Effects of excluding goat herbivory on Acacia tortilis woodland around pastoralist settlements in northwest Kenya

摘要

Browsing by goats is considered to cause poor tree regeneration and reduced tree growth around settlements throughout the arid zones of sub-Saharan Africa. This study investigated whether excluding goats from Acacia tortilis woodlands increased tree regeneration, current season's shoot growth rates and browse production over a period of 52 months between 1986 and 1990. The study also investigated the effects of climatic variability on tree growth and browse production. Excluding goal herbivory provided no advantage over continuous browsing for juvenile A. tortilis. Trees on the unbrowsed and on browsed transects increased by 22.2 (standard error [SE] +/- 0.53) cm.yr(-1) and 25.0 (SE +/- 0.58) cm.yr(-1) respectively. Fewer but longer shoots were produced by trees on the unbrowsed transects, while trees on the browsed transects invested more in shorter shoots. Net total browse production was lower on unbrowsed (1.73 [standard deviation (SD) +/- 4.3] t.ha(-1).yr(-1)) than on the browsed (3.03 [SD +/- 3.6] t.ha(-1).yr(-1)) transects. Biomass production on unbrowsed and browsed transects was closely correlated with rainfall and presumably soil moisture during wet seasons. Relative growth rates (RGR) of current season's shoots in the two treatments did not differ, implying goat herbivory at moderate stocking density (i.e. 13.0 tropical livestock units [TLU] .km(-2)) stimulated shoot growth. RGR remained positive except on the browsed transects during 1990, a dry year. Goat browsing pressure was moderate. Total biomass loss on unbrowsed transects was 15.5 %.yr(-1) compared with 27.7 %.yr(-1) on the browsed transects. These findings do not support the notion that goats always destroy young tries around settlements. Goat herbivory at moderate intensity stimulated shoot productivity. However, the results should not be used to generalize all conditions throughout sub-Saharan Africa, let alone the arid zones of northern Kenya. Rather, there is a need to emphasize individual case studies that ultimately can be used for managing degraded woodlands near pastoralist settlements. (C) Elsevier, Paris. [References: 39]