Growth Performance and Carbon Accumulation of Khaya (Khaya senegalensis) in Sri Lanka

Khaya (Khaya senegalensis) being a new forest plantation species, its growth performance under Sri Lankan conditions has not been evaluated. Further, carbon sequestration by forest plantations, which forms an important option in climate change mitigation, has not gained much attention in Sri Lanka. Hence, the present study aimed at assessing the growth performance and carbon accumulation of Khaya plantations in Sri Lanka. The tree height and tree diameter were measured non-destructively in identified age classes and biomass was estimated with allometric equations. Khaya in Kurunegala division (Intermediate Zone) has achieved 39.98 cm and 18.31 m of dbh and height, respectively, by the age of 18-20 years, whereas in Anuradhapura division (Dry Zone) it was 22.21 cm and 12.41 m, respectively. Significantly higher dbh and tree height (p<0.05) indicates the better site conditions in the Kurunegala division for superior growth of Khaya compared to those in the Anuradhapura division. The dbh and tree height of Khaya showed a strong correlation in both divisions. Significantly higher (p<0.05) biomass of Khaya was recorded in Kurunegala than in Anuradhapura division. The average carbon sequestration of K. senegalensis was 88.98 and 127.92 t/ha in Anuradhapura and Kurunegala divisions, respectively. Khaya plantations in Anuradhapura division (741.92 ha) and Kurunegala division (475.20 ha) have accumulated 21,785 t and 27,969 t of carbon, respectively. Growth performances evaluated in the present study can guide the future decision making process in establishment of forest plantations in Sri Lanka. Regional baseline carbon estimates of Khaya generated by this study could effectively be utilized for carbon budgeting programmes until validated further by increasing the frequency of measurements of variables and developing allometric relationships specific for K. senegalensis.


INTRODUCTION
Khaya (Khaya senegalensis), also known as African mahogany, is a newly introduced forest plantation species to Sri Lanka by the Forest Department of Sri Lanka in 1994. Today it has been identified as one of the promising (Tilakaratna and Weewardane, 1992) and priority species for plantation establishment in the Dry and Intermediate Zones of Sri Lanka (Tilakaratna, 2001) with the main aim of timber production. Besides timber production, they deliver environmental benefits such as site amelioration, reducing greenhouse effects by sequestering carbon dioxide from atmosphere (Shea et al., 1998) and reducing soil erosion.
Fast growing tree plantations are considered as highly efficient carbon sinks, which have potential to reduce the rate of global warming and the resultant climate change (Brown et al., 1996;Cannell, 1996;Sathaye and Ravindranath, 1998;Malhi and Grace, 2000;White et al., 2000;Schulze et al., 2000;Baker et al., 2004;Grace and Meir, 2009;Lewis et al., 2009). As the forest plantations form an important option for climate change mitigation (IPCC, 2007;Nabuurs et al., 2007), the ability of these plantations to sequester carbon has received renewed interest. In addition to their industrial timber products, the importance of forest plantations has increased substantially during the last two decades in view of the increased awareness on global climate change, and the role of forests in regulating the global carbon cycle (Dixon et al., 1994;Clark et al., 2003;Clark, 2004;Houghton, 2005).
In the Sri Lankan context few studies have focused on the concept of carbon sequestration in forest plantations. The carbon mass as indicated by the standing biomass has not been estimated yet. Estimation of carbon mass will enable economic valuation of Sri Lankan forest plantations to explore possibilities of financial gains through mechanisms such as United Nation's Collaborative Programme on Reducing Emissions from Deforestation and Degradation in Developing countries (UN-REDD) (Gibbs et al., 2007;Ravels, 2008;Schwartzman et at., 2008) and Carbon-trading under Kyoto Protocol's Clean Development Mechanism (CDM) (Fearnside, 1999). Besides, Khaya being a new introduction, evaluation of the growth performance in its predominant and recommended growing localities, i.e. Dry and Intermediate Zones, of Sri Lanka is a timely need. Thus, the present study aimed at assessing the growth performance of Khaya in Sri Lanka, while identifying the environmental benefits through carbon accumulation, to capitalize on its potential in climate change mitigation.

