A new study reveals that two tree species, the American Tulip Tree (Liriodendron tulipifera) and the Chinese Tulip Tree (Liriodendron chinense) grow a never-described type of wood that is neither hardwood nor softwood. 

Countries like China and Vietnam have been running Tulip tree plantation programs focused on utilizing their exceptional carbon storage ability. However, what made the Tulip trees so good at carbon sequestration was a secret until now. The findings from the study suggest the answer to this question lies in the microscopic structure (ultrastructure) of their wood.

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

“We analyzed some of the world’s most iconic trees like the giant sequoia, Wollemi pine, etc. This could be the largest survey, using a cryo-electron microscope, of woody plants ever done,” Raymond Whitman, one of the study authors and a researcher in plant physiology at Stanford University, said.

The unique cell wall of Tulip tree wood

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

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

“Secondary cell walls are also the largest repository of carbon in the biosphere, which makes it even more important to understand their diversity to further our carbon capture programs to help mitigate climate change,” said Jan Łyczakowski, study author and a researcher in the Plant Biotechnology Department of Jagiellonian University.

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

The cryoSEM analysis reveals that the average diameter of microfibrils in hardwood trees is 27.9 nanometers. It is typically around 16.6 nm in softwood trees. However, for L. tulipifera and L. chinense, the macrofibril diameter comes out to be 22.4 nm, and this intermediate diameter is consistent across the secondary cell wall.

“To evaluate whether this feature is confined to water-conducting tracheary elements or whether other cell types containing SCWs also have different microfibril size in Liriodendron, we decided to investigate macrofibril diameter in fiber cells. Our results indicate that the intermediate macrofibril size seen in Liriodendron is maintained in fibers,” the study authors note.

The perks of medium-sized macrofibrils  

The study suggests that changes in the properties of secondary cell walls are sensitive to macrofibril diameter. For instance, it is possible that the intermediate macrofibril size in the American and Chinese Tulip trees could be an adaptation for storing more carbon. 

Moreover, Liriodendrons originated at a time when atmospheric CO₂ underwent drastic reduction, falling from 1,000 parts per million to 500 ppm. This change may have forced the trees to develop intermediate macrofibrils for improved carbon sequestration.

“Liriodendrons diverged from Magnolia Trees around 30-50 million years ago, which coincided with a rapid reduction in atmospheric CO2. Their enlarged macrofibril structure could be an adaptation to help them more readily capture and store larger quantities of carbon when the availability of atmospheric carbon was being reduced. This might help explain why Tulip Trees are highly effective at carbon storage,” Łyczakowski notes.

These findings suggest that similar to China and Vietnam, other countries like the US (home to the American Tulip) can also consider Tulip tree plantations to enhance carbon sequestration. 

However, before they go down this path, it is important to know the current study presents a hypothesis. Further research is required to confirm the connection between intermediate macrofibril size and carbon storage

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

The study is published in the journal New Phytologist.