Functional traits of Ziziphus mucronata below mistletoe-infected trees in a semi-arid African savanna

Abstract

Mistletoes are increasingly associated with elevated leaf litter, soil nutrients, and soil moisture, resources which influence variations in plant functional traits. Despite this recognition, variations in the functional traits of understorey plants with overstorey (host) tree mistletoe infection are yet to be examined. Here, we measured the different functional traits (height, stem diameter, canopy area, leaf area, specific leaf area, whole-leaf thickness, leaf dry matter content, and chlorophyll content) of Ziziphus mucronata, a dominant woody plant beneath mistletoe-infected Vachellia karroo trees in semi-arid savanna. Two-sample t-tests were used to compare host size, mistletoe infection intensity, and trait variables between low and high mistletoe-infected trees. The relationships between Ziziphus mucronata functional traits vs. host tree diameter and the number of mistletoes per tree were explored using regression analysis and visualized using a regression biplot based on a redundancy analysis (RDA). Host tree height, canopy area, and canopy volume were strongly positively (r > 0.7, p < 0.05) related to mistletoe infection intensity. While most of the traits did not vary with mistletoe infection, the chlorophyll content and leaf area of understorey Z. mucronata increased with host tree size, being greater beneath high than low mistletoe-infected trees. These variations are linked to changes in limiting resources such as light, soil nutrients, and soil moisture due to accumulation of mistletoes and increase in size as the host tree ages. As a result, the understorey plants shifted from being resource conservative to being acquisitive as limiting resources increased. The general lack of trait plasticity in understorey Z. mucronata suggests that plastic allocation responses may not be a general consequence of mistletoe infection.