Diversity, use and resiliance of woody species in a multiple land use equatorial African savanna, central Uganda

[摘要] Savanna woodlands are vitally important in providing ecological services (e.g. erosion protection,micro-climate) and economic services (e.g. timber, food, fodder non-wood products, and wild-lifehabitats) that sustain local livelihoods and national economies. Increasing demands and the need forsustainable savanna woodland resource management requires that the ecological, economic, socialand cultural values of these resources be explored and brought to the attention of decision makers andthe general public. The identification and better understanding of the structure and dynamics ofwoodland community types, patterns of species distribution and quantitative properties of theirdiversity is important to the conservation and sustainable management of these woodlands. This studyseeks to contribute to a better understanding of Nakasongola woodland community types, speciesdiversity patterns and environment correlates, natural regeneration processes (i.e. sprouting andseedling establishment) and identifying livelihood strategies adopted by households, woody speciesutilised, and the contribution of charcoal production to household livelihoods. Data on vegetation andenvironmental variables were collected using 75 rectangular 20 x 50 m (0.1 ha) plots. Data on landuse and land cover changes, and relevant associated socio-economic parameters were collectedthrough the analysis of multi-temporal satellite imagery and field observations, as well as interviewsof local households and key informants. The basic major livelihood activities for the rural householdsin this savanna dryland are charcoal production, subsistence crop cultivation and livestock grazing.However, it, sometimes, includes various combinations of activities, i.e. charcoal production andsubsistence crop cultivation for both food and cash, and livestock keeping for income generationthrough selling the livestock products such as milk and, sometimes, the whole animal. At least 24woody species, including fruit trees (Mangifera indica and Artocarpus heterophyllus), are frequentlyharvested, including 16 species that are considered the most utilized for charcoal production. Charcoalproduction, being the major source of income to the rural households, contributes on average US$ 259± 46 (S.E.) per household annually. There were significant differences in charcoal production(Kruskal-Wallis; H = 31.42, p < 0.0001), producer sale prices per bag of charcoal (H = 35.62, p <0.0001), and annual incomes from charcoal production (H = 32.44, p < 0.0001) per households acrossthe 8 sub-counties. Most of the youth (≤ 20 years old) derive their livelihoods from charcoalproduction, a small amount of trade, offering labour services, livestock keeping, fishing, bee keepingand earth brick making. Charcoal production, livestock keeping and hunting are carried outparticularly by men, whereas, crop cultivation, and collection of fire wood, medicinal plants and fruitsare carried out, mainly, by women. However, men are also engaged in cultivation only during therainy seasons. There have been significant land cover changes in the area during the period 1984 to2001, resulting in a 64% decrease in dense woodland cover, and an 80% increase in areas undercultivation/settlements. These changes are attributed to significant spatial expansion in agriculture increased commercialisation of charcoal production, grazing and human population growth. A total of 44,195 (5,893 plants/ha) woody plants representing 99 species in 67 genera and 31 familieswere recorded. The most species rich families were Mimosaceae (13), Rubiaceae (9), Moraceae (7),Euphorbiaceae (7), Anacardiaceae (6), Combretaceae (5) and Verbenaceae (5). Density of woodyspecies differed significantly (F2, 72 = 6.3, P < 0.003) among land uses, being higher under charcoalproduction (7,131 ± 755 plants/ha) and cultivation (6,612 ± 665) areas and significantly lower undergrazing lands (4,152 ± 525). Community species composition differed significantly (Global RANOSIM =0.14, p = 0.001) among land use types. All measures of beta-diversity (spatial 'turnover” in speciescomposition) showed consistently higher beta-diversity in the grazing land use (βW = 3.1; βT = 3.1),followed by cultivation (βW = 2.8; βT = 3.0) and charcoal production (βW = 2.7; βT = 2.8), suggesting amore heterogeneous spatial distribution of species in the grazing lands. This suggests that variations inthe composition and diversity of woody species are to a great extent influenced by land use type andanthropogenic disturbances in this region. Basal area of woody species differed significantly (F2, 72 =12.0, P < 0.0001) among land uses, being highest under