Reorientation Strategies in leaning stems of young maritime pine (Pinus Pinaster) and Loblolly Pine (Pinus Taeda)

Abstract

To test for different stem reorientation strategies in leaning seedlings of maritime pine (Pinus pinaster Ait.) and loblolly pine (Pinus taeda L.), we inclined dormant plants at 0°, 45° and 90° for 35 days in optimal growthconditions. Stem angle to the vertical was measured at regular intervals. After 22 and 35 days, bending tests were carried out to determine stem elasticity and stiffness. The quantity of compression wood (CW) formed was also measured. Results showed that after 24 h, the apices of leaning maritime pine stems had completely reoriented to the vertical, whereas no such primary tropic response was observed in loblolly pine. After 22 days, the significantly stiffer loblolly pines had begun straightening up from the stem base, but not maritime pine. After 35 days, the overall secondary reorientation process was more efficient in loblolly pine, with significantly more CW produced. In both species, CW had formed immediately on tilting stems, i.e. no early wood cells had formed. As maritime pine originates from a dry to semi-arid region, having less CW will mean that water uptake will be greater for the same xylem area compared to loblolly pine which possesses more CW. In establishing maritime pine seedlings, rapid apical orientation with regard to light and gravity will compensate for a slower straightening up mechanism in the more lignified parts of the stem. Therefore, juvenile needle exposure to patches of light will be maximised at all times.