Monitoring serial changes in coastal grasslands invaded by Chromolaena odorata (L.) R.M. King and Robinson.
Loading...
Date
2000
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
The objective of this study was to describe the impacts of the density of Chromolaena odorata
(chromolaena) on species composition in coastal grasslands and to investigate serial changes in
the vegetation following the implementation of a burning programme. The thesis deals with key
ecological concepts and issues, so a comprehensive literature review is included.
Chromolaena invades coastal grasslands that are not burnt regularly (i.e. biennially). Grasslands
that were not burnt for 30 years were seral to secondary forest. The successional pathway from
open grassland to closed canopy forest varied according to soil type. Coastal grasslands on
Glenrosa soils were characterised by savanna at an intermediate stage between the grassland and
forest states. Shading ended the persistence of savanna species (e.g. Combretum molle,
Dichrostachys cinerea and Heteropyxis natalensis) in forest, whereas forest precursors (e.g.
Canthium inerme, Maytenus undata and Protorhus longifolia) only established where fire was
absent. Chromolaena infestations were characterised by multi-stemmed adult plants of variable
height (i.e. 1-3 m), depending on soil type. Regic sands did not support stratified woody
vegetation and chromolaena infestations were self-supporting, reaching a maximum height of
1.5 m. Glenrosa soils supported tree communities and chromolaena reached more than 3 m in
places. The density of chromolaena affected species composition in grasslands with moderate to
dense stands (> 5 adult plants m ¯² or >50000 shrubs ha ¯¹). Chromolaena stands became monospecific when the number of adult plants exceeded 7 m ¯². Succession to forest also ceased
once chromolaena became thicket-forming.
Fire-induced mortality of the chromolaena depended on grass fuel loads. Grass cover of 30% (c.
1 000 kg ha ¯¹) was required to achieve 80% mortality of the parent infestation after the initial
burn. Dense infestations could only be killed by running head-fires from adjacent grasslands
into thickets. Under conditions where head-fires could not be used, infestations were slashed
and burnt at the height of the dry season (July to August) to achieve an 80% kill rate. Seedlings
were killed (99%) by annual burning in sparse (≤ 10000 shrubs ha ¯¹) to moderate
< 50 000 shrubs ha ¯¹) infestations. The suppression of chromolaena and other alien species,
establishing on bare ground after clearing dense infestations, required chemical control until
grass cover was sufficient (i.e. 1 000 kg ha ¯¹) to effect uniform burning. Certain secondary alien
invaders (e.g. Lantana camara, Psidium guajava and Solanum mauritianum) persisted by
coppicing profusely after fire and herbicides need to be integrated into burning programmes
when these species occur. Grasslands on regic sands (e.g. Ischaemum fasciculatum, Panicum
dregeanum and Themeda triandra) were more resilient to the modifying effects of woody
vegetation, than grasslands on Glenrosa soils. Grasslands on Glenrosa soils did not revert to an
open state but persisted as ruderal savanna grassland (e.g. Eragrostis curvula, Hyparrhenia
tamba and Cymbopogon validus) once fire-resistant tree species (e.g. Combretum molle and
Heteropyxis natalensis) had established.
Depending the objectives for land management and the vegetation's condition, coastal
grasslands can be rehabilitated and managed in multiple states, i.e. grassland, savanna or forest
communities. A state-and-transition model based on the empirical data recorded in the study is
presented and shows chromolaena altering vegetation states from open grassland to chromolaena
dominated thicket. The model illustrates chromolaena thickets as the dominant phase of a moist
coastal forest/savanna succession, irrespective of soil type, in absence of appropriate land
management practices (e.g. control burning and integrated control of alien vegetation). This
model should aid in planning strategies for the control of chromolaena in subtropical grasslands in South Africa.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2000.
Keywords
Invasive plants--KwaZulu-Natal., Chromolaena odorata., Grassland ecology--KwaZulu-Natal., Vegetation dynamics., Vegetation--Sampling., Plant succession--KwaZulu-Natal., Theses--Grassland science.