PlantLife Volume 57.8, July 2024. Non-forest woody communities supplement plant diversity in the KwaZulu-Natal Drakensberg through fire suppression

Non-forest woody communities supplement plant diversity in the KwaZulu-Natal Drakensberg through fire suppression 

Text and photographs by Clinton Carbutt

 

Introduction

Plant diversity in the KwaZulu-Natal Drakensberg is largely accounted for by grasslands, forests, and wetlands, and especially old-growth grasslands because they are remarkably species-rich and occupy the greatest area (Carbutt and Edwards 2004). However, there is a more obscure community of non-forest woody plants, reclusive due to their sensitivity to fire and need for fire refugia, as well as the smaller areas they occupy naturally, that remains under-appreciated.

Figure 1: High topographic variability and habitat heterogeneity promotes plant species diversity. This is largely driven by natural features of the landscape that confer gradients of fire suppression promoting either open, semi-open or closed habitats. 

Although not limited to mountain regions, the complex rugged topography and high habitat heterogeneity associated with such environments shelter pockets of non-forest woody species (Figure 1). In the KwaZulu-Natal Drakensberg, they occur naturally between ca. 1800–2800 m a.s.l. on cooler, south-facing aspects where high topographic variability promotes fire protection (such as rivers and streams, rock bands and rocky outcrops, steep slopes with thin soils supporting only small fuel loads and therefore cooler fires) (Figure 2). They are particularly prevalent and well-protected in the sandstone–basalt transition zone, in the steep crests above sandstone cliffs and below the first rock bands of basalt. They sometimes occur as low as 1500 m a.s.l. where there is intentional fire suppression. They are mostly C₃ species (plants which fix carbon into 3-carbon sugars and occur in cooler and wetter environments) and evergreen (therefore assumed to transpire at varying intensities throughout the year). Their impact on ground water is considered negligible in high-rainfall areas.

Figure 2: Natural landscape features such as streams and rivers make effective barriers to fire and thereby promote fire suppression and incursions of woody floristic elements. A young cohort of Ouhout (Leucosidea sericea – Rosaceae) occupies the foreground of the image. 

These unique multi-species woody communities have historically been referred to as Afromontane fynbos, grassy fynbos, summer rainfall fynbos, Clarens heathland, and montane and sub-alpine heathlands or shrublands (Carbutt and Edwards 2001). The “fynbos” designation is solely based on the physiognomic (structural or architectural) similarities to fynbos shrublands and doesn’t reflect the dominance of Ericaceae, Proteaceae and Restionaceae that defines Cape fynbos in the Greater Cape Floristic Region. Some members do, however, share floristic connections to the Cape, mostly at the generic level, and are referred to collectively as the Cape element (Carbutt and Edwards 2012). More surprising is the faunal connection. The historical record of two Cape Grysbok shot in 1936 in Grysbuck Bush, Giant’s Castle (Barnes 2003), shows that shrublands and scrubby thickets in the KZN Drakensberg have historically supported Cape-centred mammals, no doubt due to the structurally similar low woody habitats.

The woody communities technically form part of the Grassland Biome (and not the Forest Biome) because the dominant life-forms are not tall trees in closed canopies. Instead, they are occupied by perennial shrubs and small trees that appear in open grassy landscapes when fire is suppressed (Table 1). They are generally long-lived under protection, slow-growing and slow-maturing. Some are obligate seeders (or reseeders) — these species do not resprout but rely on seeding to regenerate their populations after dying naturally or being killed by fire. Seeds may be stored in the soil or in the canopy (Pausas and Keeley 2009). They are sometimes serotinous — their seeds are held on the plant for a long time, only being released following a trigger, which is often fire. A good example is the Lip-flower Sugarbush, Protea subvestita, a fire-sensitive species requiring long fire-return intervals (FRI).

Complete fire exclusion can also be harmful in the long-term because fire is very often required to open cones (gymnosperms) and flowering heads (angiosperms) to release seeds, but this is species dependent and warrants further investigation. When fire is required, it should be at a low frequency that first allows these slow-growing species to reach sexual maturity and produce seeds necessary for recruitment. Regular hot fires will (1) kill juveniles before they have had an opportunity to mature and set seed, and/or (2) kill adults with immature flowering heads or cones holding unripe seeds that are not due for release because of long retention times. Therefore, the FRI should be longer than the species’ age to sexual maturity, at least 8 to 17 years, but this will be species dependent. While some species have physical mechanisms to protect themselves from fire [e.g., Mountain Cypress — inflammable bark and leaves due to resins (Boon 2010); Drakensberg Cycad — thick, scaled stem], these are insufficient to safeguard against regular hot fires (Figure 3).

