0760-B1

Vegetation and Soil Properties of Pinus Caribaea Plantation, Grassland and Semi-Natural Forest in Sri Lanka

S.P. Nissanka^[1] and M.B. Sakalasooriya

Abstract

The study was initiated to investigate the changes of biological diversity, productivity and soil properties of different vegetation types established naturally on degraded abandoned tea lands, namely; semi-natural forest and grassland and Pinus caribaea planted in the grassland in mid-country wet-zone of Sri Lanka.

Species diversity, stand density, vegetation structure and tree stem diameter at breast height (>10 cm) were taken using transect sampling techniques. Soil pits were cut in respective vegetations, soil profiles were described and soil samples taken from each horizon were analysed.

Vegetation diversity, canopy structure (height and stratification) and diameter class distribution were greater in semi-natural forest compared to Pinus forest and grassland. Soil profile descriptions revealed that semi-natural forest, Pinus forest and grassland had 4, 3 and 5 distinct soil horizons respectively. The soil profile depth was highest in the semi-natural forest. The soil horizon "A" was found only in the semi-natural forest and grassland. Soil was highly eroded in the Pinus forest. Soil organic matter and other nutrient contents were higher in the semi-natural forest compared to Pinus forest and grassland.

Vegetation regeneration, productivity and diversity, and soil regeneration, soil chemical and physical properties were greater in the semi-natural forest than in Pinus forest and grassland. Pinus forest soils were degraded and severely eroded mainly due to fire. Pinus forest and grassland are unsustainable land use systems that cause serious environmental and social problems under present management levels. Proper management and close monitoring is essential to improve the productivity and sustainability of Pinus forest. Grassland should be planted with suitable forestry species to enhance natural regeneration and productivity.

Introduction

Expansion of agricultural sector, colonization, infrastructure development, extraction of timber and other natural resources, illicit encroachments etc. lead to deforestation of natural forests at an alarming rate (20000-40000 ha/year) over fast few decades, where as the annual replanting during the same period has been around 2000 ha (Crisholm et al. 1999).

Because of the poor management of agricultural lands and anthropogenic disturbances, land degradation has been a severe problem in the hill country (>300 m msl) regions of the wet and intermediate zones of Sri Lanka. It was estimated that nearly 11% of the area in this region is already degraded. Most of the degraded lands are covered with grasslands and expose to frequent fire damages (Crisholm et al. 1999) and some degraded lands are undergoing secondary succession (semi-natural forests). Vulnerability to further degradation and enhancing natural regeneration of reforested lands depend on the type of tree species and management practices used.

Establishment of forest plantations mainly in degraded lands was started since1870's to reduce pressure on the rain forests and to initiate natural regeneration. At present, the total extent of plantation forests is around 84,000 ha. Exotic species of Pinus caribaea, Eucalyptus grandis, E. microcorys Swietenia macrophylla, Albizia spp. etc were used for reforestation programmes in the wet and intermediate zones (FSMP 1995). However, the general public and environmentalists concern about negative impacts of some of those exotic species such as reduce biodiversity and natural regeneration, invasiveness, soil degradation, depletion of ground water table and stream flow, and habitat change, despite their productive potentials. For example, Pinus forests are exposed to frequent fire and land degradation more compared to other broad leaves species such as Swietenia, Albizia, Alstonia etc. However, most of these issues regarding mainly on Pinus and others are not established scientifically (Gunasena et al. 1988). Therefore, it is important to investigate changes in soil properties, vegetation diversity and productivity under different vegetations to make proper species selection, land use planning and management.

The objectives of this study were to investigate the changes of vegetation diversity and productivity, and soil properties of regenerating semi-natural forest (broad-leaf forest dominated by Swietenia, Albizzia and Alstonia) and grassland (expose to annual fire) established naturally on abandon degraded tea lands and Pinus forest planted on the grassland at Hantana, in the mid country wet zone of Sri Lanka.

Materials and Methods

The experiment site is located at Hantana (approximately 720 ha) in Kandy district of Sri Lanka. The mean annual precipitation and temperature is around 2121 mm and 24 ⁰ C respectively. Elevation is around 500-700 m (msl) and the site belongs to the Agro-ecological region of wet-zone mid-country (WM3). The major soil group is Red Yellow Podzolic (Panbokke 1996). Lands at Hantana were cleared for tea cultivation in early 19^th century and abandon later on. Lands became degraded and marginal due to severe soil erosion and expose to frequent fire disturbances during the dry months. In this region, regenerating semi-natural forest and grassland are the dominant vegetation types at present. Some grassland areas were planted with Pinus species in 1980's to provide ground cover, enhance natural regeneration and to obtain production.

Vegetation analysis

Four rectangular transects each measuring 5m x 100m were marked randomly in each vegetation types, and species were identified and total number was counted and diameter at breast height (dbh) and tree height were taken only from trees having dbh > 10cm.

