干旱区生态环境知识资源中心

Zones of nutrient enrichment beneath shrubs, the so called `fertile islands', are the result of a range of interacting physical and biotic concentrating mechanisms. These concentrating mechanisms lead to specific spatial patterns of soil properties across a landscape and it has been suggested that disruption of the normal patterns into more aggregated `fertile islands' might be a useful index of desertification in semi-arid regions. The objective of this paper was to determine whether `fertile islands' are as characteristic a feature of succulent deserts as they are of shrub dominated deserts. Results indicate that, even in a community dominated by succulent species fertile islands develop under extremely arid conditions and that the degree of development is dependent on plant dynamics. Long-lived species showed greater concentration effects. However, the patterns of nutrient accretion in islands are very different to shrub invaded grasslands in North America. In particular nitrogen accretion dominated the `fertile islands' in Namaqualand, and we suggest that these islands are more ephemeral than carbon and phosphorus dominated islands reported for other systems. These more dynamic islands are a consequence of rapid community dynamics since the Namaqualand region is characterised by many short-lived perennials which turnover generations very rapidly in response to environmental conditions. Using the development of `fertile islands' to assess desertification as applied elsewhere might be inappropriate in Namaqualand because of the different patterns of community dynamics underlying the formation and characteristics of these islands.