Node frequency alters stem biomechanics and hydraulics in four deciduous woody species

Abstract

Abstract Along the distal stems of woody plants, nodes occur along the stem length separated by internode regions. Nodes typically include a leaf or leaf scar and an axillary bud that are connected to the xylem tissue within the stem through vascular leaf and bud traces. The diversion of xylem tissue into these lateral appendages creates a node gap that is typically occupied by parenchyma. We hypothesized that node-associated changes in structure within the stem tissues would result in alterations to stem biomechanics and hydraulic transport. We examined four deciduous species, Juglans californica, Populus trichocarpa, Quercus robur, and Rhus aromatica and measured node frequency, stem density, biomechanics, and hydraulic conductivity in 36 stems from each species. Vessel diameters within nodes and internodes were measured on a subset of these stems, as well as measures of xylem, pith, and node gap areas. Increased node frequency was correlated with decreased stem strength (modulus of rupture; MOR), decreased stem stiffness (modulus of elasticity; MOE), increased stem density, and decreased hydraulic conductivity. There were no differences in vessel diameter or xylem area between node and internode regions. Reduced hydraulic conductivity with increasing node frequency could have been due to increased vessel termini associated with nodes as has been found in prior research. Increased length of hydraulic pathways due to divergence of vessels around node gaps could also decrease hydraulic conductivity. Variation within the tree crown in node frequency may be an important morphological feature that has implications for crown tolerance of periodic mechanical stresses, such as wind events and fruit load.

Keywords

• Biomechanics
• Density
• Hydraulic conductivity
• Hydraulic resistance
• Nodes
• Node gaps