Journal Article Research Support, Non-U.S. Gov't Allometry maturation stress 059QF0KO0R Comparative Study Malvaceae Water anatomy axial vessel widening bark bark density bark residual strain bark thickness cell wall maturation inner bark leaf size spectrum mechanical stress mechanics mesoporosity ontogeny poplar (Populus deltoides × P. nigra) scaling sclereids secondary phloem shoot tension wood tree biomechanics tropical trees water supply