Rooted in Chemistry: Chemical Mechanisms Within Oak Trees

Keywords: oak, Quercus, mutualistic relationships, chemical signalling, summer branch drop, hormonal regulation, photosynthesis, Calvin cycle, aromatics, tannins

Abstract

Oak trees, with their exceptionally long lifespans of hundreds to over a thousand years, have evolved many internal chemical processes to withstand a range of environments and support their longevity. This paper examines the molecular and chemical mechanisms that enable oaks to adjust photosynthetic properties, regulate resource allocations, and strengthen symbiotic relationships with their environment. Central to this adaptability is the reconfiguration of the carbon metabolism of oak trees through chemical signalling with ectomycorrhizal fungi. Moreover, like other species, oak trees experience hormonal and stress changes, causing a “summer branch drop”; however, they possess tannins in many parts of the tree that act as defence mechanisms from decay caused by fungi or insects. With rising carbon dioxide levels because of climate change becoming more prevalent, oak trees learned to adjust their photosynthetic properties and continue to act as excellent carbon sinks. Finally, molecules in oak trees, such as lignin and ellagitannins, make it ideal for fermenting and aging drinks like wine or whiskey. Overall, these adaptive chemical processes within oaks have contributed to the chemical mechanisms that have allowed them to adapt and stand tall for 56 million years.

Introduction

The oak tree (Quercus) is a chemically sophisticated organism whose internal and external interactions are governed by complex molecular mechanisms. Beyond its mechanical durability, the oak has a rich chemical profile. Its chemical properties allow for greater defense, decay resistance, stress response regulation and environmental adaptation. (see Oak Physics for an extensive introduction) 

Chemical Interactions Between Oaks and Ectomycorrhizal Fungi

Oaks enhance ecosystem stability by partnering with root-associated fungi called mycorrhizae (from Greek mykēs “fungus” + rhiza “root”) in a mutualismsystem. These fungi colonize oak roots and extend vast filament networks into the soil (Fig. 1), boosting water and nutrient uptake, while the tree supplies photosynthetic sugars for fungi feeding in return (Martin & van der Heijden, 2024).

Fig. 1. Roots of a California oak (Quercus) colonized by ectomycorrhizal fungi, showing hyphal networks surrounding the root surface (Chalker-Scott, 2017).

Although mycorrhizal relationships occur across many plant families, oaks primarily form symbioses with ectomycorrhizal fungi (EMF). These fungi develop a sheath, or mantle, around the fine roots and extend into the root cortex to form the Hartig net, which is a structure first described by 19^th-century German botanist Theodor Hartig. The endomycorrhizal fungi grow within the root cells of their host (Tedersoo et al., 2010). This relationship is not just about trading nutrients; it is built on a foundation of molecular signalling and chemical exchange.

Chemical Basis of Oak-Fungus Recognition

The beginning of the oak-fungus relationship is entirely chemical. Both partners chemically signal to each other to ensure that they are recognized as beneficial and can coordinate their physiology before they physically connect (MacLean et al., 2017; Jung et al., 2012). Oak roots release an array of root exudates into the rhizosphere (layer of soil surrounding the root) to attract compatible EMF. These exudates include sugars, amino acids, and phenolic compounds, and indicate to fungi that the oak roots are open for colonization (Sebastiana et al., 2021). Studies on various plants have shown that flavonoids such as quercetin have small, aromatic, and hydroxyl-heavy structures (Fig. 2) that diffuse easily through soil water and stimulate fungal spore germination and increase branching of hyphae. This makes it easier for the fungus to locate and colonize roots (Hassan & Mathesius, 2012). These chemical signals do not guarantee a mutualistic connection, but they help the oak and fungus locate each other and begin the first stage of communication.

Fig. 2. Chemical structure of Quercetin (Magne Nde et al., 2015).

The fungi respond by releasing signalling molecules such as lipo-chito-oligosaccharides (LCOs). These have a backbone of N-acetylglucosamine units (C₈H₁₅NO₆)_n (Fig. 3), which is the same sugar found in chitin, and have lipid tails attached to aid with cell-membrane interaction (MacLean et al., 2017).

Fig. 3. Chemical structure of N-acetylglucosamine that make up the backbone of LCOs (Created by J. Neufeld, 2025).

