干旱区生态环境知识资源中心

Acacia species in arid environments are thought to only establish in years of above-average rainfall, so should exhibit cohorted or pulsed recruitment. I studied population demography of Acacia erioloba Meyer in semi-arid savanna in the Kimberley area (mean annual precipitation = 425 +/- 132 mm), South Africa, to establish whether they recruit episodically. This species was found to have a sapling bank at the sites, indicating that even though cohorted recruitment probably occurs, it is not the primary factor limiting recruitment to larger size classes. A. erioloba saplings given supplementary water and protection from herbivory showed significantly less height growth than saplings given water but not protected from herbivores, and grass within exclusion plots was taller and denser than outside of exclusion plots. The generalized linear model, although finding a significant difference between watered and protected and watered and unprotected saplings, explained only 16% of variation in growth, demonstrating the importance of factors other than competition from grass when rainfall is above-average. Average height increase across all treatments for a growing season was small (43.1 +/- 30.81 mm), suggesting most investment is belowground. A. erioloba sapling below surface stem diameter correlated positively with aboveground growth, indicating larger, and presumably older, individuals grow faster. When grass was removed around saplings, growth rates were not significantly greater than for saplings surrounded by grass, over a growing season. Thus, in these study sites, pulsed events allow seedlings to establish, and then saplings slowly accumulate over many growing seasons within the grass sward, owing to slow aboveground growth combined with competition from grass in absence of grazers. Release events, e.g., heavy grazing combined with good rainfall, may allow mass release, giving the impression of cohorted or pulsed recruitment.