The role of fire and mechanical clearing in the management of Chromolaena odorata.
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Date
2006
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Abstract
The effects of fire and mechanical clearing were investigated for their potential in assisting with
the eradication of Chromolaena odorata (previously Eupatorium odoratum). The study was
divided into two focus areas, the first focused on mechanical clearing of dense stands of C.
odorata on three sites and the second focused on the long term influences of a single burn on C.
odorata plants in the different size categories. For mechanical clearing, two key issues were
investigated; namely whether this type of clearing procedure was effective in dense C. odorata
stands and whether rehabilitation was necessary in these cleared areas. The study was conducted
from July 2002 to June 2004. The area was subject to a severe drought throughout the duration
of the study. The severe drought had a large influence on the result in both focus areas. A
bulldozer was found to be a very effective at clearing dense C. odorata stands. Results from the
mechanical clearing study showed that there was still a large viable grass seed population in the
areas that had been covered by a dense stand of C. odorata plants for over three years. Thus,
indigenous plants were able to re-colonize the area after removal of C. odorata without human
intervention, even thought the area was experiencing a severe drought. The density of C. odorata
seedlings emerging in the cleared areas was far lower than expected. The C. odorata density in
the permanent plots, for seedlings that germinated in the first season after clearing (SeptemberOctober
2002), was only 0.25,0.03 and 0.72 per 5 m2 in the three sites respectively by the end of
the study in June 2004. For the C. odorata seedlings that germinated in the second season
(September-October 2003) the density was, 0.5, 0.56 and 1.06 per 5 m2 in the three sites
respectively by the end of the study in June 2004. It was suspected that the drought influenced
seed germination. Unfortunately the number of C. odorata seedlings was so low, that no
significant relationship could be found between grass and C. odorata seedling density. By the
end of the study the grass fuel mass in all the rehabilitated sites was already over 3000 kg ha-1,
even though the area was experiencing a severe drought. This grass fuel load, when burnt, will
assist land managers in controlling C. odorata plants, especially seedlings. Very few other alien
invasive plant species emerged in the cleared areas. At the Mhlosinga site, Senna pendula made
up less than one percent of the herbaceous species composition and only a single Ricinus
communis plant was recorded. No alien plant species were recorded on the other two sites.
Results from the burning trials revealed that plants in all the size categories were affected by fire.
Greater fuel masses and fire intensities were required to kill larger C. odorata plants relative to
smaller ones. Fire was found to be very effective at eliminating small and medium size C.
odorata plants. Fire applied as a once off treatment had a significant long-term effect on the C.
odorata population. The following fuel loads were required to achieve 80% mortality in this
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study: for small plants a fuel load of over 4000 kg ha-I, for medium plants a fuel load over 4200
kg ha-I and for large plants a fuel load over 4600 kg ha-I. Little difference could be detected
between a head or a back burn, as both fire types had their own advantages and disadvantages.
Although some of the C. odorata plants in the burnt plots had not perished by the time of the
first investigation, following the burn (February 2003), by the time of the second investigation
(June 2004), many of these plants had eventually succumbed. These results highlighted the fact
that plants which are damaged by fire were more likely to persish during an extended droughts
period, than plants which were not subjected to fire. Results from the control plots, in the
burning trials, for medium and large plants showed dramatic increases in density over time.
Tagged individuals from the control plots did reveal that some of the medium and large plants
did die during the drought, although the amount was negligible when compared to the number of
new plants growing into the new size categories. A large proportion of the small plants in the
control plots also survived the drought with many of them even growing into the medium
category. The difference between the control plots and the burnt plots was obvious and
significant, especially once the fuel mass exceeded 3783 kg ha-I. Results from this study show
that fire can be used as a very effective tool in assisting land managers to control C. odorata in
open savanna bushveld.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2006.
Keywords
Chromolaena odorata--KwaZulu-Natal--Munywana Game Reserve., Fire ecology., Chromolaena odorata--KwaZulu-Natal--Control., Grassland ecology--KwaZulu-Natal., Vegetation dynamics., Savanna ecology--KwaZulu-Natal., Grasslands--Management., Restoration ecology., Theses--Grassland science.