Journal of Forest and Environmental Science
Journal of Forest and Environmental Science Vol. 33, No. 4, pp. 281-294, November, 2017 https://doi.org/10.7747/JFES.2017.33.4.281
Towards Sustainability of Tropical Forests:
Implications for Enhanced Carbon Stock and Climate Change Mitigation
Mizanur Rahman1,2,*, Mahmuda Islam1,2, Rofiqul Islam1 and Norul Alam Sobuj3
1Department of Forestry and Environmental Science, Shahjalal University of Science and Technology, Sylhet-3114, Bangladesh
2Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Institute of Geography, Wetterkreuz 15, Erlangen 91058, Germany
3Department of Biology, Faculty of Science and Forestry, University of Eastern Finland, FI-80101 Joensuu, Finland
Abstract
Tropical forests constitute almost half of the global forest cover, account for 35% of the global net primary productivity and thereby have potential to contribute substantially to sequester atmospheric CO2 and offset climate change impact.
However, deforestation and degradation lead by unsustainable management of tropical forests contribute to the un- precedented species losses and limit ecosystem services including carbon sequestration. Sustainable forest management (SFM) in the tropics may tackle and rectify such deleterious impacts of anthropogenic disturbances and climatic changes.
However, the existing dilemma on the definition of SFM and lack of understanding of how tropical forest sustainability can be achieved lead to increasing debate on whether climate change mitigation initiatives would be successful. We reviewed the available literature with a view to clarify the concept of sustainability and provide with a framework towards the sustainability of tropical forests for enhanced carbon stock and climate change mitigation. We argue that along with securing forest tenure and thereby reducing deforestation, application of reduced impact logging (RIL) and appropriate silvicultural system can enhance tropical forest carbon stock and help mitigate climate change.
Key Words: climate change, tropical forest, sustainable forest management (SFM), reduced impact logging (RIL), silviculture
Received: August 25, 2016. Revised: April 17, 2017. Accepted: June 16, 2017.
Corresponding author: Mizanur Rahman
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Institute of Geography, Wetterkreuz 15, Erlangen 91058, Germany Tel: +4991318522656, Fax: +4991318522656, E-mail: [email protected]
Introduction
The planet’s environment has been stable for the last 10000 years though at times it has been undergone sig- nificant changes (Dansgaard et al. 1993; Petit et al. 1999;
Rioual et al. 2001). This stable period is known to be Holocene which is now under threat. Since industrial revo- lution the Anthropocene era has begun (Crutzen 2002) in which mainly human activities drive the global environ- mental changes (Steffen et al. 2007) preferably known as climate change. Global warming is one of the major forms
of climate changes caused mainly by increased concen- tration of greenhouse gases (Schimel et al. 1995; Watson et al. 1996). Although all the greenhouse gases are claimed to cause global warming (Robert et al. 2008), carbon dioxide (CO2) is the most important greenhouse gas having more potential to warm the earth. Forests play a promising role in sequestrating atmospheric CO2 and thereby offset warming induced climate change impact. Tropical forests have the high potential to store terrestrial carbon because of their high biomass (Brown et al. 2009). They constitute almost half of the global forest cover (Grainger 2008), account for
35% of global net primary productivity (Saugier et al.
2001) and thereby contribute significantly to offset climate change impact and livelihood subsistence (Chomitz 2007).
Currently they contribute to half of the total forest carbon store and sequester atmospheric carbon in their above ground vegetation, belowground biomass and soils. Thus tropical forests uptake 15% of the total global carbon emit- ted from anthropogenic sources making a significant con- tribution to climate change mitigation.
Despite tropical forests are functioning as a large carbon sink having potential to mitigate climate change, defor- estation is going on throughout the tropics depleting the forest natural resources and biodiversity (FAO 2012).
Deforestation causes considerable amount of carbon emis- sion into atmosphere and causes degradation of the area.
Degradation of forests contributes further emission of al- most 0.5 Gt carbons annually into the atmosphere (Achard et al. 2004). A considerable portion of carbon is also being lost during conventional logging operation in the tropics by damaging the remaining vegetation and regenerations. Soil compaction by heavy machinery used in conventional log- ging may also have a negative effect as a result of impedi- ment of water infiltration and root penetration of seedlings and saplings (Van Rheenen et al. 2004; Mello-Ivo and Ross 2006).