When LCOs reach the oak root surface, they bind to receptors in the oak’s plasma membrane. This initiates a series of chemical signalling pathways inside the plant including waves of calcium ions (Ca²⁺) and phosphorylation reactions where enzymes activate one another by transferring phosphate groups (Bonfante & Genre, 2010). This recognition phase signals the presence of a compatible fungus and suppresses the oak’s defences by reducing localized lignin deposition and immune responses to allow fungal hyphae to colonize the root cortex and form the Hartig net.

Molecular Exchange and Metabolic Chemistry

After successful recognition, the fungus forms a mantle around the root and grows between the oak’s outermost root cells, making the Hartig net. This structure allows for the exchange of carbon, nitrogen, phosphorus, and micronutrients between both organisms (Policelli et al., 2020).

The oak tree provides the fungus with photosynthetically derived carbohydrates and fats, transported through the phloem to the roots. In general, forest trees primarily transport carbohydrates to the root-fungus interface in the form of sucrose, where it is hydrolyzed by invertase enzymes in the fungus cell wall to become glucose and fructose (Eq. 1) (Courty et al., 2016):

C₁₂H₂₂O₁₁ + H₂O → 2C₆H₁₂O₆ (1)

The fungus absorbs these simple sugars through transport proteins that make use of proton gradients in its cell membranes. Part of the sugars are used for energy, and the rest for growth and nutrient foraging (Courty et al., 2016).

In exchange for the oak tree provision of these sugars, the fungus collects essential elements such as nitrogen and phosphorus from the soil and converts them into usable forms for the tree. The EMF gathers nitrogen from the soil as nitrate (NO₃⁻), ammonium (NH₄⁺), or organic N such as amino acids. Nitrate is reduced by nitrate reductase (Eq. 2) and nitrite reductase (Eq. 3), a chemical process that adds electrons and hydrogen ions (Courty et al., 2016):

Nitrate Reductase

NO₃^- + 2H⁺ + 2e^- → NO₂ + H₂O (2)

NO₂^- + 8H⁺ + 6e^- → NH₄ + 2H₂O (3)

The fungus then uses glutamine synthetase to combine ammonium with the amino acid glutamate (C₅H₉NO₄), forming glutamine (C₅H₁₀N₂O₃) (Fig. 4);(Eq. 4), which is a form of nitrogen that can safely move into the oak’s root cells, and help it to build proteins and enzymes that it otherwise would be unable to create (Courty et al., 2016).

Fig. 4. Chemical structures of glutamine and glutamate (Created by J. Neufeld, 2025).

L-glutamate (C₅H₉NO₄) + NH₄ + ATP → L-glutamine(C₅H₁₀N₂O₃) + ADP + Pi + H⁺ (4)

Phosphorus is another key nutrient exchanged between oak trees and their fungal partners. In soil, most phosphorus is present in sparingly soluble calcium-, iron-, or aluminum-phosphate minerals, making it difficult for plants to access (Hinsinger, 2001). The fungus solves this chemically by releasing organic acids such as oxalic acid (C₂H₂O₄) and citric acid (C₆H₈O₇) that react with the inorganic compounds freeing soluble orthophosphate (mainly H₂PO₄⁻ at low pH). The fungus stores the phosphorus as polyphosphate chains, which it can break down and deliver to the oak’s roots whenever needed (Tian et al., 2021).

This exchange is essential to the nutrient uptake of plants. Mycorrhizal networks can supply as much as 80% of oak's total phosphorus and up to 20% of its nitrogen through the mutualistic relationship (Hawkins et al., 2023).

Secondary Metabolites and Chemical Defense

Although the primary purpose of oak’s mutualistic relationship with fungi is to trade nutrients, the connection also affects the tree’s secondary metabolism. This is the network of chemical pathways that produce defense compounds, antioxidants, and signaling molecules. These compounds are called secondary metabolites because they are used for communication and defense instead of basic growth (Pusztahelyi et al., 2015).

Phenolics are aromatic molecules made from the amino acid phenylalanine (C₉H₁₁NO₂). Oak trees produce many phenolics such as flavonoids and tannins, which have an abundance of hydroxyl groups that can neutralize reactive oxygen species (ROS), preventing damage to lipids and proteins during stress (like heat or drought) (Pusztahelyi et al., 2015). In oaks colonized by EMF, levels of phenolics often rise, increasing antioxidant capacity and helping oak to chemically defend its cells against oxidative stress (Jung et al., 2012). Tannins, molecules that act as chemical barriers that deter herbivores and bacteria, also change when oaks form EMF relationships. Oaks produce both condensed tannins, built from flavonoids like catechin, and hydrolysable tannins, made from gallic acid or ellagic acid. EMF can shift the balance between these two types, often increasing condensed tannins, which are a lot tougher. This adjustment strengthens the oak’s tissues and makes them less digestible to herbivores as well as more resistant to pathogen enzymes (Pusztahelyi et al., 2015).

