A thesis submitted to the Faculty of Science, University
of the Witwatersrand, in fulfilment of the academic
requirements for the degree of Doctor of Philosophy.
May 2016
Johannesburg / This thesis reveals previously unknown facts concerning the invasion, ecology and
management of the perennial alien forb Campuloclinium macrocephalum (Less.) DC.
(Asteraceae, pompom weed) in the grassland biome of South Africa. All these areas of
research are of critical importance to identify the causes of invasion and prescribe best
management practises aimed at reducing the density and spread of the weed and restoring
these ancient grasslands. Grassland biomes throughout the world are receiving international
attention because of their vulnerability to transformation, a history of ignorance regarding
their evolution and prejudice as evidenced by their exploitation.
There are many theories as to why alien plants become invasive outside of their native
range but most are controversial, except those that support the absence of natural enemies as
the primary reason for invasiveness. Few studies have attempted to empirically measure
environmental and ecological factors that facilitate invasion, not to dispute the Absence of
Predators Hypothesis (APH); APH regulates invasiveness but the external factors affecting
habitat vulnerability to invasion remain unchanged. Based on the assessments of 80 invaded
grasslands in Gauteng Province prior to the release of any host-specific biocontrol agents, it
was concluded that C. macrocephalum favours disturbed grasslands. Numerous agencies of
disturbance were identified; the most important being heavy grazing, abandonment (e.g. old
lands) and modification (e.g. draining of wetlands). Herbivory by generalist insects was
insignificant. The main drivers of native species composition in the invaded study sites were
rainfall, topography and soil texture. The weed was most problematic in grasslands with a
basal cover of <19% and in poor condition from a pastoral perspective. Other alien and
native invasive species were also found in grasslands with C. macrocephalum. High fire
frequency also appeared to exacerbate weed density.
The Novel Weapons Hypothesis postulates that some invasive species transform
vegetation for establishment, densification and expansion because they are allelopathic.
Stems and leaves of C. macrocephalum have both rigid multicellular hairs and glandular
trichomes that exude an unidentified substance. It has been speculated that allelopathy may
be an important trait aiding its rapid expansion in invaded grasslands. The importance of
allelopathy and competition was investigated under controlled conditions using Eragrostis
curvula (perennial grass), E. tef, (annual grass) and Lactuca sativa (lettuce) as test species.
Petri-dish studies proved that root and shoot extracts of adult C. macrocephalum plants had
zero inhibitory effect on the seed germination in all three test species. Stunting of radicles
was evident in treatments comprising leaf extracts at 10 and 25% w/v; with E. tef displaying a
higher tolerance than E. curvula. Eragrostis curvula, because it was the most sensitive of the
test species, was used in a pot study together with C. macrocephalum to evaluate allelopathy
and interference. The biomass and growth of E. curvula was not affected by C.
macrocephalum at densities of one or five plants per pot. The weed on the other hand
incurred density-dependant trade-offs in size, biomass and mortality. In a separate pot study,
the incorporation of weed residues into the potting medium had no impact on the growth of
E. curvula. The process went one step further by analysing the association between E.
curvula and C. macrocephalum from the 80 grassland assessments. Eragrostis curvula had a
narrower ecological niche and was only found in disturbed grasslands on well drained soils.
Campuloclinium macrocephalum invaded a broad gradient of soil types including poorly
drained wetland soils not amenable for E. curvula. Competitive exclusion between the two
species was not apparent. Pot studies and field observations support a degree of tolerance
between C. macrocephalum and E. curvula that lends support to coexistence at a range of
weed densities. The conclusions from this study were (a) C. macrocephalum is not
allelopathic and allelo-chemistry cannot be inferred as a causal mechanism for the weed’s
invasiveness, and (b) C. macrocephalum and E. curvula have different limiting resource
requirements that enables coexistence in areas where the latter can grow.
In 2006 C. macrocephalum infestations in Gauteng Province were severely damaged by
Puccinia eupatorii Dietel (Pucciniaceae), a biotrophic rust identical to the P. eupatorii strain
in quarantine that was imported into South Africa from Argentina as a potential biocontrol
agent. An investigation of the rust’s impact on C. macrocephalum was carried out in the 80
rangeland infestations previously assessed and how pathogen pressure affected the weed’s
realised niche. The rust caused premature senescence of the stems in late summer with
compensatory regrowth in autumn. In contrast disease-free plants senesced in late autumn;
the rootstocks remained dormant throughout winter and did not produce compensatory
regrowth. No significant changes in weed density were detected and C. macrocephalum
retained its realised niche. The weed remains adapted to the rust despite a 40-year separation
from the pathogen.
