Klaus Geiger, MF 20141
Land and habitat degradation, climate change, and local livelihoods are intimately related issues that continually evolve. Traditional agroforestry knowledge and practice are increasingly recognized as a sustainable land use. Agroforestry in Sri Lanka has existed for at least twenty-five centuries, but little research has quantitatively described the floristics and structures of these ‘tree garden’ systems and their relationships to human use and cultivation. This study examined the species composition and vegetative structure of tree garden systems typical of southwest Sri Lanka. Data shows as much as three times the species richness in the study site homegardens compared to other similar research in Sri Lanka. Additionally, homegardens in the study site, similar to those studied in the island’s central hill region, provide ecosystem services such as carbon sequestration that are valuable both economically and for conservation purposes. The productive activities at the site and proximity to the ecologically important Sinharaja forest reserve emphasize the crucial—but perhaps undervalued—role that local livelihoods and land management activities play in conservation.
Environmental degradation is a serious problem with global implications (UN 2013), but the urgency of this issue is often masked by increasing human well-being (Raudsepp-Hearne et al. 2013). Current global greenhouse gas (GHG) emissions are 46% above their 1990 level (UN 2013). It is widely accepted that deforestation and forest fragmentation contribute significantly to these emissions, as well as producing “edge effects,” which include stronger winds and increased forest fire susceptibility that exacerbate the impacts of climate change (Laurance 2004, Golding & Betts 2008). Agriculture alone is estimated to drive ~80% of deforestation and directly accounts for 10-15% of total anthropogenic GHGs (Van der Werf et al. 2009, Hulse et al. 2013, IPCC 2013), and this estimate is closer to 29% when including agriculture-related GHG emissions, such as transportation (Hulse et al. 2013). Timber products, palm oil, soybeans, and animal husbandry are significant drivers of deforestation, particularly in developing countries. Projections estimate that there could be a 40% reduction in global forest cover by the middle of this century, at which time the world population is estimated to exceed nine billion people (Soares-Filho 2006).
Unlike the variable effects of habitat fragmentation, habitat loss plays a clear and well-documented role in reducing biodiversity (Fahrig 2003). Protected area designations have succeeded in preventing deforestation, but often disrupted traditional livelihoods, in many cases only for the designated areas to be isolated by neighboring transformative agricultural and extractive land uses (Naughton-Treves et al. 2005). Agroforestry has been touted in recent years as a sustainable land-use alternative to combat climate change as well as the problem sometimes described as “rich forests, poor people” (Peluso 1992, Garrity 2004, Leakey et al. 2005, Verchot et al. 2007, Dawson et al. 2009).
Sri Lanka, a small island nation located off the coast of the Indian sub-continent, is biogeographically within the Indo-Malayan biome comprising a highly diverse range of fauna and flora of which 70% of the tree species in the southwest rain forest region are endemic (Myers 1990). About 44% of the island remained as native forest in 1956, the year of the first forest inventory (FAO 2007). Forest cover has declined by nearly 50% since then (Myers 1990, FAO 2002). This deforestation has significant implications for the future of species diversity—of which an estimated 25 to 30% is unique to the island—and forest-dependent local livelihoods (Erdelen 1988, Myers 1990). Rural communities have traditionally valued forests for a diversity of products and services, with timber serving a minor role. Perennial agroforestry systems called “tree gardens” result from traditional forest practices that protect biodiversity (Nair 1993): No-where is tree garden species diversity greatest than in tropical South Asia, and in particular southwest Sri Lanka (Braatz et al. 1992). While the Dravidian and Sinhala cultures of South India and Sri Lanka retained sophisticated and ancient traditions of plant use that date back over 5,000 years, the Indo-Aryans, the Mughals, and the European colonists brought new fruit species (Kosambi 1975). These introductions became incorporated into the tree gardens, which are now diverse mixtures of trees that provide fruit, medicines and spices that characterize the region but are especially diverse in the per-humid regions.
Scholars believe Sri Lanka’s tree gardens have been a cultivation practice for more than twenty-five centuries (Munasinghe 2003). In Sri Lanka, tree gardens have been studied in the central hill region, an area that was the last refuge for the Kingdom of Kandy and which traded spices from these tree gardens with the Portuguese (De Silvas 1981). They are also known as “Kandyan forest gardens,” after the city of Kandy in central Sri Lanka. Though tree gardens account for 13-15% of the total land cover of the country and constitute 30-40% of the total cultivated area, the central hill region’s Kandyan tree gardens are the only tree garden systems that have been well described (Jacob & Alles 1987, Hoogerbrugge & Fresco 1993, Wickramasinghe 1995, Pushpakumara et al. 2012, Mattsson et al. 2013, Mohri et al. 2013). Some scholars argue that this resilient management regime arose as an alternative practice to resource depletion of the once-diverse rainforest (Wickramsinghe 1995). This tradition, a mixed forest-gardening system, has been described as a “highly diversified and economically viable form of land use” (Jacob & Alles 1987). Though centuries old, the home garden system has continued to evolve from one generation to the next in order to suit socio-economic, cultural, and ecological needs (Caron 1995, Pushpakumara et al. 2012).