Indeed, logging in the tropics surprisingly follows the same economic model used for the harvesting of most of the world’s ocean fisheries: the most-valuable species are se- lectively harvested first, and when they are depleted, the next-most-valuable set is taken, until the forests are mined completely of their timber and the land becomes worth more for agriculture or ranching than for forestry (Asner et al. 2006; Karsenty and Gourlet-Fleury 2006; Laporte et al.
2007; Hall 2008; Schulze et al. 2008). This selective log- ging of high-value species for global export markets causes further expansion of logging into previously unlogged and remote areas of the Amazon, Central Africa, and Borneo (Kammesheidt et al. 2001; Laporte et al. 2007; Hall 2008;
Schulze et al. 2008; Asner et al. 2009; Bryan et al. 2010).
Selective logging has thus degraded one third of the re- maining tropical forest which is spreading menacingly through the remainder (Asner et al. 2006; Zhang et al.
2006; Nawir and Rumboko 2007; Bryan et al. 2010; Ma- tricardi et al. 2010).
Like traditional logging, many unsustainable practices still exist in tropics causing huge loss of carbon. Ensuring sustainable management of tropical forests can reduce and even some cases eradicate this carbon loss and hence sub- stantially contribute to carbon retention (Putz et al. 2008).
For instance, by using eddy covariance and ecological measurements, Miller et al. (2011) reported that improved management like reduced impact logging (RIL) minimally altered tropical rainforest carbon in an old-growth Amazo- nian forest. However, the dilemma in understanding the concept of sustainability in forestry and the challenges for its operationalization in tropics might limit this environ- mental benefits. In this context, the importance of high- lighting the key sustainability attributes of tropical forest management and their operationalization perceived much more attention to the global scientific community. Never- theless, the body of literature addressing this aspect is appa- rently inadequate and spread across a wide range of discipline. In this review paper we identified the major is- sues related to topical forest sustainability in terms of in- creasing carbon stock for climate change mitigation. We re- viewed the existing literature on recent trends in climate changes, tropical forest link to climate change, and the ways towards the sustainability of tropical forest management in relation to climate change mitigation. We used Google scholar to search the available literature with the key words:
tropical forest, climate change, sustainable management, improved forest management, silviculture in tropics, sus- tainability, afforestation, tropical farmers and carbon se- questration. We also checked the literature those are pre- liminarily not found out by Google scholar but frequently cited by the most recent literature by other means including SCOPUS. Our main objective was to highlight the im- portance of tropical forests sustainability in climate change context and provide with a framework of sustainable trop- ical forest management for enhanced carbon stock and thereby for climate change mitigation.
Tropical forests and their global dis- tribution
The tropics lie on earth between 23.5 degrees north and 23.5 degrees south of the equator (Fig. 1). A total of 56 mil- lion km2 tropical land area is divided into three main con-
Fig. 1. Map of the world terrestrial biomes (Source: UNep-GRID- Arendal 2009; modified from Ol- son et al. 2001).
Table 1. Global tropical forest area by region (source: FAO 2010;
Blaser et al. 2011) Region (number of countries)
Total forest area (million ha)
Primary forest (million ha)
Tropical Africa (26) 440 102
Tropical Asia and the Pacific (16)
317 108
Tropical Latin America and the Caribbean (23)
907 678
Global total (65) 1,664 887
tinents: Africa (52% of land area), South and Central America (32%), and South and South East Asia (17%) (Table 1). Tropical forests occupy a larger area constituting almost half of the global forest (Grainger 2008) and the highest arboreal species diversity in the world. They are found in three major tropical regions: The Neotropical, the Paleotropical, and the South East Asian tropical forests. Of those regions, Neotropical forest is the most extensive dis- tributed from Central America to Amazonia, diverse in life and portrays a magnificent world biodiversity reservoir.
Vegetation types in tropical forest include tropical rain for- est, tropical semi evergreen forest, tropical deciduous for- est, savannas, wetlands, and other vegetation types.
However, rain forests, deciduous woodlands and Savannas constitute 24%, 11% and 27% of tropical land area re- spectively (Bartholome and Belward 2005).
Annual rainfall is an important determinant factor that determines the distribution and vegetation types of tropical forests. According to Shvidenko et al. (2005), Tropical rain-
forest constitute 26% of the world’s forest area and almost 60% of the tropical forest area. Most tropical rain forests lie between 10 degrees north and south of the equator.
