A new study shows that two tree species, the American tulip tree (Tulip blossom) and the Chinese tulip tree (Chinese Liriodendron) grows a never-described species of wood that is neither hardwood nor softwood.

Countries like China and Vietnam are running tulip tree planting programs that focus on harnessing their extraordinary carbon storage ability. But what makes tulip trees so good at sequestering carbon has been a mystery until now. The study’s findings suggest that the answer to this question lies in the microscopic structure (ultrastructure) of their wood.

To reach this conclusion, the authors of the study examined the wood ultrastructure of 33 tree species using a cryo-scanning electron microscope (cryoSEM).

“We analyzed some of the most famous trees in the world, such as the giant sequoia, the Wollemi pine, etc. This is possibly the largest study of woody plants ever conducted using a cryo-electron microscope,” said Raymond Whitman, one of the study authors and a plant physiology researcher at Stanford University.

The unique cell wall of tulip tree wood

Trees generally have hardwood (such as eucalyptus, oak, birch, ash, and other flowering trees) or softwood (such as conifers and pines). Wood is made up of secondary cell walls, a thick layer between the primary cell wall (which forms bark, leaves, and growth tissues) and the cell membrane.

The secondary cell wall (SCW) is rich in cellulose, hemicellulose and lignin, organic compounds that give wood structural integrity and strength.

“Secondary cell walls are also the largest carbon store in the biosphere, so it is all the more important to understand their diversity in order to advance our carbon sequestration programs and thus contribute to mitigating climate change,” said Jan Łyczakowski, author of the study and researcher at the Faculty of Plant Biotechnology at Jagiellonian University.

The density and durability of SCW depend on the structure and arrangement of macrofibrils, which are nano- or micro-sized fibers (depending on the tree) made of cellulose and arranged in layers to form the cell wall.

CryoSEM analysis shows that the average diameter of microfibrils in deciduous trees is 27.9 nanometers. In coniferous trees, it is typically around 16.6 nanometers. L. tulipifera And L. chinense, The macrofibril diameter is 22.4 nm and this intermediate diameter is consistent throughout the secondary cell wall.

“To assess whether this property is restricted to water-conducting tracheal elements or whether other cell types containing SCWs also exhibit different microfibril sizes in Liriodendronwe decided to investigate the diameter of macrofibrils in fiber cells. Our results show that the mean macrofibril size observed in Liriodendron is also maintained in fibers,” the study authors note.

The advantages of medium-sized macrofibrils

The study suggests that secondary cell wall properties are sensitive to macrofibril diameter. For example, it is possible that the mean macrofibril size in American and Chinese tulip trees could be an adaptation to storing more carbon.

In addition, Liriodendrons emerged at a time when atmospheric CO₂ drastically reduced, from 1,000 ppm to 500 ppm. This change may have forced the trees to develop intermediate macrofibrils for better carbon sequestration.

“Liriodendrons diverged from magnolia trees about 30 to 50 million years ago, coinciding with a rapid decrease in atmospheric CO2 levels. Their enlarged macrofibril structure may be an adaptation that helps them to more easily capture and store larger amounts of carbon as the availability of atmospheric carbon decreased. This may explain why tulip trees can store carbon so effectively,” notes Łyczakowski.

These findings suggest that, similar to China and Vietnam, other countries such as the United States (home of the American tulip) could consider building tulip plantations to improve carbon sequestration.

Before going down this path, however, it is important to note that the current study represents a hypothesis. Further research is needed to confirm the link between mean macrofibril size and carbon storage.

“To further test this hypothesis, it will be important to extend the structural analysis of macrofibrils beyond the selection of organisms presented in this work, which represent only a small part of the diverse plant kingdom,” note the authors of the study.

The study was published in the journal New phytologist.