Dark opening of stomata in successional trees.

Dark opening of stomata was found in shade intolerant tree species in three different sets of experiments. In the field, leaves of eight successional species were darkened in light-proof bags and leaf resistance measured with a diffusion porometer. White ash, quaking aspen and American beech were sampled more intensively using darkened cuvettes and a dew point hygrometer. In the lab, white ash seedlings were kept in constant darkness and temperature, and their leaves also monitored with a cuvette and hygrometer. The stomata of all sampled shade intolerant species showed a dark opening response, often to leaf resistance levels typical of illuminated leaves. Shade tolerant species did not. Evidence suggests that this response is endogenous and could contribute to the morning opening of stomata of species under moderate water stress growing in well illuminated habitats.