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Interannual variability of wood density – an important plant functional trait and environmental proxy – in conifers is poorly understood. We therefore explored the anatomical basis of density. We hypothesized that earlywood density is determined by tracheid size and latewood density by wall dimensions, reflecting their different functional tasks. To determine general patterns of variability, density parameters from 27 species and 349 sites across the Northern Hemisphere were correlated to tree-ring width parameters and local climate. We performed the same analyses with density and width derived from anatomical data comprising two species and eight sites. The contributions of tracheid size and wall dimensions to density were disentangled with sensitivity analyses. Notably, correlations between density and width shifted from negative to positive moving from earlywood to latewood. Temperature responses of density varied intraseasonally in strength and sign. The sensitivity analyses revealed tracheid size as the main determinant of earlywood density, while wall dimensions become more influential for latewood density. Our novel approach of integrating detailed anatomical data with large-scale tree-ring data allowed us to contribute to an improved understanding of interannual variations of conifer growth and to illustrate how conifers balance investments in the competing xylem functions of hydraulics and mechanical support.

The flow of xylem sap through conifer bordered pits, particularly through the pores in the pit membrane, is not well understood, but is critical for an understanding of water transport through trees. Models solving the Navier–Stokes equation governing fluid flow were based on the geometry of bordered pits in black spruce (Picea mariana) and scanning electron microscopy images showing details of the pores in the margo of the pit membrane. Solutions showed that the pit canals contributed a relatively small fraction of resistance to flow, whereas the torus and margo pores formed a large fraction, which depended on the structure of the individual pit. The flow through individual pores in the margo was strongly dependent on pore area, but also on the radial location of the pore with respect to the edge of the torus. Model results suggest that only a few per cent of the pores in the margo account for nearly half of the flow and these pores tend to be located in the inner region of the margo where their contribution will be maximized. A high density of strands in outer portions of the margo (hence narrower pores) may be more significant for mechanical support of the torus.

This article is a Commentary on Björklund et al., 216: 728–740.

The vessels of various woody plants show helical sculpturing of the internal side of the secondary wall. The occurrence of these structures, termed helical thickenings (HT), is correlated with environmental parameters. Their adaptive benefit is, however, still not well understood. Suggestions for functional effects include mechanical stabilization, support of embolism refill or enhancement of water conductance. This study considers possible associations of HT with vessel wall thickness (VWT) and vessel diameter (VD) which are two xylem traits related to water transport and mechanical stabilization. Additionally, the relationship between HT and scalariform perforation plates (SPP) was studied, because a negative correlation between SPP and VWT was reported in the literature. The analysis also addressed the phylogenetic signal of HT. Available trait data for 337 species from 60 families and different biomes were used for statistical analyses. The results show (1) a significant negative correlation between HT and VWT across different biomes that likely indicates correlated evolution, (2) a tendency of HT to occur in narrower vessels (up to a diameter of about 100 µm), (3) an uneven distribution of HT and SPP among taxonomic groups of woody angiosperms, and (4) a moderate phylogenetic signal for HT which is evolutionary more labile than SPP. Based on these outcomes, the assumption of HT as a functional asset is supported which merits further study.

Both engineered hydraulic systems and plant hydraulic systems are protected against failure by resistance, reparability, and redundancy. A basic rule of reliability engineering is that the level of independent redundancy should increase with increasing risk of fatal system failure. Here we show that hydraulic systems of plants function as predicted by this engineering rule. Hydraulic systems of shrubs sampled along two transcontinental aridity gradients changed with increasing aridity from highly integrated to independently redundant modular designs. Shrubs in humid environments tend to be hydraulically integrated, with single, round basal stems, whereas dryland shrubs typically have modular hydraulic systems and multiple, segmented basal stems. Modularity is achieved anatomically at the vessel-network scale or developmentally at the whole-plant scale through asymmetric secondary growth, which results in a semiclonal or clonal shrub growth form that appears to be ubiquitous in global deserts.

