Trees and erosion

There is a mutual interaction between soils and the trees which root in them. Soils provide physical stability to the trees and enable them to absorb water and nutrients. Trees reduce soil erosion, and the decomposition of their dead parts and litter leads to forest renewal, as will be seen in Chapter 7. Soil erosion is particularly severe in areas with steep slopes, as in the Mediterranean island of Crete, where little of the original cypress and native pine forests now remain. It is also a major problem in New Zealand where it is again largely associated with land clearance, as it is in many other areas with hilly and mountainous countryside. A considerable proportion of New Zealand's pasture land is unsustainable in its present use; even when vegetated the roots of pasture species do not bind the soil strongly enough to prevent loss by surface wash and mass movement from water running over the surface, and fluvial erosion (i.e. by rivers). Erosion in exposed areas of tropical rain forests can be even more severe, sometimes leading to gullies more than 10 m deep along the lines of logging tracks.

Trees do much to reduce erosion; their roots dry out the soil due to the high water demands of the canopy and bind it far more effectively than pasture species. Hubbard Brook data suggest a 41% increase in water running from a catchment after clear felling, due to loss of the evapotranspiration component (Hornbeck et al., 1997). Soil beneath a dense tree cover will usually be drier than one where it is less dense, so the nature of the canopy can be critical where a major object of planting is to reduce erosion. There is little point in planting deciduous poplars if canopy interception of precipitation during the winter is critically important. If root strength is an important factor, a choice of Douglas fir, poplar or manuka Leptospermum scoparium (a 'tree' up to 4 m high native to New Zealand) will give roots twice as strong as those of Monterey pine. Roots can reinforce slopes in three main ways. Situations often occur in which vertical 'sinker' roots pin unstable soil horizons to the underlying substrate, but in some cases the 'failure plane' is deeper than the deepest roots. Even then afforestation can reduce soil flow rates by a factor of 10-30. This is explained by the 'rafting hypothesis': a shallow but semi-rigid raft of laterally connected root-plates (tree roots and associated soil) literally floats on the unstable underlying substrate and prevents it moving. In the third mechanism the soil mantle upslope from the tree is buttressed by the stump and larger roots. This will be effective only if sufficient trees are present and their sinker roots penetrate the stable bedrock (Maclaren, 1996).

Interaction of trees with the water cycle (see Section 3.3.1) also reduces erosion and the incidence of flooding. The canopies of trees and shrubs normally intercept 20-40% of rain which evaporates without reaching the ground and so reduces the volume of water available for runoff and erosion. Tree canopies can also be moderately effective in protecting the soil surface from splash erosion, as they intercept incoming rainfall and absorb much of its kinetic energy before it hits the ground. Similarly, a normally small proportion of the rainfall will reach the ground as stemflow, further reducing splash erosion - see Section 3.3 for more details. However, this softening effect is sometimes exaggerated and the reality is that throughfall drops are larger than raindrops and are potentially more erosive. Fortunately, overriding all this in importance is the surface litter which protects the soil surface from drop impact whatever its origin. A good layer of litter does much to reduce splash erosion.

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