Student Number : 0115350V -
MSc dissertation -
School of Plant, Animal and Environmental Science -
Faculty of Science / The effect of between-tree competition in separate even aged stands of a
Eucalyptus grandis seedling and a single Eucalyptus grandis x Eucalyptus
urophylla hybrid clone was assessed. This was done by comparing treatments
within an experiment, that was designed to test the effect planting density and
genotype had on timber production and stand dynamics through time. This
experiment is located on an escarpment in the Summer rainfall region of South
Africa, approximately 7 km North West of Albert Falls dam in the Kwa-Zulu Natal
Midlands. The terrain is flat with deep apedal soils, and the site has an estimated
mean annual precipitation of 1143 mm. The simplest method of evaluating
competition effects in plantation stands is through the use of spacing trials. Spacing
trials create stands/plots in which all silvicultural and climatic variables are constant.
Differences between treatments can then be attributed to competition alone. The
planting densities tested were 6667 stems.ha-1, 3333 stems.ha-1, 2222 stems.ha-1,
1587 stems.ha-1 and 1111 stems.ha-1 respectively. Competition was measured at
three levels, namely; (i)individual tree, (ii)size-class and (iii)stand level. Vigour and
its distribution within the population were used to explain competition processes.
Basal area was chosen as the size variable for which vigour would be analysed.
Relative Growth Rate (RGR) and Relative Production Rate (RPR) were chosen as
the measures of vigour for analysis. Morphological changes in the trees as a result
of competition were also used to understand the competitive mechanisms. It was
found that resources are not shared amongst individuals in proportion to their size,
but rather the larger individuals use disproportionately more of the resources than
the smaller individuals, and therefore competition is asymmetrical. The dominant
form of competition was found to be resource pre-emption where larger individuals
passively prevent solar resources from reaching smaller competitors. Using RPR as
the measure of vigour, competition was found to be one-sided, meaning that
smaller individuals do not reduce the vigour of larger ones. If RGR is accepted as
the measure of vigour then competition is two-sided meaning that the smaller
individuals do reduce the vigour of the larger ones to a certain extent. There was no
difference in the degree to which the smaller competitors in the clonal and seedling
treatments reduced the growth of the larger individuals. The self-thinning rate in the
seedling treatments was significantly higher than in the clonal treatments. Increases
in mean tree size and production per hectare were accompanied by increases in
mortality in the seedling treatments. In the clonal treatments it was found that even
once the mean tree size approached the self thinning line, very few or no trees
died, however there was also very little or no increase in mean tree size. When the
relationship between number of live trees and production per hectare was analysed
it was found that the clone could increase production per hectare without any
change in survivorship, which is contrary to popular theory. As time progressed the
skewness of the population progressed from negative to positive. This increase was
the most pronounced in the higher planting density treatments, and the clonal
treatments. The size variability in the population first decreased, then steadily
increased. This increase was more pronounced in the high planting density
treatments, and the clonal treatments. Clones and seedlings react in very different
ways to competition. The large differences in population dynamics observed,
indicate that we cannot just apply knowledge built up over the years regarding the
management of Eucalyptus grandis and other plantation species to Eucalyptus
hybrid clones.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/1725 |
| Date | 15 November 2006 |
| Creators | Sale, Giovanni |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 1603050 bytes, application/pdf, application/pdf |
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