Abstract
The xylem hydraulic efficiency and safety is usually measured on mechanically unstressed samples, although trees may be exposed to combined hydraulic and mechanical stress in the field. We analysed changes in hydraulic conductivity and vulnerability to drought-induced embolism during static bending of Picea abies and Pinus sylvestris branches as well as the effect of dynamic bending on the vulnerability. We hypothesized this mechanical stress to substantially impair xylem hydraulics. Intense static bending caused an only small decrease in hydraulic conductance (−19.5 ± 2.4% in P. abies) but no shift in vulnerability thresholds. Dynamic bending caused a 0.4 and 0.8 MPa decrease of the water potential at 50 and 88% loss of conductivity in P. sylvestris, but did not affect vulnerability thresholds in P. abies. With respect to applied extreme bending radii, effects on plant hydraulics were surprisingly small and are thus probably of minor eco-physiological importance. More importantly, results indicate that available xylem hydraulic analyses (of conifers) sufficiently reflect plant hydraulics under field conditions.
Highlights
Tree life is based on the formation of wood, which fulfils manifold functions
The hydraulic efficiency and safety of the transport system are based on the xylem structures, which have to provide sufficient transport capacities as well as sufficient resistance to embolism formation
There are multiple potential effects of mechanical stress on xylem hydraulics, we did not observe major implications upon bending of branches: the hydraulic efficiency was hardly affected by static bending
Summary
Tree life is based on the formation of wood, which fulfils manifold functions. Metabolic processes and storage pools are provided by parenchyma cells, while dead cells with lignified cell walls facilitate the formation of complex axis systems. The latter enable trees to withstand gravitational and dynamic forces and to transport water into the crown. The hydraulic function is provided by vessels, while fibre cells withstand self-loading and compression as. Tracheids fulfil both hydraulic and mechanical function in conifer xylem. Angiosperm and conifer conduits are connected by countless pits and are part of a complex transport network within the wood (Tyree & Zimmermann 2002)
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