Savanna Plots Research Articles

Competition between Perennial Grasses in a Neotropical Savanna: The Effects of Fire and of Hydric-Nutritional Stress

1 Pulse perturbation experiments, in which a percentage of above-ground biomass of a given species in a plot was removed on a single occasion, were performed in plots of a Trachypogon savanna in Northern Venezuela. These showed interspecific competition between the dominant C4 perennial grasses Trachypogon vestitus and Axonopus canescens in protected (i.e. unburned and ungrazed) savanna plots, regardless of the stress status. 2 Press perturbation experiments, in which a given species in a plot was continuously removed, showed that A. canescens had a higher maximum yield (defined as the density of green biomass in the permanent absence of its potential competitor), and a more deleterious influence on T. vestitus only in the protected plots. This suggests that the interaction between the two grasses is asymmetric, in agreement with the findings of the pulse experiments. 3 Press perturbation experiments revealed that the demographic importance (measured as the relative growth rate in green biomass) of interspecific interactions appears to be low, regardless of fire and stress degree. This is consistent with the low fertility of most Trachypogon savanna soils, and together with the asymmetric interaction between the two species may contribute to explain the slow replacement of T. vestitus by A. canescens in a long-term protected plot of a Trachypogon savanna in Central Venezuela. 4 Fire and stress status interacted to modify the occurrence and symmetry of the competitive interactions between the dominant grasses in Trachypogon savannas. The structure of the herbaceous community resulted from the complex interplay between biotic interactions, fire and stress status.

Ordination as an aid in determining priorities for plant community protection

CANOCO and DECORANA ordination programs were used to examine variation among vegetative and environmental variables sampled at 48 pine/graminoid savanna plots in the coastal plain of North and South Carolina, USA. Five savanna communities were delimited to encompass groupings of similar floristic composition and environmental conditions. Relative positions of ordination coordinates for protected and unprotected savannas indicated that acquisition and restoration of additional sites is needed to protect representative savanna communities throughout the study area. Such use of ordination techniques could serve to improve objectivity in determining priorities for protection of plant communities.

Factors that Control Monsoon-Rainforest Seedling Establishment and Growth in North Australian Eucalyptus Savanna

1. The seasonally deciduous tree species Bombax ceiba L. and Sterculia quadrifida R. Br. formed 9.1% and 2.3%, respectively, of the total basal area in a coastal dry monsoon rainforest in the Australian seasonally dry tropics. Neither species occurred in a nearby Eucalyptus savanna. 2. Both communities occurred on deep, iron-rich, sandy loam soils. The savanna had lower concentrations of nutrients in the surface soil than the rainforest but experienced greater maximum ground surface air temperatures than the monsoon rainforest when averaged over 1 year. Minimum air temperatures were similar. Annual mean surface soil moisture content was similar in savanna and rainforest, but savanna soils were drier in the dry season. During this time both communities had soil matric pressure potentials below 1.3 MPa. 3. S. quadrifida established more successfully in the rainforest than in the savanna but B. ceiba establishment was <2% in both communities. Nursery experiments also showed that B. ceiba had a consistently lower rate of establishment than S. quadrifida, and that seed establishment of both species was greatest on unsterilized rainforest and savanna soil. Adding 40% gravel to savanna soil also resulted in improved establishment. Eucalyptus litter, light, and rainforest or savanna soil were found to have no significant effect on seed establishment of either species. 4. After 12 months 70% of B. ceiba and 40% of S. quadrifida seedlings survived following transplantation to the rainforest. No transplants survived in the savanna, but transplants of both species survived in an irrigated savanna. Grass fires were not absolutely lethal to both species on an irrigated savanna plot. Soil sterilization inhibited the growth of both species on rainforest soils in pots, but adding fertilizer eliminated the growth inhibition caused by sterilization except when B. ceiba was grown on rainforest soil. Eucalyptus litter, light and different proportions of gravel in the soil had no effect on seedling growth rates of either species. 5. It is concluded that the long-term development of a monsoon-rainforest-savanna ecotone is critical for creating microclimatic and soil conditions favourable for monsoon-rainforest seedling establishment and growth. Experiments point toward the importance of mycorrhizas, soil fertility and soil moisture in controlling monsoon-rainforest seedling establishment. This study helps explain why so few monsoon-rainforest seedlings have been observed to establish in fire-protected Eucalyptus savannas.

