Wonders of Forest Giants: Physical Properties Governing Fir Trees

Keywords: Abies; Douglas fir; Material properties; Wood density; Growth rate; Gravitropism; Tissue mechanics; Hygroscopic movements

Abstract

Fir trees are a group of conifers widespread in the Northern hemisphere known for their needle-like leaves, cones, and incredible height. This paper examines the various design problems and solutions found in the lifecycle of fir trees and their interactions with their environment. The height of fir trees is supported by the density of their wood, which is less dense towards the center of the trunk and during early growth, resulting in a growth rate that is fast at first but then plateaus. The trunk is thickest at its base, where it connects with the taproot system, which keeps the tree anchored to the ground. The sturdiness of the tree is countered by the need for a large leaf surface area, resulting in a conical branch structure. The branches of firs are well adapted to northern climates. Their downward angle uses an initial layer of snow to deflect subsequent snowfall, while flexible reaction wood allows them to bend, shed excess weight, and snap back into shape. The growth of the trees is influenced by the factors of gravitropism and phototropism, which sense gravity and light to create physical changes. The reproduction of fir trees uses physical properties of tissues for cones to shed hygroscopic scales to release seeds, making use of the lift generated by seed wings to spread seeds as far as possible. These numerous evolutionary solutions have enabled the fir tree to be successful and widespread.

Introduction

Fir trees, also known by their scientific name, Abies, are an exceptionally interesting tree genus of many species that populate the Northern Hemisphere. With some species measuring up to 300 feet or almost 91m tall and living as long as 1000 years, fir trees are consistently the oldest and tallest trees in many forests. With a preferred habitat of mountain rocky terrain, fir trees cover a large part of the western North American forests. Despite the name, Douglas firs are members of the closely related genus Pseudotsuga, meaning false hemlock. In Figure 1, a side-by-side comparison of the Douglas fir and a common Corkbark fir is presented. In the figure, it is evident how the two are similar in appearance and shape, making their differentiation quite tricky to the untrained eye.

Fig. 1. (a) Adapted depiction of the Douglas fir tree, or Pseudotsuga menziesii (O’Brien, 2025). (b) Adapted depiction of a Corkbark fir or Abies lasiocarpa (Earle, 2025).

Douglas firs are also investigated in this paper due to their intriguing physical attributes and comparable features to fir trees. Figure 2 shows the primarily rocky and mountainous terrain that the Douglas fir inhabits.

Fig. 2. Map displaying the natural habitat of Douglas Fir trees in green. In grey is the western states of the United States of America, alongside the western and some central provinces of Canada (Hintsteiner et al., 2018).

Despite their relatively large habitat, many old-growth fir trees are disappearing quickly due to the desirability of their wood (Martin-Benito et al., 2022). At one point in time, old-growth fir trees such as the Douglas Fir tree populated large chunks of North American forest before early pioneers, and subsequent logging practices reduced this number down to a fraction of what it was once (Doll, 2024). Figure 3 shows a large piece of a fir tree on a train cart being taken away, presumably to be turned into construction material. Though now uncommon, sights like this would have been commonplace during the 19th and some of the 20th century.

Fig. 3. Image titled Douglas Fir Log Western Washington shows a large section of an old-growth Douglas fir tree being hauled away on a railroad cart (Douglas Fir Log Western Washington, n.d.).

Gymnosperms are plants that do not produce flowers or fruit. They are primarily pollinated by wind.

