For generations, scientists have categorized trees into two groups: softwoods (like pines and firs) and hardwoods (like oaks and maples). Softwoods tend to grow faster, while hardwoods take longer to mature and produce a denser wood. However, recent research has unearthed a groundbreaking discovery—a third category of wood, named ‘midwood.’ This finding could revolutionize our understanding of carbon capture and hold significant potential in the fight against climate change.
Trees are natural carbon sinks, absorbing massive amounts of carbon dioxide (CO2) from the atmosphere and storing it in their wood. Among these natural carbon-capturing giants, the tulip tree (Liriodendron tulipifera), also known as the yellow poplar, stands out as a champion. Forests dominated by tulip trees in the mid-Atlantic US store between two and six times more carbon than forests composed of other tree species. This remarkable ability has already made the tulip tree a favorite for plantations in parts of Southeast Asia and a recommended choice for carbon capture by gardeners and urban planners in the US.
The tulip tree and its close relative, the Chinese tulip tree (Liriodendron chinense), trace their ancestry back to an ancient lineage dating back 50-30 million years—a period marked by dramatic changes in atmospheric CO2 levels. Interestingly, these are the only two surviving species from this ancient lineage. Until recently, the chemistry and structure of their wood, which could reveal the secrets behind their impressive carbon-capturing capabilities, remained largely unknown.
Traditional methods for analyzing wood’s internal structure often overlook the crucial difference between living and dried wood, the latter being much easier to study. This poses a challenge, as the molecular structure of wood changes when it loses water. The key to unlocking these secrets lay in observing wood that retains its moisture. Scientists overcame this challenge by employing a technique called low-temperature scanning electron microscopy at the Sainsbury Laboratory in Cambridge University. This advanced technology allowed them to visualize wood at a nanometer scale, revealing structures over 6,000 times smaller than a human hair, while preserving the wood’s moisture for a more accurate representation of its living state.
To delve into the evolution of wood structures, researchers studied various trees at the Cambridge University Botanic Garden. They collected living samples of plants representing key milestones in evolutionary history, ensuring that these samples could be examined under the microscope without drying out.
Their investigations revealed significant variations in the size of the macrofibril, a fiber composed primarily of cellulose, the fundamental building block of wood that provides plants with the strength to grow tall. In hardwoods like oak and maple, the macrofibril measures about 16 nanometers (nm) in diameter, while in softwoods like pine and spruce, it measures approximately 28 nm. These differences could explain the distinct characteristics of softwoods and hardwoods and provide insights into why some wood types excel at carbon storage.
To gain a deeper understanding of wood’s evolution and identify plants that could potentially mitigate climate change, researchers delved further back in time, examining basal angiosperms—a group of rare and ancient flowering plants representing the earliest stages of plant evolution.
One notable member of this group is Amborella trichopoda, which exhibits the larger 28 nm macrofibrils, suggesting that hardwood macrofibrils emerged later than softwoods. To pinpoint the precise moment this transition occurred, scientists explored the magnolia family, including the stunning purple-flowered Magnolia liliiflora, some of the oldest surviving flowering plants known for their ornamental beauty. These magnolias possess hardwood-like macrofibrils with a diameter of 15-16 nm, indicating that the shift from softwood to hardwood likely happened during the evolution of magnolias.
While the tulip tree is a close relative of magnolias, its wood defies easy categorization into softwood or hardwood. Its macrofibrils had a diameter of about 22 nm, falling squarely between the ranges for hardwoods and softwoods. This unexpected intermediate structure prompted scientists to classify tulip tree wood as ‘midwood,’ a new category altogether.
The question arises: why do tulip trees possess this unique wood type? While a definitive answer remains elusive, researchers believe it’s linked to the evolutionary pressures faced by these trees millions of years ago. When tulip trees first appeared, atmospheric CO2 levels were declining from approximately 1,000 parts per million (ppm) to 500 ppm. This reduction in available CO2 might have driven tulip trees to develop a more efficient method of carbon storage, resulting in their distinctive macrofibril structure. This adaptation likely contributes to their remarkable ability to sequester carbon today.
The discovery of ‘midwood’ shatters the assumption that all trees fit neatly into the traditional softwood and hardwood categories. The tulip tree, with its ‘midwood’ structure, embodies a ‘carbon-hungry’ attitude. Researchers are now investigating whether its unique wood structure is the sole reason for its carbon-capturing prowess. They are also expanding their search to identify other potential ‘midwood’ trees and explore the existence of even more previously unknown wood types.
These groundbreaking findings highlight the significance of botanical research and the vital role that collections, like those at the Cambridge University Botanic Garden, play in uncovering new insights in plant science. The next time you stroll through a botanical garden, remember that the plant kingdom holds countless mysteries waiting to be unveiled.
