Effect of Forest Management Types on Soil Carbon Stocks in Montane Forests: A Case Study of Eastern Mau Forest in Kenya

Date

2020-10-17

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Hindawi Publishing Corporation

Abstract

Mau Forest, a major forest reserve in Kenya, has experienced anthropogenic disturbances through encroachment and forest fires. This study aimed at comparing the soil carbon stocks in different forest management types as well as how seasonal climatic changes influence its dynamics. The study was undertaken in the Eastern Mau block (Sururu) which forms part of the greater Mau Forest Complex. The forest management interventions have been in place for over 15 years with disturbed (fire) natural forest experiencing fires in 2005, 2007, and 2014 while cypress plantations were established in 1994. A nested experimental design was used in data collection, where thirty-two sample plots were nested into four blocks each measuring 100 m² delineated by forest management types (disturbed by fire, natural forest, undisturbed natural forest, plantation, and glades). In each plot, data on soil carbon stocks, soil bulk density, soil moisture, and temperature were collected for both dry and wet seasons. Data collection was carried out between November 2015 and December 2016. The results indicated that there were no significant differences in the carbon stocks among the forest management types (F4,16 = 0.61, ). However, seasonal weather changes significantly affected the amount of carbon stocks among the forest management types (F4,16 = 0.61, ). The undisturbed natural forest had the highest mean soil carbon stocks, while the plantation forest had the lowest as follows: undisturbed natural forest (135.17 ± 35.99.0 Mg·C−ha), disturbed natural forest by fire (134.52 ± 38.11 Mg·C−ha), glades (122.4 ± 64.9 Mg·C−ha), and plantation forest (116.51 ± 39.77 Mg·C−ha). Furthermore, the undisturbed natural forest management had the highest bulk density (0.66 g/cm³), while the disturbed (fire) natural forest had the lowest (0.59 g/cm³). These values were low compared to most normal mineral soils which have a bulk density of between 1.0 g/cm³ and 1.5 g/cm³. There was a significant () relationship between seasonal weather (temperature) changes and soil carbon stocks under different forest management types with the relationship being stronger in soils under glades (r² = 0.62) and weak in the undisturbed natural forest (r² = 0.26). In conclusion, forest disturbances have an impact on soil carbon stocks, and for effective management of forest towards climate stabilization, then disturbance should be minimized if not avoided.1. IntroductionForest soils are a major sink of terrestrial carbon containing more than double the amount of carbon found in forest tree biomass [1, 2] and play a fundamental role in the global carbon cycle [3–6]. The role of forest soils as either a source or sink has become vital when assessing changes in atmospheric carbon dioxide concentrations mainly due to the growing interest of reducing the greenhouse emissions [7–10].Traditionally, forest management practices focused mainly on increasing the forest productivity and growing biomass stock with little effort on managing the soil carbon. Even though the effects of forest management on soil carbon stocks are not well understood [11], consensus exists that some forest management operations have an impact on carbon budget including losses of carbon from mineral soils [12].Forest management operations have been known to affect the carbon gains and losses by changing the level of inputs to the soil carbon pools, rates of microbial decomposition, changing environmental conditions such as temperature and moisture, and changing the quality of litter [2, 11]. Variability in weather parameters especially temperature and soil moisture has been reported to influence soil organic matter decomposition [13]. In addition, wet and cold climates store more soil carbon compared to dry and hot climates with suggestions of the existing optimum climate influencing soil carbon storage [14]. Seasonal dry and wet conditions have no significant change in total soil carbon in tropical forests [15] although biomass accumulation in disturbed forests is reported to be influenced by temperature [16].The Mau Forest Complex (MFC) forms the largest closed-canopy forest ecosystem in Kenya, covering over 400,000 hectares of land [17, 18]. This forest ecosystem is comprised of several forest blocks including the Eastern Mau Forest block. MFC provides critical ecological goods and services such as river flow regulation, flood mitigation, recharge of groundwater, reduced soil erosion and siltation, water purification, conservation of biodiversity, and microclimate regulation [17, 19, 20]. Despite the above critical functions, the Mau Forest Complex has been impacted by extensive irregular and ill-planned settlements as well as illegal forest resource extraction that have reduced the cover by more than 7% in the past 21 years [21]. To address these negative trends, the government enhanced forest management through the deployment of various interventions including both policy and restoration measures.The forest reserve is being subjected to heterogeneous management to address the varied previous disturbance levels and existing resource types. These management activities are anticipated to affect the carbon dynamics such as carbon emissions and sequestration.Therefore, the aim of this study was to evaluate the impacts of the forest management on the soil carbon stocks in the Eastern Mau Forest reserve. The findings of this study contribute knowledge towards carbon management, climate change mitigation mechanisms such as Reduction of Emission from Deforestation and forest Degradation (REDD+), and the role of conservation in the sustainable management of forests and enhancement of forest carbon stocks and climate change policies.2. Materials and Methods2.1. Description of the Study Area2.1.1. LocationThis study was undertaken in Sururu forest block (Figure 1). The block is part of the larger East Mau Forest reserve. The selection of the study site was centered on the application of management objectives (plantation forest, undisturbed natural forest, disturbed (fire) natural forest, and glades). The management actions are critical in the development, conservation, and management of Kenya’s forest resources [17]. The Eastern Mau Forest is 50 km south of Nakuru Town and positioned within UTM zone 37S and the coordinate bounds (161237, 9937639); (161237, 9924748); (1773018, 9937639); (1773018, 9924748) (KFS, 2011) (Figure 1). Sururu forest block covers an area of approximately 13364.4 ha [17, 20]. Despite the complexity in the plant formation, there is a broad altitudinal zonation. The lower zone is comprised of lower montane forest which occurs below 2,300 m and then it transitions into middle montane at 2350 m. The middle montane is composed of thickets of bamboo (Arundinaria alpine) mixed with forest. The upper zone ranging from 2420 m to 2600 m at the escarpment crest forms the upper montane Sclerophyllous forest. The optimal conditions for forest growth are within the lower montane forest zone. This zone is characterized by tree species that include Aningeria adolfi-friedericii and Strombosia scheffleri [18]. Some of the major rivers and streams forming up the hydrological systems of Lake Victoria, Lake Nakuru, Lake Baringo, and Lake Natron originate from Eastern Mau, thus making it a critical watershed within the Mau Forest Complex [20].

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