Historical links between climate and fire on species dispersion and trait evolution

Abstract

The interaction effects between climate and fire regime in controlling the type of vegetation and species composition is well established among the Earth’s biomes. Climate and the associated fire regime are never stable for long, and annual temperatures, atmospheric carbon dioxide and oxygen levels, and burn probability have varied radically over the last 350 million years. At the scale of thousands of years, floras have oscillated between spreading and retracting as climate and the dependent fire regime have fluctuated. At the scale of millions of years, distinct traits have evolved along three lines: fire resistance, fire-stimulated dormancy release, and rapid postfire growth, all limited by the type of fire (as controlled by climate) and postfire weather. Eight pairings of fire- and postfire-related traits resulting from the interplay between fire and climate are noted here. Smoke-released seed dormancy is beneficial on two counts: it increases the chance of recruitment under the present fireprone climate and increases the chances of survival should the wet season shift to another time of year where temperatures are higher or lower. Four pathways can be recognized with respect to the fire regimes induced by climate changes: (1) from non-fireprone to fireprone habitats (gains fire-adapted trait, 13 studies covering the last 115 million years (My) described here); (2) from a surface fire to a crown fire-type habitat (gains a different fire-adapted trait, 13 studies); (3) from a crown fire-type to surface fire-type habitat (loss of fire-adapted trait, 12 studies); and (4) from moderately burnt (crown fire) to non-fireprone habitat, such as desert, rainforest, or alpine habitats (loss of fire-adapted trait, 6 studies). Four case studies, at decreasing taxonomic rank, are used to illustrate the intimate relationship between climate change with its associated vegetation and fire regime change as they promote adaptive trait evolution: gain then loss of heat-released seed dormancy in Dipterocarpaceae–Cistaceae–Bixaceae over 90 My, gain then loss of serotiny in Callitroid Cupressaceae over the last 65 My, gain then loss of smoke-released seed dormancy in Proteoid Proteaceae over the last 120 My, and gain then loss of resprouting and serotiny among Hakea species (Grevilleoid Proteaceae) over the last 20 My. Examples of within-species rates of migration and trait change, including a model describing increasing degree of serotiny with intensifying drought, are given. The relevance of this historical approach to current (anthropogenic) climate change and associated fire regime alteration is discussed. Despite major threats expected to species conservation status at both the macro-(biome) and micro-(population) scales, I conclude that insufficient time or opportunities remain for effective migration to less-stressed areas or suitable adaptive responses to climate/fire regime change to evolve.