Xylem physiology (and the functional implications of the structure of the xylem network) Jordi Martínez-Vilalta CREAF, Univ Autònoma Barcelona, Spain The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th

Contents… ● Basics of xylem structure and function ● The xylem is (and should be studied as) a network ● Theoretical implications of connectivity in the xylem network ● Can we measure the 3D structure of the xylem network? ● What can we learn from 2D cross-sections? ● Xylem structure and conduit packing in the xylem ● Can we measure it in tree rings? ● Final remarks Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th

Basics of xylem structure and function Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Choat et al., in review N ~250 sp

Basics of xylem structure and function ● Multifunctionality ● Possible tradeoffs ● Storage ● Construction costs ● Scaling, tapering Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Baas et al., Resistance to embolism Hydraulic conductivity (efficiency) Mechanical strength (stem and conduit levels)

Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Zimmermann, 1960s The xylem is a network

Connectivity vs. grouping vs. redundancy… ● Related concepts, relevant at different scales. Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Grouping (Carlquist et al. 1984) Connectivity (Loepfe et al. 2007) Redundancy (Ewers et al. 2007) Sectoriality (Zanne et al. 2006) Integration (Schenk et al. 2008) Segmentation (Zimmermann 1983) - + SPATIAL SCALE Aggregation (Mencuccini et al. 2010)

The xylem is a network Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Loepfe et al. (J Th Biol, 2007) Connectivity (k): average number of connected neighbors per conduit

Theoretical implications of connectivity Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Loepfe et al. (J Th Biol, 2007)

Theoretical implications of connectivity Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th k = 20.3k = 3.4

Can we measure the 3D xylem structure? ● X-ray computed microtomography (Steppe et al. 2004; Trtik et al. 2007; Brodersen et al. 2011) Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th  m Vitis vinifera

Can we measure the 3D xylem structure? ● The problem remains to measure pieces of wood that are large enough to characterize network topology at a relevant scale with a sufficient level of spatial resolution. Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Fraxinus americana Gil et al. (2011)

What can we learn from 2D cross-sections? Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Berberis vulgaris Clear aggregation Strongly uniform Buxus sempervirens Linear files Carpinus betulus

What can we learn from 2D cross-sections? Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Clustering Spatial randomness Dispersion (Ripley’s K and L functions)

What can we learn from 2D cross-sections? Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Acer pseudoplatanus

Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Mencuccini et al. (2010) STRONG AGGREGATION STRONG UNIFORMITY ● Strong phylogenetic signal. ● Accentuated conduit uniformity and accentuated aggregation must have evolved independently several times, with highly uniform and aggregated distributions being now present in all major core eudicot clades.

Xylem structure and conduit packing Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Martínez-Vilalta et al. (accepted)

Xylem structure and conduit packing Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Martínez-Vilalta et al. (accepted)

Improved version of the Loepfe et al. (2007) model: Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th

Xylem structure and conduit packing Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Martínez-Vilalta et al. (accepted)

Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Martínez-Vilalta et al. (accepted) Xylem structure and conduit packing

Can we measure it? ● yes, conduit aggregation could be added to the standard variables measured in studies of wood anatomy. ● likely to be more relevant in angiosperms. Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Fagus sylvatica, stem

Final remarks ● The topology of the xylem network matters, both in theory and practice. ● We are currently unable to measure the topology of the xylem network (in 3D) at the relevant spatial scale. ● However, reasonable estimates can be obtained from the spatial arrangement of conduits in 2D (e.g., cross-sections). ● Still unclear how the different measures of conduit spatial arrangement relate to each other and with other xylem traits and key environmental variables. ● Conduit aggregation varies across years and should be taken into account in retrospective studies of wood anatomy. Xylem physiology The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th

The significance of xylem hydraulic plasticity for reconstructing past environments: Kippel, May 15 th – 17 th Thanks for your attention... now discussion time!