Registered herbicides should provide 80% control of the targeted plants, providing label
directions concerning concentration, mixing, application and environmental considerations
are adhered to. Herbicide trials were conducted at two contrasting sites in Gauteng, a
wetland and a rocky grassland, to test the efficacy of picloram and metsulfuron-methyl on the
control of C. macrocephalum. Both sites had dense infestations of C. macrocephalum at the
beginning of the study in 2005. Herbicides were applied in either February (summer) or
April (autumn) annually for three consecutive years. Monitoring continued for an additional
three years after spraying was terminated. Puccinia eupatorii established at both study sites
from the second year of study. Both sites also experienced wildfires and drought. As these
uncontrolled factors were not anticipated in the design their individual effects could not be
factored out. Mean mortality however was <80% expected of registered herbicides which
can only be attributed to uncontrolled factors acting as natural constraints. The efficacy of
picloram was not significantly different (P<0.05) to that of metsulfuron methyl within a
season. Autumn applications are not recommended because rust damage on the leaves is too
advanced to ensure adequate herbicide uptake and translocation. Herbicide persistence in the
topsoil was not detected by gas chromatography. Three applications of herbicide were
inadequate to bring C. macrocephalum under control in plots. It is estimated that five to
seven years of herbicide application are needed to reduce the weed density to <1 plant per
plot (25 m2). This effectively renders chemical control in medium to dense infestations
uneconomic. Chemical control of C. macrocephalum will only be effective if there is a
commitment to follow-ups and remedial vegetation management practices.
The herbicide trials also looked at the impact of picloram, metsulfuron methyl and hoeing
on native species and other alien species that occurred in plots. Three plant functional groups
were analysed, namely native grasses, native forbs and alien broadleaf species. The
herbicides were applied as broadcast sprays over three years and therefore their effect on all
broadleaf species was non-selective. Hoeing twice a year for three years only targeted C.
macrocephalum. High forb richness coupled with low abundance and patchy distribution
meant there was a high turnover between replicate plots across the study sites. Herbicides had
a significant impact on the native forb functional group compared to hoeing which had a
superficial effect. Hoeing did not suppress C. macrocephalum. The parasitic forb Thesium
utile was killed in all treatments except the untreated control. The target-specific hoe
treatment also eliminated T. utile, suggesting a new association might exist between parasite
and C. macrocephalum. Hoeing also facilitated the establishment of alien annual weeds.
In South Africa the grassland biome is under considerable threat from mining,
afforestation, agriculture and urban development. Although alien plants pose a minor threat
in contrast to these land-transforming activities they remain the greatest threat to grasslands
that are not threatened by exploitation. Campuloclinium macrocephalum is not a pioneer
species, but rather a long-lived perennial herb with evolutionary adaptations to fire, herbivory
and disease. These characteristics have enabled it to establish and spread in grasslands that
have been maintained in a variety of disturbed states by a range of agencies that reduce grass
basal cover and exacerbate soil erosion. Addressing poor land use practises that foster C.
macrocephalum invasion and spread are as important as the remedial activities necessary to
control the species. Grassland restoration/rehabilitation practises have not been investigated
in the context of C. macrocephalum management and is of critical importance to the
integrated control of the weed.
Although comprehensive literature about C. macrocephalum have recently been published
gaps in our understanding of its biology, ecology and control still exist that prevent the
development of best management practices. These areas for new research make ideal
projects for post-graduate students. Future research should focus on (a) rust–herbicide
interactions, (b) fire and seedbank dynamics, (c) integrated weed management incorporating
biological control, fire, selective herbicide application techniques (e.g. spot-spraying) and
ecosystem restoration practises. Grazing strategies promoting grass species adapted to
frequent non-selective defoliation restrict pompom weed better than degraded underutilised
rangelands, as evidenced by low levels of C. macrocephalum in communal areas, and
warrants further investigation, including fence-line contrasts in invaded and un-invaded road
reserves.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/21031 |
| Date | January 2016 |
| Creators | Goodall, Jeremy Marshall |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource [142 pages], application/pdf |
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