Chemical composition of post-fire epicormic foliage affects nutritional quality for folivores

Forest fires can cause major changes in stand structure and habitat suitability for forest wildlife. For species that browse on foliage, the tree’s fire response strategy plays a major role in post-disturbance population recovery. Trees that exhibit high mortality and regrow from seed (i.e. obligate seeders) may not provide foraging habitat for tree-dwelling fauna for decades, while trees that regenerate vegetatively, such as epicormic resprouters, can provide feeding resources within days of a fire through rapidly emerging shoots. However, the chemical composition of these leaves is of critical importance for folivores. Specialist species such as koalas ( Phascolarctos cinereus ) or southern greater gliders ( Petauroides volans ) often feed on trees with relatively high available nitrogen and low concentrations of herbivore deterrent plant secondary metabolites (PSMs), which allow them to meet their energy and protein requirements. The diets of arboreal fauna are likely adapted to the chemistry of adult-phase (mature) leaves, which are predominately available year-round within their habitat. After a forest fire, surviving animals may however be reliant on epicormic regrowth for large parts of their diet and over extended periods of time as forests recover. Many studies have focused on the nutritional quality of adult leaves for folivores, but little research has been done on the nutritional quality of epicormic regrowth. We sampled Eucalyptus foliage in mixed-species eucalypt forests dominated by species from the monocalypt and symphyomyrtle subgenera a year before and after the 2019/20 ‘Black Summer’ bushfires in southeastern Australia to quantify how epicormic leaf chemistry differed from mature leaves and if epicormic foliage was nutritionally suitable for arboreal folivore foraging. Our results showed that epicormic regrowth differed chemically from mature foliage, with variation driven by species and subgenus. Epicormic leaves generally had higher nitrogen concentrations but not necessarily higher available nitrogen. Focal subgenera-specific PSM concentrations were higher in epicormic foliage of symphyomyrtle species, while monocalypt species showed lower PSMs post-fire. Our findings suggest that the suitability of epicormic foliage for folivores is highly context-dependent and shaped by local species composition. Therefore, eucalypt folivores, including koalas and greater gliders, should be able to utilise epicormic regrowth from certain species based on what we know of their tolerance for particular herbivore deterrent PSMs. In forests with a mix of resprouting species from different subgenera—a common feature in native eucalypt forests—epicormic regrowth may therefore offer sufficient nutrition to support arboreal folivore populations during post-fire recovery. • Fires reshape forest structure, influencing folivore food availability and quality. • Epicormic regrowth offers rapid food but its leaf chemistry may limit foraging. • We sampled eucalypt foliage pre/post Black Summer fires. • Epicormic leaves: more nitrogen, PSMs varied by species/subgenus. • olivore access to regrowth depends on forest species mix.