Rehabilitation and Restoration Of Degraded Forests (PDF) - IUCN

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7.2 Generating diversity at the landscape scale Chapter 7: Approaches at the landscape level Many land owners trying to balance production and biodiversity may use relatively uncomplicated planting designs rather than more complex, multi-species plantings containing high levels of diversity. This means that local or site-level diversity (known as the alpha diversity) may be relatively low, especially in small patches. On the other hand, provided land owners do opt for a variety of approaches (e.g. not every land owner just planting woodlots of Eucalyptus), the cumulative diversity present across the landscape (the gamma diversity) resulting from these many separate management decisions may be significant. The relationship between alpha and gamma diversity is illustrated in Figure 9. Figure 9. Effects of reforestation on alpha and gamma diversity A A A A B ABC ABC a: 0 1 1 3 g: 0 1 3 3 a: alpha (local) diversity g: gamma (landscape) diversity C ABC Figure 9 shows four degraded landscapes. Three of them have been partially reforested; one is in its initial degraded state. The first of the reforested sites has monocultures of species A while the second has a mosaic of monocultures of species A, B and C. This means that the alpha diversity in these two sites is similar but the gamma diversity has increased in the landscape as a result of the three different plantations. The fourth landscape has several multi-species plantations. These increase the alpha diversity but not the gamma diversity. From a silviculture perspective, monoculture plantations are easier to manage than species mixtures. If the gamma diversity is the same, is 71
REHABILITATION AND RESTORATION OF DEGRADED FORESTS there any benefit from having a higher alpha diversity at a particular site? The different forms of diversity have different functional consequences, and the answer to this question depends on the primary management objectives and the function that is most important. If the main objective is to re-establish biological diversity, then plant alpha diversity does matter and restoration must aim to restore the original plant species richness and structural complexity that will allow both plant and animal species to recolonise and reproduce at the site. If, on the other hand, the objective is re-establish site production and only some of the original biodiversity then the sacrifice of alpha diversity for the sake of gamma diversity may be acceptable. The answer to the question also depends a good deal on the landscape context. What is the degree of fragmentation? How far apart are the patches of residual forest? Some species benefit from patchiness or spatial heterogeneity and the juxtaposition of contrasting habitats (e.g. forest and grassland) because of the extent of the transition or boundary zones they provide. But the size of these patches — what Forman (1995) refers to as the landscape grain size — also affects the type of species that will establish within the patches. A fine-grained landscape or one made up of small patches suits generalist species but not those requiring larger areas of more specialised habitat. Ideally, then, a range in patch sizes is advantageous although it is the forest specialist species that are more likely to be at risk than the generalists. Most situations probably require more than one approach and Forest Landscape Restoration (as defined in Box 3) will often involve protection of forest remnants as well as reclamation, restoration and rehabilitation at the site level. These biophysical uncertainties also underpin a more difficult social dilemma: how can a variety of small, site-level decisions made by individual land managers be integrated to produce the best functional outcome at a landscape level? Very few planners or land managers appear to have successfully addressed this issue. 72
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