For fir trees, their height is a result of millions of years of development and evolution of their physical properties to cope with the effects of gravity, wind, snow and other natural forces. Being descendants of the pine family, also known as Pinaceae, fir trees are close relatives to the common pine tree. When comparing both pine and fir trees side by side, many similarities are striking, such as the presence of cones and needles rather than leaves. Fir trees, being gymnosperms, began to differentiate themselves from other tree and plant species around 23 million years ago (Zimmerman, 2020). Over many millions of years, this has resulted in distinctive structural, chemical and biological differences that make the fir tree a truly unique species of tree. For example, wood from fir trees is both its greatest strength and weakness, as it is one of the primary factors as to why the tree can grow so tall, but also the reason why the trees are cut down. Wood from the genus Abies is prized for its structural strength and ability to hold large loads of weight. At its core, the material properties of the fir wood, such as density, play a key role in ensuring that the fir tree can sustain its large size and immense weight. With trees the size of firs, it is not uncommon to see weights exceeding 4 tons (Lowry, 2025). The root structure of fir trees also allows for taller growth. Fir trees have a large central root that anchors the tree to the ground, known as a taproot, that acts as a counterweight (Fig 8.). This is contrary to what can be observed in many lower growth trees that employ a shallow, lateral root system, which can often result in uprooting during weather systems (Huang & Huang, 2021). Due to their preferred habitat, fir trees often experience immense amounts of snow. Fir trees have thus developed ways to combat this by having their branches retain a small layer of snow, which reduces snow accumulation since snow sticks less well to surfaces already covered in snow (Schmidt & Gluns, 1991). Moreover, fir trees grow tall, and depending on the species, this can be a relatively quick growth (Myers, 2024). It is important to maintain a straight growth pattern to avoid growing on an angle and subsequently inducing unnecessary stress on the tree. However, this can be difficult for young seedlings who have not yet developed a strong structure to resist wind, snow and other natural weather occurrences. To ensure straight vertical growth, the fir tree utilizes gravitropism to correct any structural deviations (Chen et al., 1999). This straight alignment maximizes the tree's potential height by ensuring gravitational force acts strictly along a vertical axis, avoiding any destabilizing horizontal weight distribution. This paper will focus on the niches in physics that fir trees employ to sustain their existence from infancy to hundreds, even thousands of years old.

Wood Density Distributions

Fir trees can achieve their impressive height in part due to the material properties that their wood possesses. However, throughout their lifetime, the wood inside of fir trees evolves to take on higher weight loads, adjusting its physical properties to account for higher levels of compression and tension force. In their infancy, fir trees face disadvantages like lower wood density, shorter fibre lengths, thinner cell walls, and higher microfibril angles (Langum et al., 2009). These disadvantageous properties prevent ultra-high growth but do allow for rapid growth during the first century of life. The growth rate of the Douglas Fir tree is presented in Figure 4, and it is evident how the growth rate evolves with the changing of the material properties of the tree as it ages.

Fig. 4. Diagram showing the rapid growth of Douglas Fir trees before a plateau of growth. Breast-height age of a tree is noted as being the age of the tree at approximately 1.3 m (Hinckley et al., 2011).

The material properties of a juvenile fir tree lead to lower strength, decreased stiffness, and increased longitudinal shrinkage issues, which prevent it from reaching the record heights for which the tree is known (Langum et al., 2009). However, over its lifetime, the fir tree matures, changing its physical properties, which allows it to further increase its reach into the sky. One way is by adjusting the relative density of the wood inside the tree trunk. This provides a crucial structural variation throughout the wood, which aids in supporting larger growth. One can make a parallel to the buttresses of medieval cathedrals when looking at the density distribution in mature fir trees. Figure 5 shows how, as the tree ages, denser mature wood develops on the exterior, leaving the interior less compact.

Fig. 5. Comparison of juvenile/mature wood ring profiles and the impact this has on the density of the wood found in the tree from the BC Ministry of Forests (Ministry of Forests, n.d.).

The distribution of denser wood in the outerings of the tree rather than the inner ones is an ingenious solution to the quest for ever taller growth. Less dense wood is what drives the height accrual in trees; however, as it grows taller, denser wood is deposited by secondary growth, providing not only a network for nutrients to cycle up and down the tree but also a structural ring around the tree that acts like a buttress-shaped cone, which tapers off with height. This distribution of wood density throughout the tree provides another physical advantage to the fir tree when dealing with wind, since stiffer and denser wood has a positive correlation to bending stiffness (Vikram et al., 2011). When examining the distribution of wood densities on the outside of the tree, we observe how the thickness of denser wood progressively lessens as the tree grows higher. The tapering of density is crucial as fir trees encounter more wind as they grow taller. It would be a disadvantage if the tree had a uniform density, since it would result in an increased risk of breakage in higher winds. The total wind force in Newtons on a tree at rest at each height interval is given by the following equation:

where u(z) is the mean wind speed (in meters per second) and A(z) is the streamlined projected area of the tree against the wind (in square meters) at some height z above the ground, C_d  is the drag coefficient and ρ is the density of the air in kilograms per cubic meter (Peltola, 2006). The wind force formula for a tree illustrates how, when a tree gets taller, the force gets larger, thus demonstrating the need to have a more elastic reaction to handle the force. Additionally, the following formula shows the contribution of gravity once any substantial bending occurs.

Where M(z) is the mass of the stem and crown, and g is the gravitational constant (in meters per square second). Together, these two formulas can be used to calculate the bending moment caused by wind, which indicates the point along the structural member where it is most likely to fail as the shear force equals zero at that point. The bending moment equation is given by:

Where x(z) is the horizontal displacement of the stem from the upright position (in meters). The Combination of formulas (1) and (2) to obtain the bending moment equations explains why the density distributions need to be the way they are (Peltola, 2006). Since denser wood is stiffer, it will have less of a tolerance for bending, and so, since wood higher up in the tree will bend more, it will need to be less stiff to tolerate the larger force. Figure 6 illustrates this as we observe how, after a certain breast-height age, fir trees switch to denser wood growth to sustain the less dense inner core, allowing it to grow further upwards.

Fig. 6. Graph of stem-wood relative density, depicting the gradual increase in wood density in the tree (Ministry of Forests, n.d., 122).

When looking at figures 3, 4 and 5 together, a picture begins to form as to why fir trees can grow so tall but also require such a lengthy period to do so. Figure 5 shows us that the average ring width of denser wood is much smaller than that of less dense juvenile wood. This means that to achieve greater height, a fir tree must undergo an initial period of rapid vertical growth, followed by secondary growth that thickens the trunk. This thickening stabilizes the existing height, allowing the tree to continue growing upward. Figure 7 presents further proof of why wood density is so critical to this structure. In Figure 7, we observe the buttress shape on the outside of the tree, but also a thin strip of denser wood that runs in the center of the tree, from the base all the way to the tip.

Fig. 7. Set of graphs depicting the tree’s density zones, highlighting especially the line of density down the middle surrounded by less dense wood (Ministry of Forests, n.d., 118).

The thin strip of denser wood acts similarly to how rebar acts in vertical columns, providing structural stability through an increase in tensile strength and structural integrity.  The added strength of the internal dense rod allows the tree to sustain upward growth while preventing internal structural weakness.

Root Structure

Every tree has a unique root structure that is designed to support its above-ground structure. The root structure of any tree is designed to act as an anchor, preventing the tree from having a structurally unstable base. The fir tree has a root structure that is known as a taproot. Figure 8 shows three different types of commonly found tree root distributions within the soil. From this figure, the taproot structure is shown as illustration C. Strikingly, the defining characteristic of the taproot structure is the large single root that descends vertically into the soil.  

Fig. 8. Three commonly found root structures in different tree species. (A) Heart root system with many angled vertical roots that extend in a semi-circle pattern. (B) Plate system with a thick horizontal root system that has seldom vertical roots descending into the ground. (C) The taproot system consists of one large vertical central root and thinner horizontal roots that extend out from the tree (Stokes, 2002).  

For the fir tree, this root structure provides an ingenious way to support its large vertical mass through episodes of turbulent weather, as Stokes et al. reported in 2002, how, in overturning tests conducted on adult fir trees, they tended to fail in the trunk or stem base rather than failing due to uprooting. Recalling the density structure of the fir tree, we note a much larger section of dense wood at the base. The large accumulation of dense wood and taproot structure work together to provide a structurally sound base for the tree, allowing it to withstand large cross forces. In Figure 9, we observe how the taproot structure handles strong cross forces like wind, confirming how to provide structural reinforcement to the overall stability of the fir tree's structure.  