Southeast Asia, Central Africa, and Amazonia are the main regions of tropical rain forests separated by oceans and comprise of different species and structure. Tropical rain forests receive consistent levels of rain and have either no or very short dry seasons, causing the trees to remain green and grow throughout the year. On the other hand com- paratively dry area with less rainfall supports the develop- ment of tropical dry forest. Most of the tropical dry forests lie between 10 and 25 degrees north and south of the equator. They cover relatively smaller area of 6 million km2 than the rain forests which cover about 17 million km2 (Ramankutty et al. 2008). Apart from tropical rain forests tropical dry forests remain green only through their rainy growing seasons and are dominated by deciduous trees that lose their leaves and go dormant during the dry season. The average tree height and presence of deciduous species de- pends primarily on average rainfall. The wetter forests con- stitute taller trees and more evergreen species compared to drier ones. In areas with even less rain and/or more frequent fires, the forests gradually transition to savannas having a few trees but are mostly covered with grasses and shrubs.
The savanna biome covers 20 million km2 (about 15 per- cent of earth’s ice-free surface (Ramankutty et al. 2008).
The long dry season along with lightning induced or hu- man-set fires prevent the trees from growing into dry forest because fires suppress tree growth but help grasses flourish.
Global climate changes and their link to tropical forests
Climate is most obviously characterized by the atmos- pheric components of climate system though it is a complex interactive system consisting of the atmosphere, land sur- face, snow and ice, oceans and other bodies of water, and living things. The planet is presently experiencing increas- ing air and ocean temperatures, widespread melting of ice and snow, and rising sea levels (IPCC 2013). Precipitation, droughts, ocean salinity, wind patterns, intensity of tropical cyclones and frequency of heat waves were also changed throughout the planet (IPCC 2013). The changes that our planet has undergone throughout its history are a result of natural factors like tiny changes in the Earth’s path around the sun, volcanic activity and fluctuations within the climate system. However, humans are having an increasing influ- ence on our climate by burning fossil fuels, cutting down rainforests and farming livestock which are directly related to the CO2 enrichment in the atmosphere. Between the pre- industrial period (c. 1750) and 2005, atmospheric CO2 in- creased from about 280 parts per million (ppm) to 379 ppm (IPCC 2007).
Forest ecosystems contain the majority (approx. 60%) of the carbon stored in terrestrial ecosystems (IPCC 2000) and account for 90% of the annual carbon flux between the atmosphere and the Earth’s land surface (Winjum et al.
1993). Despite Tropical forests cover less than 10% of the earth’s land area they are among the most valuable ecosys- tems in the world. They are rich in biological diversity hav- ing more than 50% of known plant species (Mayaux et al.
2005). Several theories exist behind this diversity which is not a topic of discussion here. However, more ecosystem services can be explained by this high species diversity in- cluding climate change regulation. World’s tropical forests are estimated to contain 428 Gt C in vegetation and soils that is almost half of the total carbon sequestered in the for- est ecosystem. They are currently considered to be carbon sinks, with recent research indicating an annual global up- take of around 1.3 Gt of carbon. Of this forests Central and South America are estimated to take up around 0.6 Gt C, African forests somewhat over 0.4 Gt and Asian forests around 0.25 Gt (Lewis et al. 2009). Nevertheless, Tropical deforestation is responsible for up to 25% of the total hu-
man-induced greenhouse gas emissions each year (Hough- ton 2005). Due to deforestation the carbon originally held in forests is released to the atmosphere, either immediately if the trees are burned, or more slowly as unburned organic matter decays. Only a small fraction of the biomass initially held in a forest ends up stored in houses or other long-last- ing structures. Most of the carbon is released to the atmos- phere as carbon dioxide, but small amounts of methane and carbon monoxide may also be released with decomposition or burning (Houghton 2005).
