Survival and life expectancy of the treeProtea roupelliaesubsp.roupelliaein a montane grassland savanna: Effects of fire regime and plant structure

Abstract

Survival and life expectancy of the tree Protea roupelliae subsp. roupelliae in a montane grassland savanna: Effects of fire regime and plant structure Abstract Survival and life expectancy are key demographic determinants of population dynamics. Using data collected in a field experiment monitored over 14 years in montane grassland of the Ukhahlamba-Drakensberg Park, South Africa, we determined the effects of components of fire regime and plant structure on the survival and life expectancy of the tree Protea roupelliae subsp. roupelliae (Proteaceae). The field experiment comprised six plots (0.2-0.5 ha in area) from which the survival and life expectancies of 1567 juveniles (non-reproductives) and 329 adults (reproductives) were estimated in response to differences in fire frequency, biennial seasonal fire, flame height, juvenile height, adult height, basal area and canopy vigour. Juvenile survival and life expectancies were highest when fires were excluded for 8 years. However, a fire after 12 years of fire exclusion and another fire 2 years later eliminated all juveniles. Over the same 14-year period of biennial fires, juvenile survival was 5%. Juvenile survival and life expectancy were higher after biennial, winter fires than after annual, winter fires. Flame height had no effect on juvenile survival and life expectancy. Both survival and life expectancy of juveniles increased as plants got older and grew taller. Adult survival was unaffected by fire frequency, flame height or tree size, but the survival of adults in response to fire seasonality was inconclusive. Adults with low canopy vigour (<25%) were negatively affected by fire. Juvenile survival and life expectancy are critical bottlenecks in the demography of P. roupelliae. This species is neither a reseeder nor a resprouter. It avoids lethal fire damage by being restricted to rocky habitats with low fire intensities. Biennial winter fires least threaten the survival and life expectancy of P. roupelliae and impact least on its role in the summer feeding and breeding of Gurney's sugarbird.

African geoxyles evolved in response to fire; frost came later

“…Receiving 60 frost-days annually in the Drakensberg Mountains, the non-SG species, Protea roupelliae subsp. roupelliae, had a 40% survival rate over eight years since establishment when unburnt, but only 4% survival when burnt annually (Smith and Granger 2017). Adult proteas are moderately resistant to frost (Rebelo 2009).…”

Fire and the persistence, decline and extirpation ofProtea roupelliaesubsp.roupelliaetrees in a montane grassland

“…In frequently burnt grasslands and savannas, adult trees of many species generally recover if damaged by fire (Frost 1984;Bond 2008;Clarke et al 2015). Conversely, tree seedling establishment in the grass layer and sapling emergence above the grass layer are critical demographic bottlenecks limiting adult tree densities (Higgins et al 2000;Bond 2008;Wakeling et al 2015;Smith and Granger 2017). Most long-term studies on the demography of tree populations in response to grass fires have been conducted in lowland savannas (e.g.…”

“…In contrast, very few such studies have ever been attempted in southern African montane grasslands (e.g. Smith and Granger 2017).…”

“…Protea roupelliae is neither serotinous nor a resprouter following fire, unlike most other Protea tree species that occur in grasslands or Cape fynbos (Rourke 1982, Lamont et al 2017. Instead, in the absence of dormant basal and epicormic buds, the survival of juveniles (seedlings and saplings) and adults of P. roupelliae depends solely on apical, winter-dormant buds (Lamont et al 2017), enclosed in a bunch of leaves, avoiding damage or death by fire (Smith and Granger 2017). In a 14-year study, juvenile survival and life expectancies were higher after biennial and winter fires than after annual and spring fires (Smith and Granger 2017).…”

“…Instead, in the absence of dormant basal and epicormic buds, the survival of juveniles (seedlings and saplings) and adults of P. roupelliae depends solely on apical, winter-dormant buds (Lamont et al 2017), enclosed in a bunch of leaves, avoiding damage or death by fire (Smith and Granger 2017). In a 14-year study, juvenile survival and life expectancies were higher after biennial and winter fires than after annual and spring fires (Smith and Granger 2017). Both survival and life expectancy increased as juveniles grew older and taller.…”