Fig. 9. (A) During large winds, taproots are pushed in the leeward direction of the tree trunk, providing a counterbalancing force that prevents uprooting. (B) In some cases, the taproot may be pushed into the windward side of the tree. However, this still provides a strong anchoring in the soil (Stokes, 2002).

The strong resilience to uprooting provided by the taproot system allows the fir tree to achieve greater heights without a shifting center of mass destabilizing the base. One can view the taproot structure as an extension of the tree stump into the ground, with the soil level acting as the center of rotation. From this perspective, the physics behind this design solution to growing higher becomes evident as the tree tends to rotate around the center of rotation, a larger force is needed since the taproot structure must displace more soil, rocks and other objects found surrounding root structures. 

Branch Angles

One of the most important physical features of fir trees is the angle of their branches, because it plays an immense role in their overall stability and ability to withstand harsh climates. Since fir trees are typically found in colder climates, they often have to bear the weight of heavy snowfall throughout winter. On account of this, fir trees employ specific branch angles to manage the weight and damage incurred by snowfall. While the natural state of these trees is at a slightly upward angle, the angle starts to descend as the snow falls. Additionally, as temperatures rise following a snowfall, the branches flex downward to shed the accumulated snow load (Figure 10). Per Figure 10, the natural mechanism that the tree uses is that it retains some of the snow on its branches, rather than trying to discard all of it immediately. The reason for this is that, as explained in a 1987 study, falling snow crystals tend to bounce off snowy surfaces, including surfaces with newly fallen snow (Schmidt & Gluns, 1991). Even more interestingly, it was found that the likelihood of this snow rebounding decreased as the temperature rose from -3°C to 0°C  (Schmidt & Gluns, 1991). Hence, the tree retains some snow to repel the new falling snow. However, this can only hold up for so long, so the branches also allow for bending downward to discard some snow. This bending descends below the horizontal to both increase the bouncing off rate of snow and to shed old snow. As seen in Figure 11, the fir tree has snow interception levels of 3.8-4.0 mm. However, it also decreases in snow catch percentage over time, thus shedding snow. This is intended to keep a minimal amount of stress on the branches of the tree.  

Fig. 10. Diagram depicting how the rise in temperature after snowfall allows the branches of the tree to bend to remove snow. The colder temperature also allows the fir tree to stiffen, per the diagram (Schmidt & Pomeroy, 1990).

Fig. 11. The top graph shows that snow interception levels off near 3.7-4 mm intercepted (more than pine). The bottom graph shows that while fir trees can intercept more snow (up to 46-47% catch), they are built to shed it as the load grows, limiting branch stress (Schmidt & Gluns, 1991).

With the environmental challenges posed to fir trees, it is hard to imagine how the tree can maintain its shape bearing so much weight. For this reason, fir trees have what’s called “reaction wood” in their branches. Essentially, this allows the tree to return to its original branch angle once snow has been shed. This mechanism is not exclusive to snow management, however. Not all fir trees deal with heavy snow loads; many face other structural challenges, such as high winds or gravitational forces that bend their branches. Hence, the reaction wood, rather than raising up the branches, simply acts to keep the tree’s optimal branch angle. Different parts of the tree’s branches have different branch angle tendencies with relation to the gravitational pull. For instance, the tip of the branch often is negatively gravitropic, meaning it grows vertically opposite to gravity. By contrast, the middle part of the branches grows in a plagiogravitropic direction, creating an angle (Groover, 2016). This allows for consistent branch angles despite harsh conditions. There is also a variety in the tree’s branch angles, as the topmost branches tend to have higher branch angles. Most notably, fir trees generally have a “leader branch” at the very top that stands directly vertical. This ortho-geotropic branch acts as an enforcer for specific angles and ensures that they maintain their angles. 

Another interesting concept in the branch angles of fir trees is what’s called “self-similar branching” (Eloy et al., 2017). As seen in Figure 12, fir trees have general symmetry in their branching around the core of the tree, keeping a degree of branch angles that form a conical shape. 

