Effects of rain, nitrogen, fire and grazing on bush encroachment in semi-arid savanna, South Africa

[摘要] ENGLISH ABSTRACT: Increases in woody plant density ('bush encroachment') reduce livestock production and biodiversity. Byconvention, soil moisture, soil nutrients, fire and herbivory are regarded as the principal factors governing thetree-grass ratio of savannas. An experiment with a completely-crossed design was employed to investigatewoody seedling (Acacia me/lifera) recruitment near Kimberley, Northern Cape, South Africa, after fire andunder conditions of maximum-recorded rainfall, nitrogen addition and grazing. The field experiment wasrepeated as a garden experiment to determine if the two experiments yield comparable results.Tree germination in the field was extremely low, probably due to below-average natural rainfall in plots thatonly received natural rain, and insufficient watering frequency in irrigated plots. As a result of lowgermination, none of the treatments (rain/nitrogen/fire/grazing) had a significant effect on tree recruitment inthe field experiment. The duration of the experiment (2000/2001 growing season) was insufficient for thetreatments to affect grass composition, although the high rainfall treatment and grazing exclusion significantlyimproved grass cover and height. The garden experiment showed that frequent watering, no nitrogenaddition and grass clipping significantly enhanced tree germination and survival (termed 'recruitment'). Therewere also significant interactions among rain, nitrogen and grazing in their effects on tree recruitment. Theeffects of rain on tree recruitment were more pronounced under nitrogen supplementation and vice versa.Similarly, high rain and high nitrogen enhanced the effect of grazing on tree recruitment.It is inferred that above-average rainfall years with frequent rainfall events are required for mass treerecruitment. Tree seedlings can further benefit from space and resources which are made available throughgrass defoliation. Conversely, nitrogen enrichment improves the competitive ability of the fast-growinggrasses relatively more than that of the N2-fixing tree component, thereby suppressing tree recruitment. Incontrast to conventional wisdom that grazing alone causes encroachment, it is suggested that there arecomplex interactions between the abovementioned factors and 'triggering' events such as unusually highrainfall.Contrary to many claims that equilibrium models are inappropriate for explaining savanna dynamics, it wasshown that consumer-resource theory has explanatory power for bush-grass dynamics of the savannastudied. The state-space approach that was used facilitated the understanding of savanna dynamics andenabled predictions about the system's response to perturbations. The applicability of consumer-resourcetheory to semi-arid nutrient-poor savannas confirmed the importance of resource competition in structuringnatural systems.

[发布机构] Stellenbosch University