Fire severity and tree size affect post-fire survival of Afrotemperate forest trees

Abstract

Abstract Background Fire is recognized as an important factor in determining the distribution and composition of Southern Cape Afrotemperate Forest in South Africa, although comprehensive understanding of the resilience of these forests to fire is lacking. We investigated effects of fire severity and tree size on post-fire survival of Afrotemperate forest tree species. Results Fire severity was estimated from observed damage to the main stem of trees, and trees were considered to have survived if they resprouted from the main stem or had green foliage in the canopy. A total of 88 plots, 40 species, and 1 378 trees were surveyed 19–24 months post-fire. We assessed the survival response of all trees collectively and for 10 species (with sample size > 40) individually, using logistic regression. Relative to other forests of the world, the post-fire survival rate (45%) of Afrotemperate forest trees collectively was comparable to that in coniferous and tropical forests but lower than that in other temperate forests and in neighboring dune thicket. Fire severity had a significant negative effect on survival and tree size had a significant positive effect. Total variance explained by the model (for species collectively) was 40.8%, of which fire severity and tree size combined explained 13.2%, and species as random factor, 27.6%. Respective tree species showed differential survival responses—four species showed high survival (> 60%), while five species showed low survival (< 40%). Further, some species exhibited strong resprouting from the base or main stem while others rarely resprouted. The survival response and resprouting abilities of species which occur in both Afrotemperate forest and neighboring, more fire-exposed dune thicket, were generally poorer in forest. Such discrepancies imply that historical fire regimes associated with vegetation types likely drive species adaptions. Conclusions Our findings suggest that fire severity and fire frequency (in terms of how tree size relates to fire frequency) are important for maintaining species richness and diversity within and between forest types. Varying resilience to fire among species supports previous assertions that fire affects species composition and diversity in these forests and suggests that potential changes in fire regimes due to global change will have consequences for forest conservation.