Survival, growth, aboveground biomass, and carbon sequestration of mono and mixed native tree species plantations on the Coromandel Coast of India

ABSTRACT In India, reforestation programs with native indigenous tree species are a recent activity. Information on experiences comparing mono- and mixed-species plantations is limited. This study aims to estimate growth, aboveground biomass, and carbon sequestration between the mixed-species plantation and mono-species plantation. The growth, survival, height, aboveground biomass, and carbon sequestration of 82 native mixed species plantations were compared with Casuarina equisetifolia an exotic species planted in this region after over a decade (2006–2016). In the mixed species plantation, 7 species showed 100% survival rate and 19 species were not survived after over a decade intervals. While in the mono plantation, C. equisetifolia showed 92% of the survival rate. The growth rate of mixed species when compared to mono plantation, it showed highly significant differences (P < 0:05) values. Simple linear regression between annual girth increment and height produced very strong positive relations (R2 0.759). The aboveground biomass estimated for the mixed native plantation was 8.007 tonnes and the mono plantation Casuarina had 5.585 tonnes. The total carbon stock estimated for the tree plantation in the two plots (both mixed native and mono) was 7.492 tonnes. A positive correlation was observed between the carbon stock and density of the top 10 species which contributed predominantly to the total carbon stock (rs = 0.773, p < 0.05). Plantation of C. equisetifolia seems to be well adapted and had more carbon stocking potential. On the other hand, mixed plantation with indigenous species would contribute more to sustainable management and they provide great shelters for many faunal communities and provide a greater range of ecological goods and ecosystem services than the mono plantations.


