Imbricacy and stomatal wax plugs reduce maximum leaf conductance in Southern Hemisphere conifers

An examination of the relationship between theoretical maximum leaf conductance as calculated from stomatal dimensions, and measured maximum leaf conductance was undertaken in a group of Southern Hemisphere conifers. The relative effects of stomatal wax plugs, found in most species of conifers in the Southern Hemisphere, and imbricate leaf arrangement were expressed as a percentage inhibition of maximum leaf conductance (g(max)) calculated from the ratio of measured g(max) to theoretical g(max). Because of the similar stomatal dimensions of all species, measured g(max) was proportional to stomatal density in plugged and unplugged species, with species without wax plugs producing maximum leaf conductances on average 91% of calculated g(max), while in species with plugged stomata measured g(max) was on average only 35% of theoretical g(max). There was no effect produced by imbricacy in itself, but when combined with epistomy, g(max) was significantly reduced to about 17% of theoretical g(max). This is clearly illustrated by comparisons of juvenile-adult foliage, and closely related imbricate and non-imbricate species. The adaptational advantages of imbricacy and wax plugs, and the potential for inferring g(max) of fossil taxa are discussed.