Determinants of community composition and diversity in KwaZulu-Natal mesic grasslands : evidence from long-term field experiments and pot and plot competition experiments.
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Date
2003
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Abstract
A predictive understanding of plant community response to various environmental
influences (e.g. type, timing and frequency of disturbance, site productivity, fertilization, etc.)
is a general goal of plant ecology. This study sought to further understanding of mesic
grassland dynamics in KwaZulu-Natal using long-term field experiments (> 50 years) and
short-term pot and plot competition experiments. The specific objectives were to: 1) examine
the effects of long-term burning of grassland on soil organic matter content because of its
potential impact on nitrogen cycling and community composition, 2) examine patterns of
community composition and species richness in response to different type, timing and
frequency of disturbance (burning and mowing) in a long-term grassland burning and
mowing experiment and to different type and amounts of fertilizer application in a long-term
grassland fertilization experiment, 3) develop hypotheses concerning the response of different
species to disturbance and fertilization, and test these hypotheses using pot and plot
competition experiments, and 4) provide a general synthesis of the results of the various field,
pot and plot experiments that may be used to develop a predictive theoretical framework for mesic grassland dynamics.
Total soil nitrogen was lowest in sites burnt annually, intermediate in sites burnt
triennially and highest in sites protected from disturbance and sites mown annually in the
dormant-period (spring or winter). Winter burning reduced soil organic carbon and total soil
nitrogen more than spring burning. Mineralizable nitrogen was reduced by burning.
The different effects of type, timing and frequency of disturbance on total soil
nitrogen appeared to be an important determinant of community composition and species
richness. Short-grass species (Themeda triandra, Eragrostis capensis, Heteropogon
contortus, Diheteropogon amplectens and Eragrostis racemosa) were most abundant in
annually burnt sites (especially winter burnt sites), whereas medium and tall-grass species
(Eragrostis curvula, Cymbopogon spp., Hyparrhenia hirta and Aristida junciformis) were
most abundant in triennially burnt sites, sites protected from disturbance and sites mown
annually in the dormant-period, all of which had higher total soil nitrogen than annually or
biennially burnt sites. Species richness and short-grass species declined with increasing levels
of nitrogen fertilization in the fertilizer experiment and declined with increasing productivity
and nitrogen availability in both the fertilizer and burning and mowing experiments. Thus, it
was hypothesized that the type, timing and frequency of disturbance resulted in different compositional states through different effects on soil resources (especially nitrogen), which
affected the competitive balance between short and tall species.
The hypothesis that composition was determined by disturbance-mediated soil
nitrogen availability was supported by competition experiments, which revealed that shortgrass
species were most competitive in low-nutrient/low-productivity treatments and tall-grass
species most competitive in high-nutrient/high-productivity treatments. The fertilizer
experiment and a competition experiment revealed that tall broad-leaved species were most
competitive in sites of highest productivity, fertilized with both nitrogen and phosphorus,
whereas tall narrow-leaved species were most competitive in sites of intermediate productivity, fertilized with nitrogen only.
It was hypothesized that summer mowing increased the abundance of short-grass
species and decreased the abundance of tall-grass species in the burning and mowing
experiment by increasing the competitive ability of short-grass relative to tall-grass species,
rather than the tall-grass species being less tolerant of mowing. A competition experiment
revealed that tall-grass species (Hyparrhenia hirta and Panicum maximum) were as tolerant
of cutting as a short-grass species (Themeda triandra). Themeda triandra was shown to
become extremely competitive in cutting treatments, reducing the biomass of most other
species relative to their monoculture biomass, showing that its dominance of mown sites in
the burning and mowing experiment was a result of its superior competitive ability rather than greater tolerance of mowing.
However, many tall erect herbaceous dicots appeared to be intolerant of summer
mowing, probably because their meristems are aerial and easily removed by mowing,
whereas short creeping herbaceous dicots were increased by summer mowing probably
because their meristems were below the mowing height. Further, these short species would be
vulnerable to shading in unmown sites. Thus, species with basal meristems
(hemicryptophytes) or meristems near the soil surface (geophytes) appear to be more tolerant
of mowing than species with aerial meristems (phanerophytes), but the tradeoff is that a low
meristem height renders them vulnerable to shading in unmown sites.
Very high litter accumulation in the sites protected from disturbance appeared to have
a direct influence on community composition and species richness. Species that dominated
these sites (e.g. Tristachya leucothrix & Aristida junciformis) initiated tillers below-ground
and had sharp erect shoots that appeared to be an adaptation for penetrating litter. Species that
initiate tillers below-ground are probably less vulnerable to the effects of shading by litter
because their tiller initiation is not dependent on high light availability. The fact that Aristida junciformis was shown to have very low competitive ability in two competition experiments,
suggests its dominance of protected sites was through tolerance of high litter levels rather
than competitive exclusion of other species. Low grass species richness in these sites was
probably a result of an inability of many species to tolerate these high litter levels.
This study has revealed that inherent site productivity and its interaction with the
effect of disturbance on soil resources and litter levels is a major determinant of community
composition and species richness. The effect of type, timing and frequency of disturbance on
soil nitrogen was able to account for the principal changes in community composition. Thus,
the influence of disturbance on soil nitrogen is a unifying principle in plant ecological theory
that enables greater understanding of disturbance-composition relationships. However,
intolerance of certain forms of disturbance (e.g. mowing) by species with aerial meristems, or
intolerance of accumulating litter in the absence of disturbance by species without sharp erect
shoots, may also have important influences on composition. In addition, this study has
revealed that plant traits (height, leaf width, position of tiller initiation, shoot morphology and
position of meristems) were well correlated with the various effects of disturbance and
fertilization on community composition, indicating that a plant trait-productivity-disturbance
framework has great potential for understanding and predicting species response to disturbance and multiple limiting nutrients.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2003.
Keywords
Grasslands--KwaZulu-Natal., Fire ecology., Prescribed burning., Soils and nutrition., Theses--Grassland science.