Influence of trees on the degradation of forest soils

Soil degradation can be a particular problem with plantation forestry and is often suspected if the growth of a subsequent similar crop is inferior to that of the original crop. The physical, biological and chemical properties of the soil, including its structure, level of organic matter, presence of toxins, and the distribution and availability of nutrients may all be involved. In forestry, gross problems are overcome by using artificial fertilizers and although in small quantities compared with agriculture and mainly involving potassium, they can still be a matter for concern, which is discussed in Section 8.6.2. However, there is evidence that not all plantations are detrimental to soils. In New Zealand, some areas have now had three cycles of Monterey (radiata) pine Pinus radiata. Maclaren (1996) concludes that there has been no decrease in productivity in subsequent rotations of planted forests and, indeed, Woollons (2000) found enhanced growth in the second rotation, though primarily because of better establishment standards and more favourable climatic conditions rather than any soil amelioration. Moreover, a global study by Evans (1999) produced similar conclusions, noting that where lower yields were found it appeared that poor silvicultural practices were responsible.

Certainly, trees can improve soils in some cases. A matter of some interest is the changed availability of nutrients when pasture is afforested. In the case of pine forests it seems that availability of nitrogen, phosphorus and sulphur in the top 10 cm of soil is usually enhanced, perhaps due to the release of enzymes by fungi associated with tree roots. In contrast, surface levels of available calcium, magnesium and potassium may decline, especially in the period before canopy closure. The implication is that there should be an improved growth of pasture species if pine forest is converted back to pasture. This was supported by a trial in which the yield of grass and clover grown on pasture soil was far less than that on soil from beneath adjacent pines. Ovington (1958) found that hardwood and coniferous trees both produce a similar reduction in soil calcium levels. Certain species, including birches, hornbeam Carpinus betulus, sweet chestnut Castanea sativa and larch Larix spp. have reputations as 'soil improvers', and earthworm numbers increase considerably as birch crops age.

Soil acidity increases if land is planted with pines, but this is not necessarily a direct cause for concern as most forest trees are more successful on acid soil. Work done on reclamation of china clay waste, development of vegetation on glacial moraines, marine and fresh water sand dunes, etc. has shown that decline in pH is a consistent phenomenon. The main problem is that of podzolization. For example, in New Zealand, earthworms (which are all introduced) drop in number to a very low level when pastures are densely planted with Monterey pine. This is probably linked to increased acidity and decreased calcium levels. Inevitably this leads to a build-up of mor humus and a locking-up of nutrients. Though improved harvesting methods and use of new machinery have lessened their impact, soil compaction, which commonly occurs when stands are harvested, reduces productivity as does erosion.

In the hot, wet climates of the tropics, intense leaching and weathering change iron, manganese and aluminium minerals to insoluble compounds, often near the surface, while silica and other minerals are carried downwards (Baillie, 1996). In consequence the effects on drainage and soil nutrient status caused by the removal of teak and other tropical forests are often very adverse.

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