An assessment of the carbon stocks and sodicity tolerance of disturbed Melaleuca forests in Southern Vietnam
© Tran et al. 2015
Received: 24 March 2015
Accepted: 3 June 2015
Published: 14 July 2015
In the lower Mekong Basin and coastal zones of Southern Vietnam, forests dominated by the genus Melaleuca have two notable features: most have been substantially disturbed by human activity and can now be considered as degraded forests; and most are subject to acute pressures from climate change, particularly in regards to changes in the hydrological and sodicity properties of forest soil.
Data was collected and analyzed from five typical Melaleuca stands including: (1) primary Melaleuca forests on sandy soil (VS1); (2) regenerating Melaleuca forests on sandy soil (VS2); (3) degraded secondary Melaleuca forests on clay soil with peat (VS3); (4) regenerating Melaleuca forests on clay soil with peat (VS4); and (5) regenerating Melaleuca forests on clay soil without peat (VS5). Carbon densities of VS1, VS2, VS3, VS4, and VS5 were found to be 275.98, 159.36, 784.68, 544.28, and 246.96 tC/ha, respectively. The exchangeable sodium percentage of Melaleuca forests on sandy soil showed high sodicity, while those on clay soil varied from low to moderate sodicity.
This paper presents the results of an assessment of the carbon stocks and sodicity tolerance of natural Melaleuca cajuputi communities in Southern Vietnam, in order to gather better information to support the improved management of forests in the region. The results provide important information for the future sustainable management of Melaleuca forests in Vietnam, particularly in regards to forest carbon conservation initiatives and the potential of Melaleuca species for reforestation initiatives on degraded sites with highly sodic soils.
KeywordsCarbon sequestration Climate change Melaleuca REED+ Sodicity
Numerous studies have shown that tropical wetlands typically contain large carbon stocks [1–7]. Protecting and restoring tropical coastal wetlands is considered a critical part of how society adapts to and mitigates global climate change .
Large areas of Melaleuca forests in Vietnam are disturbed ecosystems that experience extreme conditions, and are associated with floods and/or sodic soils. They mostly occur in the lower Mekong Basin, which has been severely impacted by climate change [9–12]. Little is known about the carbon sequestration potential of disturbed Melaleuca forests in Australasia and South-East Asia where the genus occurs. Carbon stocks of Melaleuca forests are generally considered to be low (i.e. about 27.8 tC/ha estimated by Australian Government Office ). However, Tran et al.  suggested that this has been grossly under-estimated and that Melaleuca cajuputi forests on peatland soils in Vietnam, Indonesia and Malaysia are likely to have a high potential for carbon sequestration.
Sea level rise has significant impacts on the coastal zone, where soils will become saline and/or highly sodic . Sodic soils are distinguished by an excessively high concentration of Sodium (Na) in their cation exchange complex. High sodicity causes soil instability due to poor physical and chemical properties, which affects plant growth and can have a more significant impact than excessive salinity growth [16, 17]. Sodicity impacts plant growth in three ways, including: soil dispersion, specific ion effects, and nutritional imbalance in plants [18, 19]. Excessive sodium concentrations cause clay dispersion which is the primary physical effect of the sodic soil. Sodium-induced dispersion can reduce water infiltration, decrease hydraulic conductivity, and increase soil surface crusting that strongly affect roots such as root penetration, root development, and blocking plant uptake of moisture and nutrients .
Except for those containing mangroves and other halophytes, most ecosystems are severely affected by salinity and/or sodicity. A few studies have examined saline-sodic soils in shrimp farming areas in the coastal regions of Vietnam (i.e. ECe = 29.25 dS/m and exchangeable sodium percentage ranged from 9.63 to 72.07%, which had a big impact on plant cultivation systems ).
Several studies (such as Dunn et al. , Niknam and McComb , van der Moezel et al. [23, 24]) have examined the tolerance of woody species such as Acacia, Eucalyptus, Melaleuca, and Casuarina species to salinity and/or sodicity, but more research is required. This paper examines the carbon stocks of disturbed Melaleuca forests and the sodicity tolerance of M. cajuputi forests in Southern Vietnam.
