For as long as scientists have studied trees, we have divided them into two types based on the type of wood they make. Softwoods include pine and spruce, and they generally grow faster than hardwoods, such as oak and maple, which can take decades to mature and form a denser wood.
Our recent research, however, has revealed something completely new: a third category we call “midwood.” This discovery could prove valuable in the fight against rising carbon dioxide (CO₂) levels in the Earth’s atmosphere—the primary driver of climate change.
Trees are natural carbon sinks. This means that they absorb enormous amounts of CO₂ from the air and store it in their wood. The tulip tree (Liriodendron tulipifera), also known as the yellow poplar, is a top performer in carbon capture. In the mid-Atlantic, forests dominated by tulip trees store two to six times more carbon than forests dominated by other species. The tulip tree is already popular in plantations in parts of Southeast Asia and has been touted as a good carbon capture choice by horticulturists and urban planners in the U.S.
This species, together with its close relative the Chinese tulip tree (Liriodendron chinense), belong to an ancient lineage that dates back 50-30 million years — a period marked by significant shifts in atmospheric CO₂. Only these two species survive. And until recently, their chemistry and structure, which could tell us why these trees are so good at capturing carbon, were largely unknown.
Traditional methods of analyzing the internal structure of wood ignore the differences between living and dried wood, the latter being much easier to study. This is a problem, because without water, wood changes at the molecular level. The challenge is to observe wood that still retains its water.
We overcame this by using a technique known as low-temperature scanning electron microscopy at the Sainsbury Laboratory at the University of Cambridge. This allows us to observe wood at the nanometre scale – seeing structures more than 6,000 times smaller than a single strand of human hair – while preserving the moisture in the wood to get a more accurate picture of what the wood looks like while the tree is alive.
The evolution of wood structure
We studied several trees in the Botanic Garden at the University of Cambridge to understand the evolution of wood structures. We collected living samples of plants that represent important milestones in evolutionary history. These plants are within easy walking distance of the microscope, allowing us to examine the samples without them drying out.
We found that the size of the macrofibril, a fiber composed primarily of cellulose, the basic chemical building block of wood and the strength that gives plants the power to grow tall, varies significantly between hardwoods and softwoods. In hardwoods such as oak and maple, the macrofibril is about 16 nanometers (nm) in diameter, while in softwoods such as pine and spruce, it is about 28 nm. These differences could explain why softwoods and hardwoods differ, and could help us understand why some woods are better at storing carbon than others.
Understanding how wood evolved can help us identify and exploit plants that can mitigate climate change. The tulip tree alone doesn’t tell us this, so we went further back in time and examined basal angiosperms, a group of rare and ancient flowering plants that still exist as relics of the earliest stages of plant evolution. One member of this group is Amborella trichopodathat the larger 28 nm macrofibrils, suggesting that hardwood macrofibrils evolved later than softwoods.
But when exactly did that happen?
To answer this question, we examined the magnolia family, including the purple-flowered Magnolia liliiflorawhich are among the oldest extant flowering plants known for their ornamental beauty. The plants we tested have hardwood macrofibrils with a diameter of 15-16 nm, which means that the transition from softwood to hardwood probably occurred during the evolution of magnolias.
The tulip tree is a close relative of magnolias, but its wood does not fit neatly into either softwood or hardwood categories. Instead, its macrofibrils were about 22 nm in diameter—right in the middle of the range between hardwood and softwood. This intermediate structure was completely unexpected and led us to classify tulip tree wood as “midwood,” an entirely new category.
Midwood: A Super Carbon Accumulator?
Why do tulip trees have this unique type of wood? We can’t say for sure, but we think it has to do with the evolutionary pressure these trees went through millions of years ago.
When tulip trees first emerged, atmospheric CO₂ levels dropped from about 1,000 parts per million (ppm) to 500 ppm. This reduction in available CO₂ may have prompted tulip trees to evolve a more efficient method of carbon storage, leading to their unique macrofibril structure. Today, this adaptation likely contributes to their exceptional ability to capture carbon.
We can no longer assume that a previously unstudied tree falls into the same two categories (softwood or hardwood) that scientists have been classifying trees into for years. The tulip tree, with its midwood structure, is consistent with a “carbon-hungry” attitude. We are now looking to see if its seemingly unique wood structure is the only reason it is the king of carbon sequestration, and we are expanding our search to find out if there are more midwood trees out there—or even more new wood species.
These findings underline the importance of botanical research and the role that collections such as those at Cambridge University Botanic Garden play in revealing new insights into botany. The next time you visit a botanic garden, remember that there are many mysteries still hidden in the plant kingdom, waiting to be discovered.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Raymond Wightman is based at the Sainsbury Laboratory, University of Cambridge, which receives core funding from The Gatsby Charitable Foundation.
Jan Łyczakowski receives funding from the National Science Centre of Poland.