The architecture of New Zealand's divaricate shrubs in relation to light adaptation

Abstract

The divaricating shoot habit is typified by a suite of architectural traits, quantified here using phylogenetic independent comparisons of three pairs of congeners, with or without the habit. We consider the hypothesis that the habit evolved as a structural photoprotection mechanism that maximises potential carbon fixation by minimising photoinhibition. Plants were grown in pots in full sun, or behind vertical screens transmitting c. 25, 52, or 73% sunlight. When shaded, all species shifted partitioning of biomass from stem thickening to leaf area expansion and occupied a larger crown volume for a given shoot biomass. Leaf numbers per stem length of divaricates and non-divaricates were greater in the lower and upper canopies, respectively, consistent with the view that in divaricates outer branches protect inner leaves. However, leaf numbers per stem length showed no response to variation in high light receipt. Divaricates showed some traits typical of plants adapted to sunny habitats: smaller effective leaf size, lesser fractional partitioning of biomass to leaves, and greater foliage densities. Other traits of divaricates were typical of plants adapted to shaded habitats: lesser stem diameters, stem biomass per unit stem length, leader dominance, leaf area index, and heights relative to crown diameters; and more horizontal twig orientations. Compensation for high costs of support of photosynthetic area in divaricates (leaf area per unit shoot biomass c. 1.3 m2 kg-1) compared with non-divaricates (c. 5.5 m2 kg-1) would require a larger enhancement of net canopy photosynthesis than is likely to arise from avoidance of photoinhibition.