Fire-mediated biomass loss of woody species seedlings causing demographic bottleneck in the Terai Forests of Central Nepal

Abstract

Forest fire produces varying ecological effects determining vegetation patterns in ecosystems. Plant community assemblages in forests are altered by fire influencing growth and development. Some plant communities are suppressed and some are encouraged to develop due to fire. Little empirical evidence exists revealing the condition of fire-mediated loss of small-sized individuals, and response of fire to demographic structure of woody plant species in Nepal. The aim of this research, therefore, is to quantify fire-mediated biomass loss of small-sized individuals of woody species (seedlings), and investigate the associated impacts on the demographic structure of plants in the Terai forests of central Nepal. Impacts on small-sized individuals of woody plant species were evaluated based on evidence from 25 sites with 50, 1 m × 1 m subplots for seedlings nested in 10 m × 10 m plots (n = 50) for saplings and adults. Of the 50 sampling plots, 25 represented fire-affected and the remaining 25 represented control plots. Mean density of resprouting and newly grown seedlings developed in one year after the surface fire was 46,800 ± 4071 inds. ha−1. Shorea robusta Gaertn. was the most abundant species regenerated with seedlings in the control forest sites. The estimated average yearly loss of above ground biomass (AGB) for woody plant seedlings in the forest due to surface fire was 1,651.5 ± 360.2 kg ha−1. Of the total fire-affected AGB of S. robusta, 700.0 kg ha−1 was the immediate burnt loss and 322.5 kg ha−1 was dead AGB left unburnt. Similarly, among the other most abundant species, higher proportion of immediate fresh AGB loss was observed for Mallotus philippensis (Lam.) Müll.-Arg. and Casearia graveolens Dalzell. compared to dead AGB left unburnt. The Weibull probability density function shows the existence of saplings recruitment bottleneck in the fire-prone forest sites, i.e., growth of seedlings to saplings was restricted. Surface fire impacted the development of DBH class > 0–5 cm saplings. Surface fires are one of the potential environmental filters of the plant species regeneration and composition in the forests of lowland Terai Nepal. These demographic variations and bottlenecks should be considered while formulating management actions to conserve vegetation assemblages in fire-mediated ecosystems.