These fast-growing trees could be the solution to global carbon capture

You may not think of planting a fast-growing tree more commonly seen in ornamental gardens, but this discovery could open up new opportunities for improving carbon sequestration in plantation forests.

Researchers from Jagiellonian University and the University of Cambridge have found that Tulip Trees, which are related to magnolias and can grow well over 100 feet tall, possess a unique type of wood that doesn't fit neatly into the categories of hardwood or softwood.

Using a low-temperature scanning electron microscope (cryo-SEM), the scientists imaged the nanoscale architecture of secondary cell walls (wood) in their native hydrated state. The two surviving species of the ancient Liriodendron genus, commonly known as the Tulip Tree (Liriodendron tulipifera) and Chinese Tulip Tree (Liriodendron chinense), were found to have much larger macrofibrils than their hardwood relatives. These macrofibrils are long fibers aligned in layers in the secondary cell wall.

Dr. Jan Łyczakowski, the lead author of the research published in New Phytologist, explained, “We show Liriodendrons have an intermediate macrofibril structure that is significantly different from the structure of either softwood or hardwood. Liriodendrons diverged from Magnolia Trees around 30-50 million years ago, which coincided with a rapid reduction in atmospheric CO2. This might help explain why Tulip Trees are highly effective at carbon storage.”

The team suspects that the larger macrofibrils in this “midwood” or “accumulator-wood” contribute to the Tulip Trees’ rapid growth. Łyczakowski added, “Both Tulip Tree species are known to be exceptionally efficient at locking in carbon, and 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. Tulip Trees may end up being useful for carbon capture plantations. Some east Asian countries are already using Liriodendron plantations to efficiently lock in carbon, and we now think this might be related to its novel wood structure.”

Liriodendron tulipifera is native to North America, while Liriodendron chinense is a native species of central and southern China and Vietnam.

This discovery was part of a survey of 33 tree species from the Cambridge University Botanic Garden’s Living Collections. The survey explored how wood ultrastructure evolved across softwoods (gymnosperms such as pines and conifers) and hardwoods (angiosperms including oak, ash, birch, and eucalypts).

Dr. Łyczakowski emphasized the importance of understanding wood evolution and adaptation, saying, “Despite its importance, we know little about how the structure of wood evolves and adapts to the external environment. We made some key new discoveries in this survey – an entirely novel form of wood ultrastructure never observed before and a family of gymnosperms with angiosperm-like hardwood instead of the typical gymnosperm softwood.”

Wood ultrastructure refers to the detailed microscopic architecture of wood, encompassing the arrangement and organization of its material components. The survey using cryo-SEM focused on the secondary cell wall, which is mainly composed of cellulose and other complex sugars, impregnated with lignin to make the structure rigid.

These components make up the macrofibril, forming long aligned fibers arranged in distinct layers within the secondary cell wall. The macrofibril, the smallest structure measurable using cryo-SEM, is in the order of 10-40 nanometers thick and composed of cellulose microfibrils (3-4 nanometers) plus other components.

Studying wood ultrastructure is crucial for various applications, including wood processing, material science, and understanding the ecological and evolutionary aspects of trees. Understanding the biology behind tree growth and wood deposition is also valuable for calculating carbon capture.

The wood samples for the study were collected from trees in the Cambridge University Botanic Garden, coordinated by the Garden’s Collections Coordinator Margeaux Apple. Fresh samples of wood deposited in the previous spring growing season were collected from a selection of trees to reflect the evolutionary history of gymnosperm and angiosperm populations.

Dr. Raymond Wightman, Microscopy Core Facility Manager at the Sainsbury Laboratory Cambridge University, highlighted the significance of the study: “We analyzed some of the world’s most iconic trees like the giant sequoia, Wollemi pine, and so-called ‘living fossils’ such as Amborella trichopoda, which is the sole surviving species of a family of plants that was the earliest still existing group to evolve separately from all other flowering plants. Our survey data has given us new insights into the evolutionary relationships between wood nanostructure and the cell wall composition, which differs across the lineages of angiosperm and gymnosperm plants. Angiosperm cell walls possess characteristic narrower elementary units, called macrofibrils, compared to gymnosperms, and this small macrofibril emerged after divergence from the Amborella trichopoda ancestor.”

Lyczakowski and Wightman also analyzed the cell wall macrofibrils of two gymnosperm plants in the Gnetophytes family – Gnetum gnemon and Gnetum edule – and confirmed that both have a secondary cell wall ultrastructure synonymous with the hardwood cell wall structures of angiosperms. This is an example of convergent evolution, where the Gnetophytes independently evolved a hardwood-type structure normally only seen in angiosperms.

The survey was undertaken during the UK’s fourth hottest summer on record in 2022. Wightman noted, “We think this could be the largest survey, using a cryo-electron microscope, of woody plants ever done. It was only possible to do such a large survey of fresh hydrated wood because the Sainsbury Lab is located within the grounds of the Cambridge University Botanic Garden. We collected all the samples during the summer of 2022 – collecting in the early morning, freezing the samples in ultra-cold slush nitrogen, and then imaging the samples through to midnight.”

This research underscores the continued value and impact that botanic gardens have in contributing to modern-day research. Without the diverse selection of plants represented through evolutionary time, all growing together in the Cambridge University Botanic Garden’s Collections, such a study would not have been possible.

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