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Forestry plantations 



Plantation forests will become increasingly significant in the world’s future timber supply and it is important that they be managed well from production, environmental, social and economic perspectives. Few data are available on the long-term productivity of forest plantations. In many parts of the world there are forest plantations reaching second or even third rotation, and there is evidence of a build-up in numbers of potential diseases and pest species. However, there is no evidence to suggest that yield decline, sometimes called second rotation decline, is widespread. Where growth has been depressed in second or third rotations it is as much due to bad harvesting practices as to any inherent nutrient removals causing exhaustion of the site.


Differences between perennial crops and forest plantations

Both perennial and forest plantations consist of tree crops that remain in the field for many years. Erosion may be problematic in both types of land-use when the trees are immature but no information is available on the long-term effects of soil erosion on productivity. Terracing, which is not uncommon in coffee, rubber and oil palm plantations, is rarely done in forest plantations. An advantage of forest plantations over agricultural plantations is that there are usually no people in such forests, so soil erosion of pathways is absent. Fires may be more common in forest plantations, which is an important indirect cause for soil erosion.


Some forest plantations may be planted on inferior soils compared to soils used for agricultural plantations. This is likely given the pressure on the land in the humid tropics by which the best soils will be used for commercial and food crops, and the poorer soils for growing trees. Moreover, in some areas forest plantations are established on degraded soils for watershed protection or for other purposes in which protection is more important than production. This has implications if changes in soil fertility are to be compared between the two land-use systems. The initial soil condition of forest plantations is different, which will affect the rate of change in soil chemical properties. It is therefore not surprising that in some studies soil chemical properties had improved. But there were also studies in which the soil fertility declined under forest plantations which could be due to a combination of immobilisation in the biomass, increased losses or the effect of removal of the logs at the end of the tree's cycle in the absence of nutrient replenishment.

Management of forest plantations is less subtle than management of agricultural plantations, and one could argue that the difference between forestry and agriculture is as large as the differences between agriculture and horticulture. Intensive harvesting and site preparation including maximum harvest removal of aerial biomass, can result in a serious depletion of soil organic matter and nutrient reserves from which recovery on a short-rotational basis is incomplete.  

Harvesting techniques for the logs affect the soil C and N stocks in forest plantations. The time or frequency of the harvest of the produce is an important difference with agricultural plantations. In agricultural plantations, there is a seasonal drain of nutrients after the crop is mature. This drain can be very high but is partly compensated for by regular inorganic fertiliser applications. In forest plantations, inorganic fertiliser applications may be given at the time of planting but it is uncommon that the mature trees receive inorganic fertilisers. Forest plantations mimic the natural forest in which nutrient cycling is fairly closed. Unless thinning of trees has occurred during the first years after planting, the major drain of nutrients is at harvest. If only the stem would be removed from the field nutrient losses could be restricted, but soil disturbance and complete removal of the above ground biomass is likely to induce considerable nutrient losses. This also occurs in perennial crop plantations when for example oil palm or rubber have come to the end of the economic cycle and are slashed. For the rubber this may imply an additional loss of nutrients when the wood is removed, but oil palm aboveground biomass is usually not removed.

In summary, the dynamics of soil nutrients contents in agricultural plantations is faster than in forest plantations as there is more frequent disturbance through removal, fertiliser applications, pruning etc. These differences in dynamics affect the assessment of declining soil chemical properties in agricultural and forest plantations. 



(This section is based on Chapter 7 from the book "Soil fertility decline in the tropics, with case studies on plantations".)



Hartemink, A.E. 2003  Soil fertility decline in the tropics with case studies on plantations. 360 pp. ISRIC-CABI, Wallingford. PDF of the book here