Results and discussion
Characteristics of the typical Melaleuca forests in the study areas
Major characteristics of five typical Melaleuca forests in the study areas
Primary Melaleuca on sandy soil
DBH < 5 cm
Including non-inundated, seasonal, and permanent inundation
5 cm ≤ DBH < 10 cm
10 cm ≤ DBH < 20 cm
20 cm ≤ DBH < 30 cm
30 cm ≤ DBH < 40 cm
DBH ≥ 40 cm
Regenerating Melaleuca on sandy soil
DBH < 5 cm
Including non-inundated and seasonal inundation
5 cm ≤ DBH < 10 cm
DBH ≥ 10 cm
Degraded secondary Melaleuca on clay soil with peat
DBH < 5 cm
Including seasonal and permanent inundation
5 cm ≤ DBH < 10 cm
10 cm ≤ DBH < 20 cm
20 cm ≤ DBH < 30 cm
30 cm ≤ DBH < 40 cm
DBH ≥ 40 cm
Regenerating Melaleuca on clay soil with peat
DBH < 5 cm
Including seasonal and permanent inundation
5 cm ≤ DBH < 10 cm
DBH ≥ 10 cm
Regenerating Melaleuca on clay soil without peat
DBH < 5 cm
Including seasonal and permanent inundation
5 cm ≤ DBH < 10 cm
DBH ≥ 10 cm
Average DBH of all stand classes were 16.71, 5.36, 12.93, 5.88, and 6.20 for VS1, VS2, VS3, VS4, and VS5, respectively (Figure 1b). There was a significant difference in DBH in the five Melaleuca forest types (χ2 = 446.86, p = 2.2e−16). However, post hoc test shows that there is no significant difference in tree DBH between VS1 and VS3, and between VS2, VS4, and VS5 (Additional file 1: 2b).
Average total height of all stand classes were 14.69, 7.11, 9.69, 5.68, and 7.50 m for VS1, VS2, VS3, VS4, and VS5, respectively (Figure 1c). There was a significant difference in the total height of the five Melaleuca forest types (χ2 = 11.616, p = 0.0088) (Additional file 1: 2c). Furthermore, the tree density of the five forest types was generally very high, especially of VS2, VS4 and VS5 (over 2,000 individuals/ha), which can contribute to a large biomass. The basal areas shown in Figure 1d further confirm the potential high biomass of VS2, VS4 and VS5 (BA = 28.41, 30.14, and 23.14 m2/ha, respectively). Furthermore, the basal area of VS1 is significantly greater than VS3, accounting for 41.45 and 10.29 m2/ha, respectively (F = 3.341, p = 0.0423) (Additional file 1: 2d).
Different species were found in the understorey of the various Melaleuca forest types. Key species for VS1 and VS2 include Leptocarpus sp., Lepironia sp., Hanguana sp., Eleocharis sp., Euriocaulon sp., Xyris sp., Stenochlaena sp., Melastoma sp., and Imperata cylindrica. For VS3, VS4, VS5, the following species dominate the understorey: Stenochlaenapalustris sp., Phragmitesvallatoria sp., Melastomadodecandrum sp., Diplaziumesculentum sp., Lygodiumscandens sp., Aspleniumnidus sp., Scleriasumatrensis, Cassia tora, Paederiafoetida sp., Flagellariaindica sp., and Cayratiatrifolia sp. (Table 1).
Carbon stocks of Melaleuca forests
On sandy soil, the stands and soil layers were the highest contributors to carbon density of VS1 (accounting for 41.34 and 29.11%, respectively), while VS2 has a high contribution from the soil layer, then stands (soil and stand categories contribute for carbon density of 56.15 and 28.53%, respectively) (Figure 2b). However, in the peat land, the greatest contribution of carbon densities for VS3 and VS4 are the peat and soil categories (accounting for 61.41%, 22.10% of VS3, and 57.66, and 16.72% of VS4, respectively). Separately, carbon density of VS5 is mostly linked to the soil, deadwood, and stand categories (accounting for 33.54, 32.16, and 14.66%, respectively) (Figure 2b).
