Biomass and carbon stocks of trees in tropical dry forest of East Godavari region, Andhra Pradesh, India

ABSTRACT Assessment of biomass and carbon stock (CS) of trees was carried out in three selected study sites (four 0.25 ha [50 m × 50 m] plots in each site) in East Godavari region of Eastern Ghats. Aboveground biomass (AGB) was estimated by the non-harvest method by using allometric equations. The AGB ranged from 58.04 (site I) to 368.39 (site III) Mg/ha and the total CS of trees ranged from 44.51 to 218.84 Mg/ha. The highest CS accumulation in site III could be due to more soil moisture and abundant large diameter trees while the least was obtained in the site I could be due to relatively high human disturbance. Xylia xylocarpa was a dominant biomass and carbon assimilator in the site I and II, while Terminalia arjuna was the highest contributor in site III. The present study revealed that T. arjuna and X. xylocarpa are the vital tree species to endure and sink more carbon in tropical dry forest of East Godavari region under various disturbances.


Introduction
Climate change, biodiversity loss, and global warming are some of the key environmental issues present-day society is facing. By increasing carbon storage and decreasing greenhouse gas (GHG) emissions, conserved forests can bring a drastic change in the composition of global atmospheric GHGs (Lung & Espira, 2015). Biodiversity and its connection with the carbon cycle have become current research interests for mitigating climate change (Midgley et al., 2010). In the Conference of Parties meeting (COP 21) held in 2015 in Paris, France, 195 countries agreed "to keep a global temperature rise this century well below 2°C." All the member countries of UNFCCC have acknowledged the crucial role that the resilient forests play in both climate change mitigation and development (COP21, 2015;UNFCC, 2014). The afforestation and reforestation programs in the damaged forest areas or in the barren lands can now be recognized as carbon sinks under Kyoto Protocol. Forests are important to mitigate carbon and well-managed forests increase the resilience of ecosystem services, particularly trees absorb and store large quantities of carbon (UNFAO, 2010(UNFAO, , 2015. REDD (Reducing emissions from deforestation and forest degradation) and REDD+ mechanisms enable countries or projects that avoid forest loss by compensating financial rewards through quantified carbon estimates and in order to quantify carbon benefits, assimilation of carbon has to be quantified from the forests (Ebeling & Yasue, 2008;Vieilledent et al., 2012).
Geographically tropical forests cover between 7% and 10% of land area of the planet (Lewis et al., 2009) which consists of 471 ± 93 pg of stored carbon (Pan et al., 2011). Hence, tropical forest ecosystems are essential for maintaining both carbon cycling and biodiversity (Midgley et al., 2010). Annual carbon sink in tropical forests at a global level is approximately 1.3 pg (Lewis et al., 2009;Lung & Espira, 2015). Tropical forests are clearing at an approximate rate of 15-17 M ha/y (FAO, 1995;Naveenkumar, Arunkumar, & Sundarapandian, 2017). If the same situation pertains, the carbon sink of terrestrial constituent of the planet declines while emission of GHGs will increase and thereby resulting in drastic climatic events.
In tropical forests, the quantitative studies on accumulated carbon pools, carbon fixation, and net primary production are well addressed in global scale but the regional dilemma with research gaps has been well noticed (Achard, Eva, Mayaux, Stibig, & Belward, 2004;Djomo, Knohl, & Gravenhorst, 2011;Hertel et al., 2009). Several researchers indicated that there is an uncertainty in carbon sink and stocks in spatial scale of tropical forest ecosystems which may be due to variation in topography, forest types, elevation, level of anthropogenic pressures, and microclimate (Becknell, Kucek, & Powers, 2012;Gandhi & Sundarapandian, 2017;Naveenkumar et al., 2017). In general, carbon stock (CS) was assessed in woody vegetation which has >10 cm DBH (Diameter at Breast Height); however, only a few studies were focused on juvenile population of woody species contribution in carbon sink and stocks of tropics (Chaturvedi, Raghubanshi, & Singh, 2012a;Gandhi & Sundarapandian, 2017;Mohanraj, Saravanan, & Dhanakumar, 2011;Naveenkumar et al., 2017). Generally, small stem circumference class (<10 cm) individuals in the tree population have grown rapidly than the large-diameter class trees and comprise a significant proportion in the understory woody plant biomass (Chaturvedi et al., 2012a). The requisite for estimating biomass in forest ecosystems is supported by the necessity of information on the forest carbon stocking capacity.
National level biomass inventories are carried on the basis of remote sensing. However, the uncertainty in biomass estimates persisted because CS and sequestration potential varied due to numerous factors. Biomass and CSs varied at a spatial scale in any forest type even in a small patch (1 km 2 ) due to variation in microclimate. Accurate estimation method through harvesting is not practically feasible because most of the forest ecosystems are declared as protected in several countries. Allometric equation provides an alternative estimation of biomass accepted by the scientific community and UNFCCC (Chave et al., 2003).
The Eastern Ghats are located on India's eastern coast and they form several discontinuous hill ranges from the northern Odisha to Tamil Nadu covering Andhra Pradesh. The Eastern Ghats with their geographical diversity host four major rivers (Godavari, Mahanadi, Krishna, and Kaveri) of peninsular India. Biomass and carbon assessment studies were mainly focused in the southernmost part of Eastern Ghats (Gandhi & Sundarapandian, 2017;Mohanraj et al., 2011;Naveenkumar et al., 2017;Pragasan, 2014Pragasan, , 2015Sundarapandian, Dar, Gandhi, Srinivas, & Subashree, 2013) but little is known from the northern part (Behera & Misra, 2006;Sahu, Sharma, & Ravindranath, 2015;Sahu, Suresh, & Ravindranath, 2016). However, quantitative CS assessment studies in East Godavari region of Eastern Ghats are still lacking. According to Sheikh, Kumar, Bussman, and Todaria (2011), the biomass reserves in the Eastern Ghats are being depleting continuously from 2003. Hence, the present study was a preliminary work undertaken with the objectives to assess the biomass and CS of trees in three dry tropical forest sites of the Eastern Ghats in East Godavari District, Andhra Pradesh.

