Title : Functional and species composition of understory plants varies with mistletoe-infection on host trees in a semi-arid African savanna
Abstract:
Mistletoes are a keystone species that influence plant and resource heterogeneity in semi-arid savannas. They enhance nutrients underneath tree canopies via high leaf turnover, nutrient-rich litter, and highly decomposable leaf litter, which also facilitate host litter decomposition; and through droppings and other debris from birds and animals visiting the mistletoes. Further, mistletoes reduce their hosts’ competitive edge, thus creating microhabitats that support higher understory species richness. However, little is known on how understory species and functional trait assemblages, in both canopy and intercanopy spaces, are affected by varying mistletoe intensities. This study examined whether there were variations in species composition, species and functional diversity of understory plants within high- and low mistletoe-infection canopy patches and their adjacent intercanopy spaces. A ‘mistletophily index’, which calculated the affinity of each species to each of the four different microhabitats, was developed. The four microhabitats had significantly different species compositions, and subcanopy patches had significantly higher species richness and diversity, and functional diversity, compared to their adjacent intercanopy spaces. While 34% of the recorded species had a strong affinity towards canopy patches, 9% were associated intercanopy spaces. Intercanopy spaces were mainly dominated by high grazing value grasses and V. karroo juveniles, whilst canopy patches of high mistletoe-infection had significantly higher grass (of mixed grazing value), forb, and tree diversity. High animal disturbances contributed to the elevated species richness and species and functional diversity (by reducing understory competition) within high mistletoe-infection canopy patches compared to the other microhabitats. High volumes of animals passing through to the high mistletoe-infection trees could have contributed to the different species composition of high- compared to low mistletoe-infection intercanopies. Decreaser grasses associated with low-intermediate disturbance were abundant in low mistletoe-infection canopy patches. Nonetheless, species richness and diversity, and functional diversity, increased with increasing mistletoe infection, thus grass, forb and tree species diversity were 17% to 43% higher and functional diversity indices were 0.5% to 28% greater in high- compared to low mistletoe-infection canopy patches. Consequently, 15% and 10% of the recorded species showed a strong positive affinity to high and low mistletoe infection canopy patches, respectively. Variations between high and low mistletoe-infection canopy patches, are attributed to higher decomposition rates from the relatively higher mistletoe litter turnover, leading to canopy patches with higher soil nutrients and increased nutrient cycling rates. Subsequently, high mistletoe-infection canopy patches had a higher occurrence of species that favour semi-shade, high soil moisture and nutrients and those that are prevalent on disturbed sites. Therefore, by enhancing spatial heterogeneity, variations in mistletoe infection facilitate biodiversity and to a lesser extent vegetation structural diversity in these semi-arid savannas. This study contributes to understanding the extent to which mistletoes may be more of a friend than a foe. Certainly, overly-high mistletoe densities may kill the host, but by keeping parasite densities in the mid-range, local environment enrichment can be encouraged. Therefore, it is crucial to improve our ability to harness mistletoe positive ecological effects. Thus comparative work is currently being done in Bushbuckridge South Africa to improve this understanding.