KEY MESSAGE : Greater transport capacity of diffuse- vs. ring-porous stem networks translated into greater water use by the diffuse-porous co-dominant, but similar growth indicated higher water use efficiency of the ring-porous species. Coexistence of diffuse- vs. ring-porous trees in north-temperate deciduous forests implies a complementary ecology. The contrasting stem anatomies may result in divergent patterns of water use, and consequences for growth rate are unknown. We investigated tree hydraulics and growth rates in two co-dominants: diffuse-porous Acer grandidentatum (“maple”) and ring-porous Quercus gambelii (“oak”). Our goals were (1) document any differences in seasonal water use and its basis in divergent stem anatomy and (2) compare annual growth rates and hence growth-based water use efficiencies. At maximum transpiration, maple trees used more than double the water than oak trees. Maple also had more leaf area per basal area, resulting in similar water use per leaf area between species. Maple had ca. double the tree hydraulic conductance than oak owing to greater conductance of its diffuse-porous stem network (leaf- and root system conductances were less different between species). Water use in maple increased with vapor pressure deficit (VPD), whereas in oak it decreased very slightly indicating a more sensitive stomatal response. Seasonably stable water use and xylem pressure in oak suggested a deeper water source. Although maple used more water, both species exhibited similar annual biomass growth of the above-ground shoot network, indicating greater growth-based water use efficiency of oak shoots. In sum, water use in maple exceeded that in oak and was more influenced by soil and atmospheric water status. The low and stable water use of oak was associated with a greater efficiency in exchanging water for shoot growth.

Xylem structure and function is proposed to reflect an evolutionary balance between demands for efficient movement of water to the leaf canopy and resistance to cavitation during high xylem tension. Water use efficiency (WUE) affects this balance by altering the water cost of photosynthesis. Therefore species of greater WUE, such as C₄ plants, should have altered xylem properties. To evaluate this hypothesis, we assessed the hydraulic and anatomical properties of 19 C₃ and C₄ woody species from arid regions of the American west and central Asia. Specific conductivity of stem xylem ($ K_{\\rm {s}}$) was 16%-98% lower in the C₄ than C₃ shrubs from the American west. In the Asian species, the C₃ Nitraria schoberi had similar and Halimodendron halodendron higher$ K_{\\rm {s}}$) values compared with three C₄ species. Leaf specific conductivity ($ K_{\\rm {L}};$hydraulic conductivity per leaf area) was 60%-98% lower in the C₄ than C₃ species, demonstrating that the presence of the C₄ pathway alters the relationship between leaf area and the ability of the xylem to transport water. C₄ species produced similar or smaller vessels than the C₃ shrubs except in Calligonum, and most C₄ shrubs exhibited higher wood densities than the C₃ species. Together, smaller conduit size and higher wood density indicate that in most cases, the C₄ shrubs exploited higher WUE by altering xylem structure to enhance safety from cavitation. In a minority of cases, the C₄ shrubs maintained similar xylem properties but enhanced the canopy area per branch. By establishing a link between C₄ photosynthesis and xylem structure, this study indicates that other phenomena that affect WUE, such as atmospheric COࠢ variation, may also affect the evolution of wood structure and function.

The centrifuge method for measuring the resistance of xylem to cavitation by water stress was modified to also account for any additional cavitation that might occur from a freeze-thaw cycle. A strong correlation was found between cavitation by freezing and mean conduit diameter. On the one extreme, a tracheid-bearing conifer and diffuse-porous angiosperms with small-diameter vessels (mean diameter <30 micrometers) showed no freezing-induced cavitation under modest water stress (xylem pressure = -0.5 MPa), whereas species with larger diameter vessels (mean >40 micrometers) were nearly completely cavitated under the same conditions. Species with intermediate mean diameters (30-40 micrometers) showed partial cavitation by freezing. These results are consistent with a critical diameter of 44 micrometers at or above which cavitation would occur by a freeze-thaw cycle at -0.5 MPa. As expected, vulnerability to cavitation by freezing was correlated with the hydraulic conductivity per stem transverse area. The results confirm and extend previous reports that small-diameter conduits are relatively resistant to cavitation by freezing. It appears that the centrifuge method, modified to include freeze-thaw cycles, may be useful in separating the interactive effects of xylem pressure and freezing on cavitation.

The evolution of lianas has punctuated the history of land plants, with the angiosperm lineages representing the most recent stage of liana exploration. The model of lianas as fast‐growing, disturbance‐loving plants emerges largely from the function of eudicot and magnoliid angiosperms. This chapter looks at some specific properties of ecology and function, derived from functional aspects of stem hydraulics, which appear to be restricted to lianas. It charts the broad picture of xylem structure and function of angiosperm lianescence framed by what is known about the comparative ecophysiologies of eudicot/magnoliid and monocot lianas from temperate/ tropical zones. The chapter reviews the liana hydraulic paradigm illustrated by these angiosperm climbers. Finally, it explores how other climber lineages differ from generally accepted views about how lianas function hydraulically.