Polyamines such as putrescine, spermidine, and spermine are small, positively charged molecules that help keep cell structures stable when plants are stressed. These compounds also increase in oaks with EMF, allowing the tree to handle heat and drought more effectively (Sebastiana et al., 2021). EMF also affect the oak’s hormone levels, which control defense and growth. The main hormones affected by this mutualism include jasmonic acid (JA), salicylic acid (SA) (Fig. 5), ethylene, and abscisic acid (ABA) (Jung et al., 2012).

Fig. 5. Chemical structures of plant hormones jasmonic acid and salicylic acid (Couto et al., 2016).

During EMF colonization, these hormones are chemically balanced to prevent overreaction, because the oak may mistake it for a threat and trigger defenses. The fungus releases small peptides that reduce excessive defenses while keeping the oak alert to real threats (Jung et al., 2012). In a study by Kebert et al., Quercus robur seedlings inoculated with EMF under drought stress showed significantly higher ABA levels (but not consistently higher JA) compared to non-colonized seedlings, suggesting that EMF helps the oak modulate stress hormone pathways to conserve water and protect cells (Kebert et al., 2023).

Summer Branch Drop

Summer branch drop (SBD) (Fig. 6) occurs in multiple tree species such as the elm and the ash, but its impact is more significant in oak trees due to their large size (UC Master Gardeners of Butte County, 2022). Summer branch drop is when mature trees will occasionally shed some of their branches. This phenomenon typically happens during hot summer days or during calm weather often following a heavy rain ending a period of increased soil dryness (Harris, 1983).

Fig. 6. This is a large branch dropped from a mature valley oak in the summer (UC Master Gardeners of Butte County, 2022).

Causes of Branch Failure

There is no single identified cause for summer branch drop, making hard to detect which tree is susceptible to SBD. A few possible causes include internal weakening of the branch, stress factors, added weight in the summer and wood decay. The internal weakening of the branch may be a result of a winter storm damage or the accumulation of snow leading to internal cracks or stresses (Linhares et al., 2023). A possible stress can be that of water stress which is known to cause cracks in conifer stems, and can similarly affect broadleaved trees, such as oak. As summer comes around, the added weight of new foliage and shoots, and developing fruits may cause branch failure. As decay starts it can weaken the wood and lead to fractures (Harris, 1983). A study was conducted on trees across four different Portuguese cities to evaluate summer branch drop and its causes. The trees studied in site 1, located in the city of Arcos de Valdevez, in Viana do Castelo district, were the Tilia tomentosa and the Populus nigra. In site 2, located in the city of Ponte de Lima, in Viana do Castelo district, the Platanus x hispanica and the Fagus sylvatica were observed. In site 3, located in the city of Montemor-o-Novo, in Évora district, the Ulmus carpinifolia and the Tilia cordata were studied. In site 4 (S4) located in the city of Paços de Ferreira, in Porto district, an English oak (Quercus robur) older than 150 years was being observed. The observed oak tree seemed to have no visible problems, its branches and leaves appeared dense and there was no sign of diebacks or dry branches, so the soil around was evaluated. The evaluation of the soil, done by pouring a bucket of water, showed almost complete runoff and little infiltration, indicating difficulty of gas exchange due to the fine roots developing in that last layer of soil, as this is where some air diffusion will happen (Fig. 7). This poor gas exchange affects the old age of the tree, which is the main predisposing factor of SBD, making it more prone to summer branch drop. Other predisposing factors considered were old soil compaction or sealing, injuries (biotic or abiotic), lack of light, excessive pruning, exigent gutters, and prevailing winds (Linhares et al., 2023).

Fig. 7. The water test done on the Quercus robur by pouring a bucket of water on the soil (Linhares et al., 2023).

Although winds can be a predisposing factor to SBD, the day the branch dropped was very dry and hot, thus proving that the branch failure is not necessarily due to winds. Figure 8 shows the low wind velocities on the day of the brand drop, happening sometime in the summer of 2022 on a dry day (Linhares et al., 2023).

Fig. 8. The wind velocity and direction in S4 location Porto District (Linhares et al., 2023).

