Does natural forest size really matter?

May 6, 2013

Why do people think that large forests have more conservation value than small ones? Does this apply to South Africa or only to the extensive tropical moist forests? Whether we look at mean individual patch size or total cover of natural forest, we have a very small total area of natural forest fragmented into generally very small forest patches, but with a high total species richness. Should we be concerned about the total forest area, or patch size or the number of species in those patches?

by Dr Coert J. Geldenhuys

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Western Cape Afrotemperate Forest in small fragments in fire-shadow areas at the foot of Langeberg Mountains above Swellendam. These patches survive despite regular burning of the fire-prone fynbos.

In a recent study (2009), Dr Derek Berliner assessed the conservation status of natural forests in South Africa as a basis for systematic conservation planning for the Forest Biome. Part of the study assessed the fragmentation of the forests through size range of forest patches in the 26 national forest types. His study indicated that the total forest area as 4 927 km2, but the total area and mean patch size varied much between the forest types.

Estimates of forest cover in South Africa by different studies ranged between 3 023 km2 and 5 386 km2. Experts estimated that most forest types have lost from 15% to more than 35% of their original extent, mainly through transformation to agriculture and plantation forestry. We have 72.6% of our 16 216 patches of natural forest in the size category of 1-50 ha. The study made use of mainly secondary data, but at the same time acknowledged that information was often outdated and that area estimates were subject to various problems.

Perceptions
There are many perceptions of the reasons for the small, fragmented and degraded natural forest area. People like to say how much of the forest had been fragmented, degraded or lost, but never talk about how much forest had been gained (see SA Forestry magazine May 2006)! How reliable are the statistics of actual forest cover in South Africa, and in Africa for that matter? What information goes through to become FAO statistics on the decline of world forest resources? The different area estimates of natural forest in South Africa were based on different mapping approaches, differences in patch size included and problems with differentiation of natural forest from other vegetation, particularly wattle stands, and differentiation of early forest regrowth thickets from natural mature thicket.

No approach has been consistently applied over time. Changes often reflect changes in methodology rather than changes in forest cover. For example, one mapping exercise showed natural forest cover in Limpopo Province where there is definitely no natural forest, and did not show natural forest cover between Port Elizabeth and Van Stadens River where there is probably much more than 10 000ha of natural forest. When forest mapping methodology changes, then at least there should also be one reference method maintained over time.

What would be a realistic value for a reliable estimate of total forest cover? Natural forest patches occur in areas with all-year rainfall around 500mm/year, and probably 725mm/year in summer rainfall areas. If we take the 800mm rain/year boundary then potentially 7% of South Africa could have forest – but we have less than 0.1% forest cover! Where is the rest? It is not enough to use purely physical environmental variables.

Evidence
There is enough evidence that much of this difference is attributed to effects of natural fire pathways in the landscape confining natural forests to wind/fire shadow areas (see SA Forestry magazine November 2007), and very little to clearing of forest by people (although this occurred in several areas)! We inherited very small, very fragmented forests, within the major matrix of other vegetation – we did not create that pattern over the last 500 years (current climatic period). Forest cover in South Africa has a long history of expansion and contraction. During earlier geological times, forests covered most of the southern African landscape in a different configuration than today, as evidenced by pollen records of forest species in remote, some currently very dry, areas. There were, however, also extreme cold and dry periods with much less forest than the present.

The current forest location pattern, which resulted from various periods of expansion and contraction of the fragmented forest patches, with their associated species of plants and animals of diverse life forms, incorporated changing levels of species diversity. New adaptations developed and many species have diverse forms with diverse gene pools in different landscapes within South Africa. For example, true forest species such as Rhus chirindensis, occur over a wide range inside the forest, but more species (in this case Rhus species) with more restricted distribution developed along the forest margins, and with even more restricted distribution developed outside the forest margin in more fire-prone or drought-prone areas.

Small forest patches
Natural forests currently cover diverse sites of diverse geology, climatic regimes, and other physical environmental features, with their own species assemblages. This makes the South African natural forests the most species-rich warm-temperate forests in the world. A large component of this species richness is contained in the isolated, very fragmented, and very small forest patches! Plantation forestry, with plantation stands planted in the fire-prone environments within the fire pathways, became corridors of nurse stands for the connection between former isolated natural forest patches.

High conservation value forests therefore are not only those larger forests in the landscape. They also include many very small forests within the matrix of fire-prone grasslands and shrublands (including Fynbos). Many of these patches occur in the coastal areas where in some areas of affluent societies, they expand but in other areas, they may decline because of uncontrolled resource use.

In reality, we gained forest through changed land use practices (with changed fire regimes, including exclusion of fire), and thereby fragmentation decreased (corridors developed via timber plantations, invader plant stands, woody regrowth in protected areas). This gain will cause loss in area and species of other vegetation types!

In the South African natural forest environment, we have to take a closer look at the conservation value of the small, very fragmented forest patches, with their own unique suites of species assemblages, representing specific biogeographical relationships of former periods of forest expansion and contraction. They are often not represented in the larger forests currently under conservation management. Criteria for high conservation value forests within the plantation forestry estate have to consider such dynamics and species assemblages.

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Transkei Coastal Forest along the Wild Coast, isolated yet thriving within fire-prone grassland.
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Small patch of Mpumalanga Mistbelt Forest at Mount Sheba.
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Eastern Mistbelt Forest in the Mzimkhulu area of KwaZulu-Natal severely affected by uncontrolled resource use.
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A small patch of Limpopo Mistbelt Forest in the Tzaneen lowveld area which appears to be degraded but has actually expanded. It is surrounded by fruit orchards and timber plantations.

 

* Dr Geldenhuys is a Forest Ecologist/Associate Professor in Forest Science, Department of Forest & Wood Science, University of Stellenbosch
Tel: +2782 776-1593; cgelden@mweb.co.za

Published in Feb 2013

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