Leaf area and growth of juvenile temperate evergreens in low light: Species of contrasting shade tolerance change rank during ontogeny

Abstract

1. Interspecific variation in leaf life span of woody plants should have important consequences for ontogenetic variation in biomass distribution, and hence carbon balance, in low light. This proposal was examined by measuring foliage turnover, growth, biomass allocation and biomass distribution of juveniles (25-1180 mm tall) of four evergreens differing in shade tolerance, growing in low light (2-5% canopy openness) in temperate rainforest understoreys in south-central Chile. 2. As predicted, ontogenetic trends in leaf area ratio (LAR) of two very shade-tolerant species with long-lived leaves (Aextoxicon punctatum and Myrceugenia planipes) contrasted strongly with those of mid-tolerant Eucryphia cordifolia and intolerant Aristotelia chilensis. Low-light LAR fell rapidly with increasing plant size in Eucryphia and Aristotelia, but was size-invariant in the two shade-tolerant taxa. As a result, although small seedlings of Eucryphia and Aristotelia displayed more leaf area than shade-tolerant seedlings of similar size (largely because of differences in specific leaf area), this relationship was reversed for plants over ≈500 mm tall. 3. Although the slower foliage turnover of the shade-tolerant species at least partly explains this reversal, allocational differences may also be involved. Root mass fraction of both shade-tolerant species was significantly negatively correlated with size, possibly reflecting declining allocation to roots in larger plants. In contrast, root mass fraction was not correlated with size in Eucryphia or Aristotelia. 4. Above-ground relative growth rate in low light followed similar ontogenetic trends to LAR, declining more quickly in light-demanding species than in shade-tolerant associates. As a result, large seedlings (>400 mm tall) of shade-tolerant Myrceugenia and Aextoxicon grew more quickly than their light-demanding associates at the same height. The steeply declining LAR and growth of Eucryphia and Aristotelia suggest that these taxa will eventually die of energy starvation in low light. 5. Results are consistent with the idea that accumulation of an extensive leaf area eventually gives juvenile shade-tolerant evergreens a net carbon gain advantage over their light-demanding associates in low light. Awareness of long-term ontogenetic trends will enhance understanding of relationships of shade-tolerance variation with morphology and growth of woody evergreens.