Implications of bush burning on weed species diversity , population density and organic matter content of the soil in Birnin-Yauri , Kebbi , Nigeria

The effect of fire on weed species diversity, population density and the level of organic matter content in soil in Birnin-Yauri Local Government Area, Kebbi State, Nigeria were investigated. The results obtained indicated disappearance of Panicum laetum (a grass), Cyperus rotundus (a sedge) and the two broadleaf species with procumbent habits, Ipomoea cylindrical and Tridex procumbens, but there was increase in population density of the broadleaf species that were erect with woody stems. In the control plot, C. rotundus and I. cylindrical recorded the highest increase in density and the erect broadleaf, Senna tora was more or less stable in numbers but Sida acuta doubled in soil decrease gradually with increase in frequency of burning. @JASEM Bush burning has been practiced from time immemorial and in many societies has been accepted as an integral part of traditional farming system. However, the changing pattern of life style, population increase, urbanization and farming systems that put pressure on the natural environment, suggest that traditional system of bush burning can no longer be sustained but it has been difficult to reduce or completely eliminate it (Aliero, 2004). The consequence of uncontrolled bush burning is most obvious in areas characterized by torrential rain fall, strong wind and hot solar radiation. This is because even slight disturbance of the vegetal mantle may have very considerable impact on organic matter content and vegetation biodiversity. Hopkins (1965), Onoche (1961) and Egunjobi (1971) reported on different aspects of fire on the forest and savanna vegetation in Nigeria. These works however, concentrated on forest plantations, grass reserves and cattle ranches with no attention paid on arable farms. The experiments reported in this paper were based on savanna arable crop farm with the objective of evaluating the rate of change in species diversity, population density and organic matter content of the soil due to effect of burning. MATERIALS AND METHODS The experiment was carried out in an arable crop farm that was not disturbed by fire for the preceding five years. The experimental plot was divided into two parts each measuring 50x50m in size. The species diversity and population density in each part of the plot were estimated by random throw of 1x1m quadrate at the commencement of the study in June 1998. Percentage matter content in each part of the plot was determined by collecting soil at two centimetre depth and determining the organic matter content by Walkley and Black (1979) method. In one part of the plot, herein referred to as Treatment A (TA), fire was set on the vegetation after collecting the initial data on species diversity and population density as well as the organic matter content. The other part, Treatment B (TB) was left intact. After two years (2000), another set of readings on species diversity, population density and organic matter content was taken and Treatment A set on fire again and allowed to stand for another two years after which another set of data on the parameters cited above was taken in 2002. Each time data was collected from TA, similar set of data was collected from TB, the control plot and the sets data from the two plots were compared.

Bush burning has been practiced from time immemorial and in many societies has been accepted as an integral part of traditional farming system.However, the changing pattern of life style, population increase, urbanization and farming systems that put pressure on the natural environment, suggest that traditional system of bush burning can no longer be sustained but it has been difficult to reduce or completely eliminate it (Aliero, 2004).
The consequence of uncontrolled bush burning is most obvious in areas characterized by torrential rain fall, strong wind and hot solar radiation.This is because even slight disturbance of the vegetal mantle may have very considerable impact on organic matter content and vegetation biodiversity.Hopkins (1965), Onoche (1961) and Egunjobi (1971) reported on different aspects of fire on the forest and savanna vegetation in Nigeria.
These works however, concentrated on forest plantations, grass reserves and cattle ranches with no attention paid on arable farms.The experiments reported in this paper were based on savanna arable crop farm with the objective of evaluating the rate of change in species diversity, population density and organic matter content of the soil due to effect of burning.

MATERIALS AND METHODS
The experiment was carried out in an arable crop farm that was not disturbed by fire for the preceding five years.The experimental plot was divided into two parts each measuring 50x50m in size.The species diversity and population density in each part of the plot were estimated by random throw of 1x1m quadrate at the commencement of the study in June 1998.Percentage matter content in each part of the plot was determined by collecting soil at two centimetre depth and determining the organic matter content by Walkley and Black (1979) method.In one part of the plot, herein referred to as Treatment A (TA), fire was set on the vegetation after collecting the initial data on species diversity and population density as well as the organic matter content.The other part, Treatment B (TB) was left intact.After two years (2000), another set of readings on species diversity, population density and organic matter content was taken and Treatment A set on fire again and allowed to stand for another two years after which another set of data on the parameters cited above was taken in 2002.Each time data was collected from TA, similar set of data was collected from TB, the control plot and the sets data from the two plots were compared.

RESULTS
The study revealed presence of six species of flowering plants with varied population density.Four of the weeds were broad leaf species, one grass and the other sedge.
The only grass species disappeared completely after the first burning in TA and did not appear again to the end of the experiment.The sedge survived the first burning but decreased in density from six to four and completely disappeared after the second burning.Among the broadleaf species, two were procumbent and the other two erect.The procumbent species decreased in number after the first burning and disappeared after the second.The erect species however, continued to increase in density after the successive burnings (Table 1).In TB, results obtained showed that C. rotundus and I. cylindrica tripled their original numbers, P. laetum and S. tora increased only slightly and S. accuta and T. procumbens doubled their numbers (Table 2).Organic matter content decreased progressively between the two successive burnings.In treatment A, from 2.32% before burning to 2.00 and 1.7% after the first and second burnings respectively (Fig. 1).In the unburnt plot (Treatment B), the organic matter content of soil increased from 1.52% at commencement of the experiment to 2.75 and 3.00% after the first and second burnings respectively (Fig. 2).

DICUSSION
Results obtained indicated fire on vegetation to have both depressive and stimulatory effects depending on the type of species.For grasses and sedges that are usually characterized by soft herbaceous tissues, relatively light seed and shallow roots, fire suppresses growth and lead to the disappearances of the species as recorded in case of Panicum laetum (a grass) and Cyperus rotundata (a sedge).In these species, heat generated by fire accelerate rate of transpiration, rapid drying of leaves and eventual burning of the stock plant.The light seeds are easily carried away by wind and deposited far away from the mother plant.The few seeds that are dispersed close to the mother plant are mostly on the surface and easily destroyed by fire.West (1965), Holdworth and Uhl (1997) and Aliero (2004) reported change in the direction of vegetation succession due to effect of fire on vegetation.The procumbent species, Ipomoea cylindrica and Tridax procumbens survived the first burning due probably to presence of prennating buds that may have been buried underground under soil suffered only a partial effect of fire but completely destroyed after the second burning.The erect weeds, Senna tora and Sida acuta were stimulated by burning.This is probably due to partial dormancy break of buried seeds of these two species after burning The stem of these species being woody may also sprout from the base to grow and produce new foliage at the onset of the next rainy season.
In the control plot, species of C. rotundus and I. cylindrica tripled their original numbers.S. acuta and T. procumbens doubled their numbers but P. laetum and S. tora increased in numbers only slightly (Table 2).
Burnt ash is easily blown away by wind and leached by rain water consequently the loss in percentage organic matter content.Fitzpatric (1990), Waziri and Aliero (2005) reported soil fertility as an aspect of soil-plant relationship where the plant material is removed, the relationship changes and part of the organic matter lost.
In conclusion, burning of vegetation, results in decreased biodiversity, population density of herbaceous species and loss of organic matter in soil.
Burning also has stimulatory effect and could be used as a tool to promote growth of certain desired species in a directed succession.

Table 1 :
Species diversity and density before and after successive burnings Density

Table 2 :
Species diversity and density in unburnt plot