A comparison of between-tree competition effects in separate stands of a Eucalyptus grandis seedling and a single Eucalyptus grandis x urophylla hybrid clone

[摘要] The effect of between-tree competition in separate even aged stands of aEucalyptus grandis seedling and a single Eucalyptus grandis x Eucalyptusurophylla hybrid clone was assessed. This was done by comparing treatmentswithin an experiment, that was designed to test the effect planting density andgenotype had on timber production and stand dynamics through time. Thisexperiment is located on an escarpment in the Summer rainfall region of SouthAfrica, approximately 7 km North West of Albert Falls dam in the Kwa-Zulu NatalMidlands. The terrain is flat with deep apedal soils, and the site has an estimatedmean annual precipitation of 1143 mm. The simplest method of evaluatingcompetition effects in plantation stands is through the use of spacing trials. Spacingtrials create stands/plots in which all silvicultural and climatic variables are constant.Differences between treatments can then be attributed to competition alone. Theplanting 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 atthree levels, namely; (i)individual tree, (ii)size-class and (iii)stand level. Vigour andits 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 asthe measures of vigour for analysis. Morphological changes in the trees as a resultof competition were also used to understand the competitive mechanisms. It wasfound that resources are not shared amongst individuals in proportion to their size,but rather the larger individuals use disproportionately more of the resources thanthe smaller individuals, and therefore competition is asymmetrical. The dominantform of competition was found to be resource pre-emption where larger individualspassively prevent solar resources from reaching smaller competitors. Using RPR asthe measure of vigour, competition was found to be one-sided, meaning thatsmaller individuals do not reduce the vigour of larger ones. If RGR is accepted asthe measure of vigour then competition is two-sided meaning that the smallerindividuals do reduce the vigour of the larger ones to a certain extent. There was nodifference in the degree to which the smaller competitors in the clonal and seedlingtreatments reduced the growth of the larger individuals. The self-thinning rate in theseedling treatments was significantly higher than in the clonal treatments. Increasesin mean tree size and production per hectare were accompanied by increases inmortality in the seedling treatments. In the clonal treatments it was found that evenonce the mean tree size approached the self thinning line, very few or no treesdied, however there was also very little or no increase in mean tree size. When therelationship between number of live trees and production per hectare was analysedit was found that the clone could increase production per hectare without anychange in survivorship, which is contrary to popular theory. As time progressed theskewness of the population progressed from negative to positive. This increase wasthe most pronounced in the higher planting density treatments, and the clonaltreatments. The size variability in the population first decreased, then steadilyincreased. This increase was more pronounced in the high planting densitytreatments, and the clonal treatments. Clones and seedlings react in very differentways to competition. The large differences in population dynamics observed,indicate that we cannot just apply knowledge built up over the years regarding themanagement of Eucalyptus grandis and other plantation species to Eucalyptushybrid clones.

[发布机构] University of the Witwatersrand