There has been much interest in the dynamics and stability of tropical forests, especially in the face of future climate change and uncertainty. Would tropical forests lose species or would they weaken as a carbon sink as a result of increasing temperature, changes in precipitation and climate variability? While much of the attention has been on tropical moist forest, there have been fewer investigations on these processes in tropical dry forests.

We have been studying the long-term dynamics of a tropical dry forest at Mudumalai, southern India, through monitoring of a 50-ha Forest Dynamics Plot established during 1988-89. All woody plants >1 cm dbh have been identified, tagged and mapped in the plot that is enumerated annually for recruitment and mortality, and every four years for stem growth. During the 20-year period of monitoring there have been five episodes of extensive fire (>80% of the plot burnt) while the plot has experienced one period (3-4 years) each of prolonged drought and excessive rain.

Results/Conclusions

The Mudumalai plot has shown considerable dynamism with respect to mortality and recruitment. Mortality rates in stems 1-5cm dbh stems have been >50% per annum during two fire-years and in another year following drought but the very low mortality rates of <0.5% per annum for most years in stems >30 cm dbh conferred stability to the community; this is especially the case for the four most common canopy species, Lagerstroemia microcarpa, Terminalia crenulata, Tectona grandis and Anogeissus latifolia. Heavy browsing pressure by elephants on the most abundant species, Kydia calycina (understorey tree) has followed a density-dependent pattern of reduced mortality rates with decreasing population. Following steep declines in abundance, K. calycina and the shrub Helicteres isora have experienced strong recruitment during a recent phase of no-fire and high rainfall.

The total population of stems >1 cm dbh declined from nearly 26,000 individuals (72 species) in 1988-89 to nearly 15,000 in 1996 but has since increased sharply to over 33,000 (79 species) in 2007. Aboveground biomass increased monotonously from 174 tons.ha^-1 to 193 tons.ha^-1 in 2004; even during phases of a declining population of individuals the growth of larger stems compensated for loss of smaller-sized stems. The basal area of the canopy layer (stems >30 cm) has generally increased over this period. The persistence of this tropical dry forest with respect to species richness, basal area and carbon stocks suggests considerable resilience to stress and environmental variability.