Fields with a vegetation height of under 15 cm are avoided, as are harvested and harrowed fields.
Under any of these circumstances, a crop response to under-canopy vegetation control could be expected as the crop and under-canopy vegetation are competing for the same, limited site resources.
To simulate the effects of drought on vegetation structure during a naturally wet year (2005), we removed approximately 30percentt of the under-story vegetation and 10percentt of the canopy cover from a small off-shore cay adjacent to our natural study island.
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.
In 1996/7, two trials were implemented to determine: (i) the effects of under-canopy vegetation on tree growth; and (ii) if there was a competitive influence, at what stage this occurred, in order to establish when vegetation management should recommence.
At 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.
Fig. 2 Basal area (BA; m2 ha−1) removed during two thinning operations at a Dargle (P. patula) and b Kwambonambi (P. tecunumanii), in two under-canopy vegetation control trials implemented in pine stands, South Africa.
Fig. 1 Changes in basal area (BA; m2 ha−1) over time following two thinning operations at a Dargle (P. patula) and b Kwambonambi (P. tecunumanii), in two under-canopy vegetation control trials implemented in pine stands, South Africa.
Assessments of the under-canopy vegetation occurred prior to the two thinning operations.
These differential responses to removal of under-canopy vegetation are possibly dependent on the type of vegetation that occurs as well as the different mechanisms underlying competition between the planted crop and the under-canopy vegetation.
