An assessment of the carbon stocks and sodicity tolerance of disturbed Melaleuca forests in Southern Vietnam

Abstract

Background

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.

Results

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.

Conclusions

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.

Tran et al. Carbon Balance Manage (2015) 10:15 DOI 10.1186/s13021-015-0025-6

An assessment ofthe carbon stocks andsodicity tolerance ofdisturbed Melaleuca forests inSouthern Vietnam

Da B Tran1*, Tho V Hoang1 and Paul Dargusch2

Background

Numerous studies have shown that tropical wetlands typically contain large carbon stocks [17]. Protecting and restoring tropical coastal wetlands is considered a critical part of how society adapts to and mitigates global climate change [8].

Large areas of Melaleuca forests in Vietnam are disturbed ecosystems that experience extreme conditions, and are associated with oods and/or sodic soils. They mostly occur in the lower Mekong Basin, which has been severely impacted by climate change [912]. 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 [13]). However, Tran et al. [14] 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 signicant impacts on the coastal zone, where soils will become saline and/or highly sodic [15]. 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 aects plant growth and can have a more signicant impact than excessive salinity growth [16, 17]. Sodicity impacts plant

*Correspondence: [email protected]

1 The Vietnam Forestry University, Hanoi, VietnamFull list of author information is available at the end of the article

2015 Tran et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

Web End =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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growth in three ways, including: soil dispersion, specic ion eects, and nutritional imbalance in plants [18, 19]. Excessive sodium concentrations cause clay dispersion which is the primary physical eect of the sodic soil. Sodium-induced dispersion can reduce water inltration, decrease hydraulic conductivity, and increase soil surface crusting that strongly aect roots such as root penetration, root development, and blocking plant uptake of moisture and nutrients [19].

Except for those containing mangroves and other halophytes, most ecosystems are severely aected 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 [20]).

Several studies (such as Dunn etal. [21], Niknam and McComb [22], van der Moezel etal. [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 anddiscussion

Characteristics ofthe typical Melaleuca forests inthe study areas

The major characteristics of ve Melaleuca forests types examined include standing trees, an understory, and saturated conditions (Table1). The variation in these characteristics not only distinguishes the dierent stands but also improves understanding of their carbon stocks.

The stand densities of the ve typical Melaleuca forest types varied considerably: they were 2,330, 10,950, 980, 9,833, and 6,867 trees/ha for VS1, VS2, VS3, VS4, and VS5, respectively (Table 1). Within each study site, the tree densities of regenerating forests (VS2, VS4, and VS5) were signicantly higher than primary forests (VS1) and secondary forests (VS2) (Figure1a). The increased stand densities of types VS2, VS4, and VS5 were mostly comprised of trees with a diameter at breast height (DBH) <10 cm. In contrast, VS1 was dominated by trees with DBH<20cm (accounting for 84.3%), with the balance of trees having a DBH20cm (including 4.2% of trees with

DBH 30), while VS3 was mostly dominated by trees with a 5cmDBH<20cm (accounting for 96%), with the balance having a 20cmDBH<40cm (accounting for 4%) (Table1).

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 (Figure1b). There was a signicant dierence in DBH in the ve Melaleuca forest types (2=446.86,

p=2.2e16). However, post hoc test shows that there is no signicant dierence in tree DBH between VS1 and

VS3, and between VS2, VS4, and VS5 (Additional le1: 2b).

Average total height of all stand classes were 14.69, 7.11, 9.69, 5.68, and 7.50m for VS1, VS2, VS3, VS4, and VS5, respectively (Figure1c). There was a signicant difference in the total height of the ve Melaleuca forest types (2 = 11.616, p = 0.0088) (Additional le 1: 2c).

Furthermore, the tree density of the ve 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 conrm the potential high biomass of VS2, VS4 and VS5 (BA=28.41, 30.14, and 23.14m2/ha, respectively).

Furthermore, the basal area of VS1 is signicantly greater than VS3, accounting for 41.45 and 10.29m2/ha, respectively (F=3.341, p=0.0423) (Additional le1: 2d).

Dierent 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. (Table1).

Carbon stocks ofMelaleuca forests

The carbon densities of ve typical Melaleuca forests in Southern Vietnam were 275.98, 159.36, 784.68, 544.28, and 246.96 tC/ha, respectively, for 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) (Figure2a). There is signicant dierence in carbon densities between the forest types (2=10.419, p=0.0339)

(Additional le1: 2e). On sandy soils, the carbon density of VS1 was signicantly greater (1.7 times) than VS2. The carbon density of Melaleuca forests on clay soil with peat was still high after disturbance (VS3 was 1.4 times higher than VS4). The carbon density of VS5 was lower than VS3 and VS4 because there was no peat layer.

