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Trees are very important in Buhara sub-county in the Kabale district for reasons like income generation, soil erosion control, biodiversity conservation, wood, and charcoal production among others. Eucalyptus species and Pinus species remain part of the dominant tree species planted in Buhara sub-county. Although these tree spp. are widely planted, the environmental effects of plantations comprised of exotic trees such as Eucalyptus spp. and Pinus spp. have been vigorously debated and reports such as drying up of watercourses, affecting the soil physical and chemical properties, tendency to deplete soil nutrients and fertility, suppression of other vegetation, reduction of forest biodiversity and reducing crop yield in agro-forestry systems have been made.
Although a number of reports have been made in other countries concerning the negative impacts of Eucalyptus and Pinus plantations, few studies have been done in Uganda especially in the Kabale district. Therefore, this study will assess the effect of Eucalyptus and Pinus plantations on plant diversity and soil properties in Buhara sub-county, Kabale district.
Quadrates of 1m x1m and line transects will be used in the determination of understory diversity in Eucalyptus and Pinus plantations and the control area, whereby the individual plant species in the quadrates will be directly counted and recorded. Also, soil properties such as soil pH, soil organic matter, bulky density, soil moisture, soil total nitrogen, and available phosphorous in both the Eucalyptus and Pinus plantations and the control area will be determined.
It will also involve a comparison between the individual tree species for both Eucalyptus and Pinus tree plantations for their effect on plant diversity and soil properties in the study area.
Statistical analysis of the collected data will be done using Statistical Package For Social Scientists (SPSS) version 20, PAST version 3.16, and Microsoft excess 2016. The understory diversity will be analyzed using the Shannon-Wiener index in PAST and Pearson’s correlation coefficient will also be performed to establish the relationship between the physical and chemical soil properties in SPSS at a 5% level of significance. The Eucalyptus plantations are expected to have high understory diversity, low organic matter, high bulky density, low soil moisture, and relatively high pH, total nitrogen, and available phosphorous as compared to Pinus plantations. The research design will be quantitative, the research strategy will be simple random sampling and the study will strictly assess the effect of Eucalyptus and Pinus plantations on plant diversity and soil properties in Buhara sub-county, Kabale District.
Key words: Eucalyptus plantations, Pinus plantations, Plant diversity, Soil nutrients, Buhara sub-county, Kabale district.
Biodiversity in managed plantations is an important issue for the long-term sustainability of ecosystems(Cardinale et al., 2012). The environmental effects of plantations comprised of exotic trees such as Eucalyptus spp. and Pinus spp. have been vigorously debated and many concerns about the negative ecological effects of Eucalyptus spp. and Pinus spp. from many countries all over the world have been reported. These concerns include disruption of biodiversity conservation and their allelopathic effects that inhibit undergrowth regeneration, soil fertility depletion and drying of watercourses thus lowering the water table in water catchment areas such as the wetlands(Kluthe, 2016).
In China, it was found out that the expansion of Eucalyptus spp. plantation lowers water tables and reduces water availability for irrigation on lands previously used for crops or occupied by indigenous trees and grass due to hydrophobicity and their dense and deep root network (Wen, Y. G., Zheng, X., Li, M. C., Xu, H. G., Liang, H. W., Huang, C. B., Zhu, H. G., and He, B., 2009) . It was also indicated that in China, afforestation of exotic Eucalyptus spp. and Pinus spp. plantation is reported to adversely affect the soil physical and chemical properties and the plant community biodiversity. In Africa, previous authors have reported that Eucalyptus spp. and Pinus spp. have devastating effects on the soil physical and chemical properties, depleting soil organic matter content and negatively impacting soil hydrology and plant diversity (Alemie, 2009). El-Amin et al (2001) reported that in Sudan, Eucalyptus spp. caused reduction in crop yield due to nutrient depletion and production of toxic exudates which inhibit the growth of other plants thus reducing their yield. However, little work has been done on these tree plantations in Uganda especially in Kabale district.
Forests and woodlands cover a total of 4.9 million hectares, about 24% of total land area in Uganda. Forest plantations cover 35066 hectares, tropical high forests cover 924208 hectares and woodlands cover 3974102 hectares (FAO, 2011). Over many years, there has been increased rate of deforestation in Uganda due to high demand for timber and its products hence fears of desertification. This encouraged the government to start re-afforestation programs and also encourage people to invest in forestry(Wijesinghe & Silva, 2012). Due to this, many people have invested so much in Eucalyptus and Pinus forestry due to their short maturity time hence quick economic gains. According to Banana (2009), the area cover of Eucalyptus plantations increased from 190km2 to 200km2 from 1993 to 2006 while that of Pinus increased from 160km2 to 250km2 in the same period. Forest plantations are associated with effects on plant diversity and soil organic matter. However, the type of forests may have an effect on organic matter quality like the pine needle-shaped leaves may acidify the soil leading to reduced undergrowth in Pinus plantations(A, A, A, & B, 2010). Therefore, this study will assess the effect of Eucalyptus and Pinus plantations on plant diversity and soil physical and chemical properties in Buhara sub-county, Kabale district.
Monoculture tree plantations affect the physical and chemical properties of soil which in turn affects plant diversity of a particular area. The reduction in plant diversity is mainly due to the allelopathic effects of exotic tree species like the Eucalyptus spp. and Pinus spp. due to the allelochemicals produced by those trees(Liang, Reynolds, Wassie, Collins, & Wubalem, 2016). Below are some of the soil properties which are normally affected by different tree plantations.
