The function of foliar scales in water conservation: an evaluation using tropical-mountain, evergreen shrubs of the species Rhododendron in section Schistanthe (Ericaceae)

Abstract

Leaf scales (scarious trichomes) have putative significance to leaf water relations, energy balance, and gas exchange. A survey of leaf scales, stomata, and leaf morphology across 83 species of Rhododendron section Schistanthe in a common garden, at 1207-m elevation on Moana Loa, Hawai’i, was used to probe three possible functional models for scales on abaxial leaf surfaces. Scale density was commonly 10 mm–2, but densities up to 50 mm–2 occurred. The median stomatal density was 200 mm–2, but ranged up to 500 mm–2. Stomatal dispersion was grouped into four classes based on the proportion of stomata located under scales. Stomatal pore index decreased as scale density and scale importance factor increased. Thus, maximum stomatal conductance as represented by stomatal pore index decreased as the effect of scales on the leaf surface increased. Stomatal pore index increased as the elevation of the native range increased. However, the relative effects of scales on stomata peaked at mid-elevation. Principal components analysis indicated that the most important habitat factors determining both scale importance factor and stomatal pore index were elevation and growth form (epiphyte or terrestrial). Therefore, this survey indicated that scales serve a water conservation function, which is most effective at mid-elevation of the native range.