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) (Figure2b). However, in

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Including non-inun-

dated, seasonal,

and permanent

inundation

inundated and

seasonal inunda-

tion

Including seasonal

and permanent

inundation

5 cm DBH < 10 cmVS3C1350na7.120.68nana4.570.38

Hanguana sp.

Eleocharis sp.

Euriocaulon sp.

Xyris sp.

Stenochlaena sp.

Melastoma sp.

Imperata sp.

Melastomadodecandrum

Diplaziumesculentum

Lygodiumscandens

Aspleniumnidus

Scleriasumatrensis

Cassia tora

Paederiafoetida

Flagellariaindica

Cayratiatrifolia

Density(trees/ha)DBH(cm)BA(m2 /ha)Height (m)

MeanSEMeanSEMeanSEMeanSE

Table 1 Major characteristics ofve typical Melaleucaforests inthe study areas

Forest typesTree classesCodeStand treesUnderstorySaturation levels

DBH < 5 cmVS1C0800248.33.870.11nana6.000.28Leptocarpus sp.

Lepironia sp.

5 cm DBH < 10 cmVS1C1400100.07.180.36nana9.810.68

10 cm DBH < 20 cmVS1C2750273.814.630.22nana14.800.26

20 cm DBH < 30 cmVS1C328534.024.330.49nana18.440.40

30 cm DBH < 40 cmVS1C48028.334.370.90nana20.170.97

DBH 40 cmVS1C52010.048.733.75nana22.201.77

All classesVS12,330558.016.710.5541.546.1614.690.30

DBH < 5 cmVS2C05,4502,850.03.630.07nana6.130.16Including non-

5 cm DBH < 10 cmVS2C15,500700.07.070.14nana8.080.15

DBH 10 cmnanananananananana

All classesVS210,9503,550.05.360.1428.413.147.110.13

DBH < 5 cmVS3C0150na4.410.23nana5.000.29Stenochlaenapalustris

Phragmitesvallatoria

10 cm DBH < 20 cmVS3C244020.013.110.36nana10.440.41

20 cm DBH < 30 cmVS3C330na25.001.20nana14.330.17

30 cm DBH < 40 cmVS3C410na35.35nanana12.50na

DBH 40 cmVS3C5nananananananana

All classesVS3980560.012.930.7110.294.749.690.45

DBH < 5 cmVS4C03,8672,258.63.840.06nana4.150.11Including seasonal

and permanent

inundation

5 cm DBH < 10 cmVS4C15,967176.47.200.12nana6.680.17

DBH 10 cmnanananananananana

All classesVS49,8332,265.95.880.1230.141.465.680.13

DBH < 5 cmVS5C02,133592.63.820.09nana4.950.17Including seasonal

and permanent

inundation

5 cm DBH < 10 cmVS5C14,7331,560.37.270.13nana8.650.31

DBH 10 cmnanananananananana

All classesVS56,8671,970.16.200.1423.028.537.500.25

Primary Melaleucaon

sandy soil

Regenerating

Melaleucaon sandy

soil

Degraded secondary

Melaleucaon clay soil

with peat

Regenerating

Melaleucaon clay soil

with peat

Regenerating

Melaleucaon clay soil

without peat

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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) (Figure2b).

Variability ofcarbon stocks indierent types ofMelaleuca forests

This study investigated the carbon stocks of six categories: stands, understory, deadwood, litter, root, and soil for ve types of Melaleuca forests in Southern Vietnam (Figure3).

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 signicant dierence in stand carbon density between the forest types (2 = 48.3184, p=8.1e10) (Additional le1: 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 dierence in stand carbon densities between secondary forests (VS3) and regenerating forests (VS2, VS4 and VS5) (Additional le1: 2f). These carbon stocks were lower those from other studies of dierent forests (e.g. 144 tC/ha for Asian tropical forests [25]; 200.23 tC/ha and 92.34 tC/ha of primary and secondary

swamp forests in Indonesia (involving Melaleuca vegeta

tion), respectively [26]).