Fig. 12. A set of diagrams depicting the conical branch angles among fir trees and the significance of the self-similar branching (Myers, 2024).

 The reason for this is that the tree’s branch angles are as much for wind and snow load resistance as it is for optimal light reception. If the tree’s design was intended to only maximize light reception, it would simply grow branches horizontally for a large leaf surface area. While this would absorb much light, it would also be more vulnerable to wind and snow. Inversely, the tree cannot just grow tall and slim for sturdiness, because there would be too limited light exposure. Hence, the fir tree’s design employs this “self-similar branching” to meet those needs halfway. Its conical branch angles reduce the bending and drag felt by the tree while still offering the tree enough needle-leaf surface area to intercept light for photosynthesis (Eloy et al., 2017).

Gravitropism

Gravitropism is a plant's growth response to gravity. This mechanism enables vertical orientation in trees like the fir, one of the tallest and most mechanically dependent species in boreal and temperate forests. Alongside phototropism, the orientation toward light, these two tropisms drive overall development. They are constrained by physical, mechanical, and environmental forces. Gravitropism exhibits two types of phenomena that aid the tree in achieving vertical orientation: positive and negative gravitropism. The former grows roots downwards, whereas the latter causes shoots to grow upwards. Gravitropism relies on statocytes cells that contain dense, starch-filled organelles, named statoliths. Statoliths settle under the influence of gravity, triggering a biochemical signal. In the shoots, the hormone auxin redistributes to the lower side of a tilted stem, causing the cells on that side to elongate less. In woody stems, growth rings may develop asymmetrically, and the stem will curve upward until it realigns vertically. This means that Fir trees exhibit negative gravitropism in their stem. Phototropism involves the detection of light by photoreceptors, which leads to the redistribution of auxin, promoting elongation on the shadier side of shoots. This difference in growth causes roots to bend in the direction of the light source. Phototropism allows saplings and branches to capture sunlight and is more critical when in shaded environments. Fir tree shoots grow contrary to the gravitational vector, while roots grow in the direction of gravity (Lamprecht et al., 2020).

The shoot system is the part of the tree which grows above ground. Subsequently, the root system is what grows below ground.

These two activities resemble a feedback system; gravity provides a constant downward acceleration, and light serves as an energy source. As an evolutionary design principle, fir trees exploit these phenomena by adjusting their growth to maximize energy. Just like all trees, the fir resembles a living engineering feat with self-correcting orientation, allowing them to be fit for photosynthesis. This behavior is evident in the maritime pine (Pinus), a conifer similar to the fir. Research shows that when seedlings are tilted by 30–45°, their shoot tips correct upward within hours. They only begin bending toward a lateral light source after partial vertical realignment is achieved. Vertically oriented or mildly tilted (15°) seedlings turned toward the light almost immediately without needing a strong gravitropic correction. In summary, fir shoots prioritize correcting gravitational misalignments before optimizing for photosynthesis; gravitropism sets the structural bounds, while phototropism fine-tunes light sourcing (Herrera et al., 2010).

Tropisms in firs operate on short and long timescales, involving both primary growth–the elongation of new, flexible shoots; and secondary growth–thickening by the cambium in woody tissues. In young fir seedlings, tropisms cause immediate curvature through differential cell elongation. In more mature woody stems, tropic reorientation occurs through asymmetric secondary growth, producing what is known as reaction wood or compression wood to gradually bend the trunk or branch back toward a desired orientation. In conifer trees such as firs, the reaction wood that forms on the underside of a leaning stem is called compression wood, which has thicker, lignin-rich cells that generate a pushing force to straighten the stem. From a physics perspective, the roots, which grow following gravitropism, and the root plate in the soil form an anchoring counterweight system that counterbalances the tree's tipping moments (Groover, 2016).

