Abstract
Leaf venation networks evolved along several functional axes, including resource transport, damage resistance, mechanical strength, and construction cost. Because functions may depend on architectural features at different scales, network architecture may vary across spatial scales to satisfy functional tradeoffs. We develop a framework for quantifying network architecture with multiscale statistics describing elongation ratios, circularity ratios, vein density, and minimum spanning tree ratios. We quantify vein networks for leaves of 260 southeast Asian tree species in samples of up to 2cm2 , pairing multiscale statistics with traits representing axes of resource transport, damage resistance, mechanical strength, and cost. We show that these multiscale statistics clearly differentiate species' architecture and delineate a phenotype space that shifts at larger scales; functional linkages vary with scale and are weak, with vein density, minimum spanning tree ratio, and circularity ratio linked to mechanical strength (measured by force to punch) and elongation ratio and circularity ratio linked to damage resistance (measured by tannins); and phylogenetic conservatism of network architecture is low but scale-dependent. This work provides tools to quantify the function and evolution of venation networks. Future studies including primary and secondary veins may uncover additional insights.
Highlights
Leaves have venation networks with architecture that varies widely, from a single vascular strand to purely branching structures (e.g. Ginkgo) to open net patterns to mostly parallel structures in monocots, to highly reticulate patterns in many angiosperms
Veins are implicated in photosynthesis and transpiration, as vein-mediated resource transport comprises a large portion of total leaf conductance in most species (Brodribb et al, 2007), though variation in other tissues may be important (Ohtsuka et al, 2018)
We focused on traits related to four multiscale statistics: vein density (VD), the mean elongation ratio of loops (ER), loop circularity (CR), and the minimum spanning tree ratio (MST) (Table 1)
Summary
Leaves have venation networks with architecture that varies widely, from a single vascular strand (e.g. pines) to purely branching structures (e.g. Ginkgo) to open net patterns (e.g. many ferns) to mostly parallel structures in monocots, to highly reticulate patterns in many angiosperms. Networks vary over multiple spatial scales, with several levels of branching at length scales from 10−5 m (the radius of a single vein) to 100 m (the length of some large leaves) (Roth-Nebelsick et al, 2001; Sack & Scoffoni, 2013). The presence of loops in the network (Katifori et al, 2010) could prevent the propagation of embolisms that reduce conductance under low water potential (Brodribb et al, 2016), as well as the propagation of tears or cracks (Vincent, 1982; Niklas, 1999) caused by wind or herbivores. For construction cost, lignified tissue comprising veins is costly to construct relative to other tissues (John et al, 2017), and may displace leaf volume that could be
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