The carbon densities of the understory in the Melaleuca forests of Vietnam were 2.45, 2.48, 6.23, 1.65, and 5.27tC/ha for VS1, VS2, VS3, VS4, and VS5, respectively (Figure3b). There was a statistically signicant dierence in understory carbon density between the forest types (2=30.7189, p=3.49e6) (Additional le1: 2g). However, there was no signicant dierence 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 signicantly higher than VS4.

The carbon densities of deadwood in the forest types were 30.47, 0, 67.90, 45.06, and 74.59tC/ha for VS1, VS2, VS3, VS4, and VS5, respectively (Figure 3c). There was a statistically signicant dierence in deadwood carbon density between the Melaleuca forest types (2=3.0978, p = 0.5416), but pairwise comparisons show no signicant dierences (Additional le1: 2h). Surprisingly, deadwood was not present in regenerating forests in the study sites on Phu Quoc Island. This is probably due to frequent forests res and/or fuelwood collection by people associated crop cultivation.

Some of the carbon stock of Melaleuca forests is contributed by layers of coarse and ne litter. The carbon densities of the total litter layer of the forest types were

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31.03, 14.45, 23.76, 57.35, and 39.23tC/ha for VS1, VS2, VS3, VS4, and VS, respectively (Figure3d). There was a statistically signicant dierence in overall litter carbon density between these forest types (2 = 1.5619, p=0.08156), but pairwise comparisons show no signi-cant dierences (Additional le1: 2i).

The carbon densities from peat of the Melaleuca forests were 479.62 and 294.57tC/ha for secondary forests (VS3) and regenerating forests (VS4), respectively (Figure3e). The carbon density from peat of VS3 is signicantly greater than that of VS4 (2=5.2359, p=0.0221)

(Additional le 1: 2j). This is almost certainly due to

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peat being partly burned in the regenerating forest by the severe re 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 62cm in 18 soil cores, and the peat bulk density ranged from 0.19 to 0.3. The depths of the peat layer in

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this study were much thinner than in other forests (i.e. primary peat layer in U Minh Thuong was over 90cm depth [27], and the thick peat layer in U Minh Ha was over 120cm depth [28]).

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 signicant dierence in root carbon density between the forest types (2 = 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 signicant higher than that of VS4 (Additional le1: 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 (Figure3g). There was a statistically signicant difference in organic soil carbon density between the forest types (2=1.7333, p=0.230), but pairwise comparisons showed no signicant dierences (Additional le1: 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 135.63tC/ha in the top 30cm of soil of primary and secondary forests, respectively [29]).

Overall, the carbon density of Melaleuca forests on sandy soil in Southern Vietnam ranged from 159.36tC/ ha for regenerating forests to 275.98 tC/ha for primary forests. The carbon densities of forests on clay soil ranged from 246.96tC/ha for regenerating forests without peat to 784.68 tC/ha of secondary forests with peat. Compared with the carbon stocks of other forests on peatland (e.g. the carbon density of mangrove forests in the Indo-Pacic region was 1,030 tC/ha [30]), the carbon density of disturbed Melaleuca forests on the peatland of Southern Vietnam is about one half, but the results are consistent with other studies on peat swamp forests (e.g. the carbon density of undisturbed swamp forests in South-East Asia ranged from 182 to 306tC/ha [31]). Despite this, Melaleuca forests in the peatlands of Vietnam still have high

potential as carbon stores. The case of U Minh Thuong National Park is an example. The total carbon stock of 8,038ha of Melaleuca forests in the park is about 2.69M tC (Table2), which is equivalent 9.43M tCO2e. Furthermore, there were 8,576 hectares of Melaleuca forested peatland in U Minh Ha National Park that have peat layers ranging from 40cm to over 120cm deep [32], which provides an even higher potential carbon store.

Sodicity tolerance ofMelaleuca cajuputi forests towardthe adaptation toglobal climate change

Sea-level rise is a consequence of global climate change that will severely aect coastal and wetland ecosystems.Melaleuca forests are largely located in coastal and wet-land areas that may be aected by climate change [33], 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 [34]. Salinity can also inhibit seed germination, and can even kill nonhalophytic species [35]. Both vegetative and reproductive growth of woody species are also reduced by high concentrations of sodium chloride in soil [35, 36]. The combination of ooding and salinity can create a more pronounced eect on growth and survival of plants than either stress alone [35]. High concentrations of sodium can aect the structure of sodic soils [3739]. In contrast, low sodium concentration, soil structure is not aected by salinity in saline soil [40]. 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 [17].

