INFLUENCE OF FIRE ON PLANT COMMUNITY PROCESSES AND COMPOSITION

INTRODUCTION

Today, fire is seen as a threat to the ecosystem as many ecosystems appear to be completely altered by fire. However, fire is an important ecological tool or parameter capable of maintaining ecosystem processes and functions such as nutrient cycling, succession vegetative regeneration, disease control and water cycle (Bidwell et al, 1990). A lot of research has been carried out to determine the effect of fire on the ecosystem. Fire influence is not only on plant species but also on animal species as it tends to fragment or destroy their habitat. However, this will not be covered in the scope of this essay as the aim of this essay is to review the influence of fire on plant community composition and processes. Fire is induced naturally and anthropogenically and studies have shown that a greater percentage of about 98% is anthropogenically and about 2% is natural. Although there are factors that determine the influence of fire on plant community some of which are; the pattern, frequency and intensity of the fire; vegetation type, topography or soil structure, climate and physical environment. (Neyisci, 1985; Chen et al. 2010;)

Influence of Fire on Vegetation/Succession

One of the most apparent influence of fire on plant community process and composition is the change in vegetation. Fire tends to influence vegetation in several ways such as change in the composition and structure of vegetation, loss of biomass, fuel loads e.tc. but this greatly depends on certain factors such as fire severity, species composition, soil type, moisture, physical and climatic conditions (Perez-cabello et al., 2008). Example is a study in north and south rim of Grand Canyon of ponderosa pine forest where fire played a significant change in plant composition. The area was monitored before and after the inclusion of fire. Fule, Before the inclusion of fire in the forest, there are low density of trees and after the fire regime, there was an incredible increase in tree density. (Fule et al. 1997) The example is relative to a research by Little (1974) on the northern hardwoods forest of USA where he stated the different influence of fire on species largely because of fire severity and sources of seeds as fire tends to provide suitable seedbeds for certain species. Therefore, it is established that fire of low severity should be introduced in species degraded site such as degraded ponderosa pine forest (Fule et al. 1997)

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Despite the positive report on the influence of fire, there are contrary opinions and reports on the influence of fire on vegetation. In 2000 fire burned more than 350m ha of forest and that alone constitutes about 6% of the world’s geographical area. (Food and Agriculture Organization, 2007). Fire also has a great influence on biomass and carbon accumulated by vegetation (Trees) thereby releasing or emitting unwanted gases to the environment and increasing the greenhouse gasses in the atmosphere. (Kauffman et al., 2003). Carbon stored in the forest could be lost if the forest is subjected to fire. (Vargas et al. 2008). Fire can also influence the abundance and diversity of plant species especially the seedlings species. The decrease in species abundance and diversity could be the elimination of young growing species that were shaded by other rapidly grown species. (Jhariya, 2014). This research is relative to the work of Saha and Howe (2003) in Mendha forest of central India that plant seedling diversity and abundance is seemingly lower in areas subjected to fire than areas excluded from fire. In their research, they noted a 28% difference in abundance and diversity. However, these reports are contrary to the work of Bakhtar et al (2013) that fire increase the diversity of plant species in the Oak forest.

Although, a research by Jhariya and Oraon (2012) in Bhoramdeo Wildlife Sanctuary in India on the impact of fire on herbaceous vegetation shows that ground vegetation such as herbs and grasses increase in density, abundance and diversity following the impact of fire. This is as a result of elimination or decrease in tree species that allows the penetration of light to the ground layer and less competition for nutrients to survive. Another study by Garren (1948) who worked in Pinus elliottii region reported an increase of herbs following fire. This increase in herbs includes legumes. However, in a report by Oosting (1944) who showed that any increase in herbs following fire is temporal and only for a short term.

Influence of fire on Soil Properties/Nutrients

Depending on the severity (Intensity and duration) of fire, it can damage the physical, biological and chemical properties of soil. The damage could be short, long or permanent. However, when the right type of fire is applied for example in ecosystem management, it can cause the revegetation of dormant plant species, increase the soil pH and nutrients. (Certini,2005)

Change in Soil Organic Matter

One of the changes in soil when fire occurs, is the change in organic matter. This depends on the severity of the fire which can lead to oxidation of the soil with temperature ranging from 200oC to 460oC (Giovannini et al., 1988). The gradual restoration of the affected soil starts with revegetation either naturally or artificial (ecological succession). Example is a study carried out by Fernandez et al (1997) on the top 10cm soil of Pinus sylvestris, where the soil was subjected to four different temperature namely; 150oc, 220oc, 350oc and 490oc. the first treatment shows no change hence, it is noneffective, the intermediate temperatures showed a structural change on the soil and the highest temperature shows a complete oxidation on the soil. Austin et al (1955) in his research in Western Oregon found out that the organic content of the topsoil after subjected to fire decreased by 75.5%. in a study carried out by Lutz (1984) shows that loss in the organic content of the soil depends on the severity of fire.

Change in Soil Physico-chemical properties

Several researches carried out has shown that the Physico-chemical properties of soil such as pH, Nitrogen, Calcium, Magnesium and Potassium tend to increase after fire (Certini, 2005). An Example is a study carried out by Chungu et al (2019) on how fire alters the availability of soil nutrients and enhance the growth of Eucalyptus grandis. The study showed that soil pH increased by 39% after fire which shows a reduction in soil acidity that could prevent the growth of plants. The study also shows an increase in nitrogen and other nutrient such as calcium, magnesium and potassium. However, this study contradicts the findings of Zhao et al (2015), Antoine et al (2013) and Murphy et al (2006) that soil nutrient decrease with fire. Interestingly, all studies also agreed that depending on the severity of the fire, it could increase or decrease the Physico-chemical parameters of the soil.

Change in the Biological Properties of soil

Fire has a significant effect on the biological properties of soil as it tends to affect the soil organisms such as bacteria and fungi that helps in nutrient cycling and disrupts the soil and organism relationship as seen in mycorrhiza. The organism helps in other processes such as decomposition, soil formation and energy flow (Jhariya et al., 2014). However, the influence of fire on soil biological properties can be positive or negative as seen in the work of Otsuka et al (2008) that the bacteria in a forest affected area are seen to be diverse (heterogeneous) compared to the unburn forest area.

Conclusion

Fire is an important ecological tool which plays a vital role in maintaining the structure and functions of the ecosystem, especially in plant communities if the right type of fire is applied. However, if the wrong type of fire is applied, it has a negative effect on the plant community processes and composition.  Therefore, the influence of fire on plant community processes and compositions depends on several factors such as the fire severity, intensity, frequencies, type of species compositions, moisture, soil type, physical and climatic environment.

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