The Chemistry Column

Ancient memory

Claire Gormley

There is a tree in my neighbourhood that I love to walk by. Its bark is a soft grey, and its sturdy trunk twists at the base as if it is turning its neck to look behind. Whenever I see it I always wonder what might have happened to make it grow in such a unique way. What story could this tree share?

As a society, we revere our elders for the experiences they lived through and the precious memories they hold. This is one of the reasons why dementia is so devastating, not only for those living with the disease, and their families and carers who also suffer, but for society as a whole. While we may make an effort to save our elders’ stories so they can be appreciated by future generations, seldom do we extend the same appreciation for the ancient memory of the steady giants that tower overhead.

Every year trees mark down their experiences in the form of tree rings. Dendrochronology is the study of tree rings, and has applications across a variety of fields including archaeology and climate science. Through taking core samples of trees and studying the ringed patterns, dendrochronologists can determine the tree’s age and the various environmental conditions it has experienced over time. Currently the oldest living tree known to humankind— an unnamed member of Pinus longaeva or Bristlecone Pine —has been cross-dated by scientists to be 5,070 years old as of 2020 (Guinness World Records). An ancient Bristlecone Pine family located in the White Mountains of California is widely accepted to be the oldest non-clonal species of tree, with many members sampled being over 4,000 years old and still growing (USDA Forest Service). These trees sprouted around the time the foundations of Ancient Egyptian society were being developed– between 3100 and 2686 B.C.

The process of tree ring formation is not well understood, but it is of growing importance due to the implications it may have for carbon sequestration and climate action (Rathgerber, Cuny & Fonti, 2016). Initially, scientists thought that the amount of carbon that can be sequestered into wood is limited by how much photosynthesis a plant can do; however, there is the suggestion that xylem formation could play a more significant role (ibid). Xylem is a tissue responsible for transporting water and nutrients from the roots to the leaves of vascular plants; xylogenesis is the process of producing and differentiating new xylem cells in functional wood cells. The process involves five steps: division of a cambial mother cell; enlargement of newly formed daughter cells; deposition of the secondary cell wall; lignification of the cell wall; and programmed cell death.

Many of us are familiar with the cell cycle from biology class— cell growth, followed by DNA replication, nuclear division, and separation of the cytoplasm to form two daughter cells identical to the mother cell. In the case of xylogenesis, stem cells in the vascular cambium— the main growth tissue lying beneath the bark —divide to produce new cells that will form the phloem and xylem tissue, the latter of which goes on to become wood (Fischer et al, 2019). This process is thought to be highly regulated by auxin, a key plant hormone responsible for many aspects of plant growth and development, as well as other hormones like cytokinins and gibberellins (Balzan, Johal & Carraro, 2014; Rathgerber, Cuny & Fonti, 2016). These hormones have the incredibly important role of stimulating the production of the enzymes responsible for triggering the start of the cell cycle. The timing of this stimulation varies between tree species, but studies have shown that it is influenced by temperature and could, therefore, seriously impact tree growth in our warming world (Rathgerber, Cuny & Fonti, 2016; Roibu et al, 2020).

Following cell division, the daughter cells undergo a period of enlargement, which is considered to be the first stage of xylem differentiation (Rathgerber, Cuny & Fonti, 2016). Water and solutes flow into the cell as a result of the relaxation of the primary cell wall, so as to maintain high turgor pressure and keep the plant tissue rigid. An imbalance of water and solutes, caused by water shortage, can limit cell growth because the turgor pressure is not stable. The primary cell wall is restored after the volume of the cell has increased by 10–100x its initial volume. A complex array of transcription factors then coordinate the expression of genes that control the biosynthesis, transport, deposition and assembly of the secondary cell wall. Secondary cell walls are typically composed of three layers which differ in terms of their thickness and the orientation of their cellulose microfibrils. Although the secondary cell wall does not cover the entire cell surface— ‘pits’ are left to allow for the passage of water and nutrients from cell to cell —this structure provides woody plants with mechanical support, water transport and biological resistance (ibid).

Cellulose microfibrils and hemicellulose form a dense matrix in the secondary wall, which acts as the main load-bearing network. This matrix is then infused with lignin, a complex organic polymer, that forms chemical bonds with hemicellulose to reinforce the cell wall and make it waterproof. The careful lignification of the cell walls, which starts at the cell corners of the primary wall and extends into the secondary wall, ensures that the xylem tissue is rigid and waterproof, while also maintaining the capillarity properties essential for sap ascent (Rathgerber, Cuny & Fonti, 2016).

The final step of xylem cell differentiation, and thereby tree ring formation, is apoptosis— that is, programmed cell death. Apoptosis is triggered by an influx of calcium ions into the vacuole, a membrane-bound cell organelle that helps maintain water balance. The calcium ions cause the vacuole to break up and in turn releases hydrolases. These enzymes attack and degrade cell organelles, leaving an empty space surrounded by a thick wall after a few days (Rathgerber, Cuny & Fonti, 2016). Programmed cell death is a common process in multicellular organisms, but xylem cells are notable because they only become functional after their death. Ironically, the very process that solidifies the ‘memory’ of each season’s growth has parallels with the process through which our own memories are wiped through neurodegenerative diseases like Alzheimer’s (Moujalled, Strasser & Liddell, 2021).

While researching tree ring formation, I spoke with a friend who works as Member Support Coordinator for American Conservation Experience, a non-profit based in the United States. While doing forest maintenance work, she found an unusual tree ring pattern caused by a bullet shot straight into a tree some years before. Why anyone would shoot a bullet into a tree is beyond me, but her fascination with the pattern— the memory —that remained I can understand. In time, dendrochronology may unlock the secrets to combat climate change, but until then let’s aim to appreciate not only the experiences of our elders, but the ancient memories of trees.

References: Balzan, S., Johal, G. S., & Carraro, N. (2014) ‘The role of auxin transporters in monocots development’ in Frontiers in Plant Science, 5(393):1-12 Fischer, U., Kucukoglu, M., Helariutta, Y., & Bhalerao, R. P. (2019) ‘The Dynamics of Cambial Stem Cell Activity’ in Annual Review of Plant Biology; 7:293-319 Guinness World Records. ‘Oldest Living Individual Tree.’ Available from: www.guinnessworldrecords.com/worldrecords/oldest-living-individual-tree. [Accessed: 8 October 2021] Moujalled, D., Strasser, A., & Liddell, J. R. (2021) ‘Molecular mechanisms of cell death in neurological diseases’ in Cell Death and Differentiation, 28:2029-2044. Rathgerber, C. B. K., Cuny, H. E., & Fonti, P. (2016) ‘Biological Basis of Tree-Ring Formation: A Crash Course’ in Frontiers in Plant Science, 7:734 Roibu, C., Sfecla, V., Mursa, A., Ionita, M., Nagavciuc, V., Chiriloaei, F., Lesan, I., & Popa, I. (2020) ‘The Climatic Response of Tree Ring Width Components of Ash (Fraxinus excelsior L.) and Common Oak (Quercus robur L.) from Eastern Europe’ in Forests, 11(600):2-19. USDA Forest Service. ‘Ancient Bristlecone Pine Natural History.’ Available from: www.fs.usda.gov [Accessed: 8 October 2021]