The effects of tree canopy cover on soil fertility in a Nigerian savanna

ABSTRACTThe effect of tree canopy cover on soil properties was studied over three periods: middry season (January), mid-growth period (August) and peak-growth period (October) in three 1 ha plots in savanna of north-west Nigeria. The objective was to find out whether tree canopies change the nutrient status of the soil under them relative to adjacent grasslands. Soils under tree canopies were found to have significantly higher levels of organic matter, calcium, magnesium, potassium, total exchangeable bases, cation exchange capacity and pH than those in open grasslands. Nitrogen and phosphorus were slightly higher in soils under tree canopies than those in the open grasslands. Trees 7 m and above had more influence on soil properties than smaller trees. Differences in soil properties among the study plots were due to differences in their soil texture. Seasonal trends were observed in organic matter, carbon: nitrogen ratio, cation exchange capacity, phosphorus, calcium, sodium, total exchangeable cations and percentage base saturation.

Nigerian Man and Biosphere research plots in Guinea savanna: floristics and structure of the vegetation

SummaryA study was made of the vegetation of three plots used in the Nigerian Man and Biosphere Research Programme for Savanna Studies. The aim is to provide detailed information on the vegetation of the plots which may possibly contribute to an understanding of factors influencing savanna structure and relative abundances of forbs, grasses and woody species. Results show that there are differences in the species composition of the plots. There are more forb species than grass species in each plot. The woody basal areas and crown areas of the plots do not depend entirely on the density but also on the size of the woody species. There are differences in herbage yield in the plots that could be attributed to differences in soil properties, species composition and level of human and animal activities between the plots.

Changes in Tree Density and Species Composition in a Protected Trachypogon Savanna, Venezuela

Changes were recorded in population and species density of a 3—ha permanent Trachypogon savanna plot, protected against fire and cattle grazing for 16 yr (1961—1977). This savanna is a grassland (mainly Trachypogon species), with scattered trees and groves. Under protection it is gradually changing into a denser arboreal community; trees in the grassland and in the groves increased 14.6— and 3.7—fold, respectively. In places, ironstone outcrops limit woody plant invasion and prevent the whole area from developing into a savanna woodland. Experimental explosions (using dynamite) in the ironstone outcrop led to the growth of trees in soil pockets formed in the artificial fissures.

A Re-Assessment of a Fire Protection Experiment in North-Eastern Ghana Savanna

(1) Three plots in Northern Guinea savanna were enumerated and then clearfelled in 1950. Since then one plot has been completely protected, a second has been burnt annually early in the dry season, and a third plot has been burnt annually late in the dry season. (2) In 1976-77 there were 202 trees ha-' (? 30 cm girth) on the protected plot, forty-two trees ha on the early burnt plot and twenty trees ha on the late burnt plot. Corresponding figures for basal area are 3-43, 0-51, 0.24 m2 ha-'. (3) The basal area of grass on both the burnt plots has remained constant at about 13% since 1960 whereas the basal area of grass on the protected plot has continued to decline and was 6.3% in 1976. Grass biomass at the end of the growing season in 1976 was 182 g m-2 on the protected plot, and 260 g m-2 and 144 g m-2 on the early and late burnt plots respectively. (4) There were seventy-three species of vascular plants on the protected plot in 1977, and fifty-three and forty-four respectively on the early and late burnt plots. (5) Only slight differences in the soils were observed, though the protected plot had significantly more organic matter and total nitrogen. (6) The results are compared with those of similar experiments elsewhere in Africa, and recommendations are made for improved experimental design.