Impact of elevated atmospheric humidity on anatomical and hydraulic traits of xylem in hybrid aspen

Abstract

This study was performed on hybrid aspen saplings growing at the Free Air Humidity Manipulation site in Estonia. We investigated changes in wood anatomy and hydraulic conductivity in response to increased air humidity. Two hydraulic traits (specific conductivity and leaf-specific conductivity) and four anatomical traits of stem wood – relative vessel area (VA), vessel density (VD), pit area and pit aperture area – were influenced by the humidity manipulation. Stem hydraulic traits decreased in the apical direction, whereas branch hydraulic characteristics tended to be greatest in mid-canopy, associated with branch size. A reduction in VD due to increasing humidity was accompanied by a decrease in vessel lumen diameter, hydraulically weighted mean diameter (Dh), xylem vulnerability index and theoretical hydraulic conductivity. VA and Dh combined accounted for 87.4% of the total variation in kt of branches and 85.5% of that in stems across the treatments. Characters of branch vessels were more stable, and only the vessel-grouping index (the ratio of the total number of vessels to the total number of vessel groupings) was dependent on the interactive effect of the treatment and canopy position. Our results indicate that the increasing atmospheric humidity predicted for high latitudes will result in moderate changes in the structure and functioning of the hybrid aspen xylem.

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

“…The possibility that VPD is a central selective force reflected in plant hydraulic anatomy has been little explored but does seem congruent with other studies that implicate VPD, rather than temperature per se, as a crucial agent of selection on plant hydraulics (Eamus et al, 2013;Jasi nska et al, 2015;Sellin et al, 2017). Future climates promise increasing extremes of VPD, which in turn exerts strong pressures on plants (Breshears et al, 2013;Lemordant et al, 2018;Park et al, 2013).…”

“…Instead, we show that potential conductance scales isometrically with VPD, as predicted by the scenario in Figure 1. Our results provide the first explanation accounting for the VD standtemperature relationship, and explain otherwise enigmatic results, such as finding narrower, rather than the expected wider, conduits in experiments that raise humidity levels (Jasi nska, Alber, Tullus, Rahi, & Sellin, 2015;Sellin, Alber, & Kupper, 2017).…”

“…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.…”

Plant height and hydraulic vulnerability to drought and cold

“…Previous attempts to test these predictions and distinguish between climate and plant size have limited sampling to individual clades or to scattershot sampling across flowering plants, or have not properly standardized for height (19,35,37,46). As a result, that size is the main driver of mean conduit diameter across species remains debated, theory regarding hydraulic adaptation to climate does not currently include plant size, and studies of plant hydraulics still do not routinely take plant height or distance from the stem tip into account (18,(47)(48)(49)(50).…”

Weak Tradeoff and Strong Segmentation Among Plant Hydraulic Traits During Seasonal Variation in Four Woody Species

“…*P < 0.05. T and s but not with RH, which may be because the xylem status was conservative to the latter parameter (Jasińska et al, 2015;Alber et al, 2019). In the two shrubs, water relations were significantly correlated, but in the two trees, several traits were not significantly correlated (e.g., RWC and K s of TT in Q. acutissima, md and RWC of PS in R.…”