Abstract
BackgroundIn pine plantations, shading following canopy closure reduces the growth of competing vegetation. However, canopy closure is not always achieved if low initial planting density, pruning and thinning are practised. This means that complete shading does not occur, resulting in stands with potentially competitive levels of under-canopy vegetation.MethodsAt the time of the first pruning operation, two trials were established to determine the competitive effect of under-canopy vegetation on pine tree growth in South Africa, one on a Pinus patula Schiede ex Schltdl. & Cham. (4.5 years) and one on a Pinus tecunumanii F.Schwerdtf. ex Eguiluz & J.P.Perry (2.9 years) stand. This paper documents results at the final thinning operation carried out at 17–18 years. Treatments were weedy (no vegetation control), herbaceous (complete sustained control of woody vegetation), woody (complete sustained control of herbaceous vegetation), weedfree (complete sustained control of all competing vegetation) and operational (only the woody perennial broadleaves were removed prior to any pruning/thinning event in the commercial weed control treatment). From the time vegetation management treatments were first imposed, they were maintained and tree performance monitored over 14–15 successive growing seasons, including and up to the second and final thinning to 250 stems ha−1.ResultsThe presence of vegetation for up to 15 years had no significant impact on the volume of P. patula and P. tecunumanii following two thinning operations (813 to 400 stems ha−1; 400 to 250 stems ha−1). For P. patula, but not P. tecunumanii, there was a significant and negative impact of under-canopy vegetation on the total basal area removed during two thinning operations. This may result in a reduction in the selection of stems to remain following thinning, as well as any profit that may be obtained if the thinnings were of commercial value.ConclusionsFor the species (P. patula and P. tecunumanii) and sites tested, the results indicate that if good establishment weed control is carried out (from planting until the first pruning event), the development and subsequent impact of under-canopy vegetation over the subsequent 14–15 years (between the first pruning event and the final thinning to 250 stems ha−1) on the growth of the remaining stems is negligible.
Highlights
In pine plantations, shading following canopy closure reduces the growth of competing vegetation
The pines planted were matched to the sites (Morris and Pallett 2000), with P. patula planted at Dargle plantation in the Midlands, and P. tecunumanii at Kwambonambi plantation in Zululand (Table 1)
As the treatments in these trials had limited influence on tree growth, recommendations based on results obtained at the first thinning remained valid through to the second thinning
Summary
In pine plantations, shading following canopy closure reduces the growth of competing vegetation. Canopy closure is not always achieved if low initial planting density, pruning and thinning are practised This means that complete shading does not occur, resulting in stands with potentially competitive levels of undercanopy vegetation. Many studies have shown that substantial gains in wood volume can be achieved in response to effective, early control of competitive vegetation (Wagner et al, 2006) For this reason, vegetation management during the establishment of planted forests is considered one of the most important silvicultural practices and is routinely applied to optimise resource availability (Richardson 1993; Miller et al, 2003; Rolando and Little, 2009; McCarthy et al, 2011). Fewer studies have examined the impacts of continuous vegetation control on crop tree growth, or the impact of under-canopy vegetation on final yield, despite undercanopy vegetation occurring in many planted forests
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