Tree mortality following ENSO-associated fires and drought in lowland rain forests of Eastern Nicaragua

Abstract

Occurrence of fires is an unusual event in many tropical terra firma rain forests, in particular those in Mesoamerica. As a result of the unusually intense ENSO event of 1998/1999, these forests were subjected to a prolonged drought that favored the propagation of extensive fires. In the lowland rain forests of the Atlantic Coast of Nicaragua, fires were mostly low-intensity leaf litter fires, but they produced high tree mortality. Data from 5 to 17 years of study of permanent plots/transects from four sites were used to investigate whether survival following the ENSO drought episode and subsequent fires may be affected by factors such as successional status, tree size, taxonomic identity or growth rate. The study included two sites that burned in their entirety, one that had only part of its area burned and a fourth site that was not affected by fire. Overall, tree survival was lower in burned plots, and remained low in those plots for at least 2 years after fires, especially among pioneer and mature forest species. Mortality experience between burnt and unburned plots differed widely among the 15 most abundant species. The 38 most abundant species were grouped into five guilds corresponding to their ecological successional status: (1) pioneers; (2) subcanopy/understory heliophiles; (3) canopy heliophiles; (4) subcanopy mature forests species; (5) canopy mature forest species. All guilds experienced significantly lower survival in plots that burned in mortality among guilds were between significant differences. The only the canopy mature forest species and the pioneers, which had the highest mortality. Two years after fire, survival remained significantly lower in burnt plots than in plots that had not burned, but the pattern of mortality was quite different. Both mature forests species guilds had significantly higher mortality than heliophile guilds. Tree size had an overall significant positive effect on survival after fires particularly among subcanopy and canopy mature forest species guilds. However, in unburned sites, survival was negatively correlated to tree size in mature forest guilds. While relative growth rate had no overall significant effect in plots that burned, faster growth was positively correlated with survival in particular guilds such as in pioneers and canopy heliophiles. Delayed mortality increased with relative growth rate for pioneers but decreased for subcanopy heliophiles. The effect of the ENSO event drought resulted in a significantly reduced overall survival caused by high mortality of pioneers, compared to non-drought years and to other guilds, but non-significant differences in post-drought survival were apparent among the remaining guilds. Individual tree basal area was positively correlated with survival given drought, overall, but particularly in pioneers and mature forest canopy species. Contrary to hurricanes, the delayed mortality and overall damage caused by fires is not conducive to maintaining the structure, diversity and species composition of these forests.