We showed that fire resulted in increases or maintenance of grass cover in westernmost grassy Nama-Karoo vegetation, in accordance with findings for eastern Nama-Karoo vegetation (De Klerk et al. 2001, Du Toit et al. 2015). Additionally, we found that herbaceous plants and dwarf shrubs increased significantly after fire. Du Toit et al. (2015) did not report on responses of herbs (other than grasses) to fire in the eastern Nama-Karoo, but herbs commonly abound in early successional stages of various vegetation types (Musil and De Witt 1990, Van der Merwe and Van Rooyen 2011, Bachinger et al. 2016). Most dwarf shrubs in Nama-Karoo appeared capable of resprouting after fire (Du Toit et al. 2015), whereas those in Succulent Karoo largely appeared incapable of post-fire resprouting (Rahlao et al. 2009, Van der Merwe et al. 2016). This discrepancy may reflect disparate evolutionary histories and lower fire probabilities under the drier climatic conditions of the Succulent Karoo. We encountered very few succulent dwarf shrubs in our study, but other studies suggest that succulents are incapable of sprouting, although they may establish prolifically from seed after disturbance (Du Toit et al. 2015, Van der Merwe et al. 2016). The finding that dwarf shrubs mostly recovered well after fire in Nama-Karoo is encouraging for the maintenance of these shrublands should fires become more frequent in the future.
Large shrubs were the only growth form that decreased significantly after fire. Despite all large shrub species (except Elytropappus rhinocerotis) assessed in this study exhibiting resprouting abilities (as found by Du Toit et al. 2015), resprouting success varied widely within species. Some of the variation in resprouting appeared to be related to localized differences in fire intensity (T. Kraaij, Nelson Mandela University, George, South Africa, personal observation; cf. Van Wilgen 1980). Likewise, all assessed large shrub species, except E. rhinocerotis, recruited from seed to lesser or greater extents, again showing large variation among subpopulations within species. Collectively, our results suggest that more than half of the large shrub species that were assessed (including the iconic Cliffortia arborea, Red-listed as Vulnerable; Raimondo et al. 2015) may fail to sustain their populations after fire. These large shrubs displayed low post-fire:pre-fire population ratios (Table 1) or post-fire disappearance from the majority of plots (Appendix 1). The lack of post-fire recovery of Elytropappus rhinocerotis was contrary to its prolific reseeding after fires in fire-prone renosterveld (Levyns 1972, Van der Merwe and Van Rooyen 2011). Its lack of recruitment may be due to the timing of fire in relation to seed production, the timing of post-fire rainfall (S.J. Milton, RenuKaroo, Prince Albert, South Africa, personal communication), or high intensity of burning (T. Kraaij, personal observation). We showed that several species may be vulnerable to local extinction through fires despite possessing resprouting and reseeding abilities. This was detected through quantifying, as opposed to merely categorizing, fire survival and persistence responses of large shrubs, and through conducting surveys 2 to 3 years, rather than a few months, post fire.
The above suggests that specific attributes of individual fires and fire regimes (i.e., size, patchiness, fire intensity, fire return interval, and fire season) are likely to affect persistence of species and vegetation structure in the long-term (cf. Vlok and Yeaton 2000, Parr and Andersen 2006, Kraaij and Van Wilgen 2014). Furthermore, abundances of growth forms (and herbaceous species in particular) typically vary widely among years and with post-fire vegetation age (Musil and De Witt 1990, Van der Merwe and Van Rooyen 2011, Van der Merwe et al. 2016). Our study was limited to the effects of a single fire and to single observation events pre fire and post fire. The effects of different types of fires warrant further investigation, as do post-fire Karoo vegetation dynamics in the long term.
We predict that, with increases in the frequency or intensity of fires, the structure and composition of Nama-Karoo montane shrubby grasslands will undergo increases in the smaller growth forms (graminoids, other herbs, and dwarf shrubs) but decreases or potential loss of large shrubs. Such a scenario could mirror the persistent negative feedbacks, or grass-fire cycles, observed in certain other arid and semi-arid areas (D’Antonio and Vitousek 1992, Brooks et al. 2004). Consequently, we recommend that managers largely suppress the spread of man-made fires in protected areas comprising Nama-Karoo ecosystems.