Variability of carbon stocks in different types of Melaleuca forests
The carbon densities of stands of the various forest types were 110.67, 44.27, 22.79, 48.25, and 37.20 tC/ha for VS1, VS2, VS3, VS4, and VS5, respectively (Figure 3a). There was a significant difference in stand carbon density between the forest types (χ2 = 48.3184, p = 8.1e−10) (Additional file 1: 2f). The carbon density of the stand VS1 is the highest and is 2.5, 4.9, 2.3, and 3.0 times higher than VS2, VS3, VS4, and VS5. Surprisingly, there is no statistical difference in stand carbon densities between secondary forests (VS3) and regenerating forests (VS2, VS4 and VS5) (Additional file 1: 2f). These carbon stocks were lower those from other studies of different forests (e.g. 144 tC/ha for Asian tropical forests ; 200.23 tC/ha and 92.34 tC/ha of primary and secondary swamp forests in Indonesia (involving Melaleuca vegetation), respectively ).
The carbon densities of the understory in the Melaleuca forests of Vietnam were 2.45, 2.48, 6.23, 1.65, and 5.27 tC/ha for VS1, VS2, VS3, VS4, and VS5, respectively (Figure 3b). There was a statistically significant difference in understory carbon density between the forest types (χ2 = 30.7189, p = 3.49e−6) (Additional file 1: 2g). However, there was no significant difference in understory carbon density between Melaleuca forest types on sandy soils (VS1 and VS2). On clay soils, the understory carbon densities of VS3 and VS5 were significantly higher than VS4.
The carbon densities of deadwood in the forest types were 30.47, 0, 67.90, 45.06, and 74.59 tC/ha for VS1, VS2, VS3, VS4, and VS5, respectively (Figure 3c). There was a statistically significant difference in deadwood carbon density between the Melaleuca forest types (χ2 = 3.0978, p = 0.5416), but pairwise comparisons show no significant differences (Additional file 1: 2 h). Surprisingly, deadwood was not present in regenerating forests in the study sites on Phu Quoc Island. This is probably due to frequent forests fires and/or fuelwood collection by people associated crop cultivation.
Some of the carbon stock of Melaleuca forests is contributed by layers of coarse and fine litter. The carbon densities of the total litter layer of the forest types were 31.03, 14.45, 23.76, 57.35, and 39.23 tC/ha for VS1, VS2, VS3, VS4, and VS, respectively (Figure 3d). There was a statistically significant difference in overall litter carbon density between these forest types (χ2 = 1.5619, p = 0.08156), but pairwise comparisons show no significant differences (Additional file 1: 2i).
The carbon densities from peat of the Melaleuca forests were 479.62 and 294.57 tC/ha for secondary forests (VS3) and regenerating forests (VS4), respectively (Figure 3e). The carbon density from peat of VS3 is significantly greater than that of VS4 (χ2 = 5.2359, p = 0.0221) (Additional file 1: 2j). This is almost certainly due to peat being partly burned in the regenerating forest by the severe fire of 2002. In U Minh Thuong National Park, peat comprises the top soil layer, with a deep layer of clay below. The depth of the peat layer ranged from 15 to 62 cm in 18 soil cores, and the peat bulk density ranged from 0.19 to 0.3. The depths of the peat layer in this study were much thinner than in other forests (i.e. primary peat layer in U Minh Thuong was over 90 cm depth , and the thick peat layer in U Minh Ha was over 120 cm depth ).
The carbon densities of roots in the Melaleuca forests were 22.75, 16.97, 11.97, 6.99, and 8.35 tC/ha for VS1, VS2, VS3, VS4, and VS5, respectively (Figure 3f). There was a statistically significant difference in root carbon density between the forest types (χ2 = 22.437, p = 0.00016). The carbon densities of roots in Melaleuca forests in sandy soil were higher than those in clay soil, in particular, the root carbon density of VS2 was significant higher than that of VS4 (Additional file 1: 2k).
Organic soil carbon densities to a 30 cm depth in the study areas were 75.81, 89.22, 178.93, 93.94, and 83.58 tC/ha for VS1, VS2, VS3, VS4, and VS5, respectively (Figure 3 g). There was a statistically significant difference in organic soil carbon density between the forest types (χ2 = 1.7333, p = 0.230), but pairwise comparisons showed no significant differences (Additional file 1: 2k). These results are consistent with those of other studies of soil carbon stocks in wetlands (e.g. organic soil carbon stocks in swamp forests in Indonesia (with Melaleuca vegetation) were 106.00 and 135.63 tC/ha in the top 30 cm of soil of primary and secondary forests, respectively ).
