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Exclude fire … and lose montane grasslands to forest scrub and aliens

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The author at work monitoring change in the Drakensberg.

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The most striking change over the last 40 years was the increase in woody cover on the steep slope rising from the river (Picture: Basanda Nondlazi)

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Erica woodii (Picture: Basanda Nondlazi)

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Grasslands provide vital ecosystem services (Picture: Basanda Nondlazi)

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Chimney Pot is one of the Giants Castle peaks located in the Witteberg area of the uKhahlamba Drakensberg World Heritage Site Park (Picture: Basanda Nondlazi)

Basanda Nondlazi, Field Technician, SAEON Grassland-Forests-Wetlands Node

Why bother to look at fire exclusion in the Drakensberg?

The Drakensberg supports high altitude fire-climax grasslands in Southern Africa. The area is recognised as a biodiversity hotspot and contains many endemic organisms. This unique biodiversity would be threatened by an increase in woody plants that changes grassland structure.

Fire is commonly considered the key agent for maintaining the structure and plant richness of grasslands that, in turn, provide habitat for numerous faunal organisms. For humans, grasslands provide vital ecosystem services such as water. Yet there is limited empirical evidence in support of fire as the main agent limiting the abundance of woody plants. Deeper understanding of the rate and nature of vegetation change under fire exclusion in this unique montane ecosystem would also contribute greatly to use of fire as the main management tool for maintaining grasslands.

To this end, a fire-exclusion experiment was initiated in a local catchment at Giants Castle in 1972. It was expected that these grasslands would transform to woodlands. This local catchment could be compared with adjacent biennially burnt areas in order to understand types of plants that would be promoted or become depleted by fire exclusion.

In 2011, SAEON started catchment monitoring in the exclusion registered with Ezemvelo KZN Wildlife, who are acknowledged, with appreciation, for permitting work in their protected area.

What changes are SAEON monitoring here?

The changes that are being followed are distribution and extent of some key species or communities; distribution and extent of woody vegetation communities; rate and pattern of increase in woody vegetation and, consequently, rate and pattern of loss of grassland structure and richness; rate and pattern of replacement of forest-precursor species by forest species; and presence and rate of loss of fire-adapted woody species such as Protea caffra.

Biome shift observations underway

The work has bearing on possible changes throughout the grassland biome in terms of boundary shifts if fire becomes more widely excluded. Fire-mediated change in vegetation structure and composition has implications for biome boundary shifts. For example, ingress of woody plants into grassland may change the functionality of grassland to be similar to that of a savanna or forest.

Exclude fire and lose montane grassland

Giants Castle, which derives its name from the outline of the peaks and escarpment, is part of the central Drakensberg. Chimney Pot is one of the Giants Castle peaks located in the Witteberg area of the uKhahlamba Drakensberg Park World Heritage Site. Topography over the length of Chimney Pot (Figure1) varies from east to west (about 1.2 km in length), but the western section can be characterised as a steep slope rising from the river; a gentle plateau-like slope, above this -- a steep scarp-like slope and then the gently sloping summit plateau. The eastern section has a repeated sequence of scarp-like slopes and gently sloping plateaux.

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Figures 1(a) and (b) show change in distribution and pattern of woody cover (Google image 2011) after 40 years (a); aerial photograph in1981 after 9 years of fire exclusion (b). The exclusion plot extends from the river to the summit ridge and from the green line on the far right of the plot in the 2011 picture to the firebreak on the left. For Figure 1(a) note the coarse texture of dark green granules, which represent woody vegetation. Less coarse texture on the gentle plateau-like slope is rolling grass and Erica woodii, alien bramble plus bracken. The even finer texture on the plateaux summit represents grass.

The most striking change after 40 years is the increase in woody cover (Figure 1a) from small nucleus patches (light green circles) in 1981, mostly on the steep slope rising from the river and steep scarp-like slope. The conspicuously dominant woody species appear to be Erica woodii (for dwarf shrubs), Myrsine africana and Erica drakensbergensis (for shrubs), and (for trees) Searsia dentata and Podocopus latifolius -- the national tree that claims the highest canopies.

Bold polygons on the bottom right section (Figure 1a) highlight areas of the slope along the river that are currently open, with few woody plants. On the gentle plateau-like slope in 1981 (Figure 1b), the greyish brown amorphous areas represent Erica woodii plus possibly bramble and bracken stands, which are distinct from adjacent grass-dominated (light brown) areas. Firebreaks (greyish-black) can be seen on the left, on the summit and to the right of the jeep track. Note the close correspondence between the areas dominated by Erica woodii / bracken and bramble in 1981and areas currently dominated by woody plants, especially on the steep slopes.

Preliminary observations

We observed high infestations of American bramble (Rubus cuneifolius) and bracken (Pteridium aquilinum) on areas not yet densely populated by woody plants in 2011. These species may be providing precursor conditions for the establishment of woody plants.

Woody cover on the gentle plateau-like slope is low except for that of Rubus cuneifolius, which has formed dense stands in places. There may be a strong abiotic control on the development of woody vegetation. Based on grassland structure and composition on parts of the summit plateau and western portion of the mid plateau, the plot may have been accidentally burnt during the period of attempted fire exclusion.

For already wooded areas it is expected, with continued exclusion of fire, that woody composition will change, woody biomass will increase, and forest-understory grasses and forbs will replace remaining grassland elements. Areas of bracken (and possibly bramble) infestation are expected to serve as areas for further establishment of woody elements.

It is further expected that fire-adapted trees such as Protea caffra, which is well represented on the summit and in the west of the catchment, will eventually be lost through continued successful fire exclusion. If, however, an accidental fire were to occur, the strong influence of topography on vegetation change would be expected to result in only the plateaux portions being burnt, with the greater part of the wooded areas on steep slopes remaining protected from fire. A resurvey of this project post fire events or a decade would test these predictions accordingly.

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