Aridity and competition drive fire resistance trait covariation in mountain trees

Abstract

Fire resistance traits drive tree species composition in surface‐fire ecosystems, but how they covary at different scales of variation and with the environment is not well documented. We assessed the covariation of bark thickness (BT), tree height, and crown base‐to‐height ratio across Alpine forests, after accounting for the effects of tree diameter and competition for light on individual trait variation. Traits consistently correlated across individuals and communities, although the variance of BT mainly occurred among species, whereas crown elevation traits varied mainly within species. Aridity, temperature, and competition contributed to explain the variation of fire resistance traits among and within species, driving a trade‐off between fire resistance and the ability to compete for light. Thick‐barked species (fire‐tolerant) that self‐prune their lower branches (flame‐avoiders) dominated the most fire‐prone and flammable communities in sub‐Mediterranean southern Alps, whereas thin‐barked tree species that grow tall (competition for light) dominated the least fire‐prone communities in the northern Alps. Our findings suggest a long‐term interaction between mountain tree species and fire regime. Higher allocation to trunk elongation occurs in moist and shade environments, while higher allocation to thicken the bark and distancing the crown base from surface fuels occurs in open‐canopy, dry forests where fire spreads with higher intensity.

Vulnerability of forest ecosystems to fire in the French Alps

“…To quantify fire resistance of trees, we measured bark thickness, tree height and canopy base height on dominant species for a range of tree diameters (Fréjaville et al 2018b). In addition, we used individual tree data (species, total and first branch heights) of the French National Forest Inventory (NFI, 2005(NFI, -2015 that were collected on trees with a diameter at breast height (DBH) greater than or equal to 7.5 cm in 4438 forest plots.…”

“…Each measurement is collected on circular plots with a radius of 25 m (Robert et al 2010). Bark thickness of the NFI trees was estimated from allometric equations according to DBH and tree species (Fréjaville et al 2018b).…”

“…Abies alba is characterized as a "fire intolerant" species (Tinner et al 2000) due to its relatively small bark thickness (Fréjaville et al 2018b(Fréjaville et al , 2013 and because of its low resilience ability several years after fire (Tinner et al 2005). It has even been assumed that its extinction in the mid-Holocene in the Southern Alps was due to an increase of forest fires frequency (Wick and Möhl 2006;Tinner et al 2005Tinner et al , 1999.…”

“…Larix decidua is a "fire-indifferent" species (Tinner et al 2000) due to its thick bark, especially for the biggest trees (Fréjaville et al 2013) and its high resilience characterized by a strong recruitment after fire (Moris et al 2017). Nonetheless, high mortality rates can be observed in open forests due to higher continuity between surface fuels and the base of the crown (Fréjaville et al 2018b) or in young stands defined by thinner bark (Moris et al 2017). According to the fire simulations, the immediate post-fire mortality can be relatively high in the lowest diameter class (DBH < 20 cm) especially if the flame residence time is long.…”

“…Investigating these interactions is a prerequisite for understanding the vulnerability of species and forest ecosystems to fire and anticipating the effects of global changes. In particular, it is unknown how mountain-range variability in fuel moisture and fire ignition (Zumbrunnen et al 2012), vegetation flammability (Fréjaville et al 2016), fire weather (Dupire et al 2017) and fire resistance of trees (Fréjaville et al 2018b) interact to shape broad-scale patterns in Alpine forest vulnerability to fire.…”