ProtégéZiziphus mucronata (Rhamnaceae) show no negative effects of competition with the nurse tree Acacia (Leguminaceae), even as adults

Question: A number of studies have demonstrated that the interaction between nurse plants and their protégés changes from one of facilitation to interference as life history stage progresses. Here, I aimed to assess relative fitness of the protégé plant Ziziphus mucronata (Rhamnaceae) under the subcanopy of Acacia erioloba (Leguminaceae), versus in the open, at various stages of the lifecycle. Location: Southern Kalahari, southern Africa. Methods: Germination was compared to assess the effects of shade, maternal origin (under A. erioloba or in the matrix) and soil. Seedlings were transplanted into the field under trees and in the open to ascertain establishment rates. Flower production, seed set and mistletoe infestation were compared between the two microhabitats. Results: Seeds from maternal Z. mucronata plants growing beneath A. erioloba had higher germination rates than those from maternal plants growing in the open. Germination was higher in full sun and in matrix soil, although this result was found in experimental conditions mimicking good rainfall and may not hold under dry or medium rainfall conditions. Z. mucronata demography suggested that the A. erioloba subcanopy is important for establishment, but seedlings transplanted into the matrix had higher survival rates than those under trees. Causes of seedling mortality were different: desiccation in the matrix, but invertebrate herbivory and trampling beneath trees. Mistletoe infestation was higher for adult plants growing in the open. Seed set was not significantly different between subcanopy and matrix environments, but once mistletoe infestation rates and parent plant size were accounted for, seed set was lower for adult plants growing in the open than those growing beneath trees. Conclusions: It appears that the nurse tree environment remains important for Z. mucronata throughout its lifecycle.