Forest Degradation
There are five broad levels of degradation, each of which requires a different restoration strategy. They can be distinguished by recognising six critical ‘thresholds’ of degradation; three pertain to the site being restored and three to the surrounding landscape.
Site-critical thresholds:
- The density of trees is reduced such that herbaceous weeds dominate the site and suppress tree seedling establishment
- On-site sources of forest regeneration (i.e. the seed or seedling bank, live stumps, seed trees etc.) decline below the levels needed to maintain viable populations of climax forest tree species.
- Soil degradation has proceeded to such an extent that poor soil conditions limit the establishment of tree seedlings.
Landscape-critical thresholds:
- There are only small and sparse remnants of climax forest in the landscape, such that the diversity of tree species within dispersal distance of the forest restoration site is not sufficient to represent the climax forest.
- Populations of seed-dispersing animals are reduced to the point that seeds are no longer transported to the forest restoration site in sufficiently high densities to reestablish all of the required tree species.
- Fire risk is increased such that naturally established trees are unlikely to survive because of the increased cover of combustible herbaceous weeds in the landscape immediately surrounding the restoration site.
Stage-1 Degradation
Stage-1 Degradation | Site-Critical | Landscape-Critical |
|---|---|---|
| Vegetation : Trees dominate over herbaceous weeds Sources of regeneration : Plentiful: soil seed bank viable; dense seedling bank; dense seed rain; live tree stumps Soil : Little localised disturbance; remains mostly fertile | Forest : Large remnants remain as seed sources Seed dispersers : Common; both large and small species Fire risk : Low to medium |
Recommended Restoration Strategy:
- Protection from encroachment, cattle, fire and any other further disturbances and prevention of the hunting of seed-dispersing animals
- Re-introduction of locally extirpated species
Stage-2 Degradation
Stage-2 Degradation | Site-Critical | Landscape-Critical |
|---|---|---|
| Vegetation : Mixed trees and Sources of regeneration : Seeds and seedling banks depleted live tree stumps common Soil : Remains mostly fertile: erosion low | Forest : Remnants remain as seed sources Seed dispersers : Large species becoming rare, but small species still common Fire risk : Medium to high |
Recommended Restoration Strategy:
- Protection + ANR
- Re-introduction of locally extirpated species
Stage-3 Degradation
Stage-3 Degradation | Site-Critical | Landscape-Critical |
|---|---|---|
| Vegetation : Herbaceous weeds dominate Sources of regeneration : Mostly from incoming seed rain; a few saplings and live stumps might remain Soil : Remains mostly fertile; erosion low | Forest : Remnants remain as seed sources Seed dispersers : Mostly small species dispersing small seeds Fire risk : High |
Recommended Restoration Strategy:
- Site protection + ANR + planting framework species
Stage-4 Degradation
Stage-4 Degradation | Site-Critical | Landscape-Critical |
|---|---|---|
| Vegetation : Herbaceous weeds dominate Sources of regeneration : Low Soil : Erosion risk increasing | Forest : Remnants too few or too distant to disperse tree seed to site Seed dispersers : Mostly gone Fire risk : High |
Recommended Restoration Strategy:
- Site protection + ANR + planting framework species + enrichment planting with climax species
- Maximum diversity methods such as the Miyawaki method
Stage-5 Degradation
Stage-5 Degradation | Site-Critical | Landscape-Critical |
|---|---|---|
| Vegetation : No tree cover. Poor soil might limit growth of herbaceous weeds Sources of regeneration : Very few or none Soil : Poor soil conditions limit tree establishment | Forest : Usually absent within seed dispersal distances of the site Seed dispersers : Mostly gone Fire risk : Initially low (soil conditions limit plant growth); higher as the vegetation recovers |
Recommended Restoration Strategy:
- Soil improvement by planting green mulches and the addition of compost, fertilisers or soil microorganisms
- followed by planting ‘nurse trees’ — i.e. hardy nitrogen-fixing trees that will further improve
the soil (also known as the “plantations as catalysts” method) - then thinning of nurse trees and their gradual replacement by planting a wide range of
native forest tree species