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

Tiaan Strydom,Richard M Cowling,Mark Difford,Tineke Kraaij

doi:10.7717/peerj.9240

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.

Highlights

An understanding of the ecological determinants of the boundaries between vegetation formations is central to managing and conserving biodiversity (Cowling & Potts, 2015; Hansen & Di Castri, 1992)
All the final models assessing the effects on survival, post-fire resprouting shoot count and resprouting volume of fire severity, pre-fire size, site, and their interactions, explained only small proportions of the overall deviance (r2 = 0.05, 0.09 and 0.19, respectively; Tables 2–4), suggesting that most variation was not accounted for by the factors investigated
Fire severity interacted with pre-fire size in its effect on survival (Table 2), with high fire severity possibly having more pronounced negative effects on the survival of large shrubs (Fig. 2)

Summary

Introduction

An understanding of the ecological determinants of the boundaries between vegetation formations (biomes) is central to managing and conserving biodiversity (Cowling & Potts, 2015; Hansen & Di Castri, 1992). (occurring in areas were fire seldom occur) biomes in the same mesoclimate and geology, and the extent to which fire or other factors determine that co-occurrence. Notable amongst these cases is the stable co-occurrence of fire-dependent fynbos and fire-avoiding forest and thicket in South Africa’s Cape Floristic Region (CFR) (Cowling et al, 1997; Cramer et al, 2019; Geldenhuys, 1994). On the other hand, have predominantly short-lived, bird-dispersed diaspores which produce shade tolerant seedlings that require long fire-free intervals for recruitment into fynbos (Cowling et al, 1997; Manders & Richardson, 1992). Co-occurrence of these biomes has been attributed to differences in fire regimes (Bond, Midgley & Woodward, 2003; Geldenhuys, 1994; Manders & Richardson, 1992; Pierce & Cowling, 1991; Vlok, Euston-Brown & Cowling, 2003), soil nutrients (Coetsee, Bond & Wigley, 2015; Cramer et al, 2019; Manders & Richardson, 1992), soil moisture (Manders & Richardson, 1992) and other disturbances such as wind or large herbivores (Vlok & Euston-Brown, 2002)

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