Seedling establishment of Themeda triandra Forssk. in the montane grasslands of Natal.
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Date
1994
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Abstract
Soil erosion and vegetation destruction have reached serious proportions in disturbed
areas of the montane grasslands of the Natal Drakensberg. Little is known of the
inherent ability of indigenous grasses to produce viable seed and establish seedlings for
rehabilitation programmes. This study examined the effects of fire and small-scale
disturbance on the seedling dynamics of the dominant grass, Themeda triandra Forssk.
Annual seed production of T. triandra increased with length of burning rotation, ranging
from 21 seeds m⁻² in the annual winter burn to 485 seeds m⁻² in the five-year burn. High
predation of seeds (70-98%) and low viability (37% in 15 month-old seeds) contributed
to the poor representation of T. triandra in the seed bank when compared to the aboveground
vegetation.
Seedling densities and survival were monitored by mapping quadrats at six-weekly
intervals from November 1986 to September 1989. Highest establishment of T. triandra
was recorded in the biennial spring burn treatment (< 184 seedlings m⁻²). Microclimatic
studies of the seedling environment demonstrated that increases in photosynthetically
active radiation following burning, and high soil moisture associated with accumulation
of litter, favour seedling establishment.
The main constraint to seedling establishment was high mortality in winter which
resulted in a 1% survival of seedlings of T. triandra during the three-year study. This
indicates that in the fire-climax grasslands of the study, seedlings contribute little to the
population structure. The most significant factor contributing to low survivorship is
intraspecific competition between seedlings. Spacing of seedlings in rehabilitation
programmes is therefore a critical factor in optimizing seedling establishment. Analysis of the fate of seeds of T. triandra between their production and establishment,
showed that the main constraints for seedling establishment were predation, low viability
and poor germination. A population model, based on transitions between seeds,
seedlings and mature tufts, predicts that a biennial spring burn is the optimal treatment
for maximising both seedling establishment and vegetative growth.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1994.
Keywords
Themeda triandra--Seed., Grassland ecology--Drakensberg Mountains., Theses--Botany.