Determining the causes of the tropical forest deforestation and degradation is a key prerequisite for the success of reducing emissions from deforestation and forest degradation. Study of growth rate, recruitment and mortality of tree saplings could provide useful insights into the factors influencing community dynamics in the TDF and future compositional change in the forest.
Researchers from Xishuangbanna Tropical Botanical Garden (XTBG) and Banaras Hindu University analyzed the structure of a tropical dry forest (TDF) the Vindhyan tropical range of India in terms of the composition of small-diameter trees (saplings) at five distinct sites.
The study sites were located along a gradient of soil moisture, and investigated the effects of biotic and abiotic disturbances on the diversity, aboveground biomass (AGB), AGB-accumulation, annual recruitment index (ARI) and annual mortality index (AMI) of saplings.
They hypothesized that the array of abiotic and biotic factors in the TDFs which affected the growth, recruitment and mortality of tree saplings included soil moisture, browsing by livestock and harvesting by humans, and fire.
They found that trees generally experience severe disturbance by biotic and abiotic environmental factors at every stage of the life-cycle. The disturbance would be higher during the dry period of the year. Intensity of all disturbances increased as soil moisture declined.
In their study, harvest index exhibited strongest relationship with vegetation parameters at the moister sites. Browse index exhibited strongest relationship with vegetation parameters at the drier sites.
Compared to browsing and natural mortality on account of drought and fire, damage to saplings due to harvesting was substantially higher. Variations in the sapling recruitment and mortality were mostly accounted for by harvesting.
The site which had the lowest level of soil moisture content throughout the year, contained the lowest number of species and sapling stems and also experienced the greatest level of disturbance in terms of damaged saplings.
Due to the illegal harvesting of the saplings of large canopy trees such as Terminalia tomentosa and Shorea robusta, there is a great possibility of change in floristic composition and stand dynamics of TDF in the coming decades.
The researchers thus suggest that people restrict the illegal (unauthorized) harvesting and control browsing in the forest. It’s particularly important to protect the saplings of the dominant canopy trees and those with high biomass accumulation potential.