Ethylene and Summer Branch Drop

Another cause for branch failure is the hormonal aspect. Water stress leads to a reduction in water flow, and a temperature increase within the branches and the stem. These higher temperatures are favourable for wood fungi which increases the probability of degradation. Furthermore, these high temperatures combined with high humidity can increase water pressure in the branches resulting in structural weakness and branch failure. At the same time there is an increase in ethylene concentration in tissues, which can result in weakening the cementation of cell walls. The weakening of the cementation is related to reduced transpiration, increased root pressure, and internal depression in the xylem vessels which weighs down the tree, due to water replenishment. The internal depression causing an increase in weight enhances the chances of branch failure (Linhares et al., 2023).

Ethylene Biosynthesis

Ethylene production is induced by several biotic and abiotic stressors such as wounding, flooding and drought (Xu & Zhang, 2015). Ethylene is synthesized by a pathway involving multiple conversions (Fig. 9). This pathway involves S-adenosyl-l-methionine (SAM) and 1-amino-1-carboxycyclopropane (ACC) and consists of three enzyme-catalyzed reactions. First methionine (Met) is catalyzed by methionine adenylyl transferase to form SAM. Then SAM is catalyzed by ACC synthase (ACS) to form ACC, which at the same time forms 5′-methylthioadenosine (MTA). MTA then passes through the Yang cycle to synthesize new Met. Finally, ACC is catalyzed by ACC oxidase (ACO) to synthesize ethylene (Wang et al., 2025). Understanding the ethylene biosynthesis pathway highlights the oak’s adaptive response to environmental stress during summer branch drop, as increased ethylene production influences tissue strength.

Fig. 9. The ethylene biosynthesis pathway presents methionine as Met, S-adenosyl-l-methionine as SAM and 1-amino-1-carboxycyclopropane as ACC (Wang et al., 2025).

Ganoderma Impact on Branch Failure

On other trees observed in the study done in Portugal there were signs of the biotic agent Ganoderma lucidum (Fig. 10). Ganoderma lucidum is a fungus that causes white rot which mainly degrades lignin. This is a type of internal degradation which greatly affects the structural component of the wood (Linhares et al., 2023).

Fig. 10. Ganoderma lucidum in an oak tree (LaForest, 2012).

A study was made comparing the effects of different species of Ganoderma on different wood types: silver leaf oak and white fir. In the 20 weeks of the study the results showed that the average percentage of weight loss (%WL) was greater in the oak wood than in the white wood (Table 1). Values were between 20.30 to 52.13% for the oak, and between 10.07 to 26.60% for the white fir. Within the oak itself the different species of Ganoderma resulted in different %WL (Adaskaveg et al., 1990).

Table 1. The average %WL of each Ganoderma species in oak and white fir (Adaskaveg et al., 1990).

Additionally, chemical analyses of wood loss showed that lignin, glucose and other sugars were removed from both trees, but the loss was greater in the oak than in the white fir (Adaskaveg et al., 1990).

Another fungus species, Ganoderma applanatum affects oak trees (Brazee, 2024). This type of decay is white-mottled rot found on the trunk and large roots and can extend more than 10 feet above the ground (Oak (Quercus Spp.)-Ganoderma Root and Butt Rot, 2025). In the early stages of decay, the wood still retains stiffness and there is only a small reduction in wood strength (Brazee, 2024). Thus, Ganoderma is not a direct cause of SBD, but a predisposing factor, as it reduces wood strength.

Summer Branch Drop Prevention

Wood decay by microorganisms can reduce mechanical strength causing branches to break. To avoid this, many trees put in a lot of energy to increase their wood’s durability. In wood, mechanical properties are attributable to the insoluble matrix composed of cellulose, hemicellulose and lignin (Table 2).

Table 2. A comparison of cellulose, hemicellulose, and lignin based on the molecular weight, the branching and the compactness of the molecule.

The matrix’s durability depends on the presence of extractives, whose formation and accumulation are usually linked to the transformation of sapwood into heartwood. Sapwood, the external and youngest part of the wood, contains the functional vessels and living parenchymatous cells. Sapwood is prone to decay because it has a low concentration of extractives, unlike heartwood which is the internal part of the wood containing no living cells, that is resistant to decay due to its high concentration of extractives. These extractives are formed by the parenchymatous cells as sapwood is transformed into heartwood. In trees known for their wood durability, such as oaks, these extractives are mainly tannins. The percentage of tannins concentration is greater in durable woods than in perishable woods and moderately durable woods. As shown in Table 3, durable woods tannins concentration is between 3 to 12%, in perishable woods it is less than 1% and in moderately durable woods it is between 0.5 to 2% (Scalbert, 1992).

