5.1 Genetic improvement
5.2 Role of different silvicultures
5.3 Fertilising
5.4 Site preparation establishment practices
5.5 Organic matter conservation
5.6 Holistic management
The steady transition from exploitation and management of natural forest to increasing dependence on plantation forestry is following the path of agriculture. Many of the same biological means to enhance yield are available. They are outlined here with some covered in greater detail in Working Paper FP/3.
The forester only has one opportunity per rotation to genetically change his crop. These changes include:
Silvicultural knowledge continues to increase and while large yield improvements appear unlikely, incremental gains can be expected. Important examples are:
1. Stocking level manipulation to achieve greater output by fuller site occupancy, less mortality, and greater control of individual tree growth.2. Matching rotation length to optimise yield may offer worthwhile gains.
3. In some localities prolonging the life of stands subject to windthrow by silvicultural means will increase yield over time.
4. Use of mixed crops may help in tree stability, may possibly lower pest and disease threats, but are unlikely to offer a yield gain over growing the most productive tree the site can support (FAO 1992).
5. Silvicultural systems that maintain forest cover at all times - continuous cover forestry practices - such as shelterwood and selection systems are likely to be neutral to slightly negative in production terms, while yielding gains in tree quality, aesthetics, and probably biodiversity value.
6. Crop rotation, as practised in farming, appears unlikely as a feature in plantation forestry. There are examples of plantations benefiting from a previous crop of nitrogen fixing trees such as Acacia mearnsii.
Regular application of mineral fertiliser is not presently a feature of plantation forestry. Most forest use of fertiliser is to correct known deficiencies e.g. micronutrients such as boron in much of tropics, and macronutrients such as phosphorus on impoverished sites in both the tropics and temperate zones.
Fertiliser application is likely to be the principal means of compensating for nutrient losses on those sites where plantation forestry practice does cause net nutrient export to detriment of plant growth.
Ground preparation to establish the first plantation crop will normally introduce sufficient site modification for good tree growth in the long term. Cultivation inter alia loosens soil, improves rooting, encourages drainage, limits initial weed growth, improves water percolation, may reduce frost risk and, perhaps importantly for the long-term health of the forest, brings relatively unweathered soil minerals nearer to the surface and into the main feeding zone of tree roots. Thus substantial site manipulation is unlikely for second and subsequent rotations, unless there was failure first time round, except for alleviation of soil compaction after harvesting, and measures to reduce infections and pest problems.
Weed control strategies may change from one rotation to the next owing to differing weed spectrum and whether weeds are more or less competitive to planted trees. The issue is crucial to sustainability since all the main examples of yield decline reflect worsening weed environments, especially competition from grasses and bamboos. Deliberate over-sowing of cover crops to improve fertility, reduce erosion and control weeds is employed in agricultural plantation trees such as rubber. Similar strategies are sometimes used in P. radiata plantations.
Changes between rotations in treatment of felling debris and organic matter may occur such as cessation of burning, windrowing, or removal from site in whole-tree harvesting. What is clear is that the felling, harvesting and re-establishment phase is crucial to sustainable practice and needs to be viewed as a whole. The aim should be to minimise negative impacts due to compaction, loss of organic matter, soil erosion and nutrients.
It is clear from many investigations that treatment of organic matter and care for topsoil both over the rotation and during felling and replanting is as critical to sustainability as is coping with the weed environment. While avoidance of whole tree harvesting is probably desirable on nutrition grounds, it is now evident that both prevention of systematic litter raking or gathering during the rotation and conserving organic matter at harvesting, are essential.
If all the above silvicultural features are brought together a rising trend in productivity can be expected. But if any one is neglected it is likely that the whole will suffer disproportionately. For example, operations should not exclusively minimise harvesting costs, but examine collectively harvesting, re-establishment and initial weeding as an holistic activity, so that future yield is not sacrificed for short term savings. Evidence of a rising trend reflecting the interplay of these gains is reported in Nambiar (1996) and Evans (1999b).
Holistic management also embraces active monitoring of pest and disease levels, and researching pest and disease biology and impacts will aid appropriate responses. Careful re-use of extraction routes to minimise compaction and erosion is a further example.
