Facultative hemiepiphytism as a recruitment strategy in small‐seeded tree species

Abstract

Aims: Shade-tolerant tree species often produce larger seeds with greater energetic resources to cope with light limitation. On the other hand, shade-intolerant species often produce smaller seeds with greater dispersal potential to colonise lighter, more disturbed areas of forest. We test the hypothesis that small-seeded species utilise an alternative recruitment strategy in a southern temperate rainforest by establishing epiphytically on the trunks of tree ferns. Location: Wellington region, North Island of New Zealand. Methods: To assess whether small-seeded tree species rely on tree fern epiphytism for recruitment we quantified relative abundances of both epiphytic and terrestrial subpopulations in 14 woody plant species across a range of seed sizes. We used a paired study design where we surveyed all 3,727 woody plants occurring on 322 tree ferns and in matching forest floor plots of equivalent area. Using a linear model, we then assessed the relationship between species’ seed size and their epiphytic tendency (arboreality). Results: Arboreality scores differed both between species and between life history stages. Seed size predicted arboreality regardless of life history stage, with small-seeded species more likely to occur arboreally than larger-seeded ones. However, the effect of seed size decreased predictably in later life history stages. Seed size also predicted arboreality when the model was rerun using subsets of data restricted to common understorey tree ferns. Conclusion: Interspecific differences in the epiphytic establishment of New Zealand tree species are pronounced, with only smaller-seeded species able to use this regeneration niche. Small-seeded species, especially Weinmannia racemosa, appear to be utilising an alternative recruitment strategy by establishing epiphytically on tree fern trunks.