Litter decomposition in a lowland rain forest and a deforested area in Sri Lanka

A comparative study of litter decomposition in a tropical lowland rain forest and an adjacent fernland using leaf litter from a dominant forest tree species, Cullenia ceylanica (Bombacaceae), and frond litter of the most abundant fernland species, Dicranopteris linearis (Gleicheniaceae), revealed significant differences in percentage mass lost between the two litter types and between the two sites: C. ceylanica leaf litter was reduced by 63 percent in one year in the forest vs 41.7 percent in the fernland, whereas D. linearis frond litter was reduced by 24.6 percent during the same period in the forest vs 15.7 percent in the fernland. A positive correlation with the amount of rainfall was observed for the mass loss of C. ceylanica litter in both sites; litter of D. linearis showed no such correlation with rainfall. Nutrient changes examined over one year indicated immobilization of nitrogen in both litter types in both sites and that of phosphorus only in D. linearis. Concentration and mass of K, Mg, and Ca decreased with decomposition in all four experimental treatments. A relatively slow rate of decomposition and an accumulation of organic matter in the fernland contribute to the conservation of nutrients in this aggrading ecosystem. IN FOREST ECOSYSTEMS, LITTER DECOMPOSITION REPRESENTS the major pathway for supply of plant nutrients to soil. Particularly for tropical rain forests growing in nutrient poor soils with relatively low external nutrient inputs, the turnover of bioelements is an important step (Staaf & Berg 1982). Although an increasing amount of information on litter decomposition and nutrient cycling in tropical rain forest ecosystems has been made available during the past few years (Edwards & Grubb 1977, Klinge 1977, Irmler & Firch 1980, Tanner 1981, Anderson et al. 1983, Gong & Ong 1983), comparative studies on decomposition between undisturbed and disturbed rain forests, such as sites cleared for shifting cultivation, are relatively few (Jordan et al. 1983). The extent of disruption of decomposition processes in these modified sites may have implications for the establishment and regrowth of forest seedlings. Therefore, studies into the effects of disturbance on soil biological processes in the humid tropics are urgently needed to better understand rain forest management and soil nutrient conservation (Anderson & Swift 1983). In Sri Lanka, lowland rain forests with many endemic plant species (Gunatilleke & Gunatilleke 1985) are being rapidly destroyed by logging and clearing for agriculture and human settlement. Following abandonment after shifting cultivation, many such deforested lands become almost permanent fernlands, grasslands, or scrublands; forest development is checked by recurrent fires lit by the neighboring villagers (Joachim & Kandiah 1942). In relatively undisturbed lowland rain forest areas and in an adjacent area, now a fernland dominated by D. linearis (Burm.f.), Underwood (Gleicheniaceae) (99% cover), we undertook a study to evaluate the role of soil processes in the fertility of both undisturbed and disturbed rain forest areas within the same climatic zone of Sri Lanka. In this paper we describe the results of an experiment in which we examined the mass loss and nutrient changes during decomposition of leaf litter of a dominant forest tree species, C. ceylanica (Gardn.) K. Schum. (Bombacaceae), and the frond litter of the most abundant fernland species, D. linearis, in both the forest and the fernland soils over one year from June 1980 to June 1981. STUDY SITES AND METHODS CLIMATE AND SITE CHARACTERISTICS. -The Sinharaja Man and Biosphere Reserve (8800 ha) is one of the few tracts of mostly undisturbed lowland rain forest remaining in Sri Lanka. A description of its climate, topography, geology, and soils is given by Gunatilleke and Gunatilleke (1981); the phytosociology is described by Gunatilleke and Gunatilleke (1985) and Maheswaran (1982). Some characteristic features of the forest and the fernland areas are summarized in Table 1. In disturbed areas around the perimeter of the forest, fernlands dominated by D. linearis, a highly successful secondary successional species in wet tropical climates (Joachim & Pandittesekera 1937), have replaced the once luxuriant rain forest. These fernlands have a more or less uniform height of about 1 m, and their fronds form a thickly interwoven canopy beneath IReceived 18 March 1985, revision accepted 11 September 1986. 2 Present address: School of Botany, University of Melbourne, Parkville 3052, Victoria, Australia. 3Author to whom correspondence should be sent. 90 BIOTROPICA 20(2): 90-99 1988 This content downloaded from 157.55. 39.224 on Wed, 11 May 2016 04:57:28 UTC All use subject to http://about.jstor. org/terms TABLE 1. Environmental and physico-chemical properties of the top 8 cm of soil in the forest and fernland sites in Sinharaia rain forest in Sri Lanka (from Maheswaran 1982). Forest site (Plot 4A)a Fernland site (FLD)a Area 25 x 100 m 50 x 50 m Elevation (above MSL) 518-548 m 487-518 m Soils Oxysols: Oxysols: red-yellow red-yellow podzolic podzolic Annual rainfall range 3810-5080 mm 3810-5080 mm Mean annual temperature 23.6? C n.d. b Soil pH 4.6-5.4 5.6-5.7 Height of vegetation 30 m 1 m C/N Ratio of soil 29.7 30.2 Total soil N (ppm)c 1991.5 (111.1) 2844.9 (113.0) Total soil P (ppm) 524.7 (37.3) 575.7 (6.5) Total soil K (ppm)c 578.4 (22.0) 2084.5 (103.7) Total soil Mg (ppm)c 548.6 (29.4) 1704.6 (90.0) Exchangeable soil N (ppm) 45.9 (5.0) 50.1 (5.2) Exchangeable soil K (ppm)c 39.5 (3.1) 60.9 (6.5) Exchangeable soil Mg (ppm)c 29.4 (3.5) 99.9 (5.4) Exchangeable soil Ca (ppm)c 66.1 (11.5) 300.7 (40.9) a Standard errors in parentheses. b n.d. = not determined. c Denotes significant difference at P < 0.01 (student's t-test) between sites. which is found the accumulated dead frond material. Other plant species found in these fernlands are mostly pioneer species characteristic of secondary vegetation (Joachim & Pandittesekera 1937, Maheswaran 1982). The forest site (Plot 4A, 25 x 100 m) (Gunatilleke & Gunatilleke 1985) and the fernland site (FLD, 50 x 50 m), situated near the northwestern boundary of the forest, were selected for the study (Fig. 1). The topography N SINHARAJA FOREST RESERVE