Tree fern competition reduces indigenous forest tree seedling growth within exotic Pinus radiata plantations

Abstract

Tree ferns are recognized as “keystone” species for their role in casting deep shade on forest floor environments, acting as a differential ecological filter on forest regeneration processes. Tree fern dominance is a feature of New Zealand’s central North Island exotic Pinus radiata (Pinaceae) plantations, particularly beyond 20years following plantation establishment. In circumstances where plantation forests are managed for restoration of indigenous forest species, rather than timber harvest, an understanding of the competitive role of dense tree fern stands on forest regeneration processes in these “non-harvest” plantations is necessary.We investigated the competitive effect of tree fern (Cyatheaceae and Dicksoniaceae) cover on seedling growth rates of two mature forest tree species, Podocarpus totara (Podocarpaceae) and Pittosporum eugenioides (Pittosporaceae), over two austral summers in five 36–92-year P. radiata plantations. We experimentally manipulated tree fern canopies in order to contrast heavy versus light shading. Tree fern removal significantly increased the growth rate of both P. totara and P. eugenioides, and the growth rate of the shorter-lived, faster-growing, light demanding P. eugenioides was greater than that of P. totara with tree fern removal. Damage to the seedlings by mammalian animal browsers when tree ferns were removed was greater than that occurring under tree fern cover. This damage diminished the effect of tree fern removal on seedling growth rates for P. totara, but not for the faster growing P. eugenioides.The physical clearance of tree fern stands and then the establishment of fast growing, locally representative, indigenous canopy tree species of low palatability, along with protection from animal browse, are suggested restoration strategies. Our results are consistent with existing literature in showing that where tree ferns are a dominant feature of plantation understories, deep shade is likely to reduce canopy species seedling growth rates, reducing the potential for forest regeneration.