Pioneer tree ferns influence community assembly in northern New Zealand forests

Cyathea medullaris (Cyatheaceae) is a frequent pioneer of disturbed areas (e.g. landslides) or edge environments, sometimes forming near continuous canopies. We test the hypothesis that colonisation by this species as a pioneer alters the seedling assemblage to favour more shade-tolerant broadleaved trees than that beneath another common native pioneer (Kunzea robusta, Myrtaceae) in the same landscapes. We compared vegetation and abiotic characteristics of 166 sites across the Auckland region where C. medullaris or K. robusta were abundant (≥20% basal area) along successional gradients. Using hierarchical classification and ordination, we identified distinct communities associated with the different pioneers. In the forests we consider, C. dealbata is another common understorey tree fern, which sometimes, but not always, co-occurs with C. medullaris. Cyathea medullaris / C. dealbata successions occurred on steep sites with lower annual water deficit whereas K. robusta / C. dealbata successions were located on flatter, drier sites. The prevalence of macro-charcoal in K. robusta / C. dealbata forest suggests the prominence of that community is in part an outcome of the increased importance of fire disturbance in New Zealand. Dominance of C. medullaris, with C. dealbata understoreys, influences community assembly of tree species towards dominance by shade-tolerant species, whereas seedlings of less shade-tolerant small-leaved species are more prevalent in K. robusta / C. dealbata forests. We provide evidence to suggest that, where present in early forest communities, high tree fern abundance influences the assembly of seedling communities, supporting our hypothesis. Contrary to previous suggestions, a high abundance or basal area of tree ferns on sites not historically affected by fire did not limit the establishment and growth of canopy trees including fleshy-fruited broadleaf species.