Potential carbon storage in Melaleuca peat-swamp forests: case in U Minh Thuong National Park
Land cover type
Carbon density (tC/ha)
Carbon storage tC
Mature Melaleuca forests on clay soil without peat
Mature Melaleuca forests on clay soil with peat
Regenerating Melaleuca on clay soil with peat
Regenerating Melaleuca on clay soil without peat
Others (open water, reeds and grasses)
Sodicity tolerance of Melaleuca cajuputi forests toward the adaptation to global climate change
Sea-level rise is a consequence of global climate change that will severely affect coastal and wetland ecosystems. Melaleuca forests are largely located in coastal and wetland areas that may be affected by climate change , so the risk of salinization of the region will increase. Salinity in soils can damage woody plant species by stunting buds, reducing leaf size and causing necroses in buds, roots, leaf margins and shoot tips . Salinity can also inhibit seed germination, and can even kill non-halophytic species . Both vegetative and reproductive growth of woody species are also reduced by high concentrations of sodium chloride in soil [35, 36]. The combination of flooding and salinity can create a more pronounced effect on growth and survival of plants than either stress alone . High concentrations of sodium can affect the structure of sodic soils [37–39]. In contrast, low sodium concentration, soil structure is not affected by salinity in saline soil . Sodicity and salinity always occur together and coming to have negative impacts on soil properties and plants [38, 41], but sodic soils may be either non-saline or saline .
The lower Mekong Basin and coastal regions of southern Vietnam are highly vulnerable to global climate change impacts [9, 33, 42, 43]. Most of Vietnam’s Melaleuca forests occur in these areas and will be affected projected sea-level rise. Fortunately, this study has shown that M. cajuputi has the ability to tolerant increase in sodic soils.
Chemical element concentration and sodicity levels of the Melaleuca forest soils in the study areas
Soil layers (cm)
Ca2+ (meq/100 g)
Mg2+ (meq/100 g)
Na+ (meq/100 g)
K+ (meq/100 g)
Al3+ (meq/100 g)
Fe3+ (mg/100 g)
Primary Melaleuca on sandy soil (VS1)
Regenerating Melaleuca on sandy soil (VS2)
Degraded secondary Melaleuca on clay soil with peat (VS3)
Regenerating Melaleuca on clay soil with peat (VS4)
Regenerating Melaleuca on clay soil without peat (VS5)
With the exception of mangroves, few woody species can tolerate saline and/or sodic soils. Many woody species have been examined for their tolerance of salinity and/or sodicity. For example, Eucalyptus, Melaleuca, Acacia, Casuarina [21–24], Grevillea robusta, Lophostemon confertus and Pinus caribea , and Moringa olifera  have been examined and their tolerance to salinity assessed in the field and in glasshouses. In extremely saline soils in Australia, Niknam and McComb  suggested that the land care benefit of establishing species such as Melaleuca or Casuarina is more important than their commercial value. As well as the land care value, this study has shown that M. cajuputi forests in Vietnam can adapt to climate change through their tolerance to sodicity, and other harsh conditions , and can help to mitigate climate change through their carbon storage abilities.
By undertaking original field data, this study examined the carbon sequestration potential of five types of Melaleuca forests including ‘Primary Melaleuca forests on sandy soil’ (VS1), ‘Regenerating Melaleuca forests on sandy soil’ (VS2), ‘Degraded secondary Melaleuca forests on clay soil with peat’ (VS3), ‘Regenerating Melaleuca forests on clay soil with peat’ (VS4), and ‘Regenerating Melaleuca forests on clay soil without peat’ (VS5). The study also assessed the sodicity tolerance of M. cajuputi forests in coastal and wetland regions of Vietnam.
The carbon densities of VS1, VS2, VS3, VS4, and VS5 were 275.98 (±38.62) tC/ha, 159.36 (±21.01) tC/ha, 784.68 (±54.72) tC/ha, 544.28 (±56.26) tC/ha, and 246.96 (±27.56) tC/ha, respectively. Most carbon stocks were contributed from the soil (including peat) and stands.
