Leaf out time correlates with wood anatomy across large geographic scales and within local communities

Abstract

There is a long-standing idea that the timing of leaf production in seasonally cold climates is linked to xylem anatomy, specifically vessel diameter because of the hydraulic requirements of expanding leaves.We tested for a relationship between the timing of leaf out and vessel diameter in 220 plants in three common gardens accounting for species' phylogenetic relationships. We investigated how vessel diameter related to wood porosity, plant height and leaf length. We also used dye perfusion tests to determine whether plants relied on xylem produced during the previous growing season at the time of leaf out.In all three gardens, there was later leaf out in species with wider vessels. Ring-porous species had the widest vessels, exhibited latest leaf out and relied less on xylem made during the previous growing season than diffuse-porous species. Wood anatomy and leaf phenology did not exhibit a phylogenetic signal.The timing of leaf out is correlated with wood anatomy across species regardless of species' geographic origin and phylogenetic relationships. This correlation could be a result of developmental and physiological links between leaves and wood or tied to a larger safety efficiency trade-off.

Linking xylem structure and function: the comparative method in from the cold

“…Nothing, so it seems, could be farther from the overall goal of biology to understand the living world's protean variation. To see the very real relevance of observations at small scales, let us return now to Savage et al (2022), who used the cold-deciduous forests of the north temperate zone as their study system.…”

“…In this way, comparative studies test the assumptions that give small-scale studies their relevance. Savage et al (2022) use the epistemological interdependence between small-scale and comparative studies to full advantage in connecting xylem structure to leaf phenology. Given detailed studies of few species plus models of freezing in conduits, Savage et al (2022) knew that wider conduits are more vulnerable to freezing-induced embolism.…”

“…Savage et al (2022) use the epistemological interdependence between small-scale and comparative studies to full advantage in connecting xylem structure to leaf phenology. Given detailed studies of few species plus models of freezing in conduits, Savage et al (2022) knew that wider conduits are more vulnerable to freezing-induced embolism. From this they constructed the expectation that species with wider conduits should leaf out later in the spring, as the danger of frost lowers.…”

“…Moreover, if 30 µm is a rough benchmark conduit diameter above which recovery from embolism is less likely, then species with twig conduits well above this threshold should leaf out the latest. So, Savage et al (2022) clearly needed a wide range of twig conduit diameters to test this expectation. Savage et al (2022) focused on deciduous forests of the north temperate zone; with their low diversity from almost every metric imaginable, these forests would not seem to offer the most promising substrate for finding broad variation, but Savage et al (2022) cleverly leveraged an ace that north temperate forests have up their sleeves.…”

“…953-964) is a model of how this toolbox can, and must, seamlessly integrate data and theory from across levels, from the very small scale of processes within xylem conduits to the millionsplus years scale of evolutionary time. Before highlighting just some of the inferential richness in Savage et al (2022), let me first explore why the integration of levels is such a challenge, and the empirical turn that this challenge demands.…”