cultivation and charcoal production andsignificantly lower under grazing. Woody plant density differed (F2,72 = 6.3, P = 0.003) across landuses,being highest under charcoal production and cultivation and significantly lower under grazing.The species that contributed most to both basal area and density across all the land uses wereCombretum collinum and Combretum molle. However, different species contributed the next most i.e.Piliostigma thonningii for grazing; Albizia zygia and Harrisonia abyssinica for cultivation and Veprisnobilis for charcoal production areas. For both basal area and abundance of all woody species, thetotal variance in species-environmental factor relations (for the combined first four canonical axes)was higher than 50%, suggesting a relatively strong influence of the measured environment variableson species composition and distributions. The CCA points to a significant influence of soil Ca2+ andMg2+ in association with grazing on gradients in the composition and structure of woody species in thesavanna woodland of Nakasongola.Resprouting was generally common among the woody species. A total of 2,595 stumps, representing74 species in 31 families were recorded from all plots. Of these, 98.3% resprouted and were identifiedto species level. Density of both stumps and total resprout differed significantly (p < 0.05) among theland uses, being higher in charcoal production areas than in grazing and cultivation land uses. For theoverall pooled data, resprouts per stump differed significantly among land uses (F2, 456 = 7.75, p =0.0005), being highest in charcoal production (mean ± S.E.; 14 ± 1) and cultivation (13 ± 1) land usesand lowest under grazing areas (10 ± 1). Generally, the mean number of resprouts per stump increasedwith increasing stump basal diameter (BD), being highest for BD size class > 41 cm. In relation tostump height, the highest mean resprouts/stump was found on stumps with heights ranging from 0.31-0.40 m. Based on pooled species data, regression analyses showed weakly significant negativerelationships between BD of leading resprouts and number of resprouts/stump (r2 = 0.123, p < 0.0001)and between height of leading resprouts and number of resprouts/stump (r2 = 0.068, p < 0.0001).Density of seedlings of woody species differed significantly among land use types (ANOVA; F2, 72 =5.9, p = 0.004), being highest for cultivation (3,162 ± 440 individuals ha-1), followed by charcoalproduction (2,416 ± 295 ha-1) and lowest for grazing (1,629 ± 205 ha-1). Composition of seedlingsdiffered significantly among land use types (Global RANOSIM = 0.119, p = 0.001). The distributions anddensities of some seedlings were explained by gradients in environmental variables, with edaphicfactors (i.e. Ca2+, Mg2+, K+ and organic matter) and charcoal production being the most important.The first two axes of the Canonical Correspondence Analysis (CCA) explained 41.9% of the variancein species – environmental relations and were a reflection of edaphic and charcoal production land usegradients. All of the 16 highly utilized species were well represented in both the juvenile and adultclasses, with gradually declining number of individuals with increasing stem size-class. This indicatesthat most of these species have high regeneration potential. Juvenile:adult tree ratios >1 and negativeDSCD slopes indicate good recruitment and probably successful regeneration for these species. Thestudy revealed land cover changes mainly in the dense and medium dense woodlands, reflected by theincrease in open woodland, grasslands and cultivation/settlements. These trends threaten thelivelihoods of local communities who are entirely dependent on these natural resources. Sustainablemanagement will require the establishment of suitable integrated community-based institutions andmanagement practices, with support from all key stakeholders (i.e. National Forest Authority (NFA))and local communities. Maintenance of savanna woodland resources and other ecosystem servicesessential for human well-being will require an effective legal framework to prevent over-exploitationand give incentives for the protection of the fragile savanna woodland vegetation. An appropriatesavanna woodland management policy will be required to guide changes in land use thataccommodate the requirements of land users, aided by targeted conservation efforts to all woodyplants and particularly for the highly utilized species for charcoal production as well as themultipurpose species. In addition, there is urgent need to build local capacity for improved harvestingand utilization of these tree species. This can be achieved through equipping local users with up to date information as well as observing the existing skills.

[发布机构] University of the Witwatersrand