Table 3. Tannins concentration in various tree species and wood types, including durable woods, moderately durable woods, and perishable woods. The oak is present under durable woods (Quercus alba L. and Quercus robur L) and under moderately durable woods (Quercus rubra L.) (modified from Scalbert, 1992).

Scalbert writes that tannin migration in the cell wall provides more efficient protection of polysaccharides against microbial decay. Furthermore, tannins inhibit the growth of many microorganisms, which includes several wood-decaying fungi (Scalbert, 1992). Therefore, tannins are a great way to strengthen the tree and avoid wood decay by fungi, which is a predisposing factor to SBD. So, although tannins do aid in strengthening the tree, branch drop still occurs as part of the natural pruning process of trees. This process is done to avoid decay penetration in the tree. Natural pruning happens when a ring is formed around the passive and sometimes already dead and rotting tree members. This ring represents a ‘departure’ or a branch-shedding collar which allows the rotten bark to break off at this well-defined ring notch. This allows for rapid closure of the knot hole by stress-controlled wound healing (Mattheck, 1998, pp. 59-60).

Photosynthetic Acclimation in Oak Trees under Elevated CO2

“Standing Tall: The Physics Behind the Mighty Oak Tree” examined the thermodynamic perspective of photosynthesis, introducing the concept that oak trees have the unique ability to increase their rate of photosynthesis when atmospheric carbon dioxide levels rise in response to the effects of climate change (Killi et al., 2018). When examining this fundamental process from a chemistry lens, photosynthesis is an oxidation-reduction reaction that uses light energy to convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂), as seen in Eq. 5 (Cooper, 1970). The overall chemical equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (5)

This process occurs in two main stages: the light-dependent reactions, where light energy becomes converted into chemical energy in the form of ATP and NADPH, and the light-independent reactions, otherwise referred to as the Calvin cycle, where the chemical energy from the light-dependent reactions is used to fix carbon dioxide into usable carbohydrates for the tree (Cooper, 1970).

Chemical Mechanisms of Photosynthetic Enhancement under Elevated CO2

The Calvin cycle is the starting point of carbon metabolism in plants. It is a complex series composed of 13 reactions catalyzed by 11 enzymes to synthesize carbohydrates from carbon dioxide (Fig. 11) (Heineke & Scheibe, 2009). The equation for the Calvin cycle is seen below in Eq. 6.

3CO₂ + 6NADPH + 9ATP + 5H₂O → G3P + 5NADP⁺ + 9ADP + 8Pi + 2H⁺ (6)

In elevated CO₂ conditions, the Calvin cycle equilibrium will be shifted towards the products, driving the reaction forward to increase the rates of carbon fixation and sugar production. However, the cycle can become limited by other factors, such as the availability of ATP or NADPH, or the rate at which the enzyme Ribulose-1,5-biphosphate carboxylase/oxygenase (RubisCO) can process the substrate (Fridlyand & Scheibe, 1999).

Fig. 11. Illustration of the three stages of the Calvin cycle occurring in the chloroplast. The cycle begins with carbon fixation using the enzyme RubisCO (Mukherjee, 2023).

RubisCO is the most dominant carbon dioxide-fixing enzyme found in nature, and is thus crucial for the carbon fixation stage, catalyzing the reaction of adding CO₂ to a five-carbon sugar called ribulose-1,5-biphosphate (RuBP) (Erb & Zarzycki, 2018). RubisCO is central to the light-independent reactions because it is the enzyme that converts inorganic carbon from the atmosphere into organic molecules, which subsequently initiates the process of building carbohydrates. Increased CO₂ concentrations will enhance RubisCO’s carboxylase activity, thereby increasing the rate of carbon fixation while decreasing photorespiration. Photorespiration is a light-dependent reaction where O₂ is fixed instead of CO₂, resulting in carbon being released back into the atmosphere. When considering photosynthetic efficiency, the process of photorespiration is of no apparent use to the plant and is often seen as wasteful (Horrocks & Valentine, 1999).

In an experiment conducted by Valladares et al., oak trees were seen to have a much greater RubisCO activity in full sunlight compared to beech species, as seen in Figure 12 (Valladares et al., 2002). They are able to do this through the function of RubisCO activase (Rca), a chaperone protein that removes inhibitory sugar phosphates from RubisCO’s active sites. Additionally, oak trees have a greater high-light tolerance compared to beech trees, which indicates a more efficient or robust Rca regulation mechanism under full sun conditions, leading to increased photosynthetic rates. This supports the fact that mature oak trees are better equipped to intake more carbon due to their higher RubisCO volume without experiencing photorespiration.

