Why are forest margins such a contentious management issue?
For many years the buffer width, ie. the unplanted open space between a plantation and natural forest, has been the subject of a controversial debate between plantation managers and conservationists. This space becomes invaded by invasive alien and forest species, and the forest species moves into the planted stand (see SA Forestry magazine May/June 2006). When the planted stand is felled, should the boundary be moved further away because of the expanded regrowth forest boundary (and the potential to contravene the National Forests Act)? Should there be a buffer zone between a forest and a housing development as conservation authorities require?
by Coert Geldenhuys, forest ecologist
Soft forest margins with grassland, with wider ecotone at Ngome (left) and narrow ecotone at Lekgalameetse Nature Reserve, North-eastern Escarpment.
Hard forest margins in the Southern Cape caused by felling a pine stand (Groenkop) that should have been rehabilitated to forest in this small area (left) or by road construction (right) through Bloukrans forest for new N2 Toll road (which could have been avoided with better road planning).
Soft forest margins can be managed with a road between the forest and plantation (left, Island State Forest, Port Elizabeth) and proper planning of the road alignment (right, Witteklopbos, Tsitsikamma).
Buffer zone – open space between the plantation and the natural forest margin. It is supposed to be kept open (as grassland or fynbos or just unplanted – but it quickly becomes occupied with either invader plants or forest species) – some say it should be 5 m and for a long time we argued that it should be 20 m and that it should be fixed. The plantation managers experience many problems with this buffer width. But from the understanding I developed of the forest margin dynamics, my recommendation changed towards leaving no buffer zone.
Forest margin/forest edge – this is the outer boundary of the natural forest. The transition vegetation (ecotone) between the forest and the outside natural vegetation changes from the forest to the outside vegetation – at the one end it is typical forest and the other end typical grassland, and somewhere between the two it is a mixture of the two. Some species occur only in this ecotone and not in the proper forest or in the proper grassland. That is why the ecotone vegetation is considered to have its own diversity.
The shape or profile of the ecotone vegetation determines how we see the forest margin. If the ecotone vegetation closes the gaps between the trees, we say it is a soft edge. This can be an abrupt (short ecotone) or a gradual (long) change, depending on the local landscape.
Tension zone – the area of the forest margin which changes from time to time – the area in which the forest margin expands and contracts depending on different environmental conditions – eg. wet periods with less fire versus dry periods with more fire.
The natural, very fragmented forest patches have a long margin (boundary or edge) and the smaller the patch, the larger is the ratio of forest margin to forest area. This accentuates the importance of forest margins in forest survival within the fire-adapted fynbos, grassland and woodland. Even a five-metre- wide buffer can take up a large area in any plantation or a housing development.
In the natural environment, a natural forest develops a natural ecotone (transitional vegetation) between the forest and the adjacent fire-prone fynbos, grassland or woodland. This vegetation gradually decreases in height and increases in foliage density away from the mature forest. The shape of the vertical profile of the ecotone vegetation determines how we see the forest margin. If the ecotone vegetation closes the gaps between the trees, it is a soft margin. This can be an abrupt change (short ecotone) as on the upper forest margin below a slope break, or a gradual change (long ecotone) as at the bottom end of a forest on a slope, ie. depending on the local landscape. Tree clumps in the grassland, particularly coastal grassland, form interesting dome-shaped margins from shearing by the cold and salty winds.
This forest ecotone has important functions. It has a higher biodiversity of both plants and animals than either the forest or the outside vegetation, because it includes elements of both. It consists of plants of various growth forms and sizes that close the gaps between the mature trees and as such buffers the moister micro-climate of the forest interior against the more extreme colder/hotter and drier exterior climate, similar to that of the closed forest canopy. It is mechanically strong and the aero-dynamic dome shape changes the flow of the wind to cause a fire to flow over the dome, thereby protecting the forest even during intense fires blown by strong winds.
