Ecological analysis of recovery of landslide damaged sal forest ecosystem in Nepal Himalaya

Abstract

Economically very important, sal forests (Shorea robusta Gaertn. f.) have been subjected to many catastrophic events of landslides in the eastern Siwaliks of Nepal. The present study aimed to quantify the recovery patterns in terms of changes in physico-chemical properties of soil, microbial biomass and nutrient availability in soil, species composition and forest structure, stand biomass and net production, nutrient storages and fluxes within soil-plant system, during post-landslide succession in landslide damaged moist tropical sal forest ecosystem. Temporal pattern of recovery was reconstructed from five landslide damaged sites representing a chronosequence of 1, 4, 15, 40 and 58-year-old age. A comparison was made with a nearby undisturbed mature sal forest dominated by Shorea robusta with Schima wallichii and Lagerstroemia parviflora as the main associates. The study sites are located in the catchment area of Koshi river, adjacent to Dharan, in the eastern Siwaliks (Sub- Himalayan region) of Nepal (26o 47’ to 26o 52’ N lat. and 87o 14’ to 87o 22’ E long.) within the altitude range of 450 to 750 m above mean sea level. The climate is tropical monsoon type. The mean monthly minimum temperature ranges from 11–24 oC and maximum from 19–31 oC. The annual rainfall averages 2221mm, of which 76% occurs from June to September. various components of landslide disturbed sal forest, recovered together in an interdependent manner with the passage of time. High rate of decomposition, mineralization and high nutrient uptake in the early stages may efficiently contribute to the resilience of damaged ecosystem to recover rapidly as indicated by high rate of net production of developing vegetation in the younger sites. Immobilization of N and P in litter mass and higher retranslocation of nutrients from senescing leaves for efficient use of nutrients through internal cycling, exhibited nutrient conservation mechanisms and tight cycling of nutrients in the older sites. The 58-year-old site achieved the status of a sustainable ecosystem after the landslide disturbance as it demonstrated the re-establishment of ecosystem properties in soil- plant system. However, patterns of recovery (estimated applying best fit power function models) showed that the estimated times for the 58-year-old site to reach the level of the mature sal forest would be about 20–35 years for plant biomass and microbial biomass, and about 100–150 years of organic C and total N in soil, because plant biomass and soil microbial biomass (C and N) exhibited relatively faster recovery than soil organic C and total N.