Fig. 12. Initial and total RubisCO activity and level of RubisCO activation at 08:00 hrs (left graphs) and at 14:00 hrs (right graphs) for pedunculate oak (open bars) and beech (shaded bars) in full sunlight and in 15% sunlight. Bars indicate mean ± SD of eight samples (Valladares et al., 2002).

Biochemical Acclimation and Feedback Regulation

In a controlled environment experiment conducted by Killi et al., two Mediterranean oak species, Quercus ilex and Quercus cerris, were examined to see the impact of elevated CO₂ and temperature on growth and leaf physiology. They found that in elevated CO₂ conditions, there was an accumulation of primary starch in the mesophyll cells of deciduous Q. cerris trees, but not in the evergreen Q. ilex cells, as seen in Fig. 13 (Killi et al., 2018). This indicates that the rate of photosynthetic carbon fixation in Q. cerris has outpaced the tree’s ability to use or export the sugars it is producing. Starch accumulation in the mesophyll cells can lead to feedback inhibition of photosynthesis, as the chloroplasts are overloaded with carbohydrates. This means key Calvin cycle enzymes are inhibited and the regeneration of inorganic phosphate (Pi) is reduced, affecting ATP synthesis. Thus, in Q. cerris trees, there will be a decline in photosynthetic rate, as the chloroplast is running out of usable energy carriers and space to photosynthesize. However, Q. ilex trees are able to continue photosynthesizing at a normal rate, despite the elevated CO₂ levels (Killi et al., 2018). They allocate surplus carbohydrates to other organs (e.g. roots) and utilize them for respiration. The tree will also store non-structural carbohydrates, primarily starch, in its leaves, stems, and roots for later use.

Fig. 13. (a, b, c, d); Light Microscope observations in cross sections of Quercus ilex (a, b) and Quercus cerris (c, d) leaves in control (panels a and c) and CO₂⁺ temperature treatment (panels b and d). Arrows indicate starch grains accumulation. (e, f, g, h); Transmission Electron Microscope observations in ultrastructural cross sections of Quercus cerris leaves grown in control (panels e and g) and CO2 + temperature treatment (panels f and h). Black arrows indicate starch grain accumulation in chloroplasts; white arrows indicate plastoglobuli (lipidic bodies) (Killi et al., 2018).

In high CO₂ environments, a process called nitrogen reallocation occurs in photosynthesizing organisms, in which plants will strategically move nitrogen within their leaves to balance resource use, often by chemically downregulating RubisCO and other photosynthetic proteins (Brooks & Szeto, 2024).

In an ongoing experiment conducted by Gardner et al., 175-year-old oak trees were bathed in carbon dioxide-enriched air and compared with a control group only exposed to atmospheric CO₂. After three years, they discovered that these mature oak trees had increased their rate of photosynthesis by up to a third. Specifically, they confirmed that leaf nitrogen will be reduced, and photosynthetic downregulation is observed under elevated CO₂ in oak trees (Gardner et al., 2021). This ensures that despite being under stress from high CO₂ concentrations, the oak trees can continue photosynthesizing by shifting nitrogen from less-needed proteins to other components (e.g. light-harvesting complex) in order to improve overall nitrogen, use efficiency as well as prevent a build-up of excess carbohydrates in mesophyll cells (Gardner et al., 2021).

Oak’s Exceptional Aging Power

Oak has long been prized for storing and aging liquids, not only because its dense tissue structure and tyloses make it naturally watertight, but also because its complex chemistry enhances flavor development. Structural components such as lignin and hemicellulose, along with aromatic compounds like whiskey lactones and stabilizing ellagitannins, give oak-aged drinks their distinct aromas and lasting character.

Foundations of Oak’s Watertightness

As discussed in “Standing Tall: The Physics Behind the Mighty Oak Tree,” one of oak’s most impressive traits is its watertight nature, which makes it the perfect material for barrel making. A crucial part of this property involves tyloses, which are balloon-like growths that extend into the hollow vessels of the wood. Studies have shown that these structures act as natural plugs, blocking the pathways that would normally allow water to pass through easily (Kim et al., 2024). By sealing these vessels, tyloses not only stop vertical flow but also reduce the sideways spread of moisture throughout the wood. This turns the heartwood, which was once part of the tree’s water transport system, into a compact and non-porous barrier that is ideal for containing liquids over long periods of time (Nevares & del Alamo-Sanza, 2018).