The exchangeable sodium percentage (ESP) of soil from Melaleuca forests on clay soil (VS3, VS4, and VS5) ranged from low to moderate sodicity, but those from Melaleuca forests on sandy soil (VS1 and VS2) were highly sodic.
The results provide important information for the future sustainable management of Melaleuca forests in Vietnam, particularly in regards to forest carbon conservation initiatives and the potential of Melaleuca species for reforestation initiatives on degraded sites with highly sodic soils. In Vietnam, forest carbon conservation initiatives such as REDD+ have hereto, in our view, not placed appropriate priority or consideration on the protection of carbon stocks of Melaleuca forests. The results presented in this paper suggest that Melaleuca forests in Vietnam, particularly those on peatland areas, hold globally significant carbon stocks—arguably greater than those found in upland rainforest ecosystems, which have so far been given higher priority in REDD+ planning in Vietnam. Furthermore, the results presented in this paper suggest that some Melaleuca forest species in Vietnam, particularly those on sandy soils, exhibit a tolerance for highly sodic soils. This suggests that those species might be useful in reforestation initiatives on degraded sites with highly sodic soils. As degradation pressures including climate change continue to alter the hydrological features of soil systems in areas such as the Mekong Delta in Vietnam, and the sodicity of soils in some areas increases, Melaleuca species could offer a useful option for reforestation and rehabilitation initiatives.
The results in this research provide further scientific information to support better Melaleuca ecosystem management. The results should help policy makers make better decisions in an era of global change. The results have particular relevance for the application of REED+ in the Southeast Asia.
Study sites and disturbance context
Phu Quoc National Park is located on the northern Phu Quoc Island of Vietnam (at N 10°12′07″–N 10°27′02″, E 103°50′04″–E 104°04′40″) (Figure 4). Melaleuca forest areas cover 1,667.50 ha out of the total area of 28,496.90 ha. These Melaleuca forests naturally occur on lowland regions of the island where they are seasonally inundated and/or permanent saturated, and also on permanent sand bars where no inundation occurs . The rest areas of the park are hilly and mountainous forests. Two Melaleuca forest types were found in the park: primary Melaleuca forest (VS1); and regenerating Melaleuca forest (VS2). Before the park was established in 2001, key disturbance included forest fires and human intrusion for crop cultivation. The regenerating Melaleuca forests were up to 10–12 years of age at the time this study was conducted.
U Minh Thuong National Park is located in the Kien Giang Province (at N 9° 31′–N 9° 39′, E 105° 03′–E 105° 07′) (Figure 4). Melaleuca forest on swamp peatland is an endemic ecosystem in the lower Mekong Basin of Vietnam. The core area of the park is 8,038 ha, which is surrounded by a buffer zone of 13,069 ha. Here, the key disturbance is fire, with the last major fire occurring in April 2002, which burnt the primary vegetation as well as the peat soil. The Vietnamese Environment Protection Agency  reported that 3,212 hectares of Melaleuca forests was almost destroyed, so a canal system was built as a key management solution to increase water inundation of the forest to prevent fires. Currently, there are three Melaleuca forest types in U Minh Thuong National Park: VS3, VS4, and VS5. At the time of this study, the VS4 and VS5 areas were up to 10 years old.
Field sampling and data collection
The major plots were set out as 500 m2 quadrats (20 m × 25 m), and all trees with a DBH ≥ 10 cm were measured and recorded. Sub-plots also were set out as 100 m2 quadrats (20 m × 5 m) within the major plots to measure all trees with DBH < 10 cm and a total height of >1.3 m (modified from Van et al. ). Data on DBH, alive or dead, and height were recorded for all standing trees.
Deadwood (dead fallen trees) with a diameter ≥10 cm were measured within the major plots (500 m2), while deadwood with 5 cm ≤ diameter < 10 cm were measured within the sub-plots (100 m2). Diameters at both ends of the trunk (D1 and D2), length (if ≥50 cm length), and the decay classes (involved sound, intermediate, and rotten [50, 51]) were recorded for all deadwood.