Introduction
In the tropical countries, there is increasing interest in establishing mixed native species plantations for a wide range of economic, silvicultural, and sustainability objectives (Nguyen, Vanclay, Herbohn, & Firn, 2016;Anbarashan, Padmavathy, & Alexandar, 2017). Mixed plantation systems provide native species a broader range of options for their restoration in degraded areas, protection, and biodiversity conservation (Montagnini, Gonzalez, Rheingans, & Porras, 1995;Guariguata, Rheingans, & Montagnini, 1995;Parrotta & Knowles, 1999). In the past two decades, new restoration approaches in the tropics have emphasized the establishment of highly functional plantation forests with native species in mixed stands. Recent studies suggest positive mixture effects on many ecosystem functions such as lower tree mortality, enhanced biomass productivity coupled with higher resource-use efficiency (including nutrients, water, and light) by trees, higher decomposition rates, reduced damage from pest or diseases, and better nutrient retention than the mono plantations (Forrester, Theiveyanathan, Collopy, & Marcar, 2010;Healy, Gotelli, & Potvin Partitioning, 2008;Hung, Herbohn, Lamb, & Nhan, 2011;Lawson & Michler, 2014;le Maire et al., 2013;Nichols, Bristow, & Vanclay, 2006;Puettmann & Tappeiner, 2014;Richards, Forrester, Bauhus, & Scherer-Lorenzen, 2010). Vietnam, China, and the Philippines encourage landholders to plant mixtures by their national reforestation programs (Lamb, Erskine, & Parrotta, 2005); in several countries, for smallholder and community forestry (mostly of native species) (Herbohn et al., 2014) there is often little comprehensive information. Ecological disturbance and climate change impacts can be balanced and can provide localities with more resilient forests, when mixtures of different species with differing traits are established (Rodrigues et al., 2011;Anbarashan et al., 2017). Lamb and Lawrence 1993 stated that the complete utilization of soil and water resources along different soil strata could be attained by roots of different species during plantation. Plantation of different species tends to observe more solar energy and the light requirements are broadly distributed in the vertical plane (Guariguata et al., 1995).
A major challenge of the forestry sector is to re-establish/recreate closed forest cover in deforested and degraded areas to mitigate effects associated with deforestation such as biodiversity loss, soil degradation, erosion, flooding, and salinization (Kunert & Cardenas, 2015). The establishment of managed tree plantations on suitable tropical lands currently classified as degraded could satisfy the current and projected growing demand for industrial roundwood, while limiting the harvesting pressure on the remaining natural forests (Bauhus, van der Meer, & Kanninen, 2010;Kunert & Cardenas, 2015). The overall global carbon sequestration could be substantially enhanced by reforestation in the tropical countries (Canadell & Raupach, 2008). So far, most of the reforestation responsible for a gain in forested area in the tropics has been conducted in form of industrial monocultures involving a limited number of species. Most of these species originate from few genera (i.e., Pinus, Eucalyptus, Tectona, Gmelina, Acacia, and Casuarinas) and are exotic to most of the areas where they are cultivated (ITTO, 2009). Such traditional mono-specific plantations have supplied a range of goods and services by providing a forest-like habitat connecting fragmented forests, filtration of waste water and temporally sequestering high amounts of carbon . But there is rising concern about their environmental sustainability as they make only minor contributions to the restoration of ecological functions and biodiversity compared to mixed-species plantations containing indigenous tree species (Lamb et al., 2005).
Fundamental goal of ecological research in tropical forests is about understanding the patterns of highly dynamic plant growth. Forest growth function is important for determining the size and multitude of ecological cum management applications (Vivek, Parthasarathy, & Monica, 2016). For providing practical and meaningful classification of tropical forest species, foresters in modeling growth and yield factors are needed, whereas the ecologists explain the life history of tropical forest and their diversity (Vivek et al., 2016). In prediction of forest dynamics, understanding of tree mortality is inevitable and its center to any long-term dynamics of woody plants as their biomass is regulated by the difference between gains through individual growth and losses through mortality (Scherer-Lorenzen et al., 2005). The growth and mortality of saplings of trees are dependent on impacts of various factors, such as species specific, tree vigor and size, and environmental conditions on the interactions and processes in stands (Radosevich, Hibbs, & Ghersa, 2006;Scherer-Lorenzen et al., 2005). Differences in mortality rates among species are the major determinants of ecological succession (Schneider, Ashton, Montagnini, & Milan, 2014) and forest stand structure (Semwal, Nautiyal, Maikhuri, Rao, & Saxena, 2013). Performance of a tree species was indicated by their vigor and size, as it partially reflects the competitive ability of a tree (Nakashizuka, 2001). Growth-mortality trade-off can also be predicted by their relationship in plants functional traits (Baker et al., 2004;Nguyen et al., 2016). However, the success of the establishment of mixed forest plantations depends on plantation design and an appropriate definition of the species to be used, taking into consideration ecological and silvicultural aspects (Wormald, 1992). There exists very limited information on the growth of tree species native to the tropics, and information on experiences comparing mono-and mixed-species plantations is limited. However, in the present study, we tested that the mixed forest tree species can grow/survive in the coastal sand dunes. The main objective of the present study is to determine the growth, aboveground biomass, and carbon sequestration between the mixed-species plantation and mono-species plantation after over a decade (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016). The hypotheses tested were: there is variation in growth, carbon stock, and survival among species; the growth, survival, and carbon stocking potential of native species are higher in mixtures that in monospecies plantations.

Study description and process of seedling production in the nursery
The study plots were established in 2006 in Koonimedu Coastal village on the Coromandel Coast of southern India. The mean annual maximum and minimum temperature are 33 and 24.5°C. The mean annual rainfall is 1282 mm per year with a sixmonth dry period (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016). In general, coastal sandy soils prevail in the region with poor nutrient. Prior to planting within the restoration program, seedlings were propagated from seeds of tree species native to the area. The presence and availability of local seed sources was considered to ensure the provision of genetic material for the production of seedlings to be used in the forest restoration program. Seed collection was carried out in the Tropical Dry Evergreen forests, on the Coromandel Coast, in areas containing both primary and secondary forest during the dry season between March and June. Seeds were cleaned and dried at the Pitchandikulam Forest, where seeds were stored in sacks. Periodic germination tests were carried out to test seed viability. Seeds of some species received treatments to improve germination rates, including scarification, wetting, and drying in hot, as well as cold water. Seeds were planted in shaded seedbeds with the seedlings transplanted to 6 by 12 cm polyethylene bags with perforations at the bottom to insure drainage. The trees remained in the nursery for six to one year depending on the species and its growth rate before out planting with a minimum seedling height of 30 cm.