Past studies have noted that the composition of tree gardens depends greatly on socio-economic conditions and strategies, but few architectural analyses have been employed to compare home garden characteristics with natural arrangements of flora species common in the area. In addition, there has been little evaluation of the functional diversity of Sri Lankan tree gardens (Pushpakumara et al. 2012). Lastly, highly heterogeneous research methods make it difficult to compare results and impossible to find patterns of social, economic, and ecological aspects, “on which system sustainability depends” (Pushpakumara et al. 2012). This study addresses these knowledge gaps in tree garden systems in southwestern Sri Lanka. Specifically, the following questions are asked:
How does species diversity in southwestern tree garden systems compare with previous studies in Sri Lanka?
Do the study site’s tree gardens reinforce existing evidence concerning their functional diversity?
Pitakele, Sri Lanka, is a village in the southwestern lowlands comprised of thirty households (Fig. 1). The village is adjacent to the 89km2 Sinharaja Man and the Biosphere (MAB) Reserve, a UNESCO World Heritage Site comprising the last remaining relatively undisturbed rain forest in Sri Lanka (Ishwaran 1990). The site’s underlying geology is a metamorphic undulating topography overlain by weathered in-situ ultisols that are relatively poor in fertility (Cooray 1967, Mapa et al. 1999). Elevation in the village varies between 337 and 420 m above msl. Mean annual temperature is 26°C (Ashton et al. 2001). Average annual rainfall is 4000 mm, the majority of which falls in the two annual monsoon periods from May to July and September to December (Ashton et al. 1997).
Fig. 1. Land cover types in the study site, Pitakele, Sri Lanka. Note that the bolded lines outline property boundaries and satellite farms; focal properties are indicated by numbers.
Interviews were conducted and field data collected in June and July of 2014. A head from each of the 30 households was interviewed regarding their land-use history, current plant uses (e.g., non-timber forest products like tea, rubber, cinnamon, and medicinal herbs), and the specific techniques of cultivating and maintaining their tree gardens.
In the field, all 30 property boundaries and the borders of dominant cover types throughout the tree gardens were mapped (Garmin GPSMAP 64s GPS, Garmin International Inc., Kansas, USA). Dominant species mixtures (e.g., tea plantations) were the determining factor for denoting cover type (apart from the location-based ‘patio’). Ten of the thirty houses were then selected as focal properties, based on the diversity of cover types represented and the size of the core tree garden area, in which all plant stems were mapped.
The cover types were: core tree garden, secondary shrub and forest, early seral, patio, tea plantation, rubber plantation, and rice paddy. The core tree garden area is a key productive cover type that consists of an intimate mixture of seemingly disorganized perennial crops. The secondary shrub and forest cover type differs from the core tree garden area in that it is a less intensively managed arrangement of primarily woody plants arising from partial or total disturbance of a primary cover type. Early seral areas were covered principally by grasses, sedges, and forbs. The patio was the area immediately surrounding a house. Patios had spots where the mineral soil was exposed, but always contained vegetated strips both surrounding and within them. Because tea plantations have been extensively studied (Harler 1956, Eden 1958, Fuchs 1989, Dharmasena & Hitinayake 2012), stems were not mapped, counted, or measured in this cover type. Rubber trees, like tea, were typically found growing in plantations as the dominant woody species, and as such, were listed as a unique cover type. Most rice paddies in Pitakele were shared among village residents, with the rights to cultivatation rotating among households. Thus, only those paddies cultivated by a landowner with sole rights were considered to belong to any given household. This rotation is due to the dynamic quantity of paddy land under cultivation during any given growing season (see Caron (1995) for an in-depth description of this transferable tenure system).
Every tree and shrub in each cover type (unless otherwise noted above) was mapped by GPS and identified. Structural data (e.g., height, diameter at breast height, canopy position, and horizontal area) were recorded to describe tree garden species arrangement, diversity, floristic patterns, and physical structure. Data were analyzed using ArcMap 10.2.2, Microsoft Excel 2013, and R (R Development Core Team 2014).
