Journal of Tropical Ecology (2009) 25:31–40. Copyright © 2008 Cambridge University Press doi:10.1017/S0266467408005683 Printed in the United Kingdom The savanna tree Acacia polyacantha facilitates the establishment of riparian forests in Serengeti National Park, Tanzania Gregory J. Sharam *,1 , A. R. E. Sinclair * , Roy Turkington *, † and Aerin L. Jacob * * Biodiversity Research Centre, University of British Columbia, 6270 University Blvd., Vancouver, BC, Canada, V6T 1Z4 † Department of Botany, University of British Columbia, 3529-6270 University Blvd., Vancouver, BC, Canada, V6T 1Z4 (Accepted 10 November 2008) Abstract: Forests are being converted to grasslands and croplands across Africa and natural regeneration of forests is typically poor. In Serengeti National Park, Tanzania, the savanna tree species Acacia polyacantha established in riparian grasslands and forest trees subsequently established within these stands. We examined the conditions for establishment of: (1) A. polyacantha and (2) riparian (non-Acacia) forests. Fire was excluded from three grassland areas for 5 y allowing A. polyacantha to establish during 1999 when dry-season rainfall was high. The seedlings of forest tree species did not establish in grasslands, but were found in large A. polyacantha stands (> 0.3 ha) with reduced grass cover (< 10%), higher cover of herbs (> 80%) and thorny shrubs (> 90%). Seeding survival was high in large stands (0.87 y -1 ), but declined in artificial canopy gaps due to the ingrowth of grasses (0.21 y -1 ) and subsequent fires (0.07 y -1 ). Shrub removal also reduced seedling survival (0.46 y -1 ) due to browsing by antelope. We propose that: (1) A. polyacantha establishes in pulses perhaps as infrequently as twice per century, and (2) riparian forests in Serengeti have established via facilitation under larger stands where shade excludes grass, and therefore fires and thorny shrubs exclude browsers. Key Words: Acacia, browse, elephant, fire, forest, impala, recruitment, riparian, seedling INTRODUCTION Many tropical areas can support alternative vegetation communities: open grasslands, mixtures of grasses and savanna trees, or closed-canopy forests. The establishment of forest trees into African grasslands is typically poor, despite the removal of fire, leading to ecological and conservation concerns as forests are removed and not replaced (Chapman & Chapman 2003). Kellman & Miyanishi (1982) proposed that savanna trees first establish in grasslands and subsequently facilitate the establishment of forest trees. In Serengeti National Park, Tanzania, where establishment of forest trees is effectively limited in grasslands (Sharam et al. 2006) we observed stands of Acacia polyacantha Willd., a savanna tree species, establishing as stands in grasslands adjacent to rivers during 1978. Today, regeneration of forest trees is associated with some of these stands. In this paper, we examine the conditions under which stands of savanna trees establish in grasslands and subsequently facilitate the establishment of forests. 1 Corresponding author. Email: sharam@zoology.ubc.ca The establishment of forest trees into African grasslands is poor, due to repeated fires (Sharam et al. 2006), com- petition with grasses (Chapman et al. 1999, Sharam et al. 2006), low soil fertility, poor seed dispersal (Chapman & Chapman 1999), seed predation by rodents (Duncan & Duncan 2000) and browsing by antelope (Sharam et al. 2006). In Serengeti, forest stands occur in riparian areas, composed of common species from large forests elsewhere in Africa. Establishment of these forests in Serengeti is primarily limited in grassland by fires, browsing and competition with grasses (Sharam et al. 2006). In contrast, savanna tree species such as Acacia will readily establish in grasslands when conditions are correct – when fires (Sinclair et al. 2007) and/or browsers (Prins & van der Jeugd 1993) are removed or dry-season rainfall is higher than average (Higgins et al. 2000). How- ever, the relative importance of these variables depends on the site. In recent history, Acacia trees have undergone two pulses of natural establishment in Serengeti. The first occurred from 1890–1930 when the epizootic rinderpest removed cattle and other ungulates, and thus people and fires, from the ecosystem for ∼30 y (Sinclair et al. 2007). The second occurred in the late 1970s when a