Phenotypic variation in heteroblastic woody species does not contribute to shade survival

Leaf heteroblasty involves dramatic phenotypic differences between adult and seedling leaves while leaves of homoblastic plants display only small differences. This study tested whether, in low-light environments, the marked difference in the morphology of seedling leaves that characterizes heteroblastic species confers advantages for seedling survival and growth compared with homoblastic congeners.

Four pairs of heteroblastic and homoblastic species in genera Hoheria, Aristotelia, Pseudopanax and Melicope were grown in simulated full sunlight (100 % of light, red:far red ratio (R:FR) = 1.25) or in simulated forest understorey shade (5 % of full sunlight, R:FR ratio = 0.25) in a glasshouse.

After 9 months, 100 % of seedlings of both homoblastic and heteroblastic species survived in full sun while in the understorey treatment there were 25 % fewer heteroblastic survivors than homoblastic congeners. Compared with homoblastic congeners, all heteroblastic species except for Pseudopanax crassifolius produced more and smaller leaves and branches, but grew more slowly in height, root collar diameter and total biomass both in full sun and in forest understorey treatments.

Homoblastic species survive and grow better in the forest understorey light treatment, suggesting that heteroblastic seedling leaf morphology does not give an advantage over homoblastic congeners under low light intensities.