Bulky density is defined as the density of soil in loose form, granular, nodular, structure expressed as a ratio of weight to volume. Soil bulky density is prone to destruction by forest management practices for example; forest harvesting involving the use of heavy machinery may increase soil density by packing the particles closer together and reducing soil pore space (Al-shammary et al., 2018). In addition, soil ideally has a bulky density of 1 to 1.6g/cm3. Increased soil bulky density is associated with poor soil aeration, reduced activity of soil organisms involved in nutrient cycling, poor root penetration, and reduced soil infiltration, increased surface runoff, with a consequent reduction in soil moisture, soil erosion and increased sedimentation in watersheds. When the soil bulky density is severely affected, the shallow root may fail to penetrate the soil to absorb soil nutrients. This study will also involve the determination of bulky density in the Pinus and Eucalyptus plantations and the control area.
According to (Cao, 2010), low soil moisture contents, ranging from 20.2 to 30.5% in the topsoil (0-10cm depth) of four areas under Eucalyptus spp. plantations in China was reported. Also, a phenomenon is known as “soil water mining” was occurring where by the trees’ extensive and deep root systems were able to tap into deeper soil layers that other species were not able to reach (Calder, I. R., Rosier, P. T. W., Prasanna, K. T., Parameswarappa, S. , 1997). They concluded that, the implication of this is a disturbance in the water table. (Alemie, T. C, 2009) also reported that soil moisture reduction under Eucalyptus plantations in Koga watershed in Ethiopia with the reductions being highest at distances closer to the Eucalyptus spp. There this study will find out the effect of Eucalyptus and pine plantations on soil moisture.
Soil pH is the measure of acidity or basicity of the soil and is measured by the number of hydrogen ions present in the soil solution. Soil pH affects the solubility of ions in the soil, which in turn affects microbial and plant growth. According to Mckenzie (2003), a pH range of 5.8 to 6.8 is ideal for most crops because it coincides with the optimum solubility of the most important plant nutrients. Also, litter from Pinus spp. releases phenolic acids which can be allelopathic (Duffy, 2014). According to Cao et al (2010), it was found out that the soil pH under Eucalyptus spp. plantations reduced between 0 to 10cm soil depths of four areas, which ranged from 4.2 and 4.5, in China. Also, in Koga watershed in Ethiopia, soils under Eucalyptus Spp. plantations were found to have reduced pH of 3.5 to 4.5 (Jagger & Pender, 2003). Low soil pH limits the growth and activities of decomposer soil microorganisms as soil biological activities are reduced in acidic soils (Castro-Díez P., Fierro-Brunnenmeister N., González-Muñoz, N., and Gallardo, A. , 2011). This also lowers the plant diversity in these plantations due reduced humus formation which improves soil fertility that supports plant life in a given area(Duffy, 2014). The long-term impact of pine plantations on soil involves nutrient loss and soil acidification through litter and root inputs. These impacts are as a result of slower decomposition and build upon the forest floor (Stendah, J., Johansson, M.B., Eriksson E., Nilsson, A., & LangVall, O. , 2010). This study will also involve the determination of the effect of Eucalyptus and Pinus plantations on soil pH in the study area.
Allelopathy refers to the harmful or beneficial effects of one plant on another plant, from the release of biochemicals, also known as allelochemicals, from plant parts by leaching, root exudation, residue decomposition and other processes in both agricultural and natural systems (Ferguson, J. J., Rathinasabapathi, B., and Chase, C. A., 2013). Plants like the Eucalyptus spp. and Pinus spp. are known to exhibit allelopathy effects which reduce plant diversity in these plantations(Duffy, 2014) The extracts from eucalyptus and Eucalyptus plantations of a species called Eucalyptus camaldulensis were found to have an inhibitory effect on germination, root and shoot elongation of tomato plants(Box & Box, 2011)
Benefits of improved soil organic matter include improved nutrient and water holding capacities, better soil structure which enhances root growth and increases aeration. The quantity and quality of litter production and the decomposition process play an important role in soil fertility management in terms of nutrient cycling, carbon budgeting and formation of soil organic matter under plantations (Demessie, A., Singh, B. R., Lal, R., and Strand, L. T., 2012). (Lal, R., 2005) pointed out that soil carbon content depends on complex interactions between climate, tree species and management, soils, and the chemical composition of the litter from the different tree species. Substantial changes in soil organic matter (SOM) were reported after the substitution of the natural vegetation by short-rotation Eucalyptus spp. plantations in Brazil (Lima et al., 2008). According to the study conducted by Baber et al (2006), on the effect of Eucalyptus camaldulensis on soil properties and soil fertility in Khan District in Pakistan, they found out that the organic matter content in the surface soil at 0-15cm depth, soil organic matter was low. Mature Pinus plantations keep soil fertile and also, the soils under these plantations have also been found to store as much carbon as pasture soil. The study will also involve the determination of the amount of organic matter in the Eucalyptus and Pinus plantations and the control area.
Eucalyptus spp. and Pinus spp. are part of the dominant tree species planted in Buhara sub-county, Kabale district. Although these tree species are planted in various spatial patterns in the area to meet the demand for soil erosion control, timber, fuelwood, electricity pools, charcoal production, and cushion farmers when the markets for their agricultural produce fail or are low (Oballa, P. O., Muchiri, M. N., Konuche, P. K., and Kigomo, B. N., 2010), reports such as suppression of other vegetation, reduction of forest biodiversity, drying up of water courses, affecting of soil physical and chemical properties, reducing crop yield in the nearby farms and tendency to deplete soil nutrients and fertility have been made. To maintain and safeguard the health of the soil, and plant biodiversity in Buhara sub-county, Kabale district, it’s important to study the effects of Eucalyptus and Pinus plantations on plant diversity and soil physical and chemical properties in this area.
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