A broad survey of hydraulic and mechanical safety in the xylem of conifers.

Pauline S Bouche,Sylvain Delzon,Jean-Christophe Domec,Régis Burlett,Steven Jansen,Maximilien Larter,Peter Gasson

doi:10.1093/jxb/eru218

Abstract

Drought-induced forest dieback has been widely reported over the last decades, and the evidence for a direct causal link between survival and hydraulic failure (xylem cavitation) is now well known. Because vulnerability to cavitation is intimately linked to the anatomy of the xylem, the main objective of this study was to better understand the xylem anatomical properties associated with cavitation resistance. An extensive data set of cavitation resistance traits and xylem anatomical properties was developed for 115 conifer species, with special attention given to the micro-morphology of bordered pits. The ratio of torus to pit aperture diameter, so-called torus overlap, increased with increasing cavitation resistance, while the flexibility of the margo does not seem to play a role, suggesting that air-seeding is located at the seal between the aspirated torus and pit aperture. Moreover, punctured tori were reported in various Pinaceae species. Species resistant to cavitation had thicker tracheid walls, while their lumen diameter (conduit size) was only slightly reduced, minimizing the impact on hydraulic conductance. The results also demonstrated (i) the existence of an indirect trade-off between hydraulic safety and mechanical strength; and (ii) a consistency between species distribution and xylem anatomy: species with a wide torus overlap and high valve effects are found in arid environments such as the Mediterranean region.

Highlights

Evidence for drought-induced forest dieback has been reported worldwide (Bigler et al, 2007; Van Mantgem et al, 2009; Allen et al, 2010; Peng et al, 2011; Sanchez-Salguero et al, 2012)
Effect) are the best proxy to explain the variability of cavitation resistance: species resistant to cavitation have a high valve effect
While Pittermann et al (2006b) and Sperry et al (2006) concluded that variation in the thickness to span ratio was determined by lumen diameter rather than cell wall thickness, the present study demonstrated the opposite: the wall thickness but not the lumen diameter appears to be responsible for the trade-off between hydraulic safety and mechanical strength

Summary

Introduction

Evidence for drought-induced forest dieback has been reported worldwide (Bigler et al, 2007; Van Mantgem et al, 2009; Allen et al, 2010; Peng et al, 2011; Sanchez-Salguero et al, 2012). Resistance to cavitation is a crucial trait in trees to cope with drought stress (Cochard and Delzon, 2013) in addition to, for example, rooting depth, internal water storage, and changes in biomass allocation or leaf anatomy.

Objectives

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Discussion

Conclusion