Abstract
Climate-driven increases in frequencies and intensities of wildfires will affect even relatively humid regions such as the Central Alps. During low- to moderate-intensity fires, the resistance of trees to fire is mainly determined by the insulation capability of their bark, which protects underlying tissues from lethal temperatures. However, the knowledge of fire resistance and bark heat insulation of Central Alpine species is scarce.In this study, the bark insulation capability of ten tree species was analyzed by bark surface heating experiments. Heat insulation was assessed by the time required to reach a lethal cambium temperature, linked to bark traits and analyzed with multivariate statistics.Our results revealed a strong overall relationship between bark thickness and cambial temperature responses, but also highlighted the role of bark density in insulating internal tissues. On a species level, additional bark traits, like moisture content, bark surface structure or bark thermal conductivity, contributed to the bark insulation capability.The bark insulation capability thus varied considerably between species. Identifying fire resistant species and knowledge of the traits determining species-specific bark insulation will help to better estimate impacts of fire disturbances and to face the challenge of an increased forest fire risk within the Alpine region.
Highlights
Wildfire is a major disturbance factor in many terrestrial ecosystems world-wide
We aimed to address (i) how different bark traits correlate with heat insulation capability, (ii) which traits are best predictors for heat insulation and if they vary between species, and (iii) if there are interspecific differences in heat insulation capability
The general relationships among bark traits and τcamb60 were analyzed with a principal component analysis (PCA), whereby the first two components allowed explaining 67.1% of the variance in the dataset (Fig. 2)
Summary
Wildfire is a major disturbance factor in many terrestrial ecosystems world-wide. Depending on the fire regime, tree species have evolved different traits and strategies to survive in fire-prone environments (Keeley et al, 2011; Pausas, 2015a; Pausas & Keeley, 2017; He et al, 2019). A sufficiently insulating bark is crucial during understory fires (Pausas, 2015b) to protect tissues within the stem from critical temperatures and to guarantee post-fire tree functionality. Heat is conducted through the outer bark and affects the underlying phloem, cambium and xylem (Dickinson & Johnson, 2001; Michaletz & Johnson, 2007). Heat transfer can cause the cambium and phloem to exceed lethal temperatures, resulting in tissue mortality which is generally considered to be achieved at 60 °C (Rosenberg et al, 1971; Dickinson & Johnson, 2004). Phloem/cambium necroses and dysfunctional xylem, can cause post-fire physiological limitations which may lead to tree mortality (Bär et al, 2019). A well insulating bark is a necessary prerequisite to guarantee the survival of trees in fire-prone ecosystems
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