Fruit types and seed dispersal modes of woody plants in Ahuriri Summit Bush, Port Hills, western Banks Peninsula, Canterbury, New Zealand

Abstract

In Ahuriri Summit Bush Scenic Reserve, a 5 ha area of mixed angiosperm-podocarp forest, are 56 wild woody plant species, including five adventives. Lists are given for fruit types, seed numbers per fruit, fruit sizes, degree of fleshiness of the pericarp or accessory tissues of ripe fruit, fruit colours, and dispersal modes for the seeds. Of the 22 tree species, 86% have fruit with fleshy pericarps or accessory tissues; corresponding values for the 20 shrub species are 80%, and for the 14 vine species 50%. Most of these fleshy fruit (87%) are between 2 and 8 mm in diameter. The fruit of only four species exceed 10 mm in diameter. The most frequent fleshy fruit colours are black to dark purplish-black (40%), or red (25%). Some fruit are purple, blue, orange, yellow, or white. It is assumed that birds eat the fruit and disperse the seeds of all fleshy-fruited species (and the seeds of Pittosporum spp.). Seeds of the dry-fruited species are dispersed by wind or gravity alone. One or more of kereru (Hemiphaga novae-seelandiae), bellbird (Anthornis melanura), blackbird (Turdus merula), and silvereye (Zosterops lateralis) were observed feeding on the fruit of 22 of the fleshy-fruited woody species (and one Pittosporum species) (55% of the total). The seeds of 33 (80%) of these species, with the fleshy pericarps or accessory tissues cleaned off after passage through birds, have been caught in seed traps sited in the forest. Bellbirds and blackbirds swallow fruit up to c. 10 mm diameter and are thought to be the most important seed dispersers because they live in the reserve throughout the year and move frequently as they feed. Kereru eat fruit of all sizes, are present only from summer to early winter, and move less frequently while feeding. Silvereyes are present throughout the year. They swallow fruit up to c. 6 mm diameter, but peck the flesh from larger fruit. They move frequently while feeding. The fruit properties and dispersal modes for seeds of the woody plants of Ahuriri Summit Bush are believed to be broadly representative of those of the plants of mixed angiosperm forest with scattered podocarps on Banks Peninsula and other lowland New Zealand locations where Nothofagus is absent. Possible mutualistic relationships between frugivorous birds and the woody plant flora are discussed. The evolution of the array of fruit sizes could be related to the gape sizes of the frugivorous birds present before the human invasion of New Zealand. The depletion of the native seed-dispersing birds may jeopardise the effective regeneration of the mixed forests. Naturalised blackbirds and silvereyes play important compensating roles, to some extent.