Growth rates of shrubs on different soils in Tanzania

Because little is known of growth rates of shrubs in East Africa, the growth rates of Acalypha fructicosa, Gardenia jovis-tonantis, Justicia cordata, Maerua triphylla, and Ocimum suave were measured in Lake Manyara National Park, northern Tanzania. Branch diameter increments and branch length increments were measured, based on wood-ring counts. Linear regressions fitted through the origin yielded highly significant regressions and thus growth rates. Growth rates of shrubs growing on volcanic soils were significantly higher than of those growing on nutrient-poor soils derived from Basement Complex rock. The average diameter increment for all shrubs was 2-65 mm y-1 on the volcanic soil and 1-78 mm y-1 on the nutrient-poor soil, and on average the branch length increment was 18.9 cm y-1 and 12.1 cm y-1, respectively. Implications for bush encroachment are discussed.