Herbivory by elephants is known to significantly influence the populations of many woody plant species in Africa and Asia; in the former continent there have been concerns over conversion of woodland to savanna as a result. What is the dynamics of Malvales in relation to elephant’s herbivory in a tropical dry forest during the period 1988-2007?
We have been studying the long-term dynamics of four important elephant forage species of the Order Malvales - Kydia calycina (understorey tree), Helicteres isora (shrub), Eriolaena quinquilocularis (understorey tree) and Grewia tiliifolia (canopy tree) – in a 50-ha Forest Dynamics Plot established during 1988-89 at Mudumalai, southern India. The plot is being monitored annually for recruitment and mortality while stems sizes are measured once in four years for all woody stems >1 cm dbh.
Results/Conclusions
Patterns of the dynamics of these four species showed considerable differences during the study. K. calycina, the most abundant tree in 1988-89, with 5175 individuals was also the most intensely browsed species with mortality rates up to 38% per annum initially. Its population reduced by two orders of magnitude to 63 individuals by the year 2002 mainly as a result of elephant browsing. Mortality of K. calycina also showed a density-dependent pattern of mortality with population. Following a spell of increasing rainfall with absence of fire and decreased consumption by elephants, the population recruited strongly during 2004-05 to 1180 individuals. When K. calycina abundance declined, elephants seem to have turned to E. quinquilocularis as a source of bark as seen from an increased mortality of this species during 1994-95; this species has not recovered since this phase of high mortality. Utilization of H. isora has been density-independent with a certain proportion of surviving trees killed each year. After the initial decline H.isora has recruited very strongly in recent years and is now the most abundant species in the plot. Stripping of bark by elephants of the canopy tree G. tiliifolia has resulted only in low rates of mortality but a major decline in this species can be attributed to drought during 2000-2003.
The dynamics of the Malvales in this tropical dry forest is thus driven by a complex interplay of herbivory by elephants as well as stochastic climatic and fire events resulting in long-term cycles in recruitment, mortality and population sizes. There is no evidence that elephants would convert such forests into grasslands as in semi-arid Africa.
