Reconciling species‐level vs plastic responses of evergreen leaf structure to light gradients: shade leaves punch above their weight

Abstract

Summary• When grown in a common light environment, the leaves of shade-tolerant evergreen trees have a larger leaf mass per unit area (LMA) than their light-demanding counterparts, associated with differences in lifespan. Yet plastic responses of LMA run counter to this pattern: shade leaves have smaller LMA than sun leaves, despite often living longer.• We measured LMA and cell wall content, and conducted punch and shear tests, on sun and shade leaves of 13 rainforest evergreens of differing shade tolerance, in order to understand adaptation vs plastic responses of leaf structure and biomechanics to shade.• Species shade tolerance and leaf mechanical properties correlated better with cell wall mass per unit area than with LMA. Growth light environment had less effect on leaf mechanics than on LMA: shade leaves had, on average, 40% lower LMA than sun leaves, but differences in work-to-shear, and especially force-topunch, were smaller. This was associated with a slightly larger cell wall fraction in shade leaves.• The persistence of shade leaves might reflect unattractiveness to herbivores because they yield smaller benefits (cell contents per area) per unit fracture force than sun leaves. In forest trees, cell wall fraction and force-to-punch are more robust correlates of species light requirements than LMA.

Photosynthetic ecophysiology of evergreen leaves in the woody angiosperms – a review

“…see Letts et al (2012) for data on Buxus sempervirens, Valladares et al (2005) for Ilex aquifolium, Gratani et al (2006) and Tattini et al (2006) for Quercus ilex, Phillyrea latifolia, Pistacia lentiscus and Myrtus communis]. Leaves formed in low light are thin, have low LMA and tissue density, and a low photosynthetic capacity per area, consistent with the need to adjust carbon investment costs to anticipated photosynthetic benefits in a low light environment (Poorter et al 2009;Lusk et al 2010). Extended life span, however, requires high LMA and tough leaf structure also under shaded conditions, therefore it may be hypothesized that the capacity for plastic modification of leaf structure should be lower in evergreen than in deciduous leaves (Lusk et al 2008).…”

“…In fact, at similar LMA, shade leaves were stronger than sun leaves, reflecting their slightly higher leaf density and cell wall fraction while specific cell wall strength was unchanged. Further, the decrease of LMA (a trait contributing to herbivore resistance) in shade, could be compensated for by the lower attractiveness of shade leaves to herbivores due to their lower nutritional contents per unit force needed to ingest the leaf tissue (Lusk et al 2010).…”

“…A detailed analysis of leaf plasticity in tropical evergreen species conducted by Lusk et al (2010) showed that the reduction of LMA in shade did not cause a proportional decrease in leaf mechanical properties. In fact, at similar LMA, shade leaves were stronger than sun leaves, reflecting their slightly higher leaf density and cell wall fraction while specific cell wall strength was unchanged.…”

Seedling growth rates and light requirements of subtropical rainforest trees associated with basaltic and rhyolitic soils

“…Lusk and Reich 2000;Lusk et al 2010). Shade-tolerant species such as Argyrodendron trifoliolatum have low PPFD 10% values, whereas light-demanding species such as Polyscias elegans have high PPFD 10% values (Table 2).…”

Effects of light availability and growth rate on leaf lifespan of four temperate rainforest Proteaceae

“…However, this parallel in evolutionary and plastic responses of leaf lifespan to shade is associated with divergent trends in leaf mass per area: whereas shade-tolerant evergreens have larger mass per area (LMA) than their lightdemanding associates, plastic responses to shade invariably involve a reduction in LMA (Lusk et al 2008a). Despite their lower LMA, shade leaves tend to have a larger structural fraction than sun leaves, which may enable long lifespan by reducing their attractiveness to herbivores (Lusk et al 2010).…”