Environmental influences on growth and reproductive maturation of a keystone forest tree: Implications for obligate seeder susceptibility to frequent fire

Abstract

Anthropogenic modifications to climate and natural fire regimes are occurring globally, leading to the production of environments that may be unsuitable for some species. Fire-intolerant plant species that rely on specific fire regimes for reproduction are at risk of population decline when successive fires occur in less than the time taken to produce seed. Quantifying key fire-related life history traits in such species is therefore critical for developing models of population viability, species distributions and ecosystem persistence. We studied the Australian mountain ash (Eucalyptus regnans), the world's tallest angiosperm and an ecologically and economically important keystone species. We tested whether mountain ash populations exhibit variation in susceptibility to increasing fire frequency by characterising the response of key vital rates to stand age (time since fire) under different environmental conditions. We found that the time taken to produce seed varied geographically. Mean growth rates were greater in areas receiving higher levels of solar radiation, a trend that became stronger with tree age. Tree size and age had the strongest influence on the production of fruit capsules. Mature fruit capsules were found in trees as young as 11 years old, but stands may not contain reproductively viable seed crops until they are more than 21 years old. Our results show that environmental factors influence the primary juvenile period of a keystone obligate seeder, in turn affecting the time taken for a population to develop a reproductively viable seed amount of seed. Reduced fire return intervals may therefore constrain the species’ realised niche (and geographic distribution) to areas where it can tolerate shorter fire return intervals due to faster growth and maturation. We suggest that populations of obligate seeders that reach reproductive viability faster are thus more likely to persist when exposed to multiple fires in short succession. Intra-stand variation in seed crops suggests that selection could also act on rapidly-maturing individuals, resulting in some populations exhibiting high levels of precocious reproductive activity.