Fire impacts on vegetation and soil in a forest ecosystem in Wadakkancherry forest range, Thrissur forest division, Kerala

Abstract

Fire is being used as a tool in forest management and protection in several parts of the world. A forest fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. The after effects of forest fire includes air pollution, destruction of forests and wildlife, altered species composition, outbreaks of pests and diseases etc. The frequency of wild fires around the world increases with changing climate. Considering the growing importance of tropical forest ecosystems and also because of its fire susceptibility, it is relevant to study the effects of forest fire on vegetation and soil in a forest ecosystem. The present study was undertaken in Chembikkunnu area of Poongode section of Wadakkanchery forest range in Thrissur forest division in state of Kerala. The study was carried out to assess the impacts of an unplanned forest fire on the vegetation and soil physico-chemical and biological properties in the fire burnt areas of Poongode section of Wadakkanchery forest range of Thrissur forest division. Concurrently the study also compared and contrasted the changes in vegetation and soil between burnt and control areas over a time period of six months. Physico- chemical properties of soil viz. soil moisture, bulk density, water holding capacity, soil pH, soil organic carbon, total nitrogen and total phosphorous showed marked differences between the fire affected classes themselves and between control areas. Other soil properties like soil texture, soil electrical conductivity and total potassium remained without any significant effects after the fire. Soil physico-chemical properties showed differences between pre-monsoon and post monsoon season. Soil moisture content (%) decreased with increase in frequency of fires. In the pre-monsoon conditions, the bulk density of the newly burnt area increased to 0.94g/cm3. Water holding capacity of the fire affected areas at the depth of 0-15 cm increased to 39.23% in pre-monsoon season. Soil pH of the newly burnt areas increased to 6.85 immediately after the fire. Soil organic carbon (%) followed the trend (NBP>OBP>CP). Total Nitrogen (%) and total Phosphorous (ppm) content of the burnt plots was higher compared to the control area. As vegetation intercepts rain and reduced the kinetic energy of rain water, soil erosion (Tonnes/ha/yr) of the study area followed the trend (NBP (0.13)> OBP (0.07)>CP (0.03)). Total enumeration of regeneration was carried out in the 10m x 10m plots on a monthly basis. 37 plant species representing 20 families were recorded. Among them there were 11 tree species, 2 grass species and 4 species of ferns. Hemidesmus indicus, Pycnospora lutescens and Drynaria quercifolia were found only in the fire affected plots. The Shannon- Weiner diversity index of saplings in the month of July are in the range of (1.45-2.37) while it is more for seedlings (2.76-2.96). The Sorrensen similarity indices of July (0.81(saplings), 0.91(seedlings)) points to the increased number of common species among plots. The increased evenness and reduced diversity in the fire affected plots are mainly attributed to the invasion of weeds like Chromolaena odorata, Mimosa diplotricha and grass species like Pennisetum polystachyon. Unavoidable physical, chemical and biological changes happening to the soil after every fire incident are potentially able to influence the soil fauna. Darkling beetles, broad shouldered water striders, dung beetle and centipedes were recorded only from the fire affected plots. Ants were present in all the plots in both the seasons. An increase in the number of individuals was observed after the monsoon. The hard coated beetles registered their presence in significant number in the burned classes and the soft bodied animal like earthworm, centipedes and millipedes were absent immediately after the fire but appeared after the monsoon suggesting that the temperature variation in the ecosystem might have made them to move away. The outcomes of present study necessitate longer duration study in different tropical forest types for more comprehensive understanding of the post fire dynamics of tropical forest ecosystems.