Forest structure and carbon dynamics of an intact lowland mixed dipterocarp forest in Brunei Darussalam

Sohye Lee·Jongyeol Lee·Seongjun Kim·Yujin Roh·Kamariah Abu Salim·Woo-Kyun Lee·Yowhan Son,3

Introduction

Forests are important ecosystems in terms of carbon sink and climate change,because forests sequester more carbon than any other terrestrial ecosystems(Gibbs et al.2007;Khaine and Woo 2015).In particular,tropical forests are important to the global carbon balance(Sierra et al.2007).However,estimating carbon dynamics in tropical forest ecosystems has been constrained by technical dif ficulties(Sierra et al.2007)causing high uncertainties(Chave et al.2004;Lumbres et al.2015)because of complex stand structures and high species diversity.This is especially true for mixed dipterocarp forest(MDF),a common tropical forest type in Southeast Asia,where there is wide variation in precipitation,topography,and soil nutrients across the region(Ashton and Hall 1992).In this context,a large volume of reference data is needed for intact MDFs under various conditions in order to reduce uncertainty.Slope and forest structure are two of the important factors that determine carbon dynamics,and affect one another(Vieira et al.2004;de Castilho et al.2006).Elucidation of their relationships is essential for understanding carbon dynamics in tropical forests.Determining the major factors in fluencing carbon dynamics might also contribute to more effective management of intact tropical forests.We aimed to achieve the following objectives:(1)to provide reference data for the estimation of forest structure and carbon dynamics in intact lowland MDF,and(2)to investigate the relationship between slope,forest structure and carbon dynamics in intact lowland MDF.

Materials and methods

The study site was located in a lowland Kuala Belalong MDF,part of Ulu Tembulong National Park,Brunei Darussalam(04°63′50.3′N,115°22′79.1′E).This MDF is a completely intact primary forest(Cranbrook and Edwards 1994).The study area was established by the University of Brunei Darussalam in partnership with the Center for Tropical Forest Science(CTFS).Mean monthly air temperature is 26.0–34.2 °C and mean annual precipitation is approximately 4582 mm,without a distinct dry season(Lee et al.2015).Soils are classi fied as inceptisols and ultisols.Vegetation is dominated by Dipterocarpaceae,and the main genera are Shorea (Dipterocarpaceae),Dryobalanops(Dipterocarpaceae),and Koompassia (Caesalpiniaceae)(Ashton and Hall 1992).The aboveground biomass of the study site was estimated to be 361.8 Mg ha-1(Lee et al.2015),and it is classi fied as the highest biomass class category(AGB＞350 Mg ha-1)compared with other tropical forests(Saatchi et al.2011).More detailed descriptions of climate,landform,soils,hydrology,and biodiversity are provided by Cranbrook and Edwards(1994).The study was conducted within the 25-ha CTFS plot at Kuala Belalong.Analysis presented here were based on nine 20 m×20 m quadrats demarcated on various slopes(17.3°–42.8°)with six quadrats on gentle slopes(17.3°–25.4°),and three quadrats on steep slopes(38.0°–42.8°).

To understand the forest structure,we measured diameter at breast height(DBH)for all trees≥1.0 cm DBH in 2011. Stand density was assessed for all trees(DBH≥1.0 cm)and for large trees(DBH≥20.0 cm),respectively.To identify tree family composition,we randomly selected around 150–200 trees in each 20 m×20 m quadrat.Two crown indices were used to assess crown properties:the crown form index and the crown position index(Alder and Synnott 1992),and they were categorized into five stages (1=very poor,2=poor,3=tolerable,4=good,5=perfect;Alder and Synnott 1992).The indices were determined according to the degree of symmetric shape of the crown and lateral competition of the crown.Crown form index re flects the photosynthetic potential of a tree and crown position index re flects light conditions.More detailed explanations of classi fication are provided by Alder and Synnott(1992).Crown form and crown position indices of all trees≥20.0 cm DBH were investigated in August 2014.

To estimate diameter growth rate,DBH was re-measured in 2014 for a subset of the trees.All trees with a DBH≥20.0 cm in nine 20 m×20 m quadrats were measured,and the trees with a DBH of 10.0–19.9 cm were measured in 10 m×10 m sub-quadrats installed in each 20 m×20 m quadrat.Living biomass and biomass growth rate were estimated using the biomass estimation equation developed for tropical lowland MDF in East Kalimantan,Indonesia(Basuki et al.2009).A belowground biomass to aboveground biomass ratio of 0.18,obtained from the Pasoh Forest Reserve,Malaysia(Niiyama et al.2010),was used to estimate belowground biomass.To estimate litter production and litter decomposition rates,one litter trap and one bundle of 12 litter bags were installed in each 20 m×20 m quadrat in December 2014(n=9),and samples were collected for 1 year.

Correlation analysis was used to analyze the relationships between forest structure(living biomass,basal area,stand density,and crown properties)and carbon dynamics(growth rate,litter production,and litter decomposition rate).We also compared the means of forest structure and carbon dynamics among different slopes and tree family compositions,using one-way ANOVA.Analysis was conducted using SAS 9.3 software(SAS Institute Inc.,Cary,NC,USA).

