How Do Ring-porous Trees Differ From Diffuse-porous Trees?

The problem of efficiently transporting water has been shaping the evolution of plants for as long as they have lived on dry land—the past 475 million years. As plants evolved more sophisticated water-conducting systems, they expanded into increasingly extreme environments. Today, conifers are adept at surviving in boreal climates. Hardwoods trees are the most diverse in the tropics, but many have adapted to temperate climates, where they face extreme seasonal swings in temperature and moisture.

Based on the structure of their wood, hardwood trees can be sorted into two broad groups: diffuse-porous species (Figure 1) and ring-porous species (Figure 2). These groups differ in the size and distribution of their vessel elements, which are the cells that transport water from the roots to the leaves in angiosperms (flowering plants). Diffuse-porous species have small vessel element cells (also called pores) that are evenly distributed throughout each growth ring. Ring-porous species have vessel cells in a range of sizes. Vessels in the earlywood (the first wood produced each spring) are huge and densely packed. In the latewood (the wood produced at the end of the growing season) the vessels become smaller and sparser.

The large vessels in the earlywood make the growth rings very apparent in ring-porous species, hence the name. Oaks are some of the most dramatically ring-porous species, with earlywood vessels visible to the naked eye. Despite belonging to the same family as oaks, beech trees have diffuse-porous wood.  Really, "ring-porous" and "diffuse-porous" are the extreme ends of a spectrum. Species that are in between on the spectrum (such as black walnut), are sometimes called "semi ring-porous", or "semi diffuse-porous", depending on who you ask. Conifers appear to be diffuse-porous, but conifer wood is composed of a different type of cell than the wood of angiosperms, and so they have little in common besides outward appearance (Sperry 1994, p.1737).  Appendix 1 at the end of this article is a list of ring-porous, diffuse-porous, and semi-ring-porous species commonly found in the eastern US.

Whether a tree species is ring porous or diffuse porous might seem unimportant. However, a species' hydraulic architecture explains a lot about its distribution, life history, and environmental tolerances. It's also a fascinating story of evolution, so let’s start at the beginning.

The Evolution of Xylem

The common ancestor of all plants was aquatic. Dry land was an inhospitable environment, forcing the earliest land plants to evolve traits that prevented desiccation. They evolved protective tissues that isolated their cells from the dry air, preventing water loss. However, it would be impossible for plants to completely eliminate water loss. Some water is inevitably lost during photosynthesis. Photosynthetic cells must be exposed to the atmosphere so that carbon dioxide and oxygen can move across the cell membrane. At the same time, water also diffuses out of the cells, a process called transpiration. To keep their photosynthetic tissues alive, land plants must constantly replenish the water lost during transpiration by transporting water from the soil to the leaves.  Modern-day vascular plants have evolved specialized vascular tissue to do this.

Plants first colonized land in the Ordovician geological period, 475 million years ago. The earliest land plants were similar to modern-day bryophytes in that they didn’t have a vascular system. This limited how tall they could grow and meant they could only survive in moist environments. It wasn't until the Early Devonian (~415 Mya) that plants evolved specialized vascular systems consisting of xylem and phloem cells.  Today, bryophytes such as mosses, liverworts, and hornworts are incredibly diverse and can be found on all continents. But most environments are dominated by vascular plants. Xylem enabled these first vascular plants to grow taller and colonize drier environments. The first vascular plants used xylem primarily for water transport, but the middle Devonian (~400 Mya) saw the evolution of woody plants, which used xylem for structural support as well. Wood allowed these plants to grow taller, giving them an advantage in the competition for light.  A species called Achaeopteris (Figure 3), a distant ancestor of modern gymnosperms, was one of the first recognizable trees. It had secondary xylem, enabling it to grow in diameter and form meter-wide trunks, which supported stems rising 30 meters in height.

For 60 million years, for most of their history thus far, land plants were small, and non-vascular. Then came the evolution of the first vascular plants. But only 15 million years after the evolution of vascular tissue, there were trees 30 meters tall (Savidge, 2008). This rapid change shows how important the evolution of vascular tissue was. It started an evolutionary race for better, more conductive xylem that would allow species to grow taller and faster as they competed for light and water.

