Browsing by Author "Mwanjalolo, Majaliwa J. G."

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    Responses of tree growth and biomass production to nutrient addition in a semi-deciduous tropical forest in Africa
    (Ecology, 2022) Manu, Raphael; Corre, Marife D.; Aleeje, Alfred; Mwanjalolo, Majaliwa J. G.; Babweteera, Fred; Veldkamp, Edzo; Straaten, Oliver van
    Experimental evidence of nutrient limitations on primary productivity in Afrotropical forests is rare and globally underrepresented yet are crucial for understanding constraints to terrestrial carbon uptake. In an ecosystem-scale nutrient manipulation experiment, we assessed the early responses of tree growth rates among different tree sizes, taxonomic species, and at a community level in a humid tropical forest in Uganda. Following a full factorial design, we established 32 (eight treatments four replicates) experimental plots of 40 40 m each. We added nitrogen (N), phosphorus (P), potassium (K), their combinations (NP, NK, PK, and NPK), and control at the rates of 125 kg N ha 1 year 1, 50 kg P ha 1 year 1 and 50 kg K ha 1 year 1, split into four equal applications, and measured stem growth of more than 15,000 trees with diameter at breast height (dbh) ≥1 cm. After 2 years, the response of tree stem growth to nutrient additions was dependent on tree sizes, species and leaf habit but not community wide. First, tree stem growth increased under N additions, primarily among medium-sized trees (10–30 cm dbh), and in trees of Lasiodiscus mildbraedii in the second year of the experiment. Second, K limitation was evident in semi-deciduous trees, which increased stem growth by 46% in +K than –K treatments, following a strong, prolonged dry season during the first year of the experiment. This highlights the key role of K in stomatal regulation and maintenance of water balance in trees, particularly under water-stressed conditions. Third, the role of P in promoting tree growth and carbon accumulation rates in this forest on highly weathered soils was rather not pronounced; nonetheless, mortality among saplings (1–5 cm dbh) was reduced by 30% in +P than in –P treatments. Although stem growth responses to nutrient interaction effects were positive or negative (likely depending on nutrient combinations and climate variability), our results underscore the fact that, in a highly diverse forest ecosystem, multiple nutrients and not one single nutrient regulate tree growth and aboveground carbon uptake due to varying nutrient requirements and acquisition strategies of different tree sizes, species, and leaf habits.
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    A Simplified Spatial Methodology for Assessing Land Productivity Status in Africa
    (Land, 2022) Barasa, Bernard; Mwanjalolo, Majaliwa J. G.; Banduga, Moses,; Katwere, James,; Magaya, Paul; Sadadi, Ojoatre; Wanjiru, Lydia; Walusimbi, Margaret N.
    The degradation of soil, vegetation and socio-economic transformations are a huge threat to Africa’s land production. This study aimed to (i) assess the soil and land productivity of standing biomass and (ii) determine the effect of rainfall on the standing biomass in Eastern Africa. Soil productivity was determined using the Soil Productivity Index (SPI) and a simplified model was developed to estimate the Net Primary Productivity (NPP). The SPI indicators used included soilorganic matter, texture, soil moisture, base-saturation, pH, cation-exchange-capacity, soil-depth and drainage. The inputs of the simplified model are: MODIS Soil Adjusted Vegetation Index (SAVI), soil erosion, soil nutrient content and input, rainfall, land-use/cover and agro-ecological zones. The findings reveal that the countries with the most productive soils are Mauritius, Rwanda and South Sudan—while, for standing biomass, the countries with the highest spatial extent are Mauritius (97%), Rwanda (96%), Uganda (95%), South Sudan (89%), Ethiopia (47%) and Kenya (36%). Standing biomass is dominant in biomes such as natural forests, woodlands, croplands, grasslands, wetlands and tree-plantations. High land productivity was attributed to soil quality and management, land policy reforms, favourable climatic conditions and sustainable land husbandry activities. Rainfall was significantly correlated with standing biomass in most of the studied countries (p < 0.05) except Djibouti and Rwanda. Therefore, monitoring soil health, use and land reforms are key to sustaining vegetative biomass.