Fire, an increasingly important disturbance in tropical forests, reduces the potential to manage for sustained timber yields. Although most tropical forests have historically experienced fires, until recently, fire return intervals were on the order of hundreds to thousands of years even in seasonally deciduous forests. Logging and other human activities have rendered even the wettest forests susceptible to fire especially during El Niño associated droughts. With an increasingly fragmented landscape and abundant ignition sources, the risk of destructive fires may be particularly high in forests that require intensive silviculture to achieve sustained timber yields. Such silviculture is necessary in Bolivian seasonally dry forests due to inadequate regeneration and growth of commercial species and because vine densities in these forests are among the highest in tropics.

The potential tradeoffs between timber management and fire vulnerability in such forests derive from impacts of intensive silviculture – including soil scarification, extensive vine cutting, and the opening of large canopy gaps – which include increased fuel loads and faster dry down rates compared with less intensive management regimes. We assessed fire vulnerability in a Bolivian seasonally dry forest subjected to four management intensities: a no logging control, a selective harvest treatment with no additional silviculture, and 2 treatments consisting of harvest plus additional silviculture.

To evaluate the impact of intensifying management on fire vulnerability, we first quantified the treatment effects on fuel loads using planar transects randomly located in each treatment plot. Second, we assessed the treatment impacts on vegetative cover using point sampling along 4 transects in each treatment plot. Third, we measured the dry down rates of 10-hr fuels across the range of microsites created by the different management intensities. Fourth, we set test fires in 4 m2 plots established in the same range of microsites to determine whether cover influenced the ability of fires to carry. Finally, we developed a simple model to estimate the number of fire prone days during each dry season associated with each treatment.

Results indicated that the tradeoffs between timber management and fire vulnerability in Bolivian seasonally dry forests are likely to be modest. Fuel loads were highest and vegetative cover was lowest in the most intensive treatment but these differences were not biologically significant compared to the other harvest treatments. Ten-hour fuels dried within 4 days in the most open sites, but dried within 10 days in sites with dense cover. Moreover, although sites with the least cover burned most, fire carried in nearly all cover conditions indicating that this forest is vulnerable to fire after only 4-10 rainless days. Consecutive rainless periods of 22 days are typical during the dry season. Thus, the model forecasts revealed little differences in the number of fire prone days among treatments.

The conclusion from this study is that intensifying management for silvicultural objectives will not substantially increase fire susceptibility in this forest because it is already quite fire prone. This conclusion contrasts with findings in wetter, evergreen forests where minimizing logging impacts can reduce the risk of catastrophic fire.