The balance between positive and negative interactions in a savanna
Three separate studies investigated the spatial patterns observed in central Texas savannas and the potential plant interactions (i.e. competition and facilitation) that may be causing them. The first study measured the magnitude and the direction of the spatial association between Quercus fusiformis (live oak) and Juniperus ashei (Ashe juniper) in several sites on the eastern Edwards Plateau. J. ashei individuals occurred in significantly higher than expected frequencies under woody canopies and in significantly lower than expected frequencies in the open grassland. The strength of this pattern decreased with J. ashei size, suggesting that facilitation by woody canopy species becomes less strong over the life of a J. ashei understory individual. The second study was a garden experiment testing the hypothesis that whether the effect of shade on Juniperus ashei seedlings is negative or positive depends upon water availability. The effects of three watering treatments and two shade treatments upon survival and growth were measured in a full-factorial design. The results of this study confirmed that the effect of shade upon Juniperus ashei seedlings depends upon water availability. Furthermore, instead of the effect gradually changing from negative to positive along a gradient of decreasing water availability, the effect changed abruptly from negative to positive at the point at which seedlings began to die from water shortage. The third study was a two-year experimental field study of the effects of three overstory types (‘Quercus’ overstory, ‘Juniperus’ overstory, and no overstory) and three substrate types (soil and litter from live oak clusters, from juniper clusters, and from open patches) on the germination, survival, and growth of Juniperus ashei seedlings in a factorial design. The presence of an adult Juniperus ashei facilitated J. ashei seedling germination and survival, but decreased growth rates. Evidently, facilitation of seedlings contributes to the positive spatial association between Juniperus ashei adults and J. ashei seedlings. However, the positive spatial association between adult oaks and J. ashei seedlings is evidently not due to facilitation.