Soil analysis

Soil pits were cut from representative sites in each vegetation types. Soil profiles were described and samples were taken separately from each horizon for analysis. Soil organic carbon, total nitrogen, available P, exchangeable K, pH and cation exchange capacity (CEC) were analysed as described in Van Ranst et al. 1999. The experiment was conducted during 1999-2000.

Results

Vegetation diversity

Number of species, families, total number of individuals per unit area and canopy height and number of strata's were greater in the semi-natural than in Pinus forest and grassland (Table 1 & 2). Tree species number present in Pinus forest and grassland was lower and all of them were smaller saplings. Being a planted forest, nearly 93% of trees were Pinus caribaea in the Pinus forest. No trees (>10 cm dbh) were found in the grassland. Dominant tree species in the regenerating semi-natural forest were Swietenia macrophylla, Alstonia macrophylla, Albizia lebbeck, Albizia odoratissima, Michelia champaka, Filicium decipiens, and Mangifera indica. Semi-natural forest had very high sapling population with higher diversity. Sapling population was less in both Pinus and grassland vegetation and most of the saplings found were pioneer species such as Macaranga peltata, Alstonia macrophylla, and Alstonia scholaris. The most common shrub and grass species found in the grassland and Pinus forest were the Psidium guineense and Panicum maximum, which are tolerant to fire.

Productivity

Number of trees > 10 cm dbh were greater in the Pinus, however about 86% of them were in the diameter class of 10-29 cm. However, in the semi-natural forest, trees were found in all diameter classes having more on lower diameter and less on higher diameter classes (Fig.1).

Table 1. Common families, number of individuals and percentages of trees having > 10 cm dbh (per ha) in each family (no tree species >10 c dbh were found in grassland).

Description of soil profiles

Semi-natural and Pinus forest and grassland had 4, 3 and 5 distinct soil horizons respectively (Table 4, 5 and 6). Soil profile was deepest in the semi-natural forest and it was very shallow in the Pinus forest. The grassland soil showed greater level of variation within 60 cm depth, by having 5 different horizons. The horizon "A" was found only in the semi-natural forest and grassland. This may be due to higher litter accumulation in those two vegetation types. No litter was accumulated permanently under Pinus forest, because it was destroyed due to frequent fire. The horizon "B" is very shallow (10 cm) in Pine forest.

Soil profile of semi-natural forest was dark brown in colour and the darkness increase gradually towards the topsoil. In Pinus forest, it was yellow in colour and contains relatively high gravels and quartzes at the top soil layer and the structure was relatively poor compared to other types. Presence of charcoal was visible in the soil profile of grassland, which was having light brown colour.

Soil organic carbon content was about 250% higher in the topsoil of semi-natural forest compared to other two vegetation types, and it was lowest in the Pinus forest. Litter accumulation was higher and it was relatively wet under the semi-natural forest. In the Pinus forest litter was relatively dry and decomposition was very low. Litter in the grassland and Pinus forest was subjected to frequent fire, as a result, organic carbon content and litter accumulation is very much less.

Table 2. Common families, number of individuals and percentages of trees and shrubs having < 10 cm dbh (per ha) in each family.

Fig 1. Number of trees (per ha) in different diameter classes (cm)

Table 4. Description of soil profile, chemical and physical properties of each soil horizons of semi-natural forest (units: K & P in ppm; CEC in cmol/100g).

Total nitrogen content across all profiles was highest (200%) in semi-natural forest and it was similar in the Pinus forest and grassland soils. The N content was highest in the topsoil and decline with the depth. Soil layers in the Pinus forest soil profile were having very high exchangeable potassium content compared to other soils types. However, the deeper layers in the grassland had high potassium content. Soil pH in the semi-natural forest was significantly higher in A1 horizon, and acidity gradually increases with the depth. Pinus forest and grassland soils were relatively acidic than the semi-natural forests. The CEC of semi-natural forest soils were about 400% higher compared to other soil types, especially in the upper horizons. The CEC of all horizons in the Pinus Forest soil was similar, where as it was highest in the A3 and B1 horizons in the grassland soil.

Table 5. Description of soil profile, chemical and physical properties of each soil horizons of Pinus forest (units: K & P in ppm; CEC in cmol/100g).

Table 6. Description of soil profile, chemical and physical properties of each soil horizons of grassland (units: K & P in ppm; CEC in cmol/100g).

Discussion

Vegetation analysis revealed that species diversity of Pinus forest and grassland was very much less compared to the semi-natural forest. This was mainly due to the frequent man-made fire disturbances. Only fire resistant grasses, shrubs and few trees were grown on those sites. However, in the semi-natural forest, trees were found in all diameter classes having more number of trees under lower diameter classes and less under higher diameter classes, more or less similar to a natural forest. Greater proportion of seedling and sapling populations of pioneer to mid-tolerant tree species and the distribution of diameter classes in the semi-natural forest indicates that the site was undergoing secondary succession successfully, subsequently developing into mature forest. As a planted forest, in general, Pinus forest was not having satisfactory wood biomass production or natural regeneration, due to various forms of disturbances; poor stand management and no soil conservation.

