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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".)
Publication
Hartemink,
A.E. 2003
Soil fertility decline in the tropics with case studies on
plantations. 360 pp. ISRIC-CABI, Wallingford. More info
here
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