Fir trees form compression wood on the upper side of a leaning stem or branch, pulling the stem up. This reaction wood development is a direct result of gravitropic stimuli: it is the tree’s long-term mechanical adjustment to counteract tilting. Compression wood formation allows a leaning fir to “push” itself more upright over the years, as evidenced by eccentric growth rings (Figure 13). Sustained remodelling of ‘woody tissue’ is required to maintain or restore vertical posture.

Fig. 13. A visual representation of compression wood highlighted using red arrows to showcase of the effect tropisms can play on fir tree growth correction adapted from (Darling, n.d.).

There is an inherent hydraulic ceiling to the growth of the trees. Water ascension relies on tension in the xylem; with increased height comes increased risk of hydrostatic cavitation, setting a bound typically lower than the purely mechanical constraint. The trunk can be modelled as a cantilevered hollow column with self-weight buckling, per the Euler self-weight buckling formula. This idea comes from Mattheck in his book Design in Nature (Mattheck, 1998).

When there is a strong wind, a fir tree will oscillate, which happens to be a safety factor: vibrating increases the energy damping (some of the energy is dissipated by moving the weight of the tree and by wood and soil friction), and it will dampen resonant oscillations that might otherwise increase stress. With the passage of time, the mechanical feedback system in the tree (growth response like reaction wood, tapering growth, and root reinforcement) allows it to modify its shape to accommodate the normal wind climate. Yet, if the environmental load exceeds what the tree's shape can withstand, structural failure follows. Telltale signs of long-term mechanical stress in firs include broken trunks with corrective curvatures, oval stems, and reaction wood in the rings. This can be thought of as a natural documentation showcasing the struggles the Fir overcomes in facing its environment.

Reproduction

Fir trees are characterized by their use of cones for reproduction. In fir trees cones grow upright, while in Douglas firs and pine trees they droop down. Trees grow both male and female cones, which release pollen and seeds, respectively. Female cones in the pine family, including those of fir trees, have two types of overlapping scales: sterile bract scales and seed-bearing ovuliferous scales. In pinecones, these scales are fused together, while in fir cones, they are only partially fused, shown in Figure 14 (Wiczołek et al., 2025). During the process of reproduction, smaller and shorter-lasting male cones release pollen, which is taken up by the female cones. The scales of the female cone close during fertilization, then open to release seeds when they are ready to germinate. 

Fig. 14. A. concolor scale in profile with wet and dry states. The green circles show the hinge where the partially fused bract scale attaches to the ovuliferous scale (Wiczołek et al., 2025).

Trees in the pine family primarily spread their seeds through wind, so the timing of the seeds’ release is crucial to their survival. It is advantageous for seeds to be released in dry weather. Mature cones are composed of dead cells, so scales must open and close through an entirely passive hygroscopic mechanism (Eger et al., 2022). One model of scale bending is that of a simple bilayer with one swelling and one non-swelling layer. A moist environment causes the lower sclereid layer in scales to elongate while the upper sclerenchyma fibres swell less in response to humidity. This causes an overall curving upward of the scale entirely due to the material properties of the layers, similar to the bending of a bimetallic strip as seen in Figure 15.

Fig. 15. The bending of a bimetallic strip with coefficients of linear thermal expansion ⍺ where ⍺₂ > ⍺₁ (Angel & Haritos, 2013).

The calculation for the radius of a bimetallic strip bending under thermal expansion can be given by the Timoshenko formula:

where ⍺₁ and ⍺₂ are the coefficients of linear thermal expansion, t is the total thickness of the strip, m is the ratio of thicknesses of the two metals, n is the ratio of Young’s moduli, and T_h and T_c are the hot and cold states (Angel & Haritos, 2013). Young’s modulus is a measure of the stiffness of a material, the ratio of stress to strain or deformation. Compare this with the adapted formula for the hygroscopic expansion of a cone scale:

where k is the inverse of the radius, ⍺ is the difference in linear hygrometric expansion coefficients, h is the total thickness, m is the ratio of thicknesses, n is the ratio of Young’s moduli, and Δϕ is the change in relative humidity (Reyssat & Mahadevan, 2009). Note that equations (1) and (2) are nearly identical, with one being an inverse due to measuring k rather than radius, where any temperature dependent factors are replaced with humidity dependent factors particular to the tissue layers of the scale.