Preparation of sites and planting
Once the site was prepared for planting, planting holes were dug, with dimensions of 45 cm deep by 20 cm in diameter. The planting holes were spaced at 3 m intervals along the access paths. The distances of the applied planting points were 4 m between the axes of the rows and 3 m between holes. The distance of 1.5 m gave an average planting density of 1200 trees per hectare in enrichment areas. After planting, manual clearance of grass and other herbaceous vegetation was carried out twice a year with machetes during the first three years after planting as part of maintenance to ensure that young trees are not outcompeted by weed species. Weeding was the main maintenance activity after field planting of trees, and a pruning of secondary apical shoots was conducted in the first year.

Data analysis
A total of 2055 individuals of 82 native trees and 1500 Casuarina equisetifolia were planted in 2 ha in 2006. Table 1 includes the list of species, families, and ecological importance. Species choice was based on growth rate, timber, and ecological importance. In each 1 ha plot, diameter at breast height (dbh) and total height were measured for each tree after over a decade (2016). Differences in (i) height growth rate (cm yr −1 ) and (ii) average biomass carbon gain per year (kg C yr −1 stem −1 ) for the most characteristic species within the plots were calculated. For calculation of height growth rate, the differences in height were divided by the time between measurements in years. Average biomass carbon gain per year was calculated similarly, following conversion of DAP measurements using allometric equations (given below).
Total carbon for each species was calculated using a dry tropical forest allometric equation for above and below ground biomass (Chave et al., 2005). The equation forms for estimation of carbon were as follows: Where D is the diameter and ρ is the wood specific density of tree species. The wood specific density of each tree species was taken from available literature (Mani & Parthasarathy 2007) and also from global wood density database. We used the generalized allometric equation (Pearson et al. 2005) for few species for which WSD value was not available, using diameter as the only variable. The carbon stock was estimated to be 50% of the total biomass (AGB + BGB). Above ground biomass and carbon stock were calculated for only 47 native tree species (woody species) and casuarina in the mono plantation.
The averages of total height, dbh, basal area, and survival and mortality were calculated for each 1-ha plot in each species. The differences in diameter distribution of trees between the two inventories (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016) were tested using Kolmogorov-Smirnov two sample test (Zarr 2006), and we used paired t-tests to check for the significant differences in tree variables in two different plantations using SPSS software.

Species height and growth rate
In the mixed species plantation, measurements taken at 10 years of interval resulted in Ficus benghalensis and Bauhinia racemosa demonstrating the best growth in terms of height, followed by Alibizia amara and Azadirachta indica, with no statistically significant differences (P < 0.05) between mono-and mixed native species plantations. In the mono plantation, C. equisetifolia showed a moderate growth of height (average 9.5) and girth. Simple linear regression between annual girth increment and height produced very strong positive relation (R 2 0.759; Figure 1). The growth in diameter of Ficus benghalensis was the greatest in the mixed native species plantation plots, followed by Albizia amara, Vitex leucoxylon, and Azadirachta indica with no statistically significant differences (P < 0.05) between diameter increment in the mixed plots. When compared to mono plantation, it showed highly significant differences (P < 0.05) values. In the mono plantation, Casuarina equisetifolia showed a greater diameter increment in the last 10 years when compared to the mixed species plantation. Tricalysia sphaerocarpa, Tarenna asiatica, Strychnos nux-vomica, Salvadora persica, Murraya paniculata, Glycosmis mauritiana, Cassia fistula, and Aegle marmelos showed the slowest growth rates, with no significant differences in the mixed plantation. Single species plantations of Casuarina equisetifolia were the most productive, showing significant differences (P < 0.05) in basal area, compared to all species and the mixture of native species plantations.