Results and discussion

Forest structure and carbon dynamics are listed in Table 1.Mean crown form(CF)and crown position(CP)indices were approximately 4,with no quadrats lower than 2,respectively.CF and CP did not vary by slope gradient(P=0.3).These results con firm that the crown was mostly symmetrical and the crown position was suf ficiently exposed for photosynthesis.The five dominant tree families at the study site were Euphorbiaceae(with an observed percentage composition value of 17.7%),Dipterocarpaceae(13.5%),Achariaceae(9.9%),Phyllanthaceae(8.7%),and Rubiaceae(5.4%).

Table 1 Statistics of forest structure and carbon dynamics in an intact lowland mixed dipterocarp forest,Kuala Belalong,Brunei Darussalam

The diameter growth rate(3.4±1.0 mm a-1)and the biomass growth rate(11.9±2.2 Mg ha-1a-1)were similar to previous reports(3.0 mm a-1;Manokaran and Kochummen 1987 in Malaysia,11.4±1.3 Mg ha-1a-1;Hertel et al.2009 in paleotropical primary forests).Biomass growth rates did not vary by slope gradient(P=0.3).Many studies reported the relationship between tree growth and slope in terms of soil condition.Therefore,further research on soil condition(e.g.:soil water or nutrient)might be needed to understand this contrasting result(Bellingham and Tanner 2000).Annual litter production(11.6 Mg ha-1a-1)was similar to that reported from previous studies in other tropical forests (e.g.:7.7–11.0 Mg ha-1a-1;Paoli and Curran 2007 in Kalimantan,Indonesia).Litter production did not vary by month or season.The absence of a seasonal pattern was probably related to the lack of climatic seasonality at the study site.The litter decomposition rate coef ficient was 0.11 a-1according to Olson’s(1963)constant potential weight loss model,which is relatively low in comparison with other tropical forests(e.g.0.2–0.4 a-1;Sundarapandian and Swamy 1999 in India).The soil at the study site is relatively shallow(＜2 m)with many pebbles and the topography is steep(Cranbrook and Edwards 1994).These characteristics lead to a microaggregate structure which encourages faster water in filtration compared to other tropical silty clay soils(Cranbrook and Edwards 1994;Isaac and Nair 2005).This fast draining characteristic(the average soil volumetric water content at 10 cm depth was 25%,unpublished data)might be responsible for the slow litter decomposition at the study site.

Correlations between the structural and dynamic aspects of the forest carbon cycle at Kuala Belalong are presented in Table 2.Biomass growth rate was more strongly correlated with the amount of biomass(r=0.97,P＜0.05)than tree density(r=0.39,P＞0.05).In a previous study,large trees(DBH≥20 cm)contributed greatly to the biomass on the study site(Lee et al.2015),but stand density of large trees and biomass growth rate were not correlated(r=0.32,P＞0.1).

Crown form was strongly related to biomass growth rate(r=0.66,P＜0.05),as both are related to photosynthetic capacity(Vincent et al.2002).King et al.(2005)reported that light interception is the key factor in growth rate,and that crown area and crown illumination are strongly correlated with growth.Thomas and Bazzaz(1999)reported that saplings have greater shade tolerance,therefore,they exhibit weaker correlations with growth rate than do mature trees at Pasoh Forest Reserve,West Malaysia.Because their study site was a primary forest mainly composed of mature trees,the growth rate there might be more strongly correlation with light than with other factors.Crown form was strongly correlated with forest structure(living biomass,r=0.70,P＜0.05;basal area,r=0.72,P＜0.05).These findings were similar to the review study of 43 tropical forests by Veneklaas and Poorter(1998).

Table 2 Pearson correlation coef ficients among mean values of parameters in an intact lowland mixed dipterocarp forest,Kuala Belalong,Brunei Darussalam

Categorizing each 20 m×20 m quadrat by family composition led to three groupings:mixed(individual composition percentageofalltree families＜15%),Dipterocarpaceae dominated(≥15%),and Euphorbiaceae dominated(≥15%).Tree species are usually considered an important factor in the estimation of biomass and carbon dynamics because of variation in wood density by species.However,biomass(P=0.6)and growth rate(P=0.4)were similar among these tree family composition groups.Slik et al.(2010)reported that in Borneo’s old-growth forests,the aboveground biomass was only correlated with basal area,and not with community wood speci fic density,while Amazon forests depend more on wood speci fic gravity.

Conclusion

We conclude that crown form strongly in fluences forest structure and carbon dynamics in terms of light acquisition in an intact MDF forest and that carbon accumulation by tree growth depends on the crown form.Tree family composition did not in fluence forest structure(basal area,stand density,and crown properties)or biomass growth rate.This suggests that the main driver of biomass in oldgrowth forests of Borneo is not the wood species-speci fic characteristics(e.g.gravity).Our results will provide reference data and background information for the understanding of the forest structure and carbon dynamics of an intact lowland mixed dipterocarp forest.

AcknowledgementsWe would like to thank the Universiti of Brunei Darussalam(UBD)for allowing us to conduct the study at Kuala Belalong,and the Heart of Borneo of the Ministry of Primary Resources and Tourism for granting us export permits.The establishment of the UBD-CTFS 25-ha plot was supported by HSBC,CTFS(Stuart James Davies)and UBD.

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