The differing responses to humidity between the layers can be traced back to a design solution at the cellular level, seen in Figure 16. Cellulose microfibrils are present in both sclereid cells and sclerenchyma fibres and control the swelling of the cell. In sclereid cells, the microfibrils are wound perpendicular to the direction of the cell, meaning swelling causes the cell to elongate longitudinally by resisting lateral expansion. In sclerenchyma fibres, by contrast, the microfibrils are wound longitudinally, resisting elongation (Dawson et al., 1997). This simple application of the same cellular components found in all plant cells is the main mechanism behind opening and closing cones in all the pine family.

Fig. 16. Scanning electron micrographs of (A) sclerenchyma fibres and (B) sclereids. Labelled with long axis (la) and cellulose fibre (cm) directions. Adapted from (Dawson et al., 1997).

However, the actual makeup of a conifer cone scale is more complex than that of a bimetallic strip. Figure 17 shows the structure of a cone in the pine genus, which comprises the archetypal conifer cones but does not include firs. The sclerenchyma fibres extend non-uniformly from the base of the scale, meaning that most of the bending occurs at the base. The other tissues present in the scale also play a role in the hygroscopic movement. The lower (abaxial) epidermis is responsible for a larger amount of water intake compared to the upper (adaxial) epidermis (Eger et al., 2022).

Fig. 17.  (A, B, C) P. wallichiana cone at various humidity levels. D, E) Cross sections of a single scale from Eger and al 2022 (Eger et al., 2022).

Another factor encouraging bending is the decreasing Young’s modulus of the sclerenchyma fibres with humidity (Eger et al., 2022). A higher Young’s modulus means a greater stiffness – as the humidity increases, the fibres become less stiff, intensifying the bending.

The cones of fir trees differ from those of pine trees as fir cones shed scales and fall apart upon maturity. This behaviour has a similar hygroscopic mechanism with a few key differences. The scales have three layers: adaxial, vascular bundle, and abaxial layers. The vascular bundle layer provides a scaffolding that provides resistance and sometimes bending due to a gradient in the orientation of cellulose fibres. The bending occurs throughout the scale rather than mostly at the base, though the abaxial layer is less uniform and thinner than in pinecones, which results in a greater amount of bending. Finally, the partially fused bract scale creates a hinge at the base of the ovuliferous scale, as was shown in Figure 14 (Wiczołek et al., 2025).

To travel further distances, fir pollen has air-filled sacs called sacci which lower the weight of pollen grains. The sacs also increase buoyancy in water, which allows pollen that accumulates on cones to be washed down the scale by rainwater and float into the seed opening (Owens et al., 1998). It has been suggested that the structure of a conifer cone creates eddies in the air current, which may increase the amount of pollen which is collected on the cone. Modern simulations show that the effect is present but likely does not have a large impact on the amount of pollen collected (Cresswell et al., 2007). One such simulation is shown in Figure 18.

Fig. 18.  (a,b) Computational fluid dynamics (CFD) visualizations of airflow around conifer cone models. Only a small amount of air is redirected (Cresswell et al., 2007).

Once the seeds are released, there is a balancing act in place so that they can spread as far as possible. Fir trees have seed wings that generate lift and drag in accordance with surface area. A larger seed with the same seed wing would travel less far, but larger seeds tend to have larger seed wings, partially evening out the distance (Cremer et al., 2012). Larger seeds also have the advantage of being more appealing to seed-caching animals, enabling secondary dispersal (Vander Wall et al., 2006). An issue with having larger seeds is the need to absorb water before germination. The plant embryo needs to be fully saturated with water, in other words to reach turgor, before cell growth. Due to the square-cube law, volume increases faster than surface area, so the amount of water needed increases faster than the rate of water being absorbed, which is proportional to surface area, as seed size increases. This leads to an increasing delay before germination (Norden et al., 2009). Overall, seed size varies a lot within and between fir species, with environmental conditions, and latitude in particular, being more strongly correlated (Kaliniewicz et al., 2019).

