Growing space requirement and height growth rate-related mortality risk in three fast-growing tree species of Delhi Forest

Identifying both exogenous and endogenous drivers of tree mortality is a big challenge for forest ecologists, particularly in times of climate change. The survival and mortality trends of fast-growing and slow-growing species (two distinct functional groups in nature) has been eloquently addressed and discussed in the past. This article compares height-growth rate-related mortality risk in three fast-growing Delhi- forest species viz. Eucalyptus citriodora (EC), Acacia auriculiformis (AA) and Acacia nilotica (AN). The survival curves for the three species were generated using the Kaplan-Meier function in R. Cumulative hazard curves were also built simultaneously. Results indicated that the survival probability of E. citriodora was low during most of its height development. Likewise, the cumulative hazard curve demonstrated a maximum value for EC (1.58), followed by AA (1.44) and AN (1.01). Moreover, Cox Proportional Hazard model also indicated a high risk of mortality in EC in contrast to the other two species. Covariates such as tree height suggested a high relative risk in EC as compared to the other two species. However, when crown diameter was incorporated as a predictor variable, it had a negative impact on the risk of mortality for EC in comparison to the other two species in question. With increasing crown diameter, the risk of mortality (relative risk) in EC dropped by about 32% and 11% in contrast to AA and AN, respectively, during the 11- year study period. In addition, the growing space requirement of a tree was also correlated to stem diameter, tree height, and height - to - diameter ratio (h/d). Both stem diameter and tree height had a positive correlation with the growing space requirement. h/d ratio sharply decreased with tree age and growing space having high values (> 1) in the beginning, which stabilized at later stages of growth. The investigation demonstrated that among three species evaluated, EC had the highest hazard and lowest survival time than AA and AN despite the fact that EC had attained maximum height (9.15 m) during the span of 11 years in contrast to AA (7.10 m) and AN (5.74 m).