Establishment patterns and host tree preferences of the emergent hemi‐epiphytic tree Metrosideros robusta in northern New Zealand

Abstract

The relative importance of terrestrial and epiphytic establishment of the emergent tree Metrosideros robusta (Myrtaceae) and its host tree species and sizes, were surveyed in seven different forest types with different disturbance histories in northern New Zealand. At two disturbed sites, M. robusta established terrestrially. At less disturbed sites, most M. robusta were hemi‐epiphytic, but terrestrially established individuals had colonised occasional canopy gaps. M. robusta occurred more commonly on large host trees (>50 cm diameter) than either relative density or relative basal area of host trees would predict, suggesting that large trees are intrinsically better places to establish and not just bigger targets. M. robusta grew on 20 of the 42 tree species sampled. Vitex lucens, dead trees, and the podocarps Dacrydium cupressinum, Dacrycarpus dacrydioides, and Prumnopitys ferruginea were more common as hosts than their relative densities predicted at all sites where they were present. Other host species such as Podocarpus hallii and Laurelia novae‐zelandiae were “preferred” at some sites, but not at others. M. robusta tended to establish in different sites on different host tree species. Hauturu (Little Barrier Island), where the Australian brushtail possum (Trichosurus vulpecula) has never been present, had the highest number of saplings and fewest dead M. robusta, suggesting that this introduced browser is changing the size structure of mainland New Zealand M. robusta populations.