Difference in shade tolerance affects foliage–sapwood response to thinning by two eucalypts

In eucalypt plantations in subtropical Australia, Eucalyptus dunnii exhibits greater diameter increment after thinning than more shade-tolerant Corymbia citriodora. To elucidate the mechanism underlying this difference, we investigated relationships between tree leaf area and sapwood area following thinning in 11-year-old E. dunnii and C. citriodora plantations. There was no significant difference in specific leaf area (SLA) between thinned and unthinned stands in E. dunnii and C. citriodora, whereas crown zone significantly affected SLA in the two eucalypts. At the whole-tree level non-linear leaf area (A_l)–sapwood area (A_s) relationships were measured in both eucalypts by thinning treatment. A significant increase in A_l/A_s ratio was only observed at the upper crown in thinned E. dunnii. The present results suggested the plastic nature of response of leaf characteristics in both eucalypts grown in different light environments and the species-specific pattern of crown-zone leaf responses to thinning in the two species, i.e. the top of the canopy appears to be driving greater growth response to thinning in the less shade tolerant E. dunnii compared with the more shade-tolerant C. citriodora. It is concluded that different thinning regimes should be applied in shade-tolerant and shade-intolerant eucalypt forests.