METHODOLOGY
The study was concentrated on even-aged stands of Khaya plantations distributed in the Anuradhapura and Kurunegala divisions representing the Dry and Intermediate Zones of Sri Lanka, respectively. Potential Khaya plantations of different age classes (1-3, 4-5, 6-7, 8-10, 11-12, 13-15, 16-17 and 18-20 years) were identified through a primary land survey with the use of secondary data received from the Forest Department of Sri Lanka. In each division, the age classes were stratified, where sampling was done in different age classes within a division. A minimum of two (02) random sampling plots were chosen for each age class with the size of 10 m x 12.5 m (0.0125 ha) to gather primary data non-destructively. Data at stand level were collected from 07 locations (beats) in 14 sites (blocks and sub blocks) per division ( Figure 1) having a total of 28 sites with 57 sample plots from both divisions. A total of 818 sample trees were subjected to the measurement of total tree height and diameter at breast height (dbh). Age of trees was taken from the records maintained at the Divisional Forest Office of the Forest Department of Sri Lanka. The tree height was measured using a Suunto Clinometer, while a diameter tape was used to measure the diameter at breast height (dbh), i.e. 1.3 m above the ground level. The GPS coordinates of each location was identified for mapping purpose.

Study locations (beats) in (a) Anuradhapura and (b) Kurunegala divisions
The allometric equations 1 and 2 were used to estimate the biomass and carbon (Pérez and Kanninen, 2003)  where; Ya= above ground biomass of trees (kg/tree), dbh= diameter at breast height (cm) and Yb = below ground biomass of trees (kg/tree).
The biomass carbon content was estimated by using a standard coefficient of 0.5 (IPCC, 1996). Biomass and carbon values were then scaled to hectare basis, considering thinning age at 7, 15, and 20 years with the stand stock of 1600, 800 and 400 trees per ha prior to each thinning, respectively.
Analysis of Variance (ANOVA) was used to test the divisional differences between variables, i.e. dbh, tree height, above ground biomass, below ground biomass and total biomass. Mean separation was done using the Least Significant Difference (LSD). Nonlinear regression by dynamic fitting was done to predict the missing data. Analyses were performed with SAS software system (SAS version 9.0).

RESULTS AND DISCUSSION
Khaya plantations in the Kurunegala division (Intermediate Zone) is distributed in agroecological regions* (AER) DL 1b , IL 3 , IM 3b, and IL 1a , whereas in Anuradhapura division (Dry Zone) they are in agro ecological region DL 1b (Table 1).

Growth performance of Khaya
Greater dbh and height of Khaya in Kurunegala division could be attributed to the moisture rich growing conditions in many AERs in the division and other site quality parameters.
Khaya being a deciduous tree tends to shed its leaves in the dry spell and retard its growth especially in the Dry Zone, resulting in poor dbh and height gain in the Anuradhapura division (Table 2 and (Table 2) at the same age. Forouhbakhch et al. (2006) reported that the diameter and height are good indicators of site conditions; however, they are also dependent on other factors such as inter-specific competition and stand density. As the stand density of each age class was similar for both divisions, difference in dbh and height can be attributed mainly to site conditions, in which a vast variability did exist under the climate and soil conditions prevailed in the Khaya plantations located at different AERs in Anuradhapura and Kurunegala divisions.
* DL1b: Low country Dry Zone, receives >900 mm annual rainfall, experiences bi-modal rainfall pattern and a drought/rain free conditions during Yala season, soil types are reddish brown earth and low humic glay; IL3: Least potential area belongs low country Intermediate Zone, receives 1200 mm annual rainfall, soil types are non calcic brown soil (which has low water holding capacity), reddish brown latesolic and low humic gley; IM3b: Low country Intermediate Zone, receives >1200mm annual rainfall mainly from north east monsoon, soil types are reddish brown latasolic and non-calcic brown and reddish brown earth to little extend; IL1a: Low country Intermediate Zone, receives 1400mm annual rainfall mainly through south west monsoon, experiences a weak bi-modal rainfall pattern, soil types are red yellow podsolic, reddish brown latesolic, non-calcic brown and regosols (Punyawardana, 2008)

