Xylem vessel‐diameter–shoot‐length scaling: ecological significance of porosity types and other traits

Abstract

Flowering plants predominantly conduct water in tubes known as vessels, with vessel diameter playing a crucial role in plant adaptation to climate and reactions to climate change. The importance of vessels makes it essential to understand how and why vessel diameter, plant height, and other ecological factors are interrelated. Although shoot length is by far the main driver of variation in mean vessel diameter across angiosperms, much remains to be understood regarding the factors accounting for the abundant variation around the y‐axis in plots of mean species vessel diameter against shoot length. Here, we explore the potential role of porosity types, wood density, leaf phenology, background imperforate tracheary element type, vasicentric tracheids, vascular tracheids, perforation plate type, and successive cambia in causing variation in the y‐intercept or slope of the mean species vessel‐diameter– and vessel‐density–shoot‐length associations at the shoot tip and base. We detected numerous cases of ecologically significant variation. For example, latewood vessels of ring porous species scale with a lower slope than earlywood, i.e., latewood vessels are relatively narrow in taller plants. This pattern is likely the result of selection favoring freezing‐induced embolism resistance via narrow vessels. Wood density was negatively associated with vessel diameter, with low wood density plants having wider vessels for a given height. Species with scalariform perforation plates scale with a lower shoot base vessel‐diameter–shoot‐length slope, likely reflecting selection against scalariform plates in wide vessels. In other cases, functional groups scaled similarly. For example, species with successive cambia did not differ from those with conventional single cambia in their mean vessel‐diameter–shoot‐length scaling, rejecting our prediction that species with successive cambia should have narrower vessels for a given shoot length. They did, however, have fewer vessels per unit shoot cross‐sectional area than plants of similar heights, likely because vessels have longer functional lifespans (and therefore are fewer) in species with successive cambia. Our methods illustrate how vessel diameter can be studied taking shoot length into account to detect ecologically important variation and construct theory regarding plant adaptation via the hydraulic system that includes plant size as a vital element.

Across climates and species, higher vapour pressure deficit is associated with wider vessels for plants of the same height

“…In one experiment, hybrid poplars exposed to free-air humidity enhancement produced narrower vessels rather than the wider ones that traditional ecological wood anatomy would expect, in which moister conditions would be expected to favour wider, more vulnerable but more efficient, vessels (Jasi nska et al, 2015;Sellin et al, 2017). Instead, if VD stand scales with VPD, with conductance keeping pace with atmospheric evapourative demand, then this would explain the production of narrower vessels under enhanced humidity (Jasi nska et al, 2015;Sellin et al, 2017 (Anfodillo et al, 2006;Olson et al, 2018Olson et al, , 2020. That the results of our conductance-VPD estimates coincide strikingly with the predicted isometry does suggest that the role of conduit scaling with VPD is one worth exploring in more detail, especially because our analyses clearly reject the freezing hypothesis we tested.…”

“…We then estimated per-vessel conductance as VD 4 , assuming that flow in a vessel is proportional to radius to the fourth power (Vogel, 2003). Because in addition to vessel diameter vessel density also affects bulk conductance, we multiplied VD 4 by vessel density (Vmm −2 ), also based on the model for vessel density in widening, implying that these additional sources of resistance scale in exact proportion to tip-to-base conduit widening (Lazzarin et al, 2016;Olson et al, 2018Olson et al, , 2020. We then used VPD and SL (together with the interaction term between them) to predict VD 4 Á Vmm −2 , with the prediction that, if the variation in community intercepts of the VD-SL relationship (Figure 3) is due to the scaling of VD with VPD, then VPD should predict VD 4 Á Vmm −2 with a slope whose confidence interval includes isometry.…”

Wood and bark of Buddleja: uniseriate phellem, and systematic and ecological patterns

“…Most likely our methods were not sensitive enough to detect trends. It is established that axial sampling height and individual plant height affect certain wood traits Dória et al 2019;Olson et al 2020). However, since these data were not available, we used species maximal height as a rough proxy, unavoidably making the correlations weaker.…”

Tip‐to‐base xylem conduit widening as an adaptation: causes, consequences, and empirical priorities

“…For example, for the same plant height, drought deciduous species tend to have wider vessels than evergreen species. Cold‐deciduous species, in turn, tend to have the narrowest vessels given the same plant height (Olson et al ., 2020).…”