While tyloses provide the primary physical block, they are not the only reason oak is watertight. The wood’s ability to hold liquid without any leakage also depends on two other factors: the chemical composition of its cell walls and the density of its tissue structure. Together, these characteristics cooperate at the microscopic level to minimize permeability and enhance the durability of oak.

Chemically, oak cell walls are rich in lignin, a complex aromatic polymer that binds and reinforces cellulose and hemicellulose fibers (Fig. 14).

Fig. 14. Diagram of a plant cell wall illustrating how lignin reinforces cellulose and hemicellulose fibers to add strength and water resistance (Mathews et al., 2015).

Lignin (Fig. 15) is synthesized from three alcohol-based molecules (p-coumaryl, coniferyl, and sinapyl) that link together into an intricately bound network. This structure makes the cell walls hydrophobic as many hydroxyl groups (-OH) that would normally attract water are converted into ether linkages (R-O-R’) or incorporated into aromatic rings. (Liu & Eudes, 2022). Although lignin contains many hydroxyl groups, it overall behaves hydrophobically due to its aromatic rings and methoxy regions, which contain nonpolar methoxy groups (-OCH₃). The hydroxyl groups are often involved in internal hydrogen bonding, leaving the nonpolar aromatic surfaces exposed. These hydrophobic interactions promote self-assembly of lignin units through π-π stacking and van der Waals forces, resulting in a densely linked network that resists water penetration.

Fig. 15. Structure of lignin, a complex phenolic polymer composed of cross-linked units that form an irregular three-dimensional network. The colored areas highlight substructures that can form aromatic compounds once depolymerized (refer to the legend) (adapted from Ribeiro et al., 2023).

In addition to its waterproofing role, lignin ensures rigidity by filling small spaces between cellulose microfibrils, forming a dense and continuous pattern. This sealing, combined with its hydrophobicity, is what prevents stored liquids from seeping through the wood (Salmén, 2014).

The dense grain of oak further refines its watertight properties. Its fibers and vessels are tightly packed, and many pores are closed off by tyloses, which limits capillary action (fluid movement through narrow channels in porous materials). This compact arrangement leaves very few open passages for water displacement (Livani et al., 2023). Furthermore, this fine-grained microstructure influences how oak behaves when shaped into barrels. When the wood is bent, the radial and tangential grain orientations cause overlaps in the layers that seal even more tightly once hydrated, as the cell walls slightly swell, thus making the barrel much more resistant to leakage (Arends et al., 2017). Oak’s dense structure and lignin-rich composition work together to create a natural waterproof system, which explains why oak has been the standard material for liquid storage and aging for centuries.

Aroma in Oak-Aged Liquids

Other than its watertight structure, oak possesses a unique chemical complexity that makes it ideal for fermenting and aging drinks like wine and whiskey. When shaped into barrels, the same molecules that once strengthened and waterproofed the wood, particularly lignin, hemicellulose, and lipids, become sources of distinct aromas that diffuse into the contained liquid. This occurs through heat treatment during barrel toasting (gentle heating of its interior), ethanol extraction, and slow oxygen exposure over time.

One of the main contributors to aroma development is lignin. When the barrel is toasted, some of lignin’s chemical bonds break down, forming smaller aromatic molecules (aldehydes) such as vanillin, syringaldehyde, and coniferaldehyde, which produce characteristic notes of vanilla, spice, and sweetness. Lignin can also form volatile compounds, such as guaiacol and eugenol, that generate smoky and clove-like scents depending on how much the wood is toasted. Similarly, hemicellulose lightly caramelizes under heat due to its thermosensitive nature, releasing sugars and producing almond-like aromas. These combined effects explain why different toasting levels (light, medium, or heavy) produce distinct flavors in wine and whiskey (Chira & Teissedre, 2014).

Another important group of aroma-producing molecules are the oak lactones, also known as whiskey lactones. In nature, these cyclic esters form from fatty acids in the wood’s lipids through slow oxidation or heating. According to Chiara and Teissedre (2014), there exist two stereoisomers of this compound, cis- and trans-ꞵ-methyl-γ-octalactone (Fig. 16), that dissolve easily in alcohol, and produce recognizable notes of coconut, cream, and sweet wood (Chira & Teissedre, 2014). The amount of these lactones present in the wood varies by oak species. White oak (Quercus alba) is particularly rich in them, producing more of the vanilla and coconut character of drinks like bourbon (Nevares & del Alamo-Sanza, 2018), while English oak (Quercus robur) contains fewer lactones and more tannins (which can produce slightly bitter tastes), giving off subtler spicier aromas (Cabrita et al., 2011).

