Pit and tracheid anatomy explain hydraulic safety but not hydraulic efficiency of 28 conifer species

Abstract

Conifers face increased drought mortality risks because of drought-induced embolism in their vascular system. Variation in embolism resistance may result from species differences in pit structure and function, as pits control the air seeding between water transporting conduits. This study quantifies variation in embolism resistance and hydraulic conductivity for 28 conifer species grown in a 50-year-old common garden experiment and assesses the underlying mechanisms. Conifer species with a small pit aperture, high pit aperture resistance and large valve effect were more resistant to embolism, as they all may reduce air seeding. Surprisingly, hydraulic conductivity was only negatively correlated with tracheid cell wall thickness. Embolism resistance and its underlying pit traits related to pit size and sealing were stronger phylogenetically controlled than hydraulic conductivity and anatomical tracheid traits. Conifers differed in hydraulic safety and hydraulic efficiency, but there was no trade-off between safety and efficiency because they are driven by different xylem anatomical traits that are under different phylogenetic control.

Drought resilience of conifer species is driven by leaf lifespan but not by hydraulic traits

“…short leaf lifespan) and photosynthetic rate (Table S5) and, hence, high growth rate and drought resilience for these relatively humid species. Stem diameter growth was also positively related to pit aperture, but pit aperture was not related to climatic origin, probably because it is phylogenetically conserved (Song et al ., 2022). The observed relationship between drought resilience and potential evapotranspiration is probably because evapotranspiration is strongly positively correlated with radiation (Table S2).…”

“…cavitation resistance and pit size) are under strong phylogenetic control (cf. Song et al ., 2022). Gymnosperms are therefore characterized by a strong evolutionary integration of hydraulic safety traits whereas, for example, for tropical trees P 50 and Ψ TLP have been the result of repeated evolutionary adaptation (Guillemot et al ., 2022).…”

“…R PA , MPa·s m −3 ) and pit sealing (i.e. torus overlap, margo flexibility and valve effect) were measured using electron microscopy (see Methods S3; Song et al ., 2022).…”

“…Conservative leaves, by contrast, facilitate the resistance to drought stress, but come at the cost of reduced growth (Onoda et al ., 2017). Similarly, acquisitive stem trait values such as wide pits and tracheids favor conductivity and growth rates of conifer species, but this may come at the expense of reduced drought tolerance (Song et al ., 2022).…”

Stem Trait Spectra Underpin Multiple Functions of Temperate Tree Species

“…Angiosperms can transport water efficiently through wide vessels and form thick-walled fibers for mechanical support, while gymnosperms have narrow tracheids that combine both functions ( Sperry et al, 2006 ). Angiosperms generally have higher xylem conductivity because their vessels can achieve much larger dimensions, but gymnosperms can partly compensate for their limited conduit size due to their efficient torus-margo pits ( Pittermann et al, 2006 ; Song et al, 2021 ). Many studies hypothesize that there is a general trade-off between hydraulic safety and water transport efficiency, but such a trade-off is not universal: only a quarter of the hydraulic studies have supported this trade-off hypothesis ( Gleason et al, 2015 ).…”