6% reported in primary moist evergreen broad-leaved tropical forests³⁵. The C stock contained in the debris of diameter class 10–20 cm represented the largest contributor to the total C stock of woody debris in the middle-age Eucalyptus and mature Acacia plantations, possibly partially due to the majority of standing trees having a mean DBH of 10–20 cm. This finding further supports the hypothesis that stand age affects the C stock within the woody debris of a particular diameter range.

Carbon stock at the stage of various decay class of woody debris

In subtropical natural forests, an estimated 50~90% of the woody debris is in an intermediate-to-advanced state of decay^2,3; a significantly higher proportion than that observed in the present study. This discrepancy is possibly due to the rapid growth rate of both Eucalyptus and Acacia trees limiting the time available for woody debris to decay to an advanced degree. Between the two species studies here, the share of the C stock in the woody debris (largely at an early stage of decay) was higher in the Eucalyptus than in the Acacia plantations. The outcome of the present study verifies the hypothesis that forest type exerts a significant influence on the C stock in the woody debris at particular stages of decay. Even in the mature forests, the C stock contained by material at an early decay stage made a significant contribution to the overall C stock of woody debris. The extent of this contribution was substantially greater than that reported for primary forests (~20%), but markedly lower than that associated with mature evergreen broad-leaved forests and mixed old Metrosideros polymorpha/Acacia koa forests (64 and 66%, respectively)^2,33.

In the forest ecosystem, most of the woody debris in the primary decay stage has been created by dead trees and wind damage^3,10. The snags classed in primary decay in the Acacia plantations are likely the result of a combination of typhoons and high mortality rate. A high percentage of woody debris in the plantations of the present study was allocated in decay class I, representing the effect of typhoons. In the Eucalyptus plantations, logs were the principal source of woody debris in a primary decay stage, in agreement with conclusions derived from analyses of secondary forests³.

The contribution of the litter layer and woody debris to the ecosystem’s C stock

In terms of the respective contribution of C stock by the litter layer and woody debris to the forest ecosystem, the values determined for the Acacia plantations (6.8 and 5.0%, respectively) were significantly higher than those for the Eucalyptus plantations (3.7 and 0.7%, respectively). Even in mature Acacia plantations the value reached 8.0 and 7.1%, compared with a mere 4.2 and 1.2%, respectively, for mature Eucalyptus. These findings indicate that both forest type and stand age impact on the capacity of the litter layer and woody debris to store C, in support of our third hypothesis. The contribution of the litter layer in Acacia plantations was higher than that previously reported for subtropical evergreen broad-leaved forests in the PRD region (1.5–6.7%; Table 9), as well as than the estimated worldwide mean of 5%⁴⁴. With respect to the contribution of the woody debris, the value for the Acacia plantations was greater than the estimated Guangdong province-wide mean of 3%⁹, but comparable to estimates from local evergreen broad-leaved forests (4.9–6.5%; Table 9). Therefore, both the litter layer and woody debris appear to contribute significantly to long-term C storage, particularly in Acacia plantations.

Relationship between total biomass and C stock of litter layer and woody debris

The combination of the biomass quantity and the C concentration within it determines the overall size of the C stock present in a given component. A positive correlation was obtained between the total biomass of both the litter layer and the woody debris and their respective components (Figs. 1 and 2), with the highest coefficients of determination corresponding to the total litter layer (0.89 and 0.93 for Acacia and Eucalyptus, respectively; Fig. 1A) and the total woody debris (0.87 and 0.77 for Acacia and Eucalyptus, respectively; Fig. 2A). The implication of this finding is that the C stock of the litter layer and woody debris may be restricted by the ecosystem biomass to a great extent. The conclusion is consistent with a study in which the C stock in the litter layer and the woody debris under evergreen broad-leaved trees in southern China was reported to lie in the ranges 2.13–4.37 and 1.92–8.78 t·ha⁻¹, respectively, in line with the variation found in the ecosystem biomass (66.3–391.9 t·ha⁻¹)⁷.

Figure 1

The relationship between the total biomass and the C stock in the litter layer (and its various components) in Eucalyptus and Acacia plantations.

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Figure 2

The relationship between the total biomass and the C stock in the woody debris (and its various components) in Eucalyptus and Acacia plantations.

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Source: Ecology - nature.com