The Savanna Tree Acacia polyacantha Facilitates the Establishment of Riparian Forests in Serengeti National Park, Tanzania

background

This study tests if stands of Acacia polyacantha facilitate the establishment of forest trees by examining the distribution, growth, and survival of juvenile forest trees. The influence of stand size, locations within the stand, presence of canopy trees, competition from grasses, effects of fires and effects of thorny shrubs on herbivore exclusion and conditions for establishment of A. polyacantha in grasslands were examined in Serengeti National Park of Tanzania.

Research Goals & Methods

The understory of A. polyacantha stands was surveyed for seedlings of riparian forest species. For each seedling detected they recorded the species, its height, grass height, distance to the main stem of the nearest shrub, distance to the bole of the nearest A. polyacantha canopy tree, and canopy, shrub, and herb cover in a 1-m2 plot centered on each seedling. The distance at which fire penetrated stands was measured and the seedlings scorched were tagged and recorded. Tagged seedlings were monitored for survival for 2 years.

Conclusions & Takeaways

The results showed that A. polyacantha could establish in grassland and facilitate the establishment of forest trees. This is because stands of A. polyacantha have 4 interrelated factors; reduced grass density; reduced fire frequency; dense thorny shrubs that exclude browsers and increased soil moisture during the dry season. This species can establish itself in grassland when fire is suppressed and dry-season rainfall is higher than normal. The authors’ hypothesis that the establishment of A. polyacantha followed by forest trees facilitated riparian forests along Grumeti River in the Serengeti. In conclusion, the establishment of forests through facilitation by A. polyacantha has wide reaching implications. Which is, efforts to restore forests may encounter the effects of fire and browsers, for which A. polyacantha can be used as a management tool. Managers will have to ensure that stand size is sufficient to provide appropriate conditions for the establishment of the seedlings of forest trees, creating an understory with reduced grass and fire, increased soil moisture and increased thorny shrubs that exclude browsers.