To prioritize conservation of tropical forests, it is critical to understand how sensitive they are to human impacts. The sensitivity of forests to disturbance impactsmaydifferasafunctionofother environmentalvariables.
Researchers from Xishuangbanna Tropical Botanical Garden (XTBG) and Banaras Hindu University conducted a study in the Vindhyan tropical range of India. They evaluated how the severity of four disturbance types (harvesting, browsing, fire and drought) change in tropical dry forest fragments in central India arrayed across a soil moisture gradient.
They analyzed the structure of a tropical dry forest in central India in terms of the tree composition of juveniles, saplings and adults at five distinct sites located along a gradient of soil moisture content. They also recorded the numbers of individuals in each size class killed by the four disturbance types over two years.
The researchers then recorded total stem density and recruitment at each site. They compared annual mortality index (AMI) and its four disturbance components (harvesting, browsing, drought, fire) and annual recruitment index (ARI), against the mean soil moisture content of each site using generalized linear mixed models.
They observeddifferencesinmortalitythreatsatthejuvenile,saplingandadulttree stages at all the five study sites. Juveniles died mostly due to browsing, whereas, harvesting was the most important factor for the death of saplings and adults.
Direct effects of drought and fire on mortality were negligible, because the tree species in the study region are well adapted to the harmful effects of seasonal drought and fire.
They found that total mortality rate of trees in all size classes increased as mean soil moisture content decreased. Tree recruitment decreased at the dry sites compared to the moist site.
Tree saplings and adults were mainly killed by harvesting, indicating that human impacts on tree mortality are more important than natural disturbance.Impacts of all disturbances are more severe with increasing water stress. The researchers thus regarded that changes in tropical dry forest structure due to harvesting are likely to be more rapid in more arid environments.