Survival and life expectancy of the tree Protea roupelliae subsp. roupelliae in a montane grassland savanna: Effects of fire regime and plant structure

Abstract

AbstractSurvival 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.