Interviews with the 30 household heads revealed several trends in home garden resource management. Pitakele’s households and their respective farms were established within the last 50 years, bringing with them traditional tree garden production methods. All landowners listed natural medicine, sustenance, and income as important to their tree garden management. Interviews with landowners also revealed a decline in rubber production—reportedly due to excessive rain which impedes harvesting of latex—and cinnamon production, with a concurrent surge in tea production. Cinnamon, though present in some gardens, has all but disappeared as a means of income owing to the specialized skill required for harvest. Further, landowners reportedly favored tea as a cash crop due to heightened market access from wholesale buyers arriving daily.
A total of 367 plant morphospecies in 198 genera and 86 families were found in the 30 tree gardens visited and/or reported by the landowners of Pitakele, Sri Lanka (Appendix 1). A total of 121 of the 367 distinct species documented are yet to be taxonomically identified. Thirty-five of the species reported by landowners in interviews were not observed directly by the author (Fig. 2). Four of the recorded species are known to be endangered, six are known to be near-threatened, and eight are known to be considered vulnerable (IUCN 2012). Nearly two-thirds of the recorded species had five or fewer individuals across the ten focal properties. The Simpson dominance index was 5%, meaning there was a 95% probability that two individuals randomly selected with replacement were from different species.
Fig. 2. Average species count by cover type and growth habit.
Species diversity within the 10 focal properties varied by cover type and growth habit (Fig. 2), with patio and core tree garden areas being the most diverse, and rubber areas being the least diverse. Within each strata or forest canopy layer trees and herbs accounted for the most species, whereas lianas and shrubs accounted for the least. (Fig. 5). Also, more than 80% of individual stems were under 10m tall in early seral, tree garden and patio areas. The most-recorded species across all cover types was betel nut (Areca catechu Arecaceae), which accounted for nearly 20% of all individuals. The second most-recorded species was gliricidia (Gliricidia sepium, Fabaceae) with 4.3% of all individuals; third was banana (Musa spp., Musaceae) with 3.8%; and coconut (Cocos nucifera, Arecaceae) fourth with 3.2% of individuals.
The thirty property sizes varied between 0.018 hectares to 1.34 hectares (mean = 0.34 ha, SD = 0.27). A few of the landowners cultivated additional land outside of the land immediately surrounding their household, boosting the mean landholding to 0.4 hectares. The ten focal properties ranged from 0.18 and 0.78 ha of contiguous land (mean = 0.47 ha, SD = 0.18, Fig. 3).
Fig. 3. Left: Pitakele focal property sizes by cover type (n = 10). Right: Mean area of each cover type for all 30 properties.
Patio had the smallest cover area amongst the intensively cultivated cover types (Fig. 3). The vegetation was principally herbaceous plants and small woody shrubs and so contributed little to a property’s total basal area. These plants typically served non-timber purposes, such as medicinal and ornamental uses. Many ornamental species were individuals, found once in a single garden and growing in no other gardens. Patios and the core tree garden areas had the highest species richness (total number of species found in sample area) and Simpson’s index (the probability of picking two different species at random) per unit area as compared with tea and rubber plantations.
Tea plantations were a nearly ubiquitous component of tree gardens in Pitakele, occurring on 27 of 30 properties, whether within the immediate garden or as a nearby satellite. Tea plantations and the core tree garden area made up for 76% each household’s landholding (Fig. 4). Young leaves from tea bushes were reportedly picked about every nine days, continually maintaining the bushes’ waist-high stature. The tea bushes formed the understory of each plantation, and depending on bush density, there would be little to no ground vegetation. Coconut trees and areca nut palm trees were common overstory trees in the tea plantations, and in many cases served as ladders for bulat vines (Piper betle, Piperaceae) or black pepper vines (Piper nigrum, Piperaceae). The small multi-purpose leguminous tree Gliricidia sepium was abundant in some gardens’ tea plantation, acting as a shade tree, a living fence and its leaves a natural nitrogen fertilizer. About 25% of Gliricidia individuals had bulat, black pepper vines, or another, frequently medicinal, liana species growing on them.
There were only two households with rubber plantations on the same contiguous land as the rest of their cultivated area. Existing rubber plantations did not have a cultivated understory, but they were thick with wild vegetation.
The tree gardens of Pitakele, Sri Lanka, have a high species richness compared with previous studies on Sri Lankan and other similar agroforestry gardens. With a total of 367 recorded species, it appears that Pitakele’s tree gardens hosted almost three times more than the highest total number of plant species found in similar research from Sri Lanka (Perera & Rajapaske 1991, Ranasinghe & Newman 1993, Lindara et al. 2006). The ten focal gardens in Pitakele boast an average of 100 species per landholding, whereas these same studies confirm an average total species count for a given garden to be 46, 42, and 12, respectively.