The natural forest margin area is more or less fixed in the landscape but the margin moves backward and forward within this ecotone zone, during different periods of disturbance frequency and intensity. We call this margin area a tension zone. We can demarcate the boundary of the tension zone on the forest side by the edge of the larger trees of the core forest. Within the tension zone, the trees have much smaller diameters. Sometimes an extreme fire can push the forest margin back to this forest side of the tension zone because of the natural fire movement with the wind around barriers in the landscape (see SA Forestry magazine Nov/Dec 2007).
Clearing of the ecotone vegetation creates a hard forest margin which opens the vertical stand profile with openings between larger trees, such as during removal of the forest margin vegetation or cutting of a clearing in the forest (as for a road), or felling a plantation or invader plant stand, or making a fire break on the forest margin. The hard edge exposes the forest interior to penetration by the hot, dry, and gusty winds, and even fires, ie. makes the forest more vulnerable.
In the management of the forest margin, we have to consider several issues, depending on the specific land use management objectives for natural areas versus development zones, within the same forestry or housing estate. In grassland/fynbos areas, we need to consider the natural fire regime that maintains the natural ecotone with a forest. It will be necessary to maintain a wide enough buffer between the forest margin and plantation to maintain a suitable fire regime for a healthy forest/other vegetation ecotone. Within the productive plantation area or housing/urban development, we totally change the natural disturbance regime, to the detriment of the grassland/fynbos, but to the advantage of the natural forest. A timber plantation or housing development (if a forest is not illegally cleared for that purpose) protects forest margins against frequent fires, aids forest rehabilitation and expansion in places (see SA Forestry magazine May/June 2006 and Nov/Dec 2007). Plantations reduce pressure on the forests for timber, fibre and firewood needs (see SA Forestry magazine April 2010).
We therefore, have to think about the ecotone biodiversity, potential forest expansion (gain) and ecological processes within the tension zone, alien plant control within the buffer zone, plantation productivity, and vegetation maintenance of natural 'open' areas (corridors) within the plantation and housing estates. Can we manage the fire regime cost-effectively within narrow belts of grassland/fynbos between the forest or riparian zone and a plantation/housing development to maintain healthy grassland/fynbos and a functional ecotone with natural forest?
I often ask myself how I would manage the forest margin to address the different issues raised above, and my approach would be as follows:
- If possible, put a road between the plantation or development and the forest to have a permanent boundary to allow the forest to develop a natural soft margin up to the road. The development side of the road can then be managed by best environmental principles. Plantation trees should NOT be felled across the road into the forest margin but into the plantation stand.
- If a road is not possible, then plant the plantation trees up to the forest margin but at a distance away from the forest side of the tension zone which would be 1.5 times the mature height of the plantation trees. Fix that plantation boundary and only control invasive aliens in the margin. When mature, fell the planted trees on the edge of the stand into the developing forest margin within the tension zone:
- to maintain the disturbance of that margin to maintain an own species diversity (even though it would not be the same as in a fire-controlled ecotone)
- to maintain a soft margin condition with forest tree regeneration of different heights (most forest tree species resprout after damage)
- with removal of all plantation tree debris from that margin area to reduce the fire risk
- with NO burning of tree debris inside regrowth forest margin zone.
- Manage narrow areas between planted stands and the riparian zone within the plantation as 'regrowth forest' corridors to reduce the effort of constantly having to remove invasive alien plants from the riparian zone and of maintaining difficult fire management in those narrow strips.
- Do not place a fire break adjacent to the forest margin within natural areas of the estate. If fire breaks are required, put them where they would be effective in terms of fire spread into the plantation (see SA Forestry magazine Nov/Dec 2007).
- Allow regrowth forest to expand into the housing development area but obtain the relevant permission to prune developing forest trees when they expand onto the houses. Remove alien tree species from time to time (the developing forest will control most of them).
- In a coastal development, construct a boardwalk from the beach into the scrub-forest margin very carefully and perpendicular to the prevailing winds from the coast to not cause blow-outs of the tightly structured, protective forest ecotone.
- Implement a monitoring programme to assess the costs and benefits of following this approach.
Published in August 2010