Abstract
Maintaining sufficient water transport via the xylem is crucial for tree survival under variable environmental conditions. Both efficiency and safety of the water transport are based on the anatomical structure of conduits and their connections, the pits. Yet, the plasticity of the xylem anatomy, particularly that of the pit structures, remains unclear. Also, trees adjust conduit dimensions to the water transport distance (i.e., tree size), but knowledge on respective adjustments in pit dimensions is scarce. We compared tracheid traits [mean tracheid diameter d, mean hydraulic diameter dh, cell wall reinforcement (t/b)2], pit dimensions (diameters of pit aperture Da, torus Dt, margo Dm, and pit border Dp), and pit functional properties (margo flexibility F, absolute overlap Oa, torus overlap O, and valve effect Vef) of two Scots pine (Pinus sylvestris L.) stands of similar tree heights but contrasting growth rates. Furthermore, we analyzed the trends of these xylem anatomical parameters across tree rings. Tracheid traits and pit dimensions were similar on both sites, whereas Oa, O, and F were higher at the site with a lower growth rate. On the lower growth rate site, dh and pit dimensions increased across tree rings from pith to bark, and in trees from both sites, dh scaled with pit dimensions. Adjusted pit functional properties indicate slightly higher hydraulic safety in trees with a lower growth rate, although a lack of major differences in measured traits indicated overall low plasticity of the tracheid and pit architecture. Mean hydraulic diameter and pit dimension are well coordinated to increase the hydraulic efficiency toward the outer tree rings and thus with increasing tree height. Our results contribute to a better understanding of tree hydraulics under variable environmental conditions.
Highlights
To cope with variable water availability, trees must maintain sufficient water transport (McDowell et al, 2008; Adams et al, 2017)
Tree cores from the limited site contained 160 ± 7 tree rings, whereas those from the favorable site contained 32 ± 1 (p < 0.001; all values given as mean ± standard error (SE))
The pith was missing in some cores, and the cambial age at breast height was underestimated by approximately 3 years in some trees
Summary
To cope with variable water availability, trees must maintain sufficient water transport (McDowell et al, 2008; Adams et al, 2017). Besides mechanical support and storage, the main function of the xylem and is based on the xylem’s anatomical structures (Kramer and Kozlowski, 1960; Zimmermann, 1983). Xylem conduits transport water from roots to leaves based. Water flow is restricted by resistances along the entire pathway and the cumulative resistance increases with pathway length (Ryan and Yoder, 1997). Resistance of single xylem elements decreases approximately with the fourth power of conduit lumen diameter (Hagen-Poiseuille’s law; Zimmermann, 1983), and trees counteract pathway length resistance by, respectively, widening their conduits with distance to the treetop (or branch tip; Anfodillo et al, 2013; Carrer et al, 2015; Lazzarin et al, 2016; Olson et al, 2021). With increasing conduit lumen diameter, the risk of hydraulic failure can increase (e.g., Rosner et al, 2016)
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