The lower Mekong Basin and coastal regions of southern Vietnam are highly vulnerable to global climate change impacts [9, 33, 42, 43]. Most of Vietnams Melaleuca forests occur in these areas and will be aected projected sea-level rise. Fortunately, this study has shown that M. cajuputi has the ability to tolerant increase in sodic soils.

About 28 soil samples collected from Melaleuca forests in Southern Vietnam were examined and all were

Table 2 Potential carbon storage inMelaleuca peat-swamp forests: case inU Minh Thuong National Park

Land cover type Area (ha) Carbon density (tC/ha) Carbon storage tC

Mature Melaleuca forests on clay soil without peat 1,765 305.06 538,431

Mature Melaleuca forests on clay soil with peat 601 784.68 471,593

Regenerating Melaleuca on clay soil with peat 2,106 544.28 1,146,254

Regenerating Melaleuca on clay soil without peat 1,106 246.96 273,138

Others (open water, reeds and grasses) 2,460 107.91 265,459

Total 8,038 2,694,874

The areas of Melaleuca forests in U Minh Thuong National Park are taken from a Vietnam Environment Protection Agency report [48].

Tran et al. Carbon Balance Manage (2015) 10:15

Table 3 Chemical element concentration andsodicity levels ofthe Melaleucaforest soils inthe study areas

Page 8 of 14

ESP (%)Sodicity

MeanSEMeanSEMeanSEMeanSEMeanSEMeanSEMeanSEMeanSE

Primary Melaleucaon sandy soil (VS1)0103.970.151.4130.751.7831.581.7901.560.6000.470.9100.293.3031.2032.054.28High

10304.120.171.0650.411.1381.021.7081.580.3830.330.6600.247.3102.2339.787.90High

Regenerating Melaleucaon sandy soil (VS2)0103.680.030.6900.100.3100.100.3100.150.1550.021.8600.141.6150.3621.167.82High

10303.860.040.6450.030.1750.000.1500.020.0650.021.2800.241.8100.5614.492.28Moderate

Degraded secondary Melaleucaon clay soil with peat (VS3)0104.120.257.5851.826.3201.811.7050.810.4550.190.1000.1037.15517.6310.612.37Moderate

10304.070.327.5852.965.7952.591.4700.410.7050.071.6801.6448.2457.199.451.09Low

Regenerating Melaleucaon clay soil with peat (VS4)0104.670.198.8450.556.6850.581.7600.510.5850.270.000.0047.55016.069.853.05Low

10305.000.045.8550.334.8600.421.3200.190.5750.180.000.0054.82536.4910.471.27Moderate

Regenerating Melaleucaon clay soil without peat (VS5)0104.160.2611.5804.195.5570.551.3300.240.6630.075.5335.5367.4339.036.950.49Low

10303.910.408.6031.655.1700.081.5070.240.7170.098.0007.0278.44010.379.420.37Low

(mg/100g)

Fe3+

(meq/100g)

Al3+

(meq/100g)

K+

(meq/100g)

Na+

(meq/100g)

Mg2+

(meq/100g)

2+

Forest typesSoil layers (cm)pH (KCl)Ca

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shown to be sodic (Table 3). While the exchangeable sodium percentage (ESP) of soil layers of Melaleuca forests on clay soil (VS3, VS4, and VS5) ranges from low to moderate sodicity, those of Melaleuca forests on sandy soil (VS1 and VS2) were signicantly higher, particularly VS1, which had an ESP of up to 39.78% in soil taken from depths of 1030 cm (Table 3). This indicates that both mature and young M. cajuputi forests have a high tolerance of sodic soils. Furthermore, M. cajuputi seeds can germinate and grow in highly sodic soil [e.g. M. cajuputi in forest type VS2 was able to grow in highly sodic soil with ESP up to 21.16% in the top 010cm (Table3)].

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 [2124], Grevillea robusta, Lophostemon confertus and Pinus caribea [44], and Moringa olifera [45] have been examined and their tolerance to salinity assessed in the eld and in glasshouses. In extremely saline soils in Australia, Niknam and McComb [22] suggested that the land care benet 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 [33], and can help to mitigate climate change through their carbon storage abilities.

Conclusion

By undertaking original eld data, this study examined the carbon sequestration potential of ve 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 signi-cant carbon stocksarguably 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 oer a useful option for reforestation and rehabilitation initiatives.

The results in this research provide further scientic 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.

