Fire severity effects on resprouting of subtropical dune thicket of the Cape Floristic Region

Abstract

It has been hypothesised that high-intensity fires prevent fire-dependent fynbos from being replaced by fire-avoiding subtropical thicket on dune landscapes of the Cape Floristic Region (CFR). Recent extensive fires provided an opportunity to test this hypothesis. We posit that (1) fire-related thicket shrub mortality would be size dependent, with smaller individuals suffering higher mortality than larger ones; and (2) that survival and resprouting vigour of thicket shrubs would be negatively correlated with fire severity. We assessed survival and resprouting vigour post-fire in relation to fire severity and pre-fire shrub size at two dune landscapes in the CFR. Fire severity was scored at the base of the shrub and categorised into four levels. Pre-fire size was quantified as an index of lignotuber diameter and stem count of each shrub. Resprouting vigour consisted of two variables; resprouting shoot count and resprouting canopy volume. A total of 29 species were surveyed. Post-fire survival of thicket was high (83–85%). We found that smaller shrubs did have a lower probability of post-fire survival than larger individuals but could detect no consistent relationship between shrub mortality and fire severity. Fire severity had a positive effect on resprouting shoot count but a variable effect on resprouting volume. Pre-fire size was positively related to survival and both measures of resprouting vigour. We conclude that thicket is resilient to high-severity fires but may be vulnerable to frequent fires. Prescribed high-intensity fires in dune landscapes are unlikely to reduce the extent of thicket and promote fynbos expansion.

Pre‐ and post‐fire architectural guilds of subtropical dune thicket species in the southeastern Cape Floristic Region

“…Our results are consistent with those of Strydom et al (2020) in that all species sprouted post fire. Resprouting architectural guilds showed only moderate congruence with conspecific guilds in unburnt thicket.…”

“…Mature, well-established thicket burns less frequently than fire-prone fynbos but can burn under extreme conditions (Kraaij et al, 2018;Cowling et al, 2019;Strydom et al, 2020). The area experienced a severe fire in January 2016, which burnt large patches of mature thicket along with highly flammable dune fynbos (Cowling et al, 2019;Strydom et al, 2020).…”

Fire weather effects on flammability of indigenous and invasive alien plants in coastal fynbos and thicket shrublands (Cape Floristic Region)

“…Fynbos shrublands are fire-prone and flammable while smaller areas of thicket vegetation seldom burn ( Geldenhuys, 1994 ). The persistence of fynbos-thicket mosaics requires fire at appropriate intervals (15–25 years) since thicket becomes dominant in the prolonged absence of fire ( Kraaij & Van Wilgen, 2014 ; Strydom et al, 2020 ). The area contains extensive invasions of alien shrubs and trees, commonly of the genera Acacia, Eucalyptus and Pinus , that co-occur with, and potentially replace, the native vegetation ( Fig.…”

The composition, geography, biology and assembly of the coastal flora of the Cape Floristic Region

“…We further classified woody species according to their postfire-regeneration mode, using a simplified version of Pausas et al (2016) ’s schema to match the resolution of information available to us. Species were categorized as obligate resprouters (plants exclusively resprout after fire, recruit from seeds in favourable microsites during fire-free intervals), facultative seeders (plants capable of resprouting and establishing seedlings after fire) and non-sprouters (plants killed by fire and reliant on seeds for regeneration, including obligate seeders and postfire colonizers), based in part on the literature ( Cowling & Pierce, 1988 ; Strydom et al, 2020 ; works listed under “Flora compilation”), but largely following our own observations after several wildfires at various sites along the Cape south coast between 2016 and 2021. Regeneration modes of Aspalathus species were gleaned from supplemental data provided by Cowling et al (2018) .…”

“…All habitats in these landscapes, other than the forest patches, wetlands, bypass dunes, hummock dunes and near-shore herb-and shrublands, are subject to wildfires at moderate intervals (10-30 years) (Cowling et al, 1997a(Cowling et al, , 2019. The fire ecology of calcarenite grassy-shrubland-thicket mosaics inland of Algoa Bay is poorly understood, but the incidence of fire-dependent Cape lineages (e.g., Diosmeae, Muraltia, Restionaceae) (sensu Linder, 2003) in the matrix vegetation (Taylor & Morris, 1981) and the prominence of Pterocelastrus tricuspidatus-a strong post-fire resprouter (Strydom et al, 2020)-in thicket clumps (Taylor & Morris, 1981;Carvalho & Campbell, 2021) suggest that fire is a periodic disturbance in this system. Kraaij et al (2020) have proposed that historical fire regimes on coastal lowlands in the southern Cape supported fire-prone and -dependent vegetation throughout the Pleistocene, including dune fynbos-thicket mosaics Full-size  DOI: 10.7717/peerj.11916/fig- 2 and limestone fynbos analogous to those of the contemporary CFR (Cowling et al, 2020).…”