According to the FAO (2001), the highest rates of defor- estation (in 106 ha/yr during the 1990s) occurred in Brazil (2.317), India (1.897), Indonesia (1.687), Sudan (1.003), Zambia (0.854), Mexico (0.646), the Democratic Republic of the Congo (0.538), and Myanmar (0.576). But the re- cent estimates show that the deforestation rate is slowing down in many countries though the rate is still alarming (Table S1) (FAO 2010). Both Brazil and Indonesia, which had the highest net loss of forest in the 1990s, have sig- nificantly reduced their rate of loss. Nonetheless, this is not the case in Australia where severe drought and forest fires have exacerbated the loss of forest since 2000. Forest degra- dation also causes considerable loss of carbon from the tropical forests. Estimates of carbon emissions from the degradation of forests (expressed as a percentage of the emissions from deforestation) range from 5% for the world’s humid tropics (Achard et al. 2004) to 25-42% for tropical Asia (Flint and Richards 1994; Iverson et al. 1994;
Houghton and Hackler 1999) to 132% for tropical Africa (Gaston et al. 1998). In this latter estimate, the loss of car- bon from forest degradation was larger than from deforestation.
Clarifying the concept of sustainability in terms of forest management
FAO (2005) stated forest management as the process of planning and implementing practices for the stewardship and use of forests and other wooded land aimed at achiev- ing specific environmental, economic, social and/or cultural objectives. The concept of sustainability in forest manage- ment has evolved from sustained yield and turning the sin- gle-use management of forest for timber to multipurpose approaches reflecting the wide range of goods, ecosystem
Fig. 2. Norms identified with respect to concept of sustainability in forestry (Adapted from Wiersum 1995).
services and values generated or otherwise provided by the forests. International Tropical Timber Organization (ITTO) defined SFM as: the process of managing permanent forest land to achieve one or more clearly specified objectives of management with regard to the production of a continuous flow of desired forest products and services without undue reduction in its inherent values and future productivity and without undue undesirable effects on the physical and social environment (ITTO 2005). However, the definition of SFM vary widely due either to specific field circumstances or to the particular purpose to which forest manager be- lieves a given forest should be managed (Douglas and Simula 2010). The UN’s Food and Agriculture Organiza- tion (FAO) and UN Forum on Forests (UNFF) have their own definition of SMF. FAO referred SFM to the
“application of forest management practices for the primary purpose of sustaining constant levels of carbon stocks over time” (FAO 2009). Zimmerman and kormos (2012) termed this approach as a narrow, carbon-focused approach that re- flects the FAO and ITTO definitions of SFM (ITTO 2005; FAO 2010) giving forest managers the freedom to decide which forest values to sustain. The General Assem- bly of the United Nation has adopted the most widely, inter governmentally agreed definition of SFM as: a dynamic and evolving concept aims to maintain and enhance the eco- nomic, social and environmental value of all types of forests, for the benefit of present and future generations (UN 2008, Resolution 62/98). In addition to the flow of goods and services from present to future generation SFM also main- tain forest ecological processes essential for maintaining ecosystem resilience i.e. the capacity of a forest ecosystem to recover following disturbances (Thompson et al. 2009).
ITTO has developed a set of key criteria and indicators (C&I) for the sustainable management of tropical forests (ITTO 2005) to help the monitoring, assessment and re- porting of SFM consistent with the seven thematic ele- ments of SFM specified by United Nations General Assembly 2007 (Table S2).
Many of these definitions are heavily based on the gen- eral principles of sustainable forest management. This prin- ciple has been defined as the need to maintain the pro- ductive capacity and ecological integrity of forests, the need to ensure an equitable distribution of forest management inputs and outputs, and the need to arrange for such ex-
ternal conditions that forest managers are able to sustain these management practices (Wiersum 1990). Fig. 2 illus- trates several norms reported as being involved in the prin- ciple of SFM. However, it is not unlikely that the concept of SFM will change over time in response to the dynamic and evolving needs of society. This is one of the important reason why the available definitions of SFM lack precision particularly in regard to what needs to be sustained—i.e. the objectives of SFM, the values attached by different stake- holders to various SFM objectives, the uncertainties asso- ciated with interventions in complex forest ecosystems, the timeframes and spatial boundaries involved (WCFSC 1999). Keeping the social dynamics and needs in consid- eration we can conclude that the concept of SFM should be flexible tool or mechanism ready to accept the changes in the mix of goods and services produced or preserved over long periods of time and can constantly adapt according to changing values, resources, institutions and technologies.
Ways towards sustainability of tropical forests for enhanced carbon stock
Reducing tropical deforestation
Arresting tropical deforestation is one of the major com- ponents of sustainable tropical forest management (Fig. 3) particularly in the context of climate change mitigation, pri-
Fig. 4. Changes in forest area by climatic domain from 1990 to 2005 (after: FAO 2012).