Methods

Study sites anddisturbance context

Melaleuca cajuputi is naturally distributed as scattered shrub populations along the coastal regions in the middle Provinces and up to the Northern hilly regions, and as tall forests in the Mekong Delta of Vietnam [46]. Thus, the study focussed on the sites in Southern Vietnam (involving Mekong Delta). The study investigated two sites: the Phu Quoc National Park and U Minh Thuong National Park, which both contain extensive Melaleuca forests in coastal wetlands (Figure4). A total of 14 plots were randomly selected for carbon storage assessment, covering ve types of Melaleuca stands: Primary Melaleuca forests on sandy soil(VS1), 4 plots; Regenerating Melaleuca forests on sandy soil (VS2), 2 plots; Degraded secondary Melaleuca forests on clay soil with peat (VS3), 2 plots; Regenerating Melaleuca forests on clay soil with peat (VS4), 3 plots; and Regenerating Melaleuca forests on clay soil without peat (VS5), 3 plots.

Phu Quoc National Park is located on the northern Phu Quoc Island of Vietnam (at N 101207N 102702, E 1035004E 1040440) (Figure4). Melaleuca forest areas cover 1,667.50ha out of the total area

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of 28,496.90ha. 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 [47]. 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 res and human intrusion for crop cultivation. The regenerating Melaleuca forests were up to 1012years of age at the time this study was conducted.

U Minh Thuong National Park is located in the Kien Giang Province (at N 9 31N 9 39, E 105 03E 105 07) (Figure4). Melaleuca forest on swamp peatland is an endemic ecosystem in the lower Mekong Basin of Vietnam. The core area of the park is 8,038ha, which is surrounded by a buffer zone of 13,069ha. Here, the key disturbance is fire, with the last major fire occur-ring in April 2002, which burnt the primary vegetation as well as the peat soil. The Vietnamese Environment Protection Agency [48] 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 10years old.

Field sampling anddata collection

The major plots were set out as 500 m2 quadrats (20m25m), and all trees with a DBH10cm were measured and recorded. Sub-plots also were set out as 100m2 quadrats (20m5m) within the major plots to measure all trees with DBH<10cm and a total height of >1.3m (modied from Van etal. [49]). Data on DBH, alive or dead, and height were recorded for all standing trees.

Deadwood (dead fallen trees) with a diameter 10cm were measured within the major plots (500 m2), while deadwood with 5cmdiameter<10cm were measured within the sub-plots (100m2). Diameters at both ends of the trunk (D1 and D2), length (if 50cm 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

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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 ne litter samples were collected in the major plots. The fresh weight of each litter sample was recorded. Each litter type (coarse litter and ne litter) collected in every major plot were well mixed and taken to the laboratory for drying.

Two soil samples, one from the upper (010cm) soil layer and one from the lower (1030cm) 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.

Sample analysis

Each understory and litter sample was divided into three sub-samples and dried in a drying oven at 60C to measure the moisture content, based on the Eq.(1) below:

where Rmoist = moist ratio [0:1], W = fresh weight of sub-sample i, Wdi = dry weight of sub-sample i, n = number of sub-samples. The scales used to weight sub-samples were accurate to0.01g.

Total organic carbon (C%) was measured using the WalkleyBlack method, which is commonly used to examine soil organic carbon via oxidation with K2Cr2O7

[52, 53]. Total nitrogen was measured using the Kjeldahl method, which is the standard way to determine the total organic nitrogen content of soil [54]. A standard bulk density test was used to analyze all soil bulk samples in a dryven. Bulk density was calculated using Eq.(2):

where BD=the bulk density of the oven-dry soil sample (g/cm3), Ms=the oven dry-mass of the soil sample (gram), V=the volume of the ring sample (cm3).

Exchangeable sodium percentage (ESP) was calculated using Eq.(3) [5557], and classied with four sodic levels as non-sodic soil (ESP<6), low sodic soil (ESP=610), moderately sodic soil (ESP = 1015), and highly sodic soil (ESP>15) [5557].

Basal area (BA) was calculated with Eq. (4) (modied from Jonson and Freudenberger [58]):

where BA = basal area (m2/ha), DBHi = diameter at bread height of tree i (cm), i=stand individual (i=[1:n]), n=number of trees of sample plot, Splot=area of the sample plot (m2).

