The patchy distribution of trees typical of savannas often results in a discontinuous distribution of water, nutrient resources, and microbial communities in soil, commonly referred to as "islands of fertility". We assessed how this phenomenon may affect the establishment and impact of invasive plants, using the invasion of Opuntia stricta in South Africa's Kruger National Park as case study. We established uninvaded and O. stricta-invaded plots under the most common woody tree species in the study area (Vachellia nilotica subsp. kraussiana and Spirostachys africana) and in open patches with no tree cover. We then compared soil characteristics, diversity and composition of the soil bacterial communities, and germination performance of O. stricta and native trees between soils collected in each of the established plots. We found that the presence of native trees and invasive O. stricta increases soil water content and nutrients, and the abundance and diversity of bacterial communities, and alters soil bacterial composition. Moreover, the percentage and speed of germination of O. stricta were higher in soils conditioned by native trees compared to soils collected from open patches. Finally, while S. africana and V. nilotica trees appear to germinate equally well in invaded and uninvaded soils, O. stricta had lower and slower germination in invaded soils, suggesting the potential release of phytochemicals by O. stricta to avoid intraspecific competition. These results suggest that the presence of any tree or shrub in savanna ecosystems, regardless of origin (i.e. native or alien), can create favourable conditions for the establishment and growth of other plants.

Department of Biological Sciences Macquarie University Sydney NSW 2109 Australia

Department of Botany and Zoology Centre for Invasion Biology Stellenbosch University Stellenbosch South Africa

Department of Botany and Zoology Stellenbosch University Stellenbosch South Africa

Department of Ecology Faculty of Science Charles University Viničná 7 128 44 Prague Czech Republic

Department of Invasion Ecology Institute of Botany Czech Academy of Sciences 252 43 Průhonice Czech Republic

Invasive Species Programme South African National Biodiversity Institute Kirstenbosch Research Centre Claremont South Africa

Scientific Services South African National Parks P Bag X402 Skukuza 1350 South Africa

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