Search

ABSTRACTSeed dispersal service is the proportion of plant species dispersed by a given disperser. It is important since seed dispersal enables colonisation of new habitats, and because movement of seeds even short distances away from trees can reduce vulnerability of seeds and seedlings to disease, predation and density-dependent effects that occur in the immediate vicinity of parent trees. In New Zealand, indigenous large-fruited canopy tree species are susceptible to recruitment failure because Hemiphaga novaeseelandiae(kererū; Columbidae) is the only widespread avian frugivore capable of swallowing large fruit and dispersing their seeds, and its survival relies on ongoing predator control. In view of the critical nature of this mutualism, seed dispersal service was assessed by measuring the difference in composition of seeds of indigenous large-fruited canopy tree species that were dispersed by H. novaeseelandiaetounder the canopies of co-fruiting large-fruited canopy tree species across the Auckland region in New Zealand. If seed dispersal were effective, seeds of all species should be found under the canopies of all tree species, i.e. low beta diversity of seed rain. Beta diversity was higher than expected at two locations, suggesting incomplete seed dispersal service; this pattern was attributed to interactions between frugivore behaviour and the local distribution of fruit. The third location had lower than expected beta diversity, reflecting dispersal of a complete array of tree species’ seeds to sampling sites, plus long-distance dispersal of nonlocal indigenous species of large seeds. Hemiphaga novaeseelandiaeappears to be dispersing an array of species of large seeds between co-fruiting tree species at some Auckland locations. However, incomplete seed dispersal service at two of the locations, together with evidence of low recruitment of some species of seeds, could eventually reduce tree species diversity.

ABSTRACTThe non-random selection of fruits by frugivores can affect plant demography by changing the probability of seedling recruitment. We investigated whether the New Zealand pigeon (Hemiphaga novaeseelandiae) exhibits size-based preference of fruit in two indigenous tree species, Vitex lucensand Prumnopitys ferruginea, that have large fleshy diaspores containing single seeds. We also assessed if there was an advantage for seeds that were actively dispersed since birds may select fruits that have viable seeds (although the cue for such selection remains unclear) and whether a combined scarification–deinhibition advantage was conferred on germination due to seed passage through the gut. Using seed length as a proxy for fruit size, we compared the length of passively dispersed fruits in traps with the estimated fruit length of actively dispersed seeds in six trees of each species across three areas in one fruiting season. For both species, actively dispersed seeds were significantly longer than seeds from passively dispersed fruit, suggesting frugivore preference for larger fruit. Seeds from passively dispersed P. ferrugineafruit had significantly higher viability than actively dispersed seeds and there was no measurable scarification–deinhibition effect. For V. lucens, there was no significant difference in viability between actively and passively dispersed seeds and there was a total lack of seed germination in our trial. Our results show a significant preference by H. novaeseelandiaefor larger fruit in both tree species, which was possibly associated with annual fruit production since P. ferrugineatrees that had more fruit produced larger fruit on average. That H. novaeseelandiaeselected longer fruit in our study is consistent with the prediction that birds will prefer larger fruit when beak gape limitation is absent. However, active dispersal of larger seeds may not always be advantageous since passively dispersed intact fruit germinated equally well and may even have had a viability advantage.

The productivity and decomposition of Baumea juncea and Gleichenia dicarpa were measured at two sites at Kaitoke Swamp, Great Barrier Island, New Zealand, to determine their annual biomass dynamics. These interactions, including resulting peat accumulation, are poorly understood, especially for wetland sedges. Annual productivity and maximum “standing” above-surface and below-surface biomass were measured using sequential harvest plots. Decomposition was determined using recently dead B. juncea culms and G. dicarpa fronds confined in mesh litter bags placed on, above, and below the swamp surface.The annual productivity of B. juncea (c. 1.4 kg m-2) was within worldwide estimates for swamp wetland species, but that of G. dicarpa (c. 0.5 kg m-2) was low. The order of decomposition of B. juncea leaf litter was: surface > above surface > below surface; but in the case of G. dicarpa there were no significant differences. Litter bag mesh size did not affect decomposition processes for either species, indicating that larger decomposer animals probably do not have a significant influence on biomass loss. Peat accumulation was estimated based on observations and measurements of the entrainment process. Baumea juncea and G. dicarpa both had high peat accumulation, with 71.6% (0.99 kg m-2 yr-1) and 42.8% (0.22 kg m-2 yr-1) of the annual production remaining, respectively, after all components had been underground for 5 years. Consequently, only a small amount of the annual production of biomass is lost from both sites.

The productivity and decomposition of Typha orientalis at Kaitoke Swamp on Great Barrier Island, New Zealand, were measured to determine the annual dynamics of its biomass. These interactions, including peat accumulation under nutrient-rich conditions, are often poorly understood. Annual productivity and maximum “standing” above-surface and below-surface biomass were measured using sequential harvest plots. Decomposition was determined using recently dead T. orientalis leaves in mesh litter bags placed on, above, and below the swamp surface.The annual productivity of T. orientalis (c. 3 kg m-2) is within world-wide estimates for swamp wetland species. The order of decomposition of leaf litter at Kaitoke Swamp is: below surface > surface > above surface. Litter bag mesh size did not affect decomposition processes, indicating that larger decomposer animals probably do not have a significant influence on biomass loss in the site. Peat accumulation was estimated based on observations and measurements of the entrainment process. T. orientalis had low estimated peat accumulation, with 6.5% (0.19 kg m-2 yr1) of the annual production remaining after all components had been underground for 5 years. Consequently a large amount of the annual production of biomass is lost from the site. The ratio of maximum biomass to annual productivity was 2.2, also indicating low biomass accumulation.

Whatipu Beach, north Manukau Harbour, Auckland, has prograded episodically over 1.5 centuries. Sand dune vegetation is quantitatively described, and related to successional stage, local environment, and progradation dynamics. Foredunes, with fine, mobile, infertile sand, occupy the most recently prograded areas and are partially vegetated by native species. Instead of a dune slack, a stream which changed course to flow between dune ridges has created wet “sand river” vegetation. Relict fore-dunes and a dune slack persist inland of the sand river, disrupting a lineal spatial sequence, as do rear dune wetlands. Less-mobile grass and shrub communities, derived from Ammophila arenaria dunes, occupy surfaces >50 years old. No climax communities are present. In contrast to spatial analogues, the Whatipu dunes demonstrate how rapidly succession proceeds in response to the formation of new habitat, while freshwater movements generate non-seral sand-river communities. Succession is modified by invading exotics and by the absence of many disturbance-intolerant native shrub species. Coastal progradation rates at Whatipu indicate that shrubland develops within 50 years, suggesting that spatial analogues of dune succession can mis-represent the longevity of early seral stages. These unusual successional patterns on dateable surfaces, and the presence of rare flora and fauna, make Whatipu Beach important for conservation.