4. OTHER FACTORS POTENTIALLY INFLUENCING TIMBER PRODUCTIVITY
4.4 Growth of Pines at North East Cape Forests and their Timber Properties
4.5.3 Weed Management
Vegetation management is essential during establishment of young pine plantations as competition for water, light and nutrients severely limits tree growth (Nambiar and Zed, 1980; Richardson, 1993; Smethurst
et
al., 1993). Large growth benefits can thus be achieved through the timely andlor selective removal of competing vegetation in newly established plantations (Richardsonet
al., 1996;Wagner
et
al., 1996). Since vegetation management costs may be amongst the highest associated with silvicultural operations, improvement in methods of vegetation control is an important priority for increasing overall forest productivity (Jarval, 1998).Typically, vegetation management costs are highest during the 'establishment period', viz. the time between planting and canopy closure or site capture (the time at which the planted trees dominate on the site). After canopy closure the effects of shading tend to exclude all further competing vegetation and the costs of vegetation management are reduced. The rate of canopy closure in pine trees is slower than eucalypts so there is a longer period between planting and site capture. Therefore, vegetation management costs are incurred over years, as the weed communities need to be controlled over a longer period. Final yield benefits from early silvicultural inputs such as vegetation management are thus difficult to quantify in pine plantations. Managers wanting to manipulate vegetation to increase wood production must therefore choose an appropriate degree of control, which takes into account the ultimate permissible yield loss due to competition from vegetation as well as the costs involved (Rolando and Little, 2000).
Vegetation management includes the control of any plant species that detrimentally affects plantation yield; this includes both exotic and indigenous vegetation. In any control programme individual species are seldom targeted for control, more often groups of plants are controlled. This is done as certain groups of plants may be thought to be more competitive or because the herbicides used may selectively control certain plants (Little, 1998). Vegetation control, through the use of selective herbicides, allows for the development of the following broad categories: herbaceous broadleaves, grasses, woody vegetation (perennial broadIeaves) and, in some instances, ferns. Grouping of plants into vegetation types allows for the testing of the hypothesis that due to the inherent growth morphology and competitive strategies of different vegetation types, their
competition with pines is spatially and temporally different. If these spatial and temporal zones can be determined according to the category of vegetation, then pine vegetation management programmes could be designed to exclude different vegetation types when they are most competitive (Rolando and Little, 2000).
Studies carried out in the USA, New Zealand and Australia indicate that control of herbaceous and woody plants alter the competition balance of early successional vegetation to favour pine survival and growth (Haywood and Tiarks, 1990;
Richardson, 1993; Zutter and Miller, 1998). Many of these studies have indicated that during the first three to five years after establishment, or until the trees are dominant, most competition is from grasses and broadleaf herbaceous plants, which, due to their competitive life cycle invade the plantation soon after harvesting (Tiarks and Haywood, 1986; Miller et al., 1991; Wagner et al., 1996). If unchecked, competition from this group of weeds is normally not sustained for more than two - three years, as the trees have access to deeper soil water, which is below the level of shallow rooted species (Sands and Nambiar, 1984). In addition, after three years the tree crowns are normally above the herbaceous weeds and the trees are able to compete more effectively for growing space. In a study to determine the effects of herbaceous versus woody plant control on P.
taeda establishment, Tiarks and Haywood (1986) reported grasses to be the most competitive in young P. taeda plantations, and suggested control until canopy closure. They found hardwood species to be long term competitors with pine trees, only needing to be controlled at a later stage (after canopy closure) (Rolando and Little, 2000).
Although much research has been carried out on the effects of different methods of vegetation control on pine wood production, little research has been conducted in South Africa. The noTable exceptions include the work carried out by Morris (1994), Christie (1995) and Zwolinski (1995). A structured trial base was planned and implemented by the Institute for Commercial Forestry Research during the mid-1990's to address the lack of information related to pine-vegetation interactions, particularly for those species grown in the Eastern Cape, KwaZulu- Natal and Mpumulanga (Little, 1998). The specific objectives of these establishment trials were to:
1. Determine the effect of selective control on vegetation abundance;
2. Determine which vegetation type is the most competitive during pine seedling establ ishment;
3. Determine if the degree of control required is related to site conditions, and 4. Compare tree growth on plots managed according to company practice with
weed-free control.
The results from this set of field trials have shown:
1. The importance of vegetation management at establishment on subsequent tree productivity and uniformity;
2. The diversity of sites and associated vegetation types used for pine tree growth, facilitating the development of various degrees of competition on different sites;
3. The direct and indirect effects of selective vegetation control on vegetation abundance, and
4. That growth response to vegetation management is affected by site and planted pine species.
The important outcome of this study was that specific mechanisms of competition will vary as a function of the climatic and soil conditions of the site and that the impact of different types of competitors on tree growth will change as the seedlings mature and compete in different spatial zones. Results also indicated that although grasses and herbaceous broadleaves were initially vigorous competitors, woody plants were the most detrimental to early pine growth. Tree growth was greatest where herbaceous and woody vegetation was controlled, that is on the weed free plots. Here there was no competition for light, water or nutrients, and tree growth occurred at a maximum rate. This is the ideal situation (Rolando and Little, 2000).