Conspecific negative density dependence in a long-lived conifer, yew Taxus baccata L.
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Conspecific negative density dependence in tree species can have major consequences for structuring of communities, yet in temperate forests this phenomenon remains largely unexplored. We investigated density-dependent recruitment in the long-lived conifer yew (Taxus baccata L.), for which low levels of regeneration have been linked with reduced light availability beneath conspecific canopies and speculative autotoxic effects. We combined in situ assessment of yew regeneration in a temperate forest in southwest Ireland with ex situ pot experiments to assess whether light and/or beneath conspecific canopy conditions inhibit the germination and early growth of yew. In field experiments, recruitment was related to distance from adult conspecifics, with higher levels of regeneration found in areas not beneath yew canopies. However, reduced light availability beneath the dense yew canopy was not responsible for this pattern, with abundant seedlings found in areas of equivalent or even lower light availability. Pot experiments showed that seedling survival was highest in deep shade treatments. Experiments also demonstrated that the addition of yew needles negatively affected seedling growth, but not seed germination. Together, our results suggest that under natural forest conditions, the absence of regeneration beneath mature yew canopies, often attributed to low light availability, may at least be partly related to substances in yew canopy or leaf leachates.
KeywordsJanzen–Connell Negative plant–soil feedbacks Natural regeneration Autotoxicity
The authors thank Grace Cott and Alan O’Connor for their assistance in fieldwork. Acknowledgement to National Parks and Wildlife Service, Department of the Environment, Heritage and Local Government for access to some lands. Particular thanks to Declan O’Donnell and Clare Heardman for facilitating research at the Glengarriff Wood Nature Reserve and to Killarney National Park for providing plant material. The authors gratefully acknowledge the guidance of two anonymous reviewers. This study was funded by a University College Cork departmental postgraduate fellowship.
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