Biomass allometric computation

Nine allometric equations, which are most common way to measure forest carbon stocks, were applied to calculate the above-ground and root biomass of the stands (Table 4). The selected allometric equations were tested for statistical significance using the R Statistic Program (Additional file1: 1). Using these equations, the average biomass was analyzed for five typical Melaleuca stands (VS1, VS2, VS3, VS4, and VS5). To convert from fresh to dry biomass, a moisture rate of 0.5 was applied as suggested by Van etal. [49] for the allometric equation of Finlayson et al. [59]. According to the Global Wood Density Database, the density of M. cajuputi timber ranges from 0.6 to 0.87g/cm3 [60], so 0.6g/cm3 was applied for the above-ground biomass allometric equation of Chave etal. [61].

The fallen deadwood biomass were calculated using Eq.(5) ([62], p 12):

where B=biomass (kg), r= diameter (cm), L=length (m), and =wood density (=0.6g/cm3).

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 <10cm) ([51], 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.

Soil organic carbon (SOC) was calculated using Eq.(6) [50, 51]:

where SOC=Soil organic carbon, Dep=depth of soil layer (m), BD=bulk density (g/cm3), Csample=organic

BA = [notdef]1n ~ (DBHi/200)2

Splot

10, 000

(4)

~ni=1 WWdi

W

Rmoist =

(1)

n .

B = ~ r2 L

(5)

BD = Ms

V .

(2)

SOC = Dep BD Csample 100

(6)

ESP = Na+

~[notdef]Na+[notdef]K+[notdef]Mg2+[notdef]Ca2+

100.

(3)

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Table 4 List ofallometric equations applied toexamine stand biomass ofthe Melaleuca forests

Allometric equations R2 Vegetation Sites References

log10(FW) = 2.266log10(D) 0.502 where FW = fresh

above-ground biomass (kg/tree), D = diameter at breast

height (cm)

0.98 Melaleuca spp. Northern Territory Finlayson et al. [59]

y = 0.124 DBH2.247 where y = above-ground biomass

(kg/tree), DBH = diameter at breast height (cm)

0.97 Melaleuca cajuputi Vietnam Le [63]

y = exp[2.134 + 2.53ln(D)] where y = above-ground

biomass (kg/tree), D = diameter at breast height (cm)

0.97 Mixed species Tropical, moist forest IPCC [51] or Brown [64]

ln(y) = 2.4855ln(x) 2.3267 where y = above-ground

biomass (kg/tree), x = diameter at breast height (cm)

0.96 Native sclerophyll forest NSW, ACT, VIC, TAS, and SA Keith et al. [65]

ln(AGB) = 1,554 + 2.420ln(D) + ln() where

AGB = above-ground biomass (kg/tree), D = diameter

at breast height (cm), = wood density (g/cm3)

0.99 Tropical forests America, Asian and Oceania Chave et al. [61]

ln(RBD) = 1,085 + 0.926ln(ABD) where RBD = root bio-

mass density (tons/ha), ABD = above-ground biomass

density (tons/ha)

0.83 Upland forests Worldwide IPCC [51] or Cairn et al. [66]

y = 0.27x where y = total root biomass (tons/ha), x = total

shoot biomass (tons/ha)

0.81 Natural forests Worldwide Mokany et al. [67]

Wr = 0.0214 D2.33 where Wr = coarse root biomass (kg/

tree), D = diameter at breast height (cm)

0.94 Tropical secondary forests Sarawak, Malaysia Kenzo et al. [68]

Wr = 0.023 D2.59 where Wr = coarse root biomass (kg/

tree), D = diameter at breast height (cm)

0.97 Tropical secondary forests Sarawak, Malaysia Niiyama et al. [69]

NSW New South Wales, ACT Australian Capital Territory, VIC Victoria, TAS Tasmania, SA South Australia.

carbon content of soil sample (%), and 100 is the default unit conversion factor.

Statistical analysis

One-way ANOVA tests were applied to compare stand densities, DBH, height classes, basal areas, and six categories of carbon stocks of the ve 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.

of doing eldwork 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.

Received: 24 March 2015 Accepted: 3 June 2015

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Additional le

Authors contributions

DBT conducted design of the study, eld data collection, carried out all analyses and drafted the manuscript. TVH and PD helped eld data collection, guided the research, and assisted with the writing. All authors read and approved the nal manuscript.

Author details

1 The Vietnam Forestry University, Hanoi, Vietnam. 2 School of Geography, Planning and Environmental Management, The University of Queensland, Brisbane, QLD, Australia.

Acknowledgements

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 stas 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

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