Is Fuchsia excorticata (Onagraceae) seed limited?
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Maintenance of native bird populations is necessary for the effective functioning of bird-plant mutualisms such as pollination and dispersal. The New Zealand native tree Fuchsia excorticata (Onagraceae) is declining because of possum herbivory, and is known to be pollination limited resulting in lower seed set. However, pollination limitation will not be of any consequence if F. excorticata is not seed limited. I tested whether F. excorticata is seed limited. If the species is both pollen and seed limited, it has considerable implications for one of New Zealand’s important secondary succession forest trees, as well as the honeyeating and frugivorous birds that feed on its flowers and fruit. To compare whether local bird density and the pollination service affected the seed supply, I selected two sites with high pest control, and two sites with low pest control. Visitation rates to F. excorticata flowers and bird counts were measured at each site. I measured the seed bank at increasing distance from parent F. excorticata trees, and recorded the sex of parents to determine if pollination limitation was affecting seed set according to parent gender because females are more pollen limited than hermaphrodites. A field seed addition experiment tested for seed, microsite and herbivore limitation. Natural seedling distribution was mapped to distinguish what environmental conditions favoured germination and establishment. Surprisingly, bird visitation to F. excorticata was no higher with pest control, but abundance of honeyeating birds was higher in five minute bird counts. The F. excorticata seed bank decreased with dispersal distance away from the parent tree, was larger in sites with high pest control and also significantly more abundant beneath hermaphrodite parents. On average seven times more seedlings germinated in sown plots than unsown plots, thus proving seed limitation. Caged plots had nearly twice as many seedlings as uncaged plots, which shows herbivore limitation. More seedlings germinated closer to the parent tree than further away. Therefore, F. excorticata is showing evidence of seed, microsite and herbivore limitation, and complex interactions between each. Most importantly, the pollination limitation that F. excorticata experiences due to lack of suitable pollinators causes a reduced seed set which affects the soil seed bank and level of seedling regeneration. This reduced seed supply combined with seed and microsite limitation is affecting the density of F. excorticata seedlings, and thus perhaps ultimately the density of adult trees.