Study area
The field study was performed in forest sites of East Godavari region situated in the northern part of Eastern Ghats ranged between 17°33′N and 81°45′E ( Figure 1). Average annual precipitation (for a decade) was 109 ± 6.6 cm with a peak was observed in the month of October (22.2 ± 2.9 cm) and a minimum in the month of March (0.29 ± 0.02 cm). Average monthly maximum temperature ranged from 28 to 38°C and a mean monthly minimum temperature ranged from 18 to 27°C (Figure 2).

Methods
Three study sites (each 1 ha [4 square plots of 0.25 ha with an interval of 500 m]) were selected in reserve forest of East Godavari region for the present study. Each plot has been further sub-gridded into 25 (10 m × 10 m) quadrats as workable units. The site I is nearer to the human settlements. Illegal tree felling Figure 1. Location of the study sites in East Godavari region of Eastern Ghats, Andhra Pradesh, India. and grazing are frequent in study site I even if it was demarcated as a part of the reserve forest. Site II was positioned far from the human settlements which receive relatively less disturbance than site I, and site III is nearer to rivulet which joins the tributaries of river Godavari and also experiences minor disturbances. All the live tree individuals above 1 cm GBH (Girth at Breast Height) were enumerated and its diameter at 1.37 m height was measured. The diameter of each stem was measured and summed up for multistemmed trees. Plant samples were collected, preserved, and identified with "Flora of Eastern Ghats: Hill ranges of South East India" by Pullaiah, Rao, Ramamurthy, Karuppusamy, and Rani (2011). The AGB of adults (≥10 cm DBH) and juvenile population (≥3-<10 cm DBH) of trees was estimated with allometric equation (1) given by Brown, Gillespie, and Lugo (1989). 00 Aboveground biomass AGB ð Þ¼34:4703 À 8:0671 To estimate the AGB of saplings (<3 cm DBH), the allometric equation (2) of Chaturvedi, Raghubanshi, and Singh (2012b) was employed.
Carbon of tropical dry forest is considered to be a fraction 44.53% of biomass, hence, 0.4453 was the conversion factor multiplied (Equation (4)) (Júnior et al., 2016).