Fig. 16. Multi-step synthetic pathway for the trans (top right) and cis (bottom right) whiskey lactone stereoisomers from the sugar alcohol precursor D-(+)-Mannitol (Manna et al., 2021).

Once the barrel is in use, several slow chemical processes control how these compounds move and change. Although oaks are impermeable to liquid water, its cell walls still allow small amounts of gas and vapor to pass. Studies show that this semipermeable nature makes micro-oxygenation, a slow diffusion of oxygen, possible through the difference in partial pressures in the atmosphere (high) and inside the barrel (low). This limited oxygen input helps soften harsh tannins, making the mouthfeel more pleasant, and stabilizes color by promoting oxidation reactions (Gómez-Plaza & Bautista-Ortín, 2019). For example, wine that is fermented in oak, with a controlled amount of oxygen introduced to it over time, allows it to have a more intense crimson coloration compared to its initial yellow-ish hue (Cano-López et al., 2010). Meanwhile, ethanol acts as a solvent, bringing out flavor molecules out of the wood. Because ethanol has both polar and nonpolar properties, it can dissolve a wide range of aromatic compounds from lignin-produced aldehydes to lipid-based lactones, which eases their gradual release into the liquid. According to Gómez-Plaza and Bautista-Ortín (2019), at lower ethanol concentrations, the percentage of lactones increases, whereas compounds like vanillin require a higher level of ethanol to increase.

Maintaining Balance and Flavour

While compounds from lignin and other components of the wood contribute to shaping the flavor of oak-aged beverages, another molecule called ellagitannin has a more discreet role that is just as important. Found in high amounts in the heartwood of oak, ellagitannins slowly dissolve into drinks stored in barrels. Once in the liquid, they help maintain chemical stability, influencing the smooth texture and long-lasting quality that make oak-aged drinks special (Cadahía et al., 2001).

Ellagitannins (Fig. 17) are part of a bigger group called hydrolysable tannins that can break down in water or acids. When these compounds oxidize, they can form ellagic acid, which can switch between oxidized and reduced forms (so they both give and take electrons). This property gives ellagitannins their antioxidant ability as they can neutralize unstable free radicals (Golovinskaia & Wang, 2022). Inside oak barrels, small amounts of oxygen naturally seep through the wood. Without any control, that oxygen could react too quickly with ethanol or aroma compounds, which can lead to spoilage or unpleasant flavors and mouthfeel. However, with their antioxidant property, ellagitannins help prevent this by acting as redox buffers where they neutralize reactive oxygen molecules and keep oxidation at a steady pace that supports proper aging (Vivas & Glories, 1996). Thus, ellagitannins act as chemical caretakers inside oak barrels by maintaining the long-term quality and depth of oak-aged beverages.

Fig. 17. Chemical structure of an ellagitannin (Mrabet, 2021).

Conclusion

The chemical processes of the oak tree allow it to solve multiple design challenges to ensure its survival, strength, and stability. Its mutualistic relationship with the EMF is a natural design solution where both partners benefit from the exchange of nutrients, water, and chemical signals that help the tree handle drought and nutrient-poor soil. Internally, the oak applies chemistry as a tool for construction and defense, with ligands and tannins reinforcing its tissues. This provides mechanical strength and decay resistance, while polyamines and antioxidant compounds protect cells during drought or heat stress. Hormonal regulation with abscisic acid and ethylene manages growth and water use in oak. Even SBD can be seen as a natural design solution, allowing the oak to relieve internal pressure and hormonal stress through controlled branch drop, which prevents greater structural damage over time. At the material level, oak wood has inspired real-world applications. Its watertight and durable structure is created by tyloses and lignin, and is essential to barrel making, where those chemical compounds give wine and whiskey their aroma and flavor. Oaks show how chemistry can support their ability to adapt through the process of photosynthesis. When exposed to higher levels of carbon dioxide, they can adjust by increasing RubisCO enzyme activity to fix more carbon and reduce photorespiration. They also regulate how nitrogen and starch are used within their leaves to keep energy balanced. This chemical flexibility allows oaks to keep producing sugars efficiently under changing environmental conditions. Overall, the oak’s chemistry demonstrates how chemical design solutions allow it to adapt, protect itself, and remain efficient, making it a model for chemical and structural design in nature.