Spatial patterns of tree recruitment in a montane Hawaiian wet forest after cattle removal and pig population control

Abstract

Question We measured tree recruitment over approx. 17 to 18 years after cattle removal and pig population control in a Metrosideros polymorpha (Myrtaceae) montane wet forest. We analysed how tree recruitment varies in space and between species. Location Hakalau Forest National Wildlife Refuge, Island of Hawai'i. Methods All woody plants with diameter at breast height ≥5 cm were surveyed twice at approx. 17‐ to 18‐year intervals in 200 circular plots (15‐m radius) located in two sites. We used generalized linear mixed models to analyse how the number of recruits of wind‐ and bird‐dispersed species per plot varied between plots as a function of the number and sizes of remaining trees, grass cover, distance to the forest–grassland edge, and the identity of sites. Within plots, for each species we analysed the density distribution of recruits as a function of a neighbourhood crowding index and grass cover. Observed distributions were compared to null distributions. Results About 4,000 recruits representing seven species were inventoried. Wind‐dispersed species tended to recruit randomly, including in open, grassy areas. The number of bird‐dispersed recruits was negatively associated with grass cover and positively associated with basal area of remaining trees. Bird‐dispersed species recruited more in areas with an intermediate neighbourhood crowding index. The number of recruits was strongly correlated with the number of remaining trees for each species. One species, Cheirodendron trigynum (Araliaceae), recruited twice as much as expected given the number of remnant trees. Conclusions The high levels of recruitment of all species were likely due to cattle removal as well as pig population control. Competition with grasses constitutes the main limiting factor for tree recruitment following removal of ungulates. Remnant Metrosideros polymorpha likely create favourable conditions for the recruitment of bird‐dispersed species. Higher seed production and growth rates likely allowed Cheirodendron trigynum to recruit much more than the other species.

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