Effects of simulated fire and browsing on the resprouting of subtropical dune thicket shrubs in the southeastern Cape Floristic Region

Abstract

The expansion of subtropical thicket vegetation at the expense of more species-rich, fire-prone fynbos, potentially due to lower frequency and severity of fire and browsing, is a concern in many coastal dune landscapes of the Cape Floristic Region (CFR) where these two vegetation types co-occur. We were interested in the effects of severe fire or browsing treatments (causing complete loss of aboveground biomass) on the vigour of post-fire resprouts of dune thicket shrubs. Cape St Francis, CFR, South Africa. We used an in situ experimental approach to compare the effects of simulated fire and browsing by herbivores on mortality, resprouting vigour and resprouting rate of 10 canopy-forming dune thicket shrub species from different architectural guilds, 5 years after a previous severe wildfire. Survival was significantly lower after fire (85%) than after browsing (95%), and was significantly positively related to pre-treatment shrub size. All measures of resprouting vigour were significantly lower after fire than after browsing, and were significantly positively related to pre-treatment size. Resprouting rate was significantly lower after fire than after browsing, and was significantly positively affected by pre-treatment size. Survival and measures of resprouting vigour were generally decoupled from architectural guild and species identity. Dune thicket shrubs showed high survival after both fire and browsing treatments, suggesting that these species are resilient to frequent complete loss of above-ground biomass. Our results suggest that short interval fires (5–15 years) of high severity will merely maintain the co-occurrence of dune fynbos and thicket vegetation by setting back thicket growth rather than causing large-scale mortality of thicket shrubs.