Figure 3: Managed or experimental fire suppression practiced at Giant’s Castle (right half of image) versus open, frequently burnt grassland (left half of image). The two contrasting management types are separated by the Bushman's and Mtshezana Rivers. Note the high density of the Christmas tree-like Mountain Cypress (Widdringtonia nodiflora - Cupressaceae). This species is the flagship of summer-rainfall mountain shrublands and thrives under fire suppression.

Table 1. Examples of plant species constituting non-forest woody communities particularly common under fire suppression in the KwaZulu-Natal Drakensberg.

Taxon	Family	Life-Form
Arrowsmithia spp.	Asteraceae	Shrubs
Buddleja spp.	Scrophulariaceae	Shrubs–Small Trees
Calpurnia sericea	Fabaceae	Shrub
Cliffortia spp.	Rosaceae	Shrubs
Diospyros austro-africana & D. whyteana	Ebenaceae	Shrubs–Small Trees
Encephalartos ghellinckii	Zamiaceae	Cycad
Erica aestiva, E. caffrorum & E. evansii	Ericaceae	Shrubs
Euclea spp.	Ebenaceae	Shrubs–Small Trees
Euryops spp.	Asteraceae	Shrubs
Halleria lucida	Scrophulariaceae	Tree
Heteromorpha arborescens	Apiaceae	Tree
Leucosidea sericea	Rosaceae	Small Tree
Metalasia spp.	Asteraceae	Shrubs
Morella serrata & M. pilulifera	Myricaceae	Shrubs–Small Trees
Muraltia spp.	Polygalaceae	Shrublets–Shrubs
Myrsine africana & M. pillansii	Myrsinaceae	Shrubs–Small Trees
Passerina drakensbergensis & P. montana	Thymelaeaceae	Shrubs
Phylica spp.	Rhamnaceae	Shrubs
Polemannia montana & P. simplicior	Apiaceae	Shrubs–Small Trees
Protea subvestita	Proteaceae	Shrub–Small Tree
Relhania spp.	Asteraceae	Shrubs
Searsia spp.	Anacardiaceae	Shrubs–Small Trees
Seriphium/Stoebe spp.	Asteraceae	Shrubs
Widdringtonia nodiflora	Cupressaceae	Coniferous Tree

 

Value and Significance of Woody Communities under Managed Fire Suppression

Expansion of woody species, including range expansions, is attracting an almost exclusively negative narrative. Care must be taken to distinguish bush thickening or densification (expansion of native woody species) from bush encroachment (expansion of alien woody species) (Nico Smit, personal communication). The perils associated with range expansion of alien woody plants are self-evident. But for native woody species, one must ask whether they are expanding beyond historical ranges or are densifying in areas where they already have a presence. Some native woody communities may be opportunistically expanding due to changes in key drivers such as fire regime or responding to inevitable global change drivers out of our control (e.g., global atmospheric CO₂ increase). 

Under certain circumstances and against predefined management objectives, there are a host of positive values associated with harnessing managed or experimental fire suppression to support native woody habitats in grassland-dominated mountain regions:

1.   Protection and promotion of slow-growing, slow-maturing, fire-sensitive woody species, many of which can only recruit from seed and therefore avoid open grasslands characterized by frequent fires.

2.   Promotion of phylogenetic diversity (conserving evolutionary history and the processes of speciation). This includes the conservation of gymnosperms (conifers and cycads), representing ancient phylogenetic lineages poorly represented in mountain protected areas and grassland floras.

3.   Promotion of floristic diversity in mountain floras (multi-species suites of woody taxa are represented by a wide range of genera largely absent from open grasslands and forests, occupying unique ecological niches in mountain landscapes). Some of these species are important medicinal plants (e.g., Myrsine africana).

4.   Promotion of physiognomic diversity (greater range of vegetation structural forms and architecture).

5.   Promotion of habitat heterogeneity (also beneficial to a range of fauna, including birds, mammals and invertebrates that may constitute important pollination guilds).

6.   Promotion of local representatives of the Cape floristic element (Cape-centred genera such as Cliffortia, Erica, Passerina, Phylica, and Relhania).

7.   Promotion of significant above-ground carbon storage/sequestration.

8.   Potential to host unique soil microbial communities.

Implications for Biodiversity Management in KwaZulu-Natal Drakensberg Protected Areas

Understanding the impacts of fire on flora and vegetation is important since over- or under-burning can have deleterious consequences, depending on the objectives of the burning programme and the pyric requirements of the species. The decline of Protea roupelliae seedlings because of overly frequent fires in the KwaZulu-Natal Drakensberg is a case in point (Smith and Granger 2017).