Results
The total AGB of trees was greater in study site III (390.03 Mg/ha) than that of other two study sites ( Xylia xylocarpa accumulated greater biomass and CS in both sites I and II of adult tree population, followed by Pterocarpus marsupium, Terminalia alata, Albizia odoratissima, and Bridelia retusa in site I, and Grewia tiliifolia, Dillenia pentagyna, and Lannea coromandelica in site II (Tables 2 and 3), while Terminalia arjuna stored greater biomass and CS in site III followed by X. xylocarpa, Mangifera indica, and Pongamia pinnata. In both sapling and juvenile tree populations, X. xylocarpa was the top species that accumulated greater biomass and CS compared to other species in all three study sites. Top five species of respective study sites stored 61.69-73.04% of biomass and CS in adult stage, simillarly, in the juvenile and sapling populations, these accumulated 64.28-76.31% and 31.18-60.57% respectively. The species which contributed less than 1% of the AGB in the adult population are 30, 31, and 46 in sites I, II, and III, respectively. The juvenile population of 43 species in site I, 49 species in site II, and 39 species in site III has also shown less than 1% of aboveground biomass. A similar trend was exhibited in sapling population also (54 species in site I, 55 species in site II, and 54 species in site III).  Combretaceae was the highest contributor of biomass in adult tree population of site III compared to Leguminosae, which was the family that contributed the highest biomass in all developmental stages of trees in sites I and II, and juvenile and sapling stages of site III (Table 4). Eight families in site I, 7 families in site II, and 9 families in site III contributed less than 1% of biomass and CSs. Leguminosae, Combretaceae, Anacardiaceae, Malvaceae, Phyllanthaceae, and Dilleniaceae are the major tree biomass (adults + juveniles + saplings) accumulated families in sites I and II, whereas Combretaceae, Leguminosae, Anacardiaceae, Ebenaceae, and Myrtaceae are the major contributors in site III.
Diameter class-wise distribution of adult trees showed wide variation among the study sites ( Figure 3). In all the study sites, lower diameter class (≥10-<30 cm DBH) has a greater number of individuals, whereas in mid diameter class (30-60 cm   Correlation and regression analysis indicated that adult tree basal area has shown a significant positive relationship with adult tree CS (Table 5), while juvenile basal area showed a negative relationship with saplings, adults and, total tree carbon. Similarly, adult tree density exhibited a weak negative relationship with adult tree carbon and total tree carbon, whereas it showed a positive relationship with juvenile CSs. Sapling density and juvenile density showed a significant positive relationship with saplings and juvenile carbon, respectively. Soil moisture showed a significant positive relationship with total tree CSs.

