N E W S & V I E W S • S E P T E M B E R 2 0 2 3 23 WEED CONTROL Weeds compete with plantation trees for resources. Competition control becomes critical during times of water shortage. This means spring, autumn and drought periods are times when weeds will have the biggest impact on soil water reserves. Woody weeds should be prioritised on drought prone sites. SPACING AND THINNING Wider spacing of seedlings will produce larger diameter trees but the total timber volume production per hectare will also reduce. However, if stocking is high the diameter of trees is smaller resulting in higher harvesting and extraction costs. A lower stocking will result in less intraspecific competition and therefore less stress on trees. As a rule, on good forestry sites used for pulp-wood plantations about 1500 to 1600 trees per hectare should be standing at rotation age. For average sites this can be reduced to about 1400 to 1500 stems per hectare and on poor sites stocking can be reduced to as low as 1200 trees per hectare. Thinning of plantations can be used as an effective tool for reducing stress in dry times. However, the thinning would need to be implemented timorously. Spacing and Thinning of Wattle: Wattle thinning is primarily done to reduce stand density at various stages in the rotation to reduce competition and remove inferior trees (damaged, suppressed, diseased, forked etc). Thinning can be adjusted to favour either bark or timber production: Lower stocking =bigger trees and betterquality bark. Higher stocking=higher timber tonnage but poorer quality bark. Number, timing, and intensity of thinning can be manipulated to influence the end crop and to reduce stress in the plantation. On marginal sites heavier and earlier thinning can be used to reduce stress. For example, in seedling established plantations a single thinning operation could be implemented when the trees reach four to five meters, and the stand could be reduced to 1200 stems per hectare. SALVAGE OF DROUGHT-DAMAGED COMPARTMENTS Drought damaged compartments generally consist of a mixture of dead trees; trees with dead tops and epicormic shoots on the stem; and healthy trees. The relative proportion of these three categories of trees, the distribution of damaged trees and the age of the trees will determine the method of salvaging the timber. A few general guidelines can be used to assist in making decisions relating to salvage of damaged compartments: • If there are more than 1000 (or 60%) healthy trees per hectare, then the compartment can be grown to rotation age. But you need to assess individual compartments (How is damage distributed, how severe is the damage etc). • If most trees have a diameter that is greater than 100mm then the compartment can be salvaged for pulpwood. Smaller diameter trees are not worth stripping and can be sold for charcoal production, firewood, or small diameter poles. • Most trees with epicormic shoots up the stem will strip to a certain extent (approximately half of the logs). • Dead trees and trees with damaged bark/ difficult to strip, can be felled and stacked for four to six months and the bark chipped off once dry. Logs need to be stacked in such a way to ensure maximum air movement through stacks (small gap between logs). Only stack logs with a diameter lager that 100mm, the cost to strip logs with a smaller diameter exceeds the species income. Southern Africa is a dry region and drought is a regular occurrence. A timber crop will most probably experience an extended dry period at some time in the rotation. It is therefor important to plan for these droughts at time of planting. By selecting the correct species/clone for a site and implementing sound silviculture practices, drought induced mortality can be minimised. Mulched residues improve soil moisture retention
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