The Evolution of Wood

The wood of the earliest woody plants was composed of a single type of xylem cell called tracheids. Tracheids can be found in modern day gymnosperms (conifers, and a few other primitive taxa), where they serve the dual purpose of water transport and structural support. A coniferous species with annular rings appeared in the Late Carboniferous, 300 Mya (Savidge, 2008). The presence of annular rings, and the location the fossils were found suggest that this species lived in a boreal climate (Boura, 2007). The appearance of the rings in conifers is due to differences between the cells in the earlywood and latewood. The earlywood is the xylem produced early in the growing season after the tree breaks dormancy, and the latewood is xylem produced at the end of the growing season. Thus, earlywood is the inner portion of each ring, and latewood is the outer portion. Compared to latewood, xylem cells of the earlywood have thinner cell walls, giving them greater capacity to transport water. This allows the tree to sustain high rates of photosynthesis in the early growing season. Later in the growing season, the plant produces xylem cells that have thicker walls. The thick-walled cells in the latewood provide better structural support, but can't transport as much water.

The wood of angiosperms (flowering plants) is more complex than that of gymnosperms.  Angiosperms wood is composed of different types of xylem cells, with some dedicated to structural support and others dedicated to water transport. Fiber cells are solid, and only provide structural support.  The presence of fiber cells is part of the reason that angiosperm wood is so dense, and why angiosperm tree species are collectively called "hardwoods". The most striking difference between angiosperm vessels and tracheids is their size. Vessels are much wider. Wider pipes can transport water more efficiently, which gives trees with vessels an obvious advantage.

However, as vessel-size increases, the risk of embolism also increases. Embolism happens when air bubbles form in the xylem. This can be due to freeze-thaw cycles, or due to drought. Embolism renders a xylem cell permanently useless for conducting water.  So, there is a tradeoff between safety and efficiency of water transport. Embolisms are rarer in diffuse porous angiosperms. And conifers, with their tiny tracheids, rarely suffer freeze-thaw embolism. While this does happen in conifers, it is so rare that researchers have trouble inducing embolism in the laboratory (Sperry, 1994, p. 1744). This is how conifers had adapted to boreal climates 300 million years ago, before angiosperms had even arisen.

Early angiosperms had diffuse-porous wood, with small vessels. They also had evergreen foliage. The first deciduous angiosperms evolved in the Early Cretaceous 125 Mya. The first ring-porous wood appears at the same time in the fossil record. This was probably not a coincidence. 94% of ring-porous trees today are deciduous. Deciduous foliage is an adaptation for temperate climates, so perhaps ring-porous wood is too.  That would explain why ring-porous species are more prevalent in temperate regions. Most species of tree with described wood are diffuse-porous.  Worldwide, about 4% of species have ring-porous wood, but in temperate regions 17.7% of species are ring-porous, while in the tropics only 1% of species are ring-porous (Boura 2007). There is a clear correlation between temperate habitat, deciduous foliage, and ring-porous wood.

In temperate regions, freezing winter conditions lead to embolism in vessels. Deciduous trees will have lost a lot of hydraulic conductivity once it comes time to break dormancy in spring. Ring-porous wood may have evolved as a way to rapidly recover hydraulic conductivity at the start of the growing season. As soon as they break dormancy, ring porous species produce a ring of huge, dense vessels which supports rapid photosynthesis.  But these large vessels are only a temporary solution. They switch to producing smaller vessels later in the growing season. These small vessels can take over if the larger earlywood vessels embolize.

Ecological Implications

Ring-porous and diffuse-porous are functional groups. Rather than grouping species based on shared genetics, functional groups encompass species with shared ecological roles. A species' hydraulic architecture affects its phenology and water-use, which is why ring-porous and diffuse-porous considered separate functional groups.   While conifers appear diffuse-porous at first glance, they do not have much in common anatomically or ecologically with diffuse-porous angiosperms, so it doesn't make sense to put them in the same functional group. The following information applies only to angiosperms.

