Orthotropic lateral branches contribute to shade tolerance and survival ofThujopsis dolabratavar.hondaisaplings by altering crown architecture and promoting layering

Abstract

We describe observed long-term adaptive responses to light conditions in the morphology of saplings of Thujopsis dolabrata var. hondai Makino, a shade-tolerant conifer species. Under stably bright conditions, the crown shape was slender and conical. In contrast, in stably dark conditions, the crowns had a highly distinctive broad, shallow, bowl-like shape, due to the formation of characteristic orthotropic lateral branches with higher apices than the stem. Leaf display by these branches may reduce self-shading and stem elongation requirements for leaf arrangement in dark conditions, thereby minimizing increases in maintenance costs associated with increasing tree size. Epicormic terminal leaders were frequently present on long branches in the lower part of the crowns. These branches did not show vigorous growth or contribute to crown expansion. However, the low-hanging branches in the lowest quarter of the crown may frequently touch the ground, particularly under snow pressure, and produce adventitious roots under these leaders, thereby increasing numbers of ramets. The snow pressure may also cause stem layering and restrain saplings from increasing their height to more than 1 m. These morphological and reproductive traits of lateral branches may contribute to shade tolerance and improve survival of the saplings.Key words: apical control, epicormic shoot, light intensity, longevity in shade, orthotropic end of a branch, vegetative reproduction. Résumé :Les auteurs décrivent les réponses adaptatives aux conditions de lumière observées à long-terme sur la morphologie des gaules de Thujopsis dolabrata var. hondai Makino, une espèce de conifère tolérante à l'ombre. En condition d'éclairage stable, la taille de la couronne était plus mince et conique. Par contre, en condition de noirceur stable, les couronnes présentaient une forme distinctive large et concave à cause de la formation de branches latérales orthotropes caractéristiques possédant des apex plus hauts que la tige. L'organisation des feuilles sur ces branches pourrait réduire l'auto-ombrage et les exigences relatives à l'allongement des tiges pour l'arrangement des feuilles en condition de noirceur, minimisant ainsi les augmentations de coûts de maintien associés à l'accroissement de la taille de l'arbre. Des pousses apicales épicormiques terminales étaient fréquemment présentes sur les branches longues de la partie inférieure des couronnes. Ces branches ne montraient pas de croissance vigoureuse ou ne contribuaient pas à l'expansion de la couronne. Cependant, les branches basses du quart inférieur de la couronne pouvaient souvent toucher le sol, particulièrement sous la pression de la neige et produire des racines adventices sous ces pousses apicales, augmentant ainsi le nombre de ramets. La pression de la neige pouvait aussi provoquer le marcottage des tiges et restreindre l'augmentation de la hauteur des gaules à plus qu'un mètre. Ces traits morphologiques et reproductifs des branches latérales pourraient contribuer à la tolérance à l'ombre et améliorer ...

Morphological plasticity of six tree species with different light demands growing in multi-layered deciduous forests in Central Europe

“…We assumed that at crowns better correspond with the strategy of persistence assigned to shade tolerant speciesbecausethis crown shape facilitates light capture and minimizes self-shading (Horn 1971;Niinemets 2010). This pattern of morphological response is well-documented in shade tolerant conifers (Leibindgut 1945; Gratzer et al 2004;Vencurik et al 2015;Hitsuma et al 2015).However, in the lower layer of the studied stands, light adapted alders and oakshad the most attened crowns, mid shade tolerant ashes hadthe least attened crowns and shade tolerant species showed intermediate properties. Thus,the registered crown widthtocrown length ratios did not re ect the light demand of the species.…”

“…Long-living and large organisms such as treeshavedeveloped numerous mechanismsto modify their architecture in response tochanges linkedwith intrinsic growth processes, disturbancesoccurring in their neighborhood and interactions with other organisms (Horn 1971). Tree longevity relies on vegetative vigor and disease resistance together with plastic crown branching and root readjustments (Piovesan and Biondi 2021).Trees may change the pattern of resource allocation and in the longer term also the structure of their organism to reduce selfshading and stem elongation requirements for leaf arrangement, maintainmechanical resistance and improve survival (Messier and Nikinmaa 2000;Hitsuma et al 2015). The architecture of trees is a very plastic feature and may considerablyvary depending on the conditions and environments in which treesgrow (Seidel et al 2011;Schröter et al 2012).Therefore, scaling parameters may strongly vary and may not be centered on the values predicted by geometric or metabolic scaling theories (West et al 2009;Pretzsch2009).…”