Studies in the ecology of Nothofagus cunninghamii Oerst. III. Two limiting factors : light intensity and water stress

The observation that Nothofagus cunninghamii seedlings survive only under canopy gaps in closed Nothofagus forest led to an investigation of the light compensation point for this species. The light compensation point of 6-month-old N. cunninghamii seedlings was determined as 1 .15 % of full summer sunlight. This is considerably higher than that for Eugenia smithii (0.1 %), and explains the difference in seedling regeneration patterns in these structurally and floristically similar forest types. It is closer to that for Eucalyptus regnans (1.5-1.7 %), but the slight difference may be sufficient to explain the ability of Nothofagus to invade an E. regnans forest where E. regnans seedlings themselves are unable to survive. N. cunninghamii has failed to spread onto drier sites at Mt. Donna Buang, which may be due to its low drought tolerance. However, experiments indicate that N. cunninghamii seedlings are more drought-resistant than might be anticipated from their wet cool habitat. Two-year-old seedlings in the glasshouse survived up to 7 weeks without watering, as did some younger ones. The relative turgor of leaves at incipient death was 70 %. Observations on planted seedlings in the field and natural coppice Nothofagus during the severe drought of the 1967-68 summer suggested that summer water stress is one of the factors confining Nothofagus to cool moist positions, though such severe selection may only be effected once or twice in a century.