Correlates of tree species sorting along a temperature gradient in New Zealand rain forests: seedling functional traits, growth and shade tolerance

It is widely believed that species sorting on environmental gradients reflects trade-offs between competitive ability and physiological tolerance of stresses such as frost and desiccation. One specific expression of this general idea is the hypothesis that tree species sorting on temperature gradients in temperate regions involves a trade-off between growth rate and frost resistance, because adaptations to frost reduce light interception and carbon gain potential. We measured seedling growth of 17 New Zealand rain forest angiosperm trees in a glasshouse, as well as biomass partitioning, gas exchange and hydraulic traits. We then related these variables to the mean July (winter) minimum temperatures most frequently experienced throughout the range of each species. Species associated with mild winters on average had wider vessels and more conductive stems and were leafier (i.e. developed more foliage area per unit sapwood area) than species from frostier sites. Species' positions on the temperature gradient were not significantly correlated with relative growth rates of seedlings; they were, however, negatively correlated with two measures of species light requirements in the field: the light compensation point for growth, and low-light mortality rates obtained from the literature.Synthesis. Although seedling growth rates of warm- and cool-temperate New Zealand angiosperm trees were similar on average, the former are more shade tolerant. Competitive hierarchies associated with tree species sorting on temperature gradients thus probably involve a shift in the relationship between shade tolerance and growth rate, rather than a simple trade-off of growth with cold tolerance. This shift is associated with variation in light interception potential per unit of seedling biomass, possibly reflecting a trade-off between stem conductivity and resistance to freeze-thaw embolism.