Phototropism and gravitropism in development of tree form

Abstract

This study examined contribution of phototropism in development of tree form on forest slopes, where spatial gradient in light environment exists. Main stem inclination (MSI) of mature trees on forest slopes were measured for Cryptomeria japonica (Cj), Pinus densiflora (Pd), Quercus myrsinaefolia (Qm), and Quercus serrata (Qs). MSI was larger in Qs, intermediate in Pd and Qm, and smaller in Cj. Intensity of phototropism and gravitropism in main stem of seedlings were determined on the four species. Change in MSI caused by phototropic response to unilateral illumination was larger in Qs, intermediate in Qm, and smaller in Pd and Cj. Change in MSI caused by gravitropic erection to artificial inclination was larger in Pd, intermediate in Cj, and smaller in Qm and Qs. Gravitropic erection to artificial inclination was inhibited by unilateral illumination. Intensity of the inhibition corresponded with the intensity of phototropism in each species. The intensity of phototropism in the seedlings and the MSI in the mature trees on the forest slopes were significantly correlated among those species, while the intensity of gravitropism did not showed significant correlation with the latter. Resultant stem inclination on forest slopes depends more on intensity of phototropism responding to spatial gradient of light environment than on that of gravitropic erection responding to passive inclination cased by such as slope falure.