Total aboveground biomass and contribution per species
The total aboveground biomass estimated for the two different plantations (mono and mixed) was 14.98 tonnes. The aboveground biomass estimated for the mixed native plantation was 8.007 tonnes and the mono plantation Casuarina had 5.585 tonnes of aboveground biomass. Among the 47 native mixed species, Bauhinia racemosa shared a maximum of 2.855 tonnes (35.7%) to total biomass followed by Albizia amara (17.8%), Lepisanthes tetraphylla (5.2%), and Wrightia tinctoria (5.1%; Table 3; Figure 2). A positive correlation was observed between height and biomass of all 47 species (rs = 0.773, p < 0.05; Figure 3).

Total carbon stock
The total carbon stock estimated for the tree plantation in the two plots (both mixed native and mono) was 7.492 tonnes. The carbon stock estimated for the mixed native plantation was 4.003 tonnes and the mono plantation Casuarina had 2.792 tonnes of total carbon (Table 3).

Contribution of different tree species to total carbon stock
Total carbon was significantly different between species (F = 6.6, p < .0001). Bauhinia racemosa had significantly greater total biomass gain per year than all other species in the mixed species plantations (p < 0.05). Among the 47 tree species in the mixed species plantation, the contribution of Bauhinia racemosa to the total carbon stock was high (1427.75 kg) followed by Albizia amara (712.52 kg), Lepisanthes tetraphylla (207 kg), Wrightia tinctoria (204.51 kg), and Azadirachta indica (181.32 kg) (Table 3). A positive correlation was observed between the carbon stock and density of the top 10 species which contributed predominantly to the total carbon stock (rs = 0.773, p < 0.05).