Fig. 2. (a) Age vs individual tree height and (b) age vs individual dbh of Khaya in Anuradhapura and Kurunegala divisions
The dbh and tree height of Khaya showed a strong degree of correlation in both Anuradhapura and Kurunegala divisions ( Figure 3). As the relationship of dbh and tree height is used to estimate the site quality (Stout and Shamway, 1982), it can be concluded that locations of Khaya plantations in Kurunegala division is superior to that of Anuradhapura. However, a huge site variation did exist in the divisions as they were located in two agro-climatic zones (Intermediate and Dry) while the Khaya plantations were distributed solely in AERs DL 1b in Anuradhapura and DL 1b , IL 3 , IL 1a , and IM 3b in Kurunegala divisions.

Above ground, below ground and total biomass
A significantly higher (p<.05) above ground biomass (AGB), below ground biomass (BGB) and total biomass (TB) of Khaya at tree level were recorded in the Kurunegala, attributing to moisture-rich growing conditions (Table 3). In contrast, less annual rainfall and prolonged droughts resulted in a significantly lower (p<0.05) tree level biomass of Khaya in Anuradhapura. Irregular biomass distribution through age classes was observed from the Kurunegala division (Figure 4). This reflected the climate variation of the different agro ecological regions (DL 1b , IL 3 , IL 1a , IM 3b ) within the Kurunegala division, whereas DL 1b was the sole agro-ecological region in the Anuradhapura division ( Figure 4) resulting in less variation. Similarly, De Costa and Suranga (2012) reported that carbon stocks of forest plantations vary with their age, while biomass accumulation would be slow in the young plantations until they establish their canopy cover to maximize radiation interception and photosynthesis.
Khaya plantations representing the age classes 6-7 and 11-12 years in the Kurunegala division were located in the agro-ecological regions DL 1b, and IL 3 , which are comparatively drier. The plantations representing the other age classes in the Kurunegala division were distributed in the agro-ecological regions with higher annual rainfall. This explains the lower AGB, BGB and TB per tree recorded from age classes 6-7 and 11-12 years in the Kurunegala division (Table 3). Further, Khaya plantations representing the age classes 4-5 and 18-20 years in Kurunegala division were located only in the agro-ecological region IL 1a , which experiences an annual rainfall >1400 mm, thus contributing to significantly greater biomass at tree level (Table 3).

Fig. 4. Biomass distribution in Anuradhapura and Kurunegala divisions
De Costa and Suranga (2012) reported that carbon estimates are related to the environmental conditions of the respective sites, inferior forest management and/or inferior quality of sites. Further, a combination of higher precipitation that promotes greater photosynthesis and lowered temperature that reduces respiration could also have resulted in high carbon sequestration rates in plants. Khaya plantations in Kurunegala division recording a greater AGB, BGB and TB for many age classes explain this aspect. Biomass and carbon values estimated for each age class of Khaya in the present study can be considered as the first overall estimation of biomass and carbon mass carried out in Sri Lanka (Table 4). Despite their approximate nature, these estimates can be used as basic data in carbon budgeting programmes. Further, these estimates can be fine-tuned and made more accurate by increasing the frequency of measurements and by developing specific allometric relationship for K. senegalansis.

CONCLUSIONS
Khaya records significantly higher growth performances and biomass accumulation in the Kurunegala division than in Anuradhapura division, owing to moisture rich growing conditions. Growth performance of Khaya evaluated in the present study can be utilized for decision making on future forest plantation establishment in Sri Lanka and to design a management plan for K. senegalensis.
The average carbon sequestration of K. senegalensis in Anuradhapura (88.98 t/ha) and Kurunegala (127.92 t/ha) divisions are well ahead of IPCC's benchmark of plantation forests and major plantation forest species, confirming its potential in climate change mitigation. Biomass and carbon mass generated through present study for each age class of K. senegalensis in Anuradhapura and Kurunegala divisions could be considered as regional baselines for forestry carbon projects, until new information is generated. They can be used for carbon estimations in future carbon budgeting programmes with the existing inventory data of the Forest Department of Sri Lanka in a cost effective manner.