Diversity, use and resiliance of woody species in a multiple land use equatorial African savanna, central Uganda
Date
2011-03-17
Authors
Kalema, Vettes Neckemiah
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Abstract
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-life
habitats) that sustain local livelihoods and national economies. Increasing demands and the need for
sustainable savanna woodland resource management requires that the ecological, economic, social
and cultural values of these resources be explored and brought to the attention of decision makers and
the general public. The identification and better understanding of the structure and dynamics of
woodland community types, patterns of species distribution and quantitative properties of their
diversity is important to the conservation and sustainable management of these woodlands. This study
seeks to contribute to a better understanding of Nakasongola woodland community types, species
diversity patterns and environment correlates, natural regeneration processes (i.e. sprouting and
seedling establishment) and identifying livelihood strategies adopted by households, woody species
utilised, and the contribution of charcoal production to household livelihoods. Data on vegetation and
environmental variables were collected using 75 rectangular 20 x 50 m (0.1 ha) plots. Data on land
use and land cover changes, and relevant associated socio-economic parameters were collected
through the analysis of multi-temporal satellite imagery and field observations, as well as interviews
of local households and key informants. The basic major livelihood activities for the rural households
in this savanna dryland are charcoal production, subsistence crop cultivation and livestock grazing.
However, it, sometimes, includes various combinations of activities, i.e. charcoal production and
subsistence crop cultivation for both food and cash, and livestock keeping for income generation
through selling the livestock products such as milk and, sometimes, the whole animal. At least 24
woody species, including fruit trees (Mangifera indica and Artocarpus heterophyllus), are frequently
harvested, including 16 species that are considered the most utilized for charcoal production. Charcoal
production, 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 across
the 8 sub-counties. Most of the youth (≤ 20 years old) derive their livelihoods from charcoal
production, a small amount of trade, offering labour services, livestock keeping, fishing, bee keeping
and earth brick making. Charcoal production, livestock keeping and hunting are carried out
particularly by men, whereas, crop cultivation, and collection of fire wood, medicinal plants and fruits
are carried out, mainly, by women. However, men are also engaged in cultivation only during the
rainy seasons. There have been significant land cover changes in the area during the period 1984 to
2001, resulting in a 64% decrease in dense woodland cover, and an 80% increase in areas under
cultivation/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 families
were recorded. The most species rich families were Mimosaceae (13), Rubiaceae (9), Moraceae (7),
Euphorbiaceae (7), Anacardiaceae (6), Combretaceae (5) and Verbenaceae (5). Density of woody
species differed significantly (F2, 72 = 6.3, P < 0.003) among land uses, being higher under charcoal
production (7,131 ± 755 plants/ha) and cultivation (6,612 ± 665) areas and significantly lower under
grazing 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 species
composition) 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 a
more heterogeneous spatial distribution of species in the grazing lands. This suggests that variations in
the composition and diversity of woody species are to a great extent influenced by land use type and
anthropogenic 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 and
significantly 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 were
Combretum 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 Vepris
nobilis for charcoal production areas. For both basal area and abundance of all woody species, the
total 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 variables
on species composition and distributions. The CCA points to a significant influence of soil Ca2+ and
Mg2+ in association with grazing on gradients in the composition and structure of woody species in the
savanna woodland of Nakasongola.
Resprouting was generally common among the woody species. A total of 2,595 stumps, representing
74 species in 31 families were recorded from all plots. Of these, 98.3% resprouted and were identified
to species level. Density of both stumps and total resprout differed significantly (p < 0.05) among the
land uses, being higher in charcoal production areas than in grazing and cultivation land uses. For the
overall 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 uses
and lowest under grazing areas (10 ± 1). Generally, the mean number of resprouts per stump increased
with increasing stump basal diameter (BD), being highest for BD size class > 41 cm. In relation to
stump 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 negative
relationships 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 charcoal
production (2,416 ± 295 ha-1) and lowest for grazing (1,629 ± 205 ha-1). Composition of seedlings
differed significantly among land use types (Global RANOSIM = 0.119, p = 0.001). The distributions and
densities of some seedlings were explained by gradients in environmental variables, with edaphic
factors (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 variance
in species – environmental relations and were a reflection of edaphic and charcoal production land use
gradients. All of the 16 highly utilized species were well represented in both the juvenile and adult
classes, with gradually declining number of individuals with increasing stem size-class. This indicates
that most of these species have high regeneration potential. Juvenile:adult tree ratios >1 and negative
DSCD slopes indicate good recruitment and probably successful regeneration for these species. The
study revealed land cover changes mainly in the dense and medium dense woodlands, reflected by the
increase in open woodland, grasslands and cultivation/settlements. These trends threaten the
livelihoods of local communities who are entirely dependent on these natural resources. Sustainable
management will require the establishment of suitable integrated community-based institutions and
management practices, with support from all key stakeholders (i.e. National Forest Authority (NFA))
and local communities. Maintenance of savanna woodland resources and other ecosystem services
essential for human well-being will require an effective legal framework to prevent over-exploitation
and give incentives for the protection of the fragile savanna woodland vegetation. An appropriate
savanna woodland management policy will be required to guide changes in land use that
accommodate the requirements of land users, aided by targeted conservation efforts to all woody
plants and particularly for the highly utilized species for charcoal production as well as the
multipurpose species. In addition, there is urgent need to build local capacity for improved harvesting
and 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.
Description
Keywords
Alpha diversity, ANOSIM, beta-diversity, Canonical Correspondence Analysis (CCA), diameter size-class distributions (DSCD), land cover change, land use, livelihood strategies, resprouting, seedling recruitment, basal diameter (BD)