Discussions
The results of the present study provide valuable information to support the establishment of plantations with native mixed species and pure design. Introducing new species, however, is not without risks. Many reforestation projects fail due to inappropriate species choice, a consequence of inadequate knowledge about the potential of species and their growth and survival rates under different site and environmental conditions (Corlett, 1999;Rodrigues, de Castro, Cancho, & Balakrishnan, 2009;Wuethrich, 2007). The use of a greater variety of native indigenous species in reforestation may improve the resilience of ecosystems, decrease sensitivity to pest and diseases, and increase functional diversity (Benayas, Newton, Diaz, & Bullock, 2009;Hooper et al., 2005;Rodrigues et al., 2009). Creation of forests in the tropics takes place across a wide variety of non-climatic and climatic conditions. Different reforestation experiments have elucidated the strong effects that environmental conditions may have on species growth and survival (Butterfield, 1996;Calvo-Alvarado, Arias, & Richter, 2007;Park et al., 2010). Local climate conditions also have a larger impact on plantations success. The development in height and girth of the crown is mainly determined during growth in the monsoon (rainy season), while, minimal growth occurring during dry seasons. Initial  growth of the tree species vary according the species and the local seasonal weather conditions, including the amount of rainfall generated in a given year. In general, mortality rate are determined by the amount of rainfall in a given year. Tree species in their first three years of growth are especially vulnerable to the drying out the soil. On the other hand, the finding that 23% of the species may have high initial mortality and unsatisfactory early growth is critical information for avoiding early failure of reforestation projects. Several species showed poor performance and seemed to be unsuitable for large-scale planting in open plantation sites. Ashton, Gunatilleke, Singhakumara, and Gunatilleke (2001) reported that some of these species might do better when planted after site amelioration by earlier planted or extant nurse trees. Overall, species in mixed plantings had higher values of carbon sequestration than the mono plantation. According to our results, it seems that fast-growing species (i.e., B. racemosa, A. Amara, L. tetraphylla) accumulate biomass and carbon very fast in the first stage of their lifespan, before they are about 10 years old. On the other hand, tree plantations that include slower-growing species (i.e., Aglaia elaeagnoidea, Memecylon umbellatum) may accumulate more biomass and carbon within the system in the long term, compared to stands or mixtures of fast-growing species only. This shift in the accumulation of biomass and carbon may be related to differences in the wood specific gravity and growth patterns among fast and slow growing species (Elias & Potvin, 2003;Redondo-Brenes & Montagnni, 2006;Thomas, 1996). Wood specific gravity varies widely between tropical forest tree species, and it is closely related to differences in diameter growth rates and life history strategies (Baker et al., 2004;Redondo-Brenes & Montagnni, 2006;Whitmore, 1998). The values of aboveground biomass and carbon sequestration in mono plantings from this study are lower than values found in other regions of tropical humid climate, such as in 8.5year-old mono plantings of Casuarina equisetifolia, Eucalyptus robusta, and Leucaena leucocephala in Puerto Rico (Parrotta & Knowles, 1999). Values of this study are also higher than those reported for pure plantation of Pinus caribaea, Leucaena spp., Casuarina spp., Pinus patula, Cupressus lusitanica, Acacia nitolica, Senna siamea, Azadirachta indica, Gmelina arborea (Brown, Lugo, & Chapman, 1986;Lugo & Brown, 1992;Schroeder, 1992;Silver, Ostertag, & Lugo, 2000;Subak, 2000).
The present study revealed that the variation of GBH increment was also found on trees from similar species. This might be due to the response of each species to the growth process, which is different among species, as well as among trees of similar species. Many research showed that the internal and external factors had affected tree growth and development (Breugel et al., 2011). The internal factors comprised genetic factor, plant growth process, internal growth property, and physiological process. On the other hand, the soil parameters, micro climatic factors, and response plant to the environment could be the external factors. Miya, Yoshida, Noguchi, and Nakamura (2009) reported that variation in diameter growth of different saplings of different species in an uneven-aged mixed stand was influenced by individual growth conditions, but it was negatively related to the wood density (Keeling, Baker, Martinez, Monteagudo, & Phillips, 2008). Overall, the findings indicated that raising plantations on degraded lands or open land, particularly where seedbanks of native forest species are lacking, initiates the process of forest succession with nurse effect for woody native species regeneration. The plantations C. equisetifolia in this area would have been thinned out on a rotational basis to facilitate native species establishment. The  numbers of vascular plant species in the native species mixed plantation plot were much higher than C. equisetifolia (mono plantation), indicating that reforestation of open areas with native species might indeed speed up the recolonization of some other native flora through their influence on understorey microclimate and soil fertility improvement, and provision of habitats for seed-dispersing animals.

Conclusions
In conclusion, the present study shows that both mono and mixed native species can perform well in the plantation sites. Although the plantations are still young and it may be too soon to determine the behavior of the species studied, it is evident that best growth for these species was demonstrated in mixed native species systems. A majority of species planted here were shade-tolerant mature forest species whose survival appeared to be consistent in the mixed species plot. The species that performed poorly were mature forest species that may  require special efforts in restoration programs, such as provision of tree guards to prevent browsing, selection of appropriate microsites (e.g., shaded areas), and the use of older or hardened saplings that can better tolerate drought or browsing. The higher mortality of shadeintolerant species appears to be the results due to high intensity of light in coastal dune ecosystem. This result in not surprising because most of the species that were planted are late-successional trees found in mature forest and included few pioneer species. Management practices such as pruning and thinning could favor the development of these species in mixed plantations and provide revenues at earlier ages, when an appropriate group of species is used. Plantation of C. equisetifolia seems to be well adapted and had more carbon stocking potential. On the other hand, mixed plantation with indigenous species would contribute more to sustainable management and they provide great shelters for many faunal communities and provide a greater range of ecological goods and ecosystem services than the mono plantations.  Figure 3. Simple linear regression between height increment and total aboveground biomass in mixed species plantation (height class were distrusted like 1-3, 3-6, 6-9, 9-12, 12-15, and 15-18).