Ring-porous species rely on their large earlywood vessels in the early growing season. This means that ring porous species are especially vulnerable to spring drought or late-spring frost. However, most of the time this isn't an issue. Spring is usually wet in temperate regions, and trees "wait" to produce new xylem until there is minimal risk of frost (Chistman 2012).

In ring-porous species, it is only once the earlywood vessels are formed that the tree has enough hydraulic conductivity to support photosynthesis.  This means they must begin forming new xylem before they grow new leaves.  Since they have no leaves, they must tap into carbohydrate reserves to form their first vessels each year. This makes ring-porous species vulnerable to repeated dry years. Drought prevents trees from replenishing as much of their carbohydrate reserves. So if there are dry conditions year after year, eventually the tree won't have enough stored energy to produce its first earlywood vessels, meaning it can't leaf-out, and dies (Butto 2021).

In the summer, the tree switches to producing small latewood vessels, as the large earlywood vessels begin to embolize. These smaller vessels are less vulnerable to drought-embolism, an adaptive trait in temperate regions which tend to have dry summers. Additionally, the stomata of ring porous species are very responsive to changes in humidity. When the air is dry, ring porous trees begin closing stomata. Even if they are rooted in wet soil, as long as the air is dry, they downregulate their water use in this way. This effectively rations water in the soil, and protects trees from embolism, but further slows photosynthesis. This trait is less adaptive on wet sites, where the water supply is safe even during extreme droughts (Bush 2008, Bader 2022).

Diffuse-porous trees, on the other hand, get a slower start each spring.  Their many small vessels are resistant to freeze-thaw embolism. This allows them to survive in colder climates than ring-porous species. It also means that diffuse-porous species don't lose as much hydraulic conductivity over the winter. They are able to grow leaves before beginning to grow new xylem. They begin growing wood comparatively late, and it takes a while to fully recover hydraulic conductivity, so diffuse porous species don't reach peak growth rates until fairly late in the season. Their peak growth rate often coincides with some of the driest months of the year in temperate regions. This makes diffuse porous species vulnerable to summer drought (D'Orangeville 2021). To make things worse, they don't regulate their water like ring-porous species do. Unlike in ring-porous species, the stomata of diffuse porous species don't respond to dry conditions in the air. Even during drought, diffuse porous trees continue to transpire, and risk depleting their water supply (Butto 2021, p.12). They are less vulnerable to drought on very wet sites. And since diffuse porous species don't reduce their water use during droughts, but ring porous species do, this inadvertently benefits the diffuse-porous trees when both are growing together. This means that on wet sites, diffuse porous trees can outcompete ring-porous trees (Bush 2008).

The earlywood of diffuse porous species retains some hydraulic conductivity over the winter. This means that diffuse porous species can partially rely on the previous years' xylem to transport water. To a ring-porous tree, the older rings of xylem are less useful for conducting water; they rely mostly on the current-year's earlywood for most of their water transport. This explains why ring-porous species have thinner sapwood than diffuse-porous species.  Sapwood refers to the xylem that actually conducts water. Old sapwood eventually gets converted to heartwood; the hollow xylem tubes get filled in and solidified, meaning the cells can't conduct water, but they provide better structural support. Diffuse-porous trees have many rings of sapwood. A ring-porous tree of the same diameter might have only a few rings of sapwood. Ring-porous Catalpas have as few as 1 or 2 rings of sapwood, while most diffuse-porous species have dozens, and black gum (Nyssa sylvatica) can have up to 100 rings of sapwood (Hoadley 1980).

Ring-porous wood most likely evolved as an adaptation to temperate climates. Yet diffuse-porous species are still found in temperate regions. Sometimes this is because they get relegated to slightly different sites. But ring-porous oaks and diffuse-porous maples often grow side by side. In order to compete with ring-porous trees, diffuse-porous species have evolved an opposite approach to solve the same problem of how to live in an environment that freezes every year. Ring-porous species "accept" the fact that their hydraulic conductivity will be seriously compromised each winter, since they "know" they can make up those losses by growing huge and efficient earlywood vessels. On the other hand, temperate diffuse-porous species took the opposite approach. They evolved to be "risk-averse"; they "accept" that their hydraulic conductivity will be low, since that means they will be relatively safe from embolism. They have also evolved other features to improve their conductivity and prevent embolism, such as root pressure. Root pressure happens when sugars and solutes are actively transported into the root cells, causing an osmotic gradient that pulls in water from the soil. This even allows them to reverse some embolisms (Boura, 2007).

