Orthotropic lateral branches contribute to shade tolerance and survival of Thujopsis dolabrata var. hondai saplings by altering crown architecture and promoting layering
2015
Hitsuma, Gaku | Morisawa, Takeshi | Yagihashi, Tsutomu
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.
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