Cold tolerance of photosynthesis as a determinant of tree species regeneration patterns in an evergreen temperate forest

Niche partitioning of light among seedling species is a key mechanism supporting coexistence in forests. Species sort along light gradients through direct responses to light and through indirect responses mediated by other environmental factors. Canopy gaps in temperate evergreen rainforests experience sub-zero temperatures and thus gap-dependent species are vulnerable to cold photoinhibition from exposure to high light at low temperatures. We used a shadehouse experiment to test two hypotheses: (1) that gap-dependent species are resistant to cold photoinhibition; and (2) that gap-dependence observed in the field may be driven by the interaction between high light and low temperatures. Specifically, we predicted that some species restricted to shade in the field are excluded from gaps because of low resistance to cold photoinhibition. Gap dependence of angiosperm and conifer seedlings was estimated from expert opinion, and from experimental growth and survival responses to light treatments representing a forest understorey and forest gap. Additional seedlings were used to evaluate resistance to cold photoinhibition (sub-zero temperatures at dawn). Gap-dependent species were resistant to cold photoinhibition. Our second hypothesis was supported by Beilschmiedia tawa (Lauraceae), which had low resistance to photoinhibition, a strong positive growth response to the light treatments, and is restricted to shade in the field. Seedling regeneration niches in temperate rainforest are shaped in part by the interaction between light and low temperatures, and this interaction will be crucial for determining seedling responses to climate warming.