Natural Towers of the Southern Hemisphere: The Biomechanical Properties of Araucaria

Keywords: Araucaria, Xylem, Hydraulic System, Drought Resilience, Leaf