Stem-righting mechanism in gymnosperm trees deduced from limitations in compression wood development

Abstract

Background and aims: In response to inclination stimuli, gymnosperm trees undergo corrective growth during which compression wood develops on the lower side of the inclined stem. High compressive growth stress is generated in the compression wood region and is an important factor in righting the stem. The aims of the study were to elucidate how the generation of compressive growth stress in the compression wood region is involved in the righting response and thus to determine a righting mechanism for tree saplings.

Methods: Cryptomeria japonica saplings were grown at inclinations of 0 degrees (vertical) to 50 degrees. At each inclination angle, the growth stress on the lower side of the inclined stem was investigated, together with the degree of compression-wood development such as the width of the current growth layer and lignin content, and the upward bending moment.

Key results: Growth stress, the degree of compression wood development, and the upward moment grew as the stem inclination angle increased from 0 to 30 degrees, but did not rise further at inclinations > 30 degrees.

Conclusions: The results suggest the following righting mechanism for gymnosperm saplings. As the stem inclination is elevated from 0 to 30 degrees, the degree of compression wood development increases to force the sapling back to its original orientation; at inclinations > 30 degrees, the maximum degree of compression wood is formed and additional time is needed for the stem to reorient itself.

Fig. 1.

Experimental set-up. (A) The stem was wired to a stake to maintain the inclination angle of the stem. (B) Above the point of fixation to the stake, the stem was free to return to its original position. Measurements were taken from the stem below the point of fixation. The inclination angles (θ) were 0, 10, 20, 30, 40 and 50°.

Fig. 2.

Estimation of the bending moment from growth stress. The shaded region below the neutral axis indicates the compression wood region of the current growth layer (A). Compressive growth stress (σ) was generated in this area; l is the distance from the neutral axis to the centroid of the compression wood region.

Fig. 3.

The relationship between stem inclination and longitudinal released strain. Each solid square is the mean of 20 strain measurements (circles); they are positioned at fixed 10° intervals for clarity. Error bars indicate s.d. Different letters indicate significant differences at P < 0·05.

Fig. 4.

Representative cross-sectional images of a stem from each inclination angle. The lower side of each stem is the lower side of the inclined stem. Scale bar = 1 mm.

Fig. 5.

The relationships between the stem inclination and (A) width of the current growth layer on the lower side of the inclined stem, (B) percentage of cell wall in cross-section, (C) microfibril angle (MFA) and (D) Klason lignin content. Each symbol indicates the mean of five saplings, except for MFA in which it denotes the mean of 30 tracheids. Error bars indicate s.d. Different letters denote significant differences. The absence of letters indicates that no significant differences were observed.

Fig. 6.

The relationship between stem inclination and the absolute values of upward and downward bending moments. Each column for the upward moment is the mean of five saplings. Different letters indicate significant differences.