Fig. 3. Schematic presentation of the components of sustainable forest management in the tropics and various factors influencing those components and thereby carbon storage in tropical forests.
marily because deforestation is responsible for 90% of all greenhouse gas emissions related to Land use, land use change, and Forestry since 1850 (Houghton et al. 2001).
There is considerable evidence that carbon emissions from deforestation underestimate total emissions which meant that the carbon stocks in many forests are decreasing with- out a change in forest area. Examples include losses of bio- mass associated with selective wood harvest, forest frag- mentation, ground fires, shifting cultivation, browsing, and grazing (Laurance et al. 1998; Nepstad et al. 1999;
Laurance et al. 2000; Barlow et al. 2003), and accumu- lations of biomass in growing and recovering (or secon- dary) forests. Nonetheless, the tropical forest area is de- creasing almost all over the world particularly in Africa and South America (FAO 2012) (Fig. 4).
In spite of the different approaches and methods, many studies indicate that future deforestation will remain high in the tropics. Sathaye et al. (2007) estimated that defor- estation rates will continue in almost all regions of the
world. Africa and South America have high rates of loss, cumulatively about 600 M ha by 2050. Thus reducing de- forestation is a high-priority mitigation option within the tropical regions. Jung (2005) noted that in the short term (2008-2012), avoided deforestation will result in 93% of the total mitigation potential in the tropics where as in the long run 27.2 US$ /tCO2 is needed to virtually eliminate poten- tial deforestation (Sohngen and Sedjo 2006).
A net cumulative gain of 278,000 Mt CO2 is expected over 50 years. The largest gains in carbon would occur in Southeast Asia, which gains nearly 109,000 MtCO2 for 27.2 US$/tCO2, followed by South America, Africa, and Central America, which would gain 80,000, 70,000, and 22,000 MtCO2 for 27.2 US$/tCO2, respectively (Sohngen and Sedjo 2006; Nabuurs et al. 2007) (Fig. 5). In addition to the significant carbon gains, substantive environmental and other benefits could be obtained from this option. To counteract the loss of tropical forests, an understanding of the underlying and direct causes of deforestation is utmost
Fig. 5. Forest tenure distribution by tenure category in 30 tropical forest countries with complete data for 2002 and 2008 in all tenure cat- egories (after RRI 2009).
important because deforestation is not caused by a single factor rather it is governed by a set of socio economic and socioecological factors (Chomitz et al. 2006).
Poor socioeconomic condition was identified as one of the important causes of deforestation in the tropics. Provid- ing alternative sources of income and employment oppor- tunities were reported to be an effective way of reducing tropical deforestation. For example, promoting off-farm employment, education and social networking reduced en- vironmental resource extraction in the least developed country, Cambonia (Nguyen et al. 2015). Changes in man- agement strategies may also contribute to reduce defo- restation. Land privatization was found to be an incentive for afforestation by forest dependent people in Vietnam which is supposed to reduce deforestation (Nguyen et al.
2010).
Reducing carbon loss by following Reduced Impact Loggin (RIL)
Most of the researches and discussions about REDD＋
concentrated on the tropical deforestation, neglecting the potential of carbon saving from reducing forest damages during harvesting (da Fonseca et al. 2007; Gullison et al.
2007). It is assumed that forest degradation causes the same magnitude of carbon losses as those from deforestation (Nepstad et al. 1999; Asner et al. 2005). The production forest of the world comprises a total area of 403 million hec- tares of tropical forests (Blaser et al. 2011), about a quarter
of which is managed by rural communities and indigenous people (White and Martin 2002). The unsustainable ex- ploitation of forest products and values by local people for their livelihood subsistence lead to most degradation of tropical forests (ITTO 2002; WRI 2009). Further degra- dation of tropical forests has been resulted from commercial selective logging practiced on around 130 million ha area (ITTO 2006). A selective logging is frequently applied in the tropics which focussed mainly on timber with high mar- ket value. Selective logging is suitable for maintaining a mixed forest at one hand; on the other hand, it has several disadvantages including damage of the residual stand which further degrade the forest.
