Proximity signal and shade avoidance differences between early and late successional trees

23 February 2005

Competitive interactions between plants determine the success of individuals and species. In developing forests, competition for light is the predominant factor. Shade tolerators acclimate photosynthetically to low light1±3 and are capable of long-term survival under the shade cast by others, whereas shade avoiders rapidly dominate gaps but are overtaken in due course by shade-tolerant, later successional species. Shade avoidance4±6 results from the phytochrome-mediated perception of far-red radiation (700± 800 nm) scattered from the leaves of neighbours, provides early warning of shading7, and induces developmental responses that, when successful, result in the overgrowth of those neighbours8. Shade tolerators cast a deep shade, whereas less-tolerant species cast light shade9, and saplings tend to have high survivorship in shade cast by conspecific adults, but high rates of mortality when shaded by more-tolerant species9. Here we report a parallel relationship in which the shade-avoidance responses of three tree species are inversely proportional to proximity signals generated by those species. On this basis, early successional species generate small proximity signals but react strongly to them, whereas late successional species react weakly but generate strong signals.