A number of reasons might explain the high species diversity in Pitakele. Lower elevation, greater commercialization, low urbanization, and lower fragmentation were found to be the driving factors for high plant diversity in homegardens (Arifin et al. 1997, Kehlenbeck et al. 2007). Pitakele is both rural and located at lower elevation, though properties are not particularly large by comparison. Further, the village’s proximity to the Sinharaja forest reserve may contribute to the greater species diversity via seed dispersal from the forest.
Fig. 4. Strata distribution by cover type.
Though protected areas generally prevent deforestation and, thus, habitat loss, in many cases they are increasingly isolated geographically and genetically by surrounding land converted to non-forest cover (Naughton-Treves et al. 2005). However, various authors have suggested that Kandyan forest gardens’ multi-strata vegetative structure, >70% canopy cover, and irregular horizontal distribution contribute to their resemblance to undisturbed forests (Perera 1991). As a system that has evolved over several centuries, it is probable that there is an established and proven logic behind site placement and spacing (Jacob & Alles 1987). Like other Sri Lankan homegardens, Pitakele’s can be divided in to 3-5 strata: a groundstory layer (<1m tall), a shrub layer (1-2.5 m), a mid-story layer (2.5-10 m), and an overstory layer (>10 m, which may be subdivided in to two; Nair & Fernandes 1986, Perera & Rajapakse 1991, Fig. 4). Curiously, jak fruit (Artocarpus heterophyllus, Moraceae) was absent as a dominant overstory species, contradicting previous studies that describe jak fruit as a prominent feature in the Kandyan forest garden canopy (Perera & Rajapakse 1991). As expected, the cover types with a greater percentage of individuals in the mid- and overstory strata also had greater basal area of vegetation per hectare (Fig. 5). This physical similarity to natural forest underscores Pitakele’s Kandyan forest gardens’ importance in conservation, and their proximity to the Sinharaja forest reserve.
Fig. 5. Total basal area by cover type.
The Kandyan forest gardens’ physical similarity to natural forest permits it to provide many of the same ecosystem services that the natural forest provides (Kehlenbeck et al. 2007, Mohri 2013). The ecosystem services provided by homegardens, such as those in Pitakele, generally fall in to four categories: provision (e.g., food, medicine, fuel), regulation (e.g., climatic, carbon storage), cultural (e.g., religious purposes, such as the sacred ‘bo’ tree, Ficus religiosa), and support (e.g., nutrient cycling). Through provision, homegardens play a critically important role in conservation by mitigating “the fuelwood problem”—extraction of fuelwood from forests that is currently a primary cause for forest degradation on the island (Erdelen 1988). Additionally, tree gardens’ regulatory effect on local climate are potentially significant, as the carbon stocks in a wet-zone garden like in Pitakele may amount to between 48 and 145 Mg C ha-1 with a mean of 87 Mg C ha-1 (Mattsson et al. 2013). Applying the average quantity of sequestered carbon in wet-zone tree gardens in Pitakele this amounts to a total of nearly 1,343 Mg C stored across all thirty properties and satellite farms.
With growing emphasis on tea production as the primary local income, the existence of tree gardens in Pitakele near the Sinharaja forest reserve may be undervalued for their conservation and economic importance. Nevertheless, although homegarden species diversity is cited as crucial to productive sustainability, some scholars argue that the benefits of biodiversity for local forest-dependent communities are exaggerated (Gunatilake 1998, Kehlenbeck 2007). What is overlooked is that land in developing countries is the most important productive asset and that there are differences in technology, preferences, and discount rates at different levels of development (Panayotou 1994). It follows that biodiversity conservation is a development issue. Thus, it is encouraging that Sri Lanka’s forest policy evolved from a preservationist reaction to colonial denudation in the decades following their 1948 independence to a more nuanced approach with greater stakeholder involvement in recent decades (De Zoysa 2001).
I would like to thank A. Keerthiratne, K.D. Tandula Jayaratne, Pradeep Rajathewa, for their assistance in fieldwork and help in plant identification. I am grateful for guidance and support from Dr. Mark Ashton, Dr. B.M.P. Singhakumara, and Dr. Simon Queenborough. A very special thank you to the Tropical Resources Institute of the Yale School of Forestry and Environmental Studies, and especially donors to the F&ES Sri Lanka Program for Forest Conservation, whose generous support made this study possible.
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Klaus Geiger is a Carbon Services Staff Auditor with Rainforest Alliance. Klaus assists and leads field audits for carbon sequestration projects in Latin America under six different carbon standards. Klaus received his Masters of Forestry from the Yale University School of Forestry and Environmental Studies in 2014, and his Bachelors of Forestry from University of Missouri-Columbia in 2008.↩