Conclusion

A tree's hydraulic architecture determines what climate it is suited for.  Ring-porous wood probably evolved along with deciduous foliage as an adaptation to temperate climates. The large vessels of ring-porous wood are more efficient at conducting water. However, they are also more prone to embolism. For this reason, diffuse-porous trees are still competitive in temperate climates, especially on wet sites. Furthermore, diffuse-porous species have evolved other traits, such as root pressure, which enable them to compete against ring-porous trees. Appendix 2 is a table summarizing major differences between ring-porous and diffuse-porous species.

It is easy to take for granted the fact that the Appalachians are covered in lush deciduous forests. But this is a harsh climate from the perspective of a plant. Conifers were the dominant trees outside of the tropics as early as 300 Mya. It took until 125 Mya for angiosperms, with their unique porous wood, to adapt to temperate climates.  Hopefully this gives you a deeper appreciation of the hardwoods of the Appalachians, and their complex and diverse hydraulic architecture. The story of how they got here stretches millions of years into the past.

Appendix 1: Common ring-porous, diffuse-porous, and semi ring-porous species.

Ring-porous species:

• Oaks (Quercus spp.)
• Catalpa (Catalpa speciosa)
• Ash (Fraxinus spp.)
• Chestnuts and chinkapins (Castanea spp.)
• Black locust (Robinia pseudoacacia)
• Honeylocust (Gleditsia triacanthos)
• Elms (Ulmus spp.)

Diffuse-porous species:

• American beech (Fagus grandidentata)
• Birches (Betula ssp.)
• Maples (Acer spp.)
• American sycamore (Platanus occidentalis)
• Sweet gum (Liquidambar styraciflua)
• Sour gum (Nyssa sylvatica)
• Aspens (Populus spp.)
• Willows (Salix spp.)
• Dogwoods (Cornus spp.)
• Tulip-poplar (Liriodendron tulipifera)

Semi ring-porous species:

• Black walnut (Juglans nigra)
• Black cherry (Prunus serotina)
• Hickories (Carya spp.)

Appendix 2: major differences between ring-porous and diffuse porous species

Works cited

Bader, M.K et al (2022) Less pronounced drought response in ring porous than in diffuse-porous temperate tree species.  Agricultural and Forest Meteorology 327 (2022). 

Boura, A. and Franceschi, D. (2007) Is porous wood structure exclusive to deciduous trees? C. R. Palevol 6 (2007) 385–391 Doi.org/10.1016/j.crpv.2007.09.009

Bush S.E et al (2008) Wood anatomy constrains stomatal responses to atmospheric VPD in irrigated, urban trees.  Oecologia 156, pages13–20 (2008) DOI: 10.1007/s00442-008-0966-5

Butto, V. et al (2021) Contrasting carbon allocation strategies of ring-porous and diffuse-porous species converge towards similar growth responses to drought.  Frontiers in Plant Science, vol 12.

Christman, M. A. at al inc. Sperry, J.S (2012) Rare pits, large vessels, and extreme vulnerability to cavitation in a ring porous tree species.  The New Phytologist 193

D’Orangeville, L. et al (2021) Peak radial growth of diffuse porous spp occurs during periods of lower water availability than for ring porous and coniferous trees.

Hoadley, B.R. (1980) Understanding wood: a craftsmans guide to wood technology.  Taunton Press.

Savidge, R. A. (2008) Learning from the past – the origin of wood.  The Forestry Chronicle, Vol 84, No. 4. 

Sperry et al (1994) Xylem embolism in ring porous, diffuse porous, and coniferous trees of northern Utah and interior Alaska  Ecology, Vol. 75, No. 6 (September, 1994) Doi.org/10.2307/1939633