There comes a time when every fir’s life must end, but its death sets in motion a striking chain of ecological changes that often go far beyond a mere gap in the forest. Since the roots grow downwards, the bottom section of the fir is quite wide-spreading. When the fir’s time comes and it must be taken by nature, it will fall, and its root plate will be lifted and turned. A raised root mound will be formed, in addition to a pit left behind where the roots used to be. These new structures will alter the environment by introducing new microhabitats with differences in light, shelter, and soil exposure. The ecological outcomes are tied to the fir’s gravitropism trait, as without it the shape and expansion of the roots would be different. The root plate and pit are a direct consequence of how the fir tree’s root growth is directed over time as a response to gravity. The raised root plate lies exposed to the sun and the air, causing it to dry rapidly. The habitat of the plate encourages the colonization of mosses, lichens, and fungi, which provide nutrients to many insects. In turn, these insects will feed small vertebrates in the forest, promoting nutrient cycles in the ecosystem. Beneath the root plate forms a small cavity, which can serve as a small “den” as shelter for birds and mammals such as foxes, rabbits, and squirrels, especially during dangerous weather conditions. On the other hand, the pit left behind often becomes moist and depressed and can easily collect rainwater. Its conditions are perfect for hosting insects. Additionally, the decay of the wood acts as a substrate for beetles and ants, which speed up the decomposition of the wood and also generate vital food sources for higher trophic levels. For example, woodpeckers tend to forage on beetle larvae that nest within the rotting wood of the tree. It is fortunate that our wonderful Earth has such mechanisms that protect the prosperous and charming wildlife in its ecosystems.

Conclusion

From an engineering perspective, fir trees are ideal natural structures for both static (self-weight, gravity) and dynamic (wind, snow) loads through growth adaptations. They work within stability margins for most of their lifetime, perfecting the balance of static and dynamic forces, developing engineering design solutions that result in their mammoth height and lengthy existence. A first design solution is the transformation of density in new wood growth as the fir tree matures to allow an ever-greater height. Over successive growth cycles, the tree develops dense wood layers on the outer rings to support the tree’s height and stability, while leaving a less dense interior for internal functions. The distribution of dense wood in a mature fir tree resembles that of buttresses on the exterior of a medieval cathedral, and ingeniously, this distribution of density provides support in the same manner. Hence, this design solution serves to fight the challenges of natural destructive forces like the bending moment caused by wind and gravity. A second design solution that was explored in this paper was the taproot structure of a fir tree. The taproot structure supports the large mass and profile of the tree by providing a large central sinking root that counteracts wind and other natural challenges posed against the tree. The taproot structure allows the fir tree to grow continuously taller without risking uprooting like other tree species with shallower, heart root systems. Furthermore, another design solution that was explored in this paper was the physical adaptation of branch angles. It is observed that branch angling creates a conical shape for the tree that offers advantages for light reception and wind resistance while maximizing snow-load bearing to minimize the stress on the branches. Additionally, alongside the other design solutions to sustain its large height, fir trees use gravitropism to maintain vertical growth. Namely, the positive and negative gravitropic properties of the tree work together in a feedback system in response to external physical influences. Drawing an analogy to Maslow’s hierarchy of needs, the tree can be said to require a solid gravitropic foundation before phototropic micro correction takes charge. Lastly, the issue of reproduction has a wonderful solution, which ensures the reproduction and continuation of the species. Fir trees utilize physical factors such as wind and gravity to optimize seed production and growth, given often harsh environmental situations. The cones of a fir tree repurpose basic cellular components to change the physical properties of tissues in response to moisture, allowing them to passively open. Together, these design solutions permit fir trees to grow to gigantic heights and sustain an existence that can be many centuries old. Without many of these design solutions, fir trees would become victim of the forces of nature found in their harsh habitats, limiting them to shorter heights, lopsided growth patterns and reduced reproductive success. Taking away these design solutions, fir trees would not be the tree species that we know and see today.