0503-B2

Forest floor dynamics and the carbon balance in tropical forests in Western Ghats, India

SM. Sundarapandian* & P.S. Swamy ¹

Abstract

Forest floor live (aboveground herbaceous and belowground) biomass and net primary productivity, soil respiration and carbon balance were studied in tropical forests at Kodayar in Western Ghats of Tamil Nadu. Aboveground (herbaceous community includes tree seedlings and saplings up to 1 m height) net primary productivity was significantly greater in site I (where tree density is lower) compared to other study sites. Similarly, very fine root ( _1 mm) biomass was greater here than in other study sites. The greater values on this site (I) could be attributed to recurrence of annual fire, which leads to the dominance of grasses in herbaceous community. However, fine (>1 - _ 3 mm) root biomass and net primary productivity were significantly greater in evergreen forest sites (III and IV) compared to deciduous forest sites (I and II) which could be attributed to greater tree density and basal area along with greater litter accumulation. Greater values of soil respiration in site I may be due to greater herbaceous cover. A partial carbon balance of these forests based on carbon input through litter accumulation on the soil and carbon output through soil respiration revealed that annual CO₂ - C output was 2.15 - 2.8 times higher than the C input (aboveground litterfall + residing litter). The excess of carbon output may be attributed to respiration of root based inputs.

Introduction

The Western Ghats of India is one of the biologically richest areas in South Asia and is characterized by high levels of biodiversity. During the recent past these forests were subjected to unscientific exploitation for agriculture, construction of hydroelectric project, raising monoculture plantations and other developmental activities. Successful conservation of these forests will ultimately depend upon an understanding of forest ecosystem dynamics. Herbaceous ground vegetation, though it constitutes only a small proportion of the total biomass in a forest ecosystem, plays an important role in ecological characteristics and classification of forests (Ovington 1955). Chaturvedi & Singh (1987) was also reported that herbaceous vegetation of the forest floor contributes around 15% of total net primary production because of the higher turnover of the herbaceous litter, herbaceous stratum is also expected to have major influence on ecosystem processes. Only in the last two decades some attempts were made to understand roots as part of the entire forest ecosystem (Santantonio et al. 1977). Soil respiration is an important functional process of the decomposition subsystem and also a useful parameter for studying soil biological activity; carbon cycling and energy flow in an ecosystem (MacFadyen 1963, Reiners 1968). Large amount of carbon is released to the atmosphere as CO₂ during decomposition of litter added to the soil from above- and below- ground sources and additional CO₂ is released by respiration of living roots (Behera et al. 1990, Rout & Gupta 1989). However, published information on the ecosystem processes at the forest floor is limited. Therefore, the present study aims at understanding the forest floor dynamics. In order to achieve this objective an attempt was made (1) to estimate above ground herbaceous net primary productivity, (2) to quantify belowground net primary productivity in the soil layers to a depth of 25 cm, (3) to measure the rates of carbon dioxide evolution through soil respiration and (4) to prepare a carbon balance of the ecosystem on the basis of carbon input and output.

Study area

The study area at Kodayar (77_15' E, 8_29' N) is at 250-650 m elevation in Kanyakumari district of Tamil Nadu, South India. The mean annual rainfall recorded in the study sites were 2338 mm, of which 81% occurred from June to November. Average monthly maximum and minimum temperatures were 30_C and 26_C in summer and 28_C and 24_C in winter, respectively.

Four study sites were selected in natural (climax) tropical forest ecosystems at Kodayar village within the radius of 10 km distance. The species that dominate in site I are Terminalia paniculata Roth, Careya arborea Roxb. and Buchanania lanzan Spr., etc.. The herbaceous community is mostly dominated by monocotyledons such as Themeda cymbaria Hack., Themeda sp. and Globba orixensis Roxb., etc. Site I has been subjected to wild fire during summer. Site II is dominated by T. paniculata followed by Aporosa lindleyana Baill., etc. Understorey vegetation is dominated by Helicteres isora L., Eupatorium odoratum L., etc. Site II has also been subjected to anthropogenic perturbations. Sites III and IV are undisturbed evergreen forests. The species that dominate these sites are Hopea parviflora Bedd., Syzygium laetum Gandhi, followed by Artocarpus heterophyllus Lam., etc. Understorey vegetation was dominated by Psychotria nigra L., Calamus sp., etc.

Methods

Aboveground (includes tree seedlings and saplings up to 1 m height) net primary productivity was calculated using an incremental biomass harvest method (Sims & Singh 1978) by 21 randomly placed quadrats of 1 m x 1 m at monthly intervals in each study site. Litter production was estimated by standard method (Sundarapandian and Swamy 1996a).. Standing crop (residence) of forest floor litter (up to the surface of mineral soil) was estimated in the month of June in all the sub-sites. Fine roots were sampled from soil monoliths of 15x15x25 cm, excavated. On each sampling date, 20 monoliths were collected randomly from each site every month. Standing crop root biomass of each sample was determined in the laboratory by wet-sieve method The rates of soil respiration were measured by using alkali absorption method. The measurements were made at monthly intervals. Simultaneously, measurements were made on litter respiration. The values of carbon input and output parameters have been calculated in terms of carbon mass. For the dry matter values of litter accumulation, carbon mass has been calculated on the assumption that carbon content is 0.45 of the dry matter (annual litter production + residence litter; Coleman 1973, Woodwell et al. 1978). The CO₂ output values were calculated in terms of carbon weight. Student-Newman-Keuls test was used to compare the means of density, basal area, above- below- ground biomass, litterfall, standing crop of litter (residence litter) and soil respiration among the sites Regression analysis was used to correlate soil moisture, rainfall, and air temperature and litter moisture with soil respiration.