According to the most recent high-resolution remo- te-sensing analysis in the Brazilian Amazon forests, se- lective logging accounted for 15%-19% higher carbon emissions than that reported from deforestation alone (Huang and Asner 2010). Reduced impact logging (RIL), one of the components of sustainable forest management can reduce this loss of carbon from the tropical forest degradation. About 30% carbon losses could be reduced by applying reduced-impact logging techniques as compared to conventional logging operations (Pinard and Cropper 2000; Putz et al. 2008). ITTO defined reduced impact log- ging as the intensively planned and carefully controlled im- plementation of timber harvesting operations to minimise the environmental impact on forest stands and soils. A number of practical measures are involved in RIL
(http://www.itto.int/feature15/).
1. a pre-harvest inventory and the mapping of individual crop trees;
2. the pre-harvesting planning of roads, skid trails and landings to minimise soil disturbance and to protect streams and waterways with appropriate crossings;
3. pre-harvest vine-cutting in areas where heavy vines connect tree crowns;
4. the construction of roads, landings and skid trails fol- lowing environmentally friendly design guidelines;
5. the use of appropriate felling and bucking techniques including directional felling, cutting stumps low to the ground to avoid waste, and the optimal crosscutting of tree stems into logs in a way that maximises the recovery of use- ful wood;
6. the winching of logs to planned skid trails and ensur- ing that skidding machines remain on the trails at all times;
7. where feasible, using yarding systems that protect soils and residual vegetation by suspending logs above the ground or by otherwise minimising soil disturbance; and
8. conducting a post-harvest assessment in order to pro- vide feedback to the resource manager and logging crews and to evaluate the degree to which the RIL guidelines were successfully applied.
Reduced impact logging practices in the tropical forests would retain at least 0.16 gigatons of carbon per year (Gt C y-1) (Putz et al. 2008). A Meta analysis conducted by Putz et al. (2012) reported a high variation in carbon retention (47-97%) reflects wide range of harvest intensities and the care with which harvests were performed. A detailed har- vest plan and logging performed by trained and supervised crews substantially retain more biomass through the first harvest than in matched areas where conventional logging is followed. (Pinard and Putz 1996; da Fonseca et al. 2007;
Bryan et al. 2010; Medjibe et al. 2011; Miller et al. 2011;
Medjibe 2012). However, like any traditional logging RIL also follows reduction in above ground biomass. Mazzei et al. (2010) reported that after RIL logging at an Amazonian site the aboveground biomass (AGB) reduced by 23% with an additional 10% reduction in AGB due to high mortality rates of damaged trees within the first year. In a 30-year cut- ting cycle, logging intensities should be reduced by at least 40%-50% for regaining this AGB given that more large trees left unfelled (Mazzei et al. 2010). Therefore, appro-
priate logging intensity coupled with suitable felling cycle determines the success of RIL practices in the tropics.
Improving current stock by applying appropriate sil- vicultural system
Silviculture is the practice and theory of controlling the establishment, composition, structure and growth of the forest to satisfy specific objectives. It is heavily based upon silvics “the principles underlying the growth and develop- ment of single trees and of the forest as a biological unit”. A silvicultural system is a planned program of silvicultural treatments extending throughout the life of a stand to sus- tain a particular set of values. It includes the regeneration treatments and any tending operations, protective treat- ments or intermediate cuttings. There are different silvicul- tural systems practiced in various forests in tropical region.
Some are used in high forest which is originated from seed and some are applied in low forest which is originated from vegetative propagation. Whatever their type, appropriate silvicultural systems have potential to ensure a healthy for- est and thereby mitigate climate change. This is not only be- cause forests and the soils beneath them are Earth’s largest terrestrial sinks for atmospheric carbon (C) but also be- cause healthy forests provide a partial check against atmos- pheric rises in CO2 (Powers et al. 2012).
Silvicultural treatments can enhance the regeneration of tropical trees and stimulate population growth rates of trop- ical forest. Verwer et al. (2008) reported a positive effect of silvicultural treatments on seedling and sapling survival and tree growth rates. Based on the data gathered over a 4-year period in 326 ha plots of the Long Term Silvicultural Research Program in Bolivia they demonstrated that the re- covery of tropical tree population can be enhanced by the application of intermediate level of silvicultural treatment.
Various silvicultural treatments may include (Penã-Claros et al. 2008; Verwer et al. 2008).