Seventy random quadrats (1 m × 1 m) were located in the main plots to collect and record the ‘fresh weight’ of the understory. Samples of all species from the understory were collected in each major plot and taken back to the Vietnam Forestry University laboratory for drying.
Seventy random coarse litter samples and seventy random fine litter samples were collected in the major plots. The fresh weight of each litter sample was recorded. Each litter type (coarse litter and fine litter) collected in every major plot were well mixed and taken to the laboratory for drying.
Two soil samples, one from the upper (0–10 cm) soil layer and one from the lower (10–30 cm) soil layer, were taken from each of 14 plots, giving a total of 28 soil samples. The 28 soil samples were taken back to the National Institute of Agricultural Planning and Projection laboratory for further analysis. Various soil chemical properties of the 28 samples were tested including: pHKCl, total C, total N, Ca2+, Mg2+, Na+, K+, Al3+, and Fe3+. Twenty-eight duplicate soil samples were collected and analyzed for bulk density.
Biomass allometric computation
List of allometric equations applied to examine stand biomass of the Melaleuca forests
log10(FW) = 2.266log10(D) − 0.502 where FW = fresh above-ground biomass (kg/tree), D = diameter at breast height (cm)
Finlayson et al. 
y = 0.124 × DBH 2.247 where y = above-ground biomass (kg/tree), DBH = diameter at breast height (cm)
y = exp[–2.134 + 2.53ln(D)] where y = above-ground biomass (kg/tree), D = diameter at breast height (cm)
Tropical, moist forest
ln(y) = 2.4855ln(x) − 2.3267 where y = above-ground biomass (kg/tree), x = diameter at breast height (cm)
Native sclerophyll forest
NSW, ACT, VIC, TAS, and SA
Keith et al. 
ln(AGB) = –1,554 + 2.420ln(D) + ln(ρ) where AGB = above-ground biomass (kg/tree), D = diameter at breast height (cm), ρ = wood density (g/cm3)
America, Asian and Oceania
Chave et al. 
ln(RBD) = – 1,085 + 0.926ln(ABD) where RBD = root biomass density (tons/ha), ABD = above-ground biomass density (tons/ha)
y = 0.27x where y = total root biomass (tons/ha), x = total shoot biomass (tons/ha)
Mokany et al. 
Wr = 0.0214 × D 2.33 where W r = coarse root biomass (kg/tree), D = diameter at breast height (cm)
Tropical secondary forests
Kenzo et al. 
W r = 0.023 × D 2.59 where Wr = coarse root biomass (kg/tree), D = diameter at breast height (cm)
Tropical secondary forests
Niiyama et al. 
Then, the biomass of the fallen deadwood was determined using the IPCC [50, 51] density reduction factors (sound = 1, intermediate = 0.6, and rotten = 0.45). The biomass of standing dead trees was measured using the same criteria as live trees, but a reduction factor of 0.975 is applied to dead trees that have lost leaves and twigs, and 0.8 for dead trees that have lost leaves, twigs, and small branches (diameter <10 cm) (, p 4.105).
To convert biomass to carbon mass for all categories (stands, roots, deadwood, understory, and litter), a factor of 0.45 was applied.
One-way ANOVA tests were applied to compare stand densities, DBH, height classes, basal areas, and six categories of carbon stocks of the five Melaleuca forest types. LSD post hoc tests were also used for all pairwise comparisons between group means. Statistical analysis was undertaken using Microsoft Excel 2010 and the R Statistic Program.
DBT conducted design of the study, field data collection, carried out all analyses and drafted the manuscript. TVH and PD helped field data collection, guided the research, and assisted with the writing. All authors read and approved the final manuscript.
This study was authorized to access and collect vegetation and soil samples by the director boards of two national parks including the Phu Quoc National Park and U Minh Thuong National Park. All work was approved by the Vietnam Forestry University. We would like to thank the staffs of Phu Quoc National Park; U Minh Thuong National Park; the National Institute of Agricultural Planning and Projection; and the Vietnam Forestry University for their association of doing fieldwork and laboratory work. We also specially thank the anonymous reviewers for their excellent comments on the earlier version of this manuscript. We gratefully thank International Foundation for Science (IFS) for research funds.
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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