Discussion
The AGB estimated in the present study ranged from 58.04 to 368.39 Mg/ha has exceeded the range (39-334 Mg/ha) given by Becknell et al. (2012) who obtained the values while reviewing 40 published papers on seasonally dry tropical forests and 95-214 Mg/ha reported in Javadi hills of Eastern Ghats (Naveenkumar et al., 2017). Mean AGB in the present study (168.99 Mg/ ha) is lower than the mean AGB of 260 African tropical forests (395.7 Mg/ha) estimated by Lewis et al. (2013), in the forests of Borneo (445 Mg/ha, Slik et al., 2010), and Amazonian forests average of 289 Mg/ha (Malhi et al., 2006). The adult tree basal area showing a significant positive relationship with adult tree CS and productivity has been proven by many small-scale research and observations in natural ecosystems (Midgley et al., 2010). The mean aboveground CS was 75.26 Mg/ha in the present study (ranged 25.84-164.05 Mg/ha) that lies within the global range value (14-123 Mg C/ha) reported by Murphy and Lugo (1986) in tropical deciduous forests and studies reported in the recent past (Becknell, 2012;Becknell & Powers, 2014;Gandhi & Sundarapandian, 2017). Brown and Lugo (1990) concluded that total amount of accumulated biomass in forest ecosystems may vary with variation in biophysical characteristics, microclimate, and level of anthropogenic disturbances. The present result pointed out that site I was facing relatively high disturbance over a period of time which influences the reduction in large  diameter trees which in turn resulted in lower biomass than that of other study sites. The highest biomass in study site III (five and four-fold, respectively in sites I and II) could be attributed to numerous larger diameter trees in the study site which constituted significant biomass. The study site III is nearer to rivulet which could provide a conducive environment for the extensive growth of trees that resulted in the accumulation of greater biomass. The water availability due to rivulets influences the increase in the volume of trees and thereby accumulating greater AGB. Alvarez-davila et al. (2017) found that water availability strongly influences forest structure and tree growth. Similarly, Becknell et al. (2012) also stated that water availability is one of the main factors which regulates biomass accumulation. The larger diameter trees contribute majority of forest biomass and it was also confirmed by other researchers (Brown & Lugo, 1992;Brown et al., 1995;Brown, 1996;Chave et al., 2003;Culmsee, Leuschner, Moser, & Pitopang, 2010;Gandhi & Sundarapandian, 2017;Lung & Espira, 2015;Midgley & Niklas, 2004;Slik et al., 2010). The relative differences in anthropogenic pressures among the study sites could be the reason for low CSs in study sites I and II. Several studies enlightened that human disturbances could alter species composition and structure of forest ecosystem (Anbarashan & Parthasarathy, 2012;Htun, Mizoue, Kajisa, & Yosida, 2011;Sheil & Burslam, 2003;Srinivas & Sundarapandian, 2018) that resulted more stem density and lower basal area in site I than that of least disturbed study site (III) where less stem density and higher basal area were recorded. Even though the abundance of trees was more in sites I and II, their diameter and canopy sizes were low that creates canopy openings and light penetration to the ground level and thereby providing favorable conditions for tree regeneration and understory vegetation development.
The juvenile tree population CSs (4.68-6.70 Mg/ha) assessed in the present study were higher to the values (0.6-3.6 Mg/ha) reported by Chaturvedi et al. (2012a) in tropical dry forests of Uttar Pradesh, India. The contribution of tree juvenile population (15.05% of CSs) to the total tree CSs was high in study site I while the least (2.16%) was observed in study site III. Human activates and cutting of trees might have resulted as coppicing which enhances sapling and juvenile tree populations in site I that inferences greater juvenile biomass contribution. However, tree sapling contribution to total CS was greater in study site III than other study sites. This may be due to water availability. Ceccon, Sanchéz, and Campo (2004) and Marod, Kutintara, Tanaka, and Nakashizuka (2004) stated that the early stages of tree growth may be regulated by sunlight availability, adequate soil nutrients, soil texture, and soil moisture content. The adult trees contributed significantly greater CS (73.2-94.5%) to total tree CS compared to tree juveniles and saplings contribution (5.5-26.8%). However, future carbon accumulation will depend on the present status of the tree sapling and juvenile populations. The significant variation observed in the biomass and CSs among the study sites could be attributed to variations in species composition, soil characteristics, level of anthropogenic pressures, etc.
X xylocarpa was a dominant contributor of biomass and CS in sites I and II, while T. arjuna was the highest contributor in site III. The present study revealed that T. arjuna and X. xylocarpa are the important adult tree species to endure and sink more carbon in the dry tropical forest in East Godavari region even under anthropogenic pressure. T. arjuna accumulated significant biomass carbon that could be due to favorable conditions, i.e., the rivulets are nearer to the study plots. Several studies also reported that the T. arjuna prefer water-rich areas for sustenance (Datta & Goyal, 1996;Johnsingh & Joshua, 1989;Kundu & Schmidt, 2015). The present study also has drawn the need for conservation of T. arjuna, X. xylocarpa, and other major contributors in order to fix significant CS in the tropical dry forest in the Eastern Ghats. Thus, the present study concludes that the tropical dry forests in East Godavari region stored a substantial amount of carbon as similar to the tropical dry forest of other parts of India as well as elsewhere. The CS of trees in the tropical dry forest could be attributed to soil moisture, the density of various stages of development of trees and anthropogenic pressure.

Conclusion
The dry tropical forest ecosystem is becoming particularly important because of facing high anthropogenic pressures which resulted in the loss of forest cover and biodiversity. Present work gave an emphasis on the tree biomass and CS contribution of trees at various stages of development, showing the necessity of further research work to understand the factors influencing biomass and CS contributions of other woody vegetation and nonwoody vegetation in the regional scale. Results from the present study indicate the utmost requirement of formal actions to curtail anthropogenic pressure and to take conservation measures in the tropical forest of East Godavari region, Andhra Pradesh for increase carbon sink in the climate change scenario. permission to execute fieldwork. We are also grateful to Dr. S. Karuppusamy, Department of Botany, The Madura College, Madurai, India for identification of plant species.

Disclosure statement
No potential conflict of interest was reported by the authors.