∙ Pre-harvest inventory of merchantable commercial trees, using specific minimum cutting diameters (50–70 cm DBH)
∙ Lianas cut on merchantable trees 6 months before logging
∙ Skid trail planning
∙ Retention of 20% merchantable commercial trees as seed trees
∙ Directional felling
∙ Merchantable trees harvested using species-specific minimum cutting diameters (50-70 cm in DBH)
∙ Pre-harvest marking of future crop trees (FCTs)＞10 cm DBH
∙ Lianas cut on FCTs 2-5 months before logging
∙ Post-harvest liberation of FCTs from overtopping non-commercial trees by girdling
∙ Soil scarification in felling gaps during logging (1.1 gaps ha-1)
∙ Post-harvest girdling of non-commercial trees＞40 cm DBH (0.13 trees ha-1)
In another study, Penã-Claros et al. (2008) analyzed the effects of three different sets of silvicultural treatments on the densities and growth rates of seedlings saplings and poles of 23 commercial tree species in a moist tropical forest in Bolivia. They showed that, silvicultural treatment pos- itively affected seedling density and growth rates. However, apart from the effect of silvicultural treatment they also not- ed the effect of ecological guild. They found stronger effect of silvicultural treatment on long-lived pioneer than on shade tolerant species suggest that different silvicultural treatments have different effects on tropical tree regene- ration and that ecological guilds-specific treatments should be considered for sustainable management of tropical low- land forests. However, the effect of silvicultural system on long term soil C storage is still on debate. Jurgensen et al.
(2012) evaluated the impact of periodic thinning on soil C and N pools in a 134-yr-old red pine (Pinus resinosa Ait.) forest in Minnesota, and a 104 yr-old northern hardwood forest in Wisconsin. They did not find any significant effect of multiple thinning on C and N pool size in the forest floor and surface mineral soil (30-cm depth) in either red pine or hardwood stands where as the heaviest-thinned (13.8 m2ha-1) and uncut control red pine stands had higher C and N contents in the mineral A horizon, as compared to the other four thinning treatments. Therefore, this issue need to be further studied in a wide range of forest and treatment type.
Forest restoration in the degraded forest
The world has an estimated 850 million hectares of de- graded forests most of which lies in the tropics. ITTO (2002) classified 60% of the world’s tropical forest as de-
graded forest, including secondary forests, degraded pri- mary forests and degraded forest land. Afforestation and re- forestation have a promising role in rehabilitating degraded tropical ecosystems (Parrotla 1992). Sang et al. (2013) demonstrated that appropriate reforestation enhances soil fertility and promotes carbon sequestration on degraded tropical lands. Increasing forest area and density through afforestation, reforestation and forest restoration thus result in increased absorption of carbon dioxide from the atmo- sphere. Once the trees are harvested, new trees can grow in their place and continue to sequester carbon. According to FAO (2011), planted forests cover around 264 million hec- tares and absorb an estimated 1.5 G tonnes of carbon from the atmosphere each year. The rates of carbon sequestration on forest land depend on the management practices adopt- ed, the tree species involved, and the geographic area covered. For the conversion of agricultural land to forests by the way of afforestation, for example, sequestration rates will vary considerably depending on the region and species involved. In accordance with tree growth, carbon sequestra- tion rates share a standard pattern of initially rising rates followed by gradually declining rates.
Choice of rotation age is another determinant of carbon storage in afforested plantations in the degraded sites.
Fixation of rotation is determined by the objectives of plantation. If biodiversity and associated benefits are in- cluded in the objectives the rotation period is fixed at higher age than the plantation which was raised upon only timber production or carbon sequestration. By a set of empirical analyses, Nghiem (2014) showed that the inclusion of bio- diversity conservation into the optimization model for planted forests in Vietnam induced a longer optimal rota- tion age compared to the period that maximizes the joint value from timber and carbon sequestration. However, such a longer rotation period resulted in slightly lower NPV for forest owners (Nguyen and Nghiem 2016). Given this differential, governments in such tropical countries may need to consider additional monetary incentives to for- est owners if they are to encourage longer rotation ages and therefore maximize biodiversity and its associated benefits (Nghiem 2014).
Securing land and forest tenure
Land and forest tenure is a central issue of concern for
sustainable forest management in the tropics. Deforestation and forest degradation have a series of direct and under- lying causes (Geist and Lambin 2001; Kanninen et al.
