Fire and browsing are an evolutionary pressure in many vegetation types for driving trait acquisition, such as plant architecture features and resprouting vigour (Bellingham and Sparrow 2000; Bond and Midgley 2001; Clarke et al. 2013; Lamont et al. 2017). During the early Miocene (ca. 20 Myr), subtropical thicket and dry forest became the dominant vegetation over large parts of southern Africa (Cowling et al. 2005; Neumann and Bamford 2015) and mammalian megaherbivores started to diversify (Janis 1993; Pennington et al. 2010). Top-down browsing by megaherbivores (e.g. Sivatheres and Gomphotheres) in these ecosystems likely drove trait acquisition such as resprouting, fine branching and spinescence (Hendey 1982; Janis et al. 2000; Bond and Silander 2007; Onstein et al. 2022). Consequently, the ancestral selective force for resprouting was likely defoliation by browsing. Since the late Miocene (10 Myr), cooling and drying (Janis 1993; Westerhold et al. 2005) produced fire-supporting climates, which promoted the growth and expansion of grass- and shrub-dominated fire-prone ecosystems in the subtropics (Janis 1993; Jacob et al. 1999; Keeley and Rundel 2005; Edwards et al. 2010; Rundel et al. 2018). Subsequently, fire likely emerged as a significant evolutionary factor for trait acquisition in thicket lineages.
Here we report on the effects of experimental fire and browsing treatments on post-fire resprouting of subtropical dune thicket (hereafter ‘dune thicket’) shrub species in the eastern Cape Floristic Region (CFR) of South Africa. Specifically, we investigate their resilience, in terms of survival and resprouting vigour, to these types of defoliation. While dune thicket is not a fire-dependent ecosystem, it is exposed to fire where it co-occurs with fire-dependent and highly flammable dune fynbos (Calitz et al. 2015; Msweli et al. 2021), which can burn every 10–20 years (Cowling et al. 1997; van Wilgen et al. 2010; Kraaij et al. 2013a). The position of thicket in the landscape determines the levels of fire exposure, influencing thicket species composition and structure (Strydom et al. 2022). Dune thicket has relatively low flammability (Clarke et al. 2015; Msweli et al. 2021) and is seldom exposed to fire, owing to fire suppression to protect coastal developments and alteration of fire spread patterns. Recent wildfires in dune thicket around the towns of Cape St Francis (2016) and Knysna (2017) showed dominant canopy species, for example Sideroxylon inerme Forssk., Pterocelastrus tricuspidatus Walp., Mystroxylon aethiopicum (Thunb.) Loes., Euclea racemosa L. and Searsia glauca (Thunb.) Moffett., to have high survival rates (75%-90%) and strong resprouting abilities from below ground bud banks after severe burning (Strydom et al. 2020).
The architectural composition of dune thicket is largely determined by the level of landscape-scale fire exposure (Strydom et al. 2022). Thicket that has not burnt in over ca. 100 years has an abundance of vertical growers, which tend to show weak resprouting after fire (Strydom et al. 2021). With moderate fire exposure (50–100 year fire intervals), lateral spreaders, which are strong post-fire resprouters, dominate the thicket canopies. With high fire exposure (10–50 year fire intervals), hedge formers are most abundant. Species with this architecture (e g. Euclea racemosa, Olea exasperata Jacq., Rapanea gilliana Mez and Searsia laevigata (Moffet) Moffett) are strong post-fire resprouters and include many geoxylic shrubs with enlarged woody stems belowground (White 1977; Cowling 1983; Maurin et al. 2014; Grobler and Cowling 2021). It seems likely that geoxyles evolved as a result of the emergence since the late Neogene of climates that could support recurrent fire, as has been demonstrated for African savanna geoxyles (Hoetzel et al. 2013; Maurin et al. 2014; Lamont et al. 2017).
Browsing regimes have influenced survival and resprouting vigour of many woody plants across multiple ecosystems, such as thicket, savanna, and forest (Cowling and Kerley 2002; MacGregor and O’Connor 2004; Fornara and Du Toit 2008; Lagendijk et al. 2011). African elephant (Loxodonta africana) browsing on trees can be destructive (Sharam et al. 2006; Edkins et al. 2008; Morrison et al. 2016); these megaherbivores are the cause of highest levels of savanna tree mortality when compared to other browsers (MacGregor and O’Connor 2004; Sharam et al. 2006; Morrison et al. 2016). Furthermore, by reducing the stature of trees, they expose tree shoots to browsing by mesoherbivores such as kudu, Tragelaphus strepsiceros, and impala, Aepyceros melampus (Makhabu et al. 2006). In the unbroken forms of succulent thicket of the interior CFR that are not exposed to fire, elephants at high density reduce substantively the cover and stature of the vegetation when compared to elephant-free thicket, and induce the proliferation of branching via epicormic resprouting in many shrub species (Cowling and Kerley, 2002; Kerley and Landman 2006).
Other than Sharam et al. (2006), who studied broad-leaved thickets in the Serengeti of Tanzania, we know of no studies that have investigated the differential effects of fire and browsing on the survival and resprouting vigour of African thicket species. In dune thicket, the effect of fire severity on mortality and resprouting vigour of canopy-forming shrubs has been assessed by Strydom et al. (2020), who showed that smaller shrubs had higher mortality and weaker resprouting vigour than larger shrubs. While elephant impacts have been assessed in inland thicket of South Africa (Cowling and Kerley 2002; Kerley and Landman 2006), little is known about the effects of megaherbivore browsing on dune thicket species.
The need to understand the resilience of thicket shrub species and architectural guilds to fire and browsing provided stimulus to this study. We used an experimental approach to compare the effects of simulated severe fire and simulated severe browsing by herbivores on shrub survival and resprouting vigour of ten dune thicket shrub species from different architectural guilds (sensu Strydom et al. 2021) five years after a previous severe wildfire. Understanding thicket resilience to disturbance can also inform the conservation and management of dune thicket in the event of future environmental change, such as severe fires and shorter return interval fires potentially caused by climate change, urban interface expansion and increases in invasive alien plant cover (Moriondo et al. 2006; Kraaij et al. 2011, 2013b; Kelly et al. 2020). Short return interval fire and severe fires, as were simulated in our study, could potentially limit the expansion of thicket at the expense of species- and endemic-rich dune fynbos (Hoffman and Cowling 2021). Moreover, the increase of nature reserves, game farms and eco-estates in the coastal landscapes of the CFR, comes with the stocking of large herbivores, some extralimital to thicket (Cornelius 2011), and will likely impact thicket shrub structure and composition (Cowling and Kerley 2002; Kerley and Landman 2006). Thus, knowing the resilience of thicket shrub species to severe browsing will aid our understanding of thicket resilience.
We were also interested to explore whether resilience to fire and browsing defoliation regimes differed among architectural guilds in relation to their differential abundance in sites with different fire exposure (Strydom et al. 2022). We expect vertical growers and lateral spreaders, which grow in sites subject only to occasional fires, to have higher survival and resprouting vigour after severe browsing than after severe fire, and the hedge formers, which are most abundant in frequently burnt sites, to have a more vigorous response to fire than browsing.