Soil profile information showed that the soils were not much deep, but well drained, with dark brown to yellow in colour. Surface "A" horizons were present in grassland and semi-natural forest. It was noted that fine and medium roots were abundant in the upper soil horizons resulting fairly good soil structure in both those vegetations. The surface soils of the grassland developed very good crumb structure than semi-natural forest. This may be due to the thick fibrous roots of grasses. However, grassland soils are subjected to very heavy erosion when the rainy season starts after dry period, where grasses are set on fire. Sometimes these grasslands are subjected fire damages several times a year. A recent study done at Upper Mahawelli watershed areas in Sri Lanka, revealed that soil erosion was over 30 t/ha./year in grasslands (Gunasena, 1988). Pinus forests are damaged even more due to repeated burning of Pinus litter, which are dry and flammable. As a result, Pinus forests are heavily eroded, degraded and surface horizons are completely damaged. Gunasena 1988 reported that Pinus land lose valuable surface soil well over 40 t/ha./year.

The upper horizons in semi-natural forest are rich with most of essential elements, such as N, P, and K and organic matter. Higher organic matter content has resulted greater CEC in semi-natural forest soils improving the soil physical, chemical and biological properties. Because of the high CEC, nutrient retaining and cycling is better and soils were less acidic compared to other soil types. Nutrient losses in Pinus forest and grassland were very much higher due to soil erosion and repeated fire. Consequently, outcome of the burning is the loss of nutrients through volatilization, leaching and migration (Kreeft 1992).

Overall results indicate that, in the semi-natural forest, vegetation regeneration and improvements of productivity, biological diversity, soil physical, chemical and biological properties are superior to Pinus forest and grassland. Pinus forest and grassland vegetations are unsustainable land use systems, which cause severe environmental and social problem under present management levels. Even though the soil properties were depleted under Pinus forest, it accumulated reasonable amount of biomass even with frequent fire disturbances and in very poor soil, indicating very good production potential if manage properly. Since there is huge demand for timber at present in the country, these Pinus and the semi-natural forests could be developed into production forests with better management, while improving the protection and aesthetic properties of sites.

Conclusion

Overall comparative results on three different vegetation types exhibit that, performance of semi-natural forest dominated by Swietenia, Albizia and Alstonia is far exceeds than Pinus forest and grassland regarding the capability of vegetation regeneration, improvement of productivity and diversity, soil regeneration and improving soil physical and chemical properties of the degraded soils at Hantana. There were no significant differences between Pinus forest and grassland in their fertility levels, however, Pinus forest soils are becoming a problematic soil, owing to a greater soil erosion and shallow depth of the soil. The physical characteristics of the soils in grassland are comparatively better than the Pinus forest soils. Management of the organic matter in the tropical soils is paramount importance to ensure eco-system sustainability. Therefore, it is necessary to increase quantity of litter and residues retain in the site. Pinus forest and grassland are unsustainable land use systems, which cause severe environmental and social problem under present situation. Proper management and close monitoring is essential to improve the productivity and sustainability of Pinus forest. Grassland should be planted with suitable forestry species and manage well to enhance natural regeneration and productivity.

References

Bird, P., 1995. A management plan for the upper Hantana catchment forests, University of Peradeniya - Oxford Forestry Institute link project. Peradeniya, Sri Lanka.

Crisholm, A., A. Ekanayake and S. Jayasuriya. 1999. Economic policy reforms and the environment: land degradation in Sri Lanka. Ministry of Public Administration, Home Affairs & Plantation Industries, Planning division, 55/75, Vauxhall lane, Colombo 2.

FSMP, 1995. Forestry sector master plan. Forests Department of Sri Lanka, Ministry of Environment and Forestry, Sampathpaya, Battaramulla, Sri Lanka.

Gunasena, H.P.M., Savitri Gunatilleke and A.H. Perera, 1988. Reforestation with Pinus in Sri Lanka, Proceeding of a symposium organized by the University of Peradeniya and British High Commission on behalf of the Overseas Development Administration, UK.

Kreeft, R. 1992. Conservation of Pinus forests via Introduction of broad leaf species, Ed. J.H. Sandom, OP-OFI link project, Faculty of Agriculture, University of Peradeniya.

Panbokke, C.R., !996. Soils and agro-ecological environments of Sri Lanka. Natural resources, energy and science authority of Sri Lanka, 47/5, Maitland Place, Colombo 7.

Van Ranst, E., M. Verloo, A. Demeyer and J.M. Pauwels. 1999. Manual for the soil chemistry and fertility laboratory. International Training Center for Post-graduate Soil Scientists, Krijgslaan, 281/58, B-9000 Gent, Belgium.