2007), many of them can be resolved if land and forest ten- ure are secured (Suyanto et al. 2002; Walters et al. 2005;
Nawir et al. 2007; Corbera et al. 2011). Securing tenure thereby help enhance carbon stock in tropical forests. Land tenure has been defined as the right (whether customary or statutory) that determines who can hold and use land (including forests and other landscapes) and resources, for how long, and under what conditions (Sunderlin et al.
2009). Customary tenure systems are determined at the lo- cal level and are often based on oral agreements where as statutory tenure systems are applied by governments and are codified in state law (RRI 2009).
Defining property rights to forestland and determining the rights and responsibilities of landowners, communities and loggers at local and national level is the key to effective forest management (Stern 2006; Nguyen et al. 2010;
Lamibini and Nguyen 2016). Longer-term investments in sustainable management become worthwhile only when property rights are secure, on paper and in practice (Eliasch 2008). Nonetheless, a fundamental reality of contemporary forest tenure particularly in the developing countries is that it still involves conflicts between the state and civil society (Ellsworth and White 2004; Fitzpatrick 2006). Local com- munity living in and around the forests continue to claim customary rights. In the same way, indigenous people and other traditional forest dwellers reject state control over for- ests they view as their own (Lynch and Talbott 1995; RRI 2008; Sunderlin et al. 2008). However, colonial and post- colonial state policies failed to recognise the rights of forest dwellers (Peluso 1995; Pulhin et al. 2010).
In recent decades attempts have been made to recognise or restore tenure rights to forest dependent peoples. Many tropical countries are now partially recognizing local peo- ple’s rights to manage forests to meet diverse needs and to combat illegal logging as a part of the revision of their legal frameworks. Adopting community forest management is an approach which addresses the right and responsibilities of the community residing around the forests. This approach has been reported to be successful in some countries in tropical regions. However, in 25 of the 30 tropical countries the area of the forest administered by governments de-
creased by 14% between 2002 and 2008 (Fig. 5; RRI 2009) suggests that the initiatives are still limited. In most devel- oping countries, clarifying forest tenure and tenure reform have been bogged down due to multiple factors. According to Sunderlin et al. (2008), efforts to resolve tenure issues have been blocked by special interest groups, and hampered by insufficient funding and a lack of technical capacity. A strong political commitment, among other transparent and equitable process, may resolve this land and forest tenure issues.
Concluding remarks
There is now widespread acceptance among forest in- dustry oficials, professional foresters, governmental agen- cies and environmental groups that the ecological, social, and economic functions of the world’s forests are under stress. Despite an array of governmental, intergovern- mental, and non-governmental efforts to address global for- est deterioration, the gravity and hastening of most prob- lems are increasing (Levin et al. 2008). Deforestation, de- sertification, unsustainable management practices and con- sequent forest degradation and climate-related impacts on forest ecosystems are combining to contribute to un- precedented species losses and limit ecosystem services par- ticularly in the tropics (Leakey and Lewin 1995; Pimm and Brooks 2000). These devastating processes ultimately re- sult in greenhouse gas emissions and carbon stock reduc- tions in tropical forest ecosystem along with other impact on livelihood, environmental functions and other socio-eco- nomic values. Our view is that sustainable management can tackle and rectify such deleterious impacts. The question narrowly focused in existing literature is what sustainable forest management means in tropical forests and how sus- tainable management of tropical forest can be achieved. In this paper, we clarified the concept of sustainability in trop- ical forest context and highlighted the key attributes of for- est management which are directly or indirectly linked to the carbon pool in tropical forests ecosystem. We argue that tropical forest carbon pool can be enhanced by the way of reducing deforestation, reducing carbon loss by following Reduced impact logging (RIL), improving current stock by applying appropriate silvicultural system, restoring for- est in the degraded forest and securing land and forest
tenure. Arresting deforestation calls for review of national and global initiatives to consider the issues related to land and forest tenure which is insufficiently addressed to date.
Along with this, wide spread application of RIL and appro- priate silvicultural practices can ensure sustainable tropical forest management and enhanced carbon stock in the trop- ics which in turn will help mitigate climate change.
Acknowledgements
The authors thank Curtis F. Barrett of Wageningen University, Netherlands for his insightful comments on the earlier version of the manuscript. The anonymous re- viewers are also acknowledged for their valuable comments which substantially improved the contents of the manu- script.
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