Drakensberg protected areas, such as the expansive Maloti-Drakensberg Park World Heritage Site (MDP WHS), can play an important role in the conservation of non-forest woody communities. The Park’s fire management plan can be adapted to preclude frequent fire from designated areas. These areas, larger than those protected by natural fire refugia, will support larger populations of woody species. Important areas in this regard are located at Giant’s Castle (Figure 3), Sani Pass and Catchment 9 at Cathedral Peak. To manage the MDP WHS optimally for non-forest plant biodiversity, a “sum of the parts” approach is necessary. This involves maintaining stable grasslands through frequent to variable fire regimes to promote grass and forb diversity requiring a range of FRI (Gordijn et al. 2018) and practising managed or experimental fire suppression in other areas to promote woody diversity (species requiring much longer FRI). This combined approach will maximize plant diversity in the MDP WHS. Although many of these woody species are not rare or threatened, or endemic, there is greater value in the “collective” as a unique woody community and habitat promoting floristic diversity and habitat heterogeneity in the MDP WHS, the KwaZulu-Natal Drakensberg and the Grassland Biome.

Conclusions

The Grassland Biome is synonymous with herbaceous diversity, however, it also includes woody shrubland diversity that supplements diversity further. Not all native woody expansion is bad for biodiversity. Against set objectives and in designated areas, mountain shrublands expanding under managed fire suppression, for example, make significant yet unappreciated contributions to plant diversity (Figures 4 & 5). They harbour non-forest, fire-sensitive species requiring refuge from frequent fire. Gradients of fire protection, ranging from more frequently burning areas housing fire-tolerant species to very infrequently burning areas sheltering more fire-averse species, are a key driver of fine-scale patterns of vegetation and habitats in summer-rainfall mountains. The collective is a desirable mosaic of heterogeneous habitats that optimize plant diversity and landscape-scale diversity.

Figure 4A.

Figure 4B.
Figures 4A and 4B: Mountain shrublands densifying and expanding under fire suppression. 

Figure 5: A dense thicket of Erica evansii (Ericaceae), an arborescent erica that flourishes under fire suppression. 

Fire can have positive and negative impacts on flora and vegetation and should therefore be applied at frequencies that, among other things, aim to promote species and habitat diversity. The burning requirements of summer-rainfall mountain shrublands require further research, particularly for certain species. Long-term management and monitoring are essential to safeguard these unique habitats making a significant contribution to mountain plant diversity. Managed or experimental fire suppression requires decisive planning and proactive management. It necessitates skilled staff, budgets, alien plant control and protective measures such as wide fire breaks to exclude incidental fires from these areas. Sites earmarked for future fire suppression should make better use of areas naturally predisposed to some degree of fire protection to create shrubland mosaics supplementing plant diversity in special places such as the KwaZulu-Natal Drakensberg.

References:

Barnes B. 2003. Giant’s Castle: A Personal History. Bill Barnes; pp. 1–282.

Boon R. 2010. Pooley's Trees of Eastern South Africa. Flora and Fauna Publications Trust, Durban; pp. 1–624.

Carbutt C, Edwards TJ. 2001. Cape elements on high-altitude corridors and edaphic islands: Historical aspects and preliminary phytogeography. Systematics and Geography of Plants 71: 1033–1061.

Carbutt C, Edwards TJ. 2004. The flora of the Drakensberg Alpine Centre. Edinburgh Journal of Botany 60(3): 581–607.

Carbutt C, Edwards TJ. 2012. An Eden in exile: outliers of the Cape Floristic Region. PlantLife 41 & 42: 4–11.

Gordijn PJ, Everson TM, O’Connor TG. 2018. Resistance of Drakensberg grasslands to compositional change depends on the influence of fire-return interval and grassland structure on richness and spatial turnover. Perspectives in Plant Ecology, Evolution and Systematics 34: 26–36.

Pausas JG, Keeley JE. 2009. A burning story: the role of fire in the history of life. BioScience 59(7): 593–601.

Smith FR, Granger JE. 2017. Survival and life expectancy of the tree Protea roupelliae subsp. roupelliae in a montane grassland savanna: Effects of fire regime and plant structure. Austral Ecology 42: 422–432.

About the author: Clinton Carbutt is the Plant Scientist for Ezemvelo KZN Wildlife with an interest in the floras of the Drakensberg Mountain Centre and KwaZulu-Natal Midlands, as well as the conservation of temperate grasslands. He is passionate about mountains and mountain floras. He has a PhD in Botany from the University of Natal and is the Southern African representative on the IUCN Mountains Specialist Group.