Some trees can save effort when it comes to adding growth rings

Sometimes it’s easy to oversimplify plant anatomy. For example, trees don’t have blood. They have sap. It might be tempting to say that they carry sap in their xylem, the way we have blood in their veins, but it’s a shallow comparison. There’s no heart pumping sap round a tree. And unlike humans, trees grow a new ring of xylem to carry sap each year. Or so it was thought.

A few trees are known to have connections between concentric growth rings. New research by Jay Wason and colleagues has identified anatomical structures that allow for certain tree species to pass water across annual growth rings. These structures haven’t been identified conclusively in many species previously. Wason explained: “This can be a huge carbon saving strategy for many trees in stressful environments because they can reuse the xylem from last year and may be an important aspect in woody plants that are able to live many years without appearing to grow in diameter.”

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Representative cross sections of dye staining in 2- to 3-year-old stem sections of (A) Acer rubrum, (B) Fagus grandifolia, (C) Fraxinus americana and (D) Quercus rubra. Stems were perfused with stain solution basipetally from the current-year shoot to determine if the dyecould reach previous growth rings. In each image, the pith and previous growth rings are to the left and the newer rings and epidermis are to the right. Arrows indicate growth ring boundaries. All vessels near the dye injection point in the current-year shoot were stained, and flow appeared to preferentially move through the largest diameter vessels. Image: Wason et al. 2019.

The findings will help botanists understand the linkages between xylem anatomy and function. This will help botanists, discover more about how trees cope with environmental extremes.

The research concentrates on sapwood, the living layer of wood between the dead heartwood and the bark. This size of this layer varies from species to species. Wason said: “Most trees produce new wood every year in a very predictable pattern, which can be seen in the annual rings in the cross section of a tree. The amount of active sapwood of a tree can vary quite a bit, both between different trees of one species or between different species. The amount of wood moving water up the tree can be anywhere from a single growth ring just a few millimeters wide, to multiple growth rings 10 cm or more into the tree.

“Importantly, however, even if there is a large area of active sapwood, the majority of water (>90%) still tends to flow through the current year growth ring. But, there’s a lot of variability between species which is related to wood anatomy. Our work highlights that some trees can tap into older growth rings when conditions are stressful or if there is damage to more current year growth ring while other trees do not.”

I thought the xylem might act a bit like straws but, as Wason explained, it’s a bit more complicated than that. “Tree rings can be thought of as a network of interconnected straws, but this generalization oversimplifies the amazing anatomical diversity at the microscopic level. The straws carry water up a tree and can pass water from one straw to another to create a continuous water column. Most of the straws are less than the diameter of a human hair and are only a few centimeters long.”

“When two straws are adjacent, they can form connections that allow water to pass between them. In many species, the straws from one year do not connect to the straws of the next year. This can be advantageous because damaged xylem from last year won’t affect the new xylem. For example, one of the big problems plants face is that during drought air-bubbles can get stuck in the straws, which block water flow to the leaves. The lack of cross-ring connections means that the tree relies completely on the current year growth ring to transport water, which could be risky if that ring is damaged. So, trees are apparently using different strategies to balance the risk and reward of connecting the xylem across many years of growth.”

Our work identifies how certain species have connections between straws that neighbor each other across growth rings, allowing the networks in each ring to connect. Importantly, the trees that have those connections, also seem to have other adaptations to help limit the likelihood of air traveling between growth rings.”

Cross-connections between years of growth rings can be important for coping with stress like drought. Stress can decide how a growth ring grows. The tree doesn’t simply grow a new ring out. It starts from a point. Wason said: “When a tree forms a growth ring, it starts growing at the branch tips and, if conditions for growth are good, growth cascades down the trunk to the base of the tree. However, in stressful conditions, growth can stop before the ring is formed near the base of the tree. Therefore, the partial ring that forms in the branches needs to connect back to the previous year’s growth ring to access the water in the soil. That’s where cross-ring connections become advantageous.”

The tree doesn’t actively decide how to route water through the xylem. This is down to physics. Wason said: “As water evaporates from the canopy, it is drawn up from the roots through the entire network. Most of the water, however, will preferentially flow through the path of least resistance, which is usually the shortest path with the largest straws and the fewest connections that need to be crossed. This is often through the current year ring but, if there are connections across rings, some water will be flowing through the older rings as well. However, with enough blockages in the current year ring, more water could start flowing through older growth rings.”

Given that these cross-connections provide routes around damage, you might expect any tree to want them. However, when that damage is an embolism, an air bubble that blocks the flow, cross-connections risk allowing these bubbles to move into more important parts of the tree. These bubbles block flow of sap, like the bends blocks blood flow for a diver. Wason notes not all trees are as likely to have these bubbles.

“Many trees, particularly ones that have large straws (xylem vessels) are almost guaranteed to have air bubbles form in them over the winter as the sap freezes and gases come out of solution. Therefore, we think that many trees, like oak, don’t have any connections to older growth rings so they can isolate the air bubbles and be sure they don’t damage the newly forming growth ring. However, species like maple have small enough vessels that they can make it through winter without too many air bubbles forming. Therefore, it may be less risky for them to have connections to previous year rings.”

“What’s really interesting is that you can find both oaks and maples growing in the same forest. So it isn’t clear yet if there’s an advantage to one strategy versus another.”

As you might expect, these findings explain that, for some trees, you cannot age them by counting tree rings. This does make them less useful for archaeology or climate reconstruction. But don’t expect any redating of material due to this study. Wason said: “Dendrochronologists are very careful to only include data from tree rings that they can assign a date with confidence. They have a suite of statistical tools, sampling techniques, and validation protocols that they use to be sure that their data are accurate. Our improved understanding of cross-ring connections can help dendrochronologists better understand why some tree species are more likely to have missing rings and what conditions lead to that.”

Wason said: “Our research is relevant to better understanding the anatomical underpinnings of how tree rings function for water transport. This work will be relevant to scientists studying water transport in woody plants, dendrochronologists, and forest ecologists.” The missing rings matter. At the microscopic scale, understanding these rings could help scientists keep trees alive, as climates grow more extreme. At a bigger scale, knowing how and why these rings go missing will help scientists understand carbon budgets for forests.

What might seem like a simple pattern of lines can be a sophisticated network. Getting a clear view of how sap flows should improve our understanding of biodiversity in forests.