Architectural differences in saplings of temperate versus tropical angiosperms; consequences of the deciduous habit?

Abstract

Abstract:The architecture of saplings of temperate deciduous species of the southeastern United States was compared with that of tropical evergreen species of Central America, Borneo, and northeastern Australia. The deciduous species were more planar in the understory than were the tropical species, because of (i) more planar leaf displays within branches, (ii) a high frequency of arching, plagiotropic main stems (associated with greater plasticity in crown symmetry in relation to light), and (iii) a lower height of first branching. The deciduous species also had more planar branches than did subtropical and temperate evergreen angiosperms. This greater planarity in temperate deciduous understories may be associated with the simultaneous positioning of most leaves during a single flush in the spring. In contrast, saplings in tropical understories typically bear multiple leaf cohorts and position new leaves at the peripheries of existing leaf displays. These results and those of other studies suggest that there are adaptive links between plant architecture and phenology. Other factors, such as latitudinal variation in sun angles, may influence crown shape in overstory trees, but did not seem to be involved here, possibly because the filtering effect of the canopy results in smaller latitudinal shifts in understory illumination angles during the growing season. Thus, by favouring the deciduous habit, the cold winters and warm, humid summers of the eastern deciduous biome of North America appear to have had a notable influence on sapling architecture.Key words: morphological plasticity, plagiotropy, stem orientation, tree architecture.Résumé : Les auteurs ont comparé l'architecture de jeunes arbres d'espèces décidues tempérées du sud-est des É tats-Unis, avec celle d'espèces sempervirentes tropicales de l'Amérique Centrale, de Bornéo et du nord-est de l'Australie. Les espèces décidues de sous-étages sont plus planaires que ne le sont les espèces tropicales, dû à : (i) une disposition plus planaire des feuilles, sur la branche; (ii) une fréquence plus grande de tiges principales voûtées, plagiotropiques (associée avec une plus grande plasticité dans la symétrie du houppier, en relation avec la lumière) et; (iii) des premières branches plus basses. Les espèces décidues ont également plus de branches planaires que les angiospermes subtropicales sempervirentes tempérées. Cette planarité plus forte, dans les sous-étages des espèces décidues tempérées, peut être associée au positionnement simultané de la plupart des feuilles au cours d'une même poussée au printemps. D'autre part, les jeunes arbres des sous-étages des forêts tropicales portent typiquement de nombreuses cohortes foliaires, et placent les nouvelles feuilles aux périphéries de l'étalage des feuilles existantes. Ces résultats et ceux de d'autres études suggèrent qu'il y aurait des liens adaptatifs entre l'architecture des plantes et la phénologie. D'autres facteurs, comme la variation latitudinale des angles solaires, peuvent influencer la forme du houppier c...

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