Results

Aboveground net primary productivity of herbaceous community was significantly (P<0.01) greater in site I compared to other study sites (Table 1). Net primary productivity of very fine (<1 cm) root was significantly grater in the site I compared to other study sites. Net primary productivity of fine root(>1-<3 cm) in evergreen forests (III and IV) were significantly (P<0.05) greater than deciduous forest sites (I and II).

The relationship between CO₂ evolution rates and abiotic factors showed significant positive correlation. (Rainfall r = 0.83-0.90 P<0.001; Soil moisture r = 0.66-0.83 P<0.05; Litter moisture r = 0.83-0.89 P<0.001). Monthly carbon (CO₂-C) output was calculated from the measurements of soil respiration and soil litter systems in Fig. 1. The total annual carbon output through soil respiration was greater in deciduous forests than in evergreen forests. However, greater carbon output of the litter system was recorded in evergreen forests compared to deciduous forest except for site II. Lowest carbon output through litter respiration was recorded in site I. Annual carbon balance of the forest sites is presented in Table 1. Carbon input was taken in terms of litter accumulation on the surface soil. CO₂ - C evolution from soil litter system represents carbon output. But litter disappearance data is used for carbon output analysis. Carbon output and input ratio for all the four study sites indicated that the CO₂ - C output was 2.1 to 2.8 times greater than the input of carbon.

Discussion

The quantity of forest floor depends on the canopy closure and the climate (Chaturvedi & Singh 1987). Forest floor aboveground net primary productivity (NPP) was significantly (P<0.01) greater in site I compared to other study sites. The greater values in site I could be attributed to the positive effects of annual fire (Ramakrishnan & Ram 1988). Grasses recover faster following fire with rapid refoliation. This could be one of the reasons for greater value. Here in site I, herbaceous production is inversely correlated with tree density and basal area (c.f. Table 1). Lowest aboveground NPP in evergreen forests could be attributed to thick canopy cover and domination of dicot species in the herbaceous community and no anthropogenic perturbations to the forest floor. However, lower density and basal area of the herbaceous vegetation (Table 1) reflects in lower NPP values in evergreen forests. It generally occurs as root mats on soil surface or is concentrated on and in the first few centimetres of the soil (Sundarapandian & Swamy 1996a) and can change rapidly due to perturbations. Fine root NPP values obtained in this study are comparable with those of moist semi-deciduous forest of Ghana (Lawson et al. 1970). The significant variation in belowground NPP in the study sites may also be due to differences in site quality and species composition (Sundarapandian & Swamy 1996b). In site I, greater values of very fine root NPP may be attributed to the effect of annual fire, because apart from the scorching effects, fire alters the microenvironment through increased insulation, higher soil fertility status and release from inhibitory allelochemics and competition (Ramakrishnan & Ram 1988). Similarly, higher fine root NPP was reported in savannah compared to forests by Saterson & Vitousek (1984). Greater fine root NPP in site II, III & IV compared to site I may be attributed to high tree density as well as basal area (Sundarapandian & Swamy 1996a, Sundarapandian & Swamy 1996c) and also greater influence on their environment by accumulation of more litter.

Under similar climatic conditions, the effect of habitat differences on CO₂ evolution rates have been reported by Rout & Gupta (1989) and Singh & Gupta (1977). In the present study, comparatively greater carbon output in site I compared to other sites could be due to variations in vegetal cover, edaphic conditions, substrate quality of overlying litter and physical conditions of forest floor litter as suggested by Rout & Gupta (1989). The calculated annual CO₂ evolution from soil-litter system exceeded the input of carbon in litter accumulation on the surface of the soil by 2.15 to 2.8 times. The greater carbon output through soil-litter system is in conformity to the results of several other studies available (Anderson 1973, Rajvanshi & Gupta 1986, Rout & Gupta 1989). Greater carbon output through soil respiration could be due to root respiration, which has been said to contribute 33-55% of total soil respiration in the forest ecosystems (MacFadyen 1970). The contribution of roots to total CO₂ output might be as high as 67-82% reported by Medina et al. (1980) for root mat in an Amazonian forests. While, in a temperate hard wood forest, soil respiration composed of approximately equal contribution of CO₂ from the aboveground litter (37%), belowground litter (30%) and root respiration (33%) (Bowden et al. 1993). Greater carbon output through soil respiration in the tropical forests at Kodayar could be largely due to belowground processes such as root respiration and decomposition as also reported by Raich & Nadelhoffer (1989) and Nadelhoffer & Raich (1992).

The present study reveals that above- and below- ground NPP varied greatly with respect forest type, tree density and basal area, herbaceous species composition, litter on soil surface and soil moisture. Similarly carbon output through soil respiration also varied greatly with respect to soil moisture, litter moisture, season and root biomass.

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Table 1. Annual aboveground, belowground and litter production and carbon balance of the study sites in the tropical forests at Kodayar in the Western Ghats of Tamil Nadu. Different letter(s) on the same rows indicates significant differences (Student-Newman-Kuels test * P<0.05; ** P<0.01).

Fig.1. Monthly carbon (CO2-C g/m2) output through soil respiration from soil litter system in the tropical forests at Kodayar in the Western Ghats of Tamil Nadu.

¹ Department of Plant Sciences, School of Biological Sciences, Madurai Kamaraj University, Madurai - 625021, India.
*Centre for Research Studies in Botany, Saraswathi Narayanan College, Madurai - 625021, India
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