Fire‐induced tree mortality in a neotropical forest: the roles of bark traits, tree size, wood density and fire behavior

Abstract

Large-scale wildfires are expected to accelerate forest dieback in Amazô nia, but the fire vulnerability of tree species remains uncertain, in part due to the lack of studies relating fire-induced mortality to both fire behavior and plant traits. To address this gap, we established two sets of experiments in southern Amazonia. First, we tested which bark traits best predict heat transfer rates (R) through bark during experimental bole heating. Second, using data from a large-scale fire experiment, we tested the effects of tree wood density (WD), size, and estimated R (inverse of cambium insulation) on tree mortality after one to five fires. In the first experiment, bark thickness explained 82% of the variance in R, while the presence of water in the bark reduced the difference in temperature between the heat source and the vascular cambium, perhaps because of high latent heat of vaporization. This novel finding provides an important insight for improving mechanistic models of fire-induced cambium damage from tropical to temperate regions. In the second experiment, tree mortality increased with increasing fire intensity (i.e. as indicated by bark char height on tree boles), which was higher along the forest edge, during the 2007 drought, and when the fire return interval was 3 years instead of one. Contrary to other tropical studies, the relationship between mortality and fire intensity was strongest in the year following the fires, but continued for 3 years afterwards. Tree mortality was low ( 20%) for thick-barked individuals ( ! 18 mm) subjected to medium-intensity fires, and significantly decreased as a function of increasing tree diameter, height and wood density. Hence, fire-induced tree mortality was influenced not only by cambium insulation but also by other traits that reduce the indirect effects of fire. These results can be used to improve assessments of fire vulnerability of tropical forests.

Early recruitment responses to interactions between frequent fires, nutrients, and herbivory in the southern Amazon

“…We assessed the impact of these variables on woody species' regeneration and survival, diversity, and community composition. The results reported here build on long-term research in the area, testing the hypothesis that repeated fires may push forests to a tipping point beyond which they are converted to savanna-like ecosystems (Balch et al , 2011Brando et al 2012;Massad et al 2013). This work extends the findings of Massad et al (2013) which reported on regeneration during the first 6 months of seedling establishment after repeated fire.…”

Post-Fire Changes in Forest Biomass Retrieved by Airborne LiDAR in Amazonia

“…These findings confirm that fire-related impacts in Amazonian forests are more persistent than previously recorded in the literature (up to three years; [15]). However, very little is still known about the fate of forest biomass and carbon stocks following forest fire events considering the long-term dynamics of vegetation that remain in these areas [15,16]. LiDAR, therefore, is a powerful technology to further explore these impacts.…”

“…Moreover, mortality rates in forests affected by fires are significantly different from zero, two to three years after the fire event [16]. Nevertheless, capturing these long-term impacts of fires on tropical forests in a systematic way remains a challenge because of the difficulty in obtaining complex forest structure information over large areas, as well as the frequency of fire events and ecological processes [15,17].…”

Interactions between repeated fire, nutrients, and insect herbivores affect the recovery of diversity in the southern Amazon

“…Fire is naturally important in this region as it helps to maintain the cerrado ecosystem, but, with increasing demands for agricultural and pasture land, fire is used as a tool to remove unwanted biomass, and it often degrades forests via escaped fires (Nepstad et al 1999). This has increased fire frequencies in the southern Amazon above historical levels, particularly during drought years (Bush et al 2008). These human impacts and a lack of protected areas combine to place the seasonally-dry forest of the southern Amazon among the most vulnerable of the Amazonian ecoregions (SoaresFilho et al 2006), particularly because predictions for this area suggest temperature will increase and precipitation will decrease-a combination that will result in increased fire potential (Malhi et al 2008).…”

“…Many fires escape into the forest, where they tend to burn the surface litter layer (i.e. they do not become crown fires) and cause tree mortality by exposing the base of the tree to a low-intensity fireline (Brando et al 2011). The region experiences a dry season between May and September (Nimer 1989), and more than 95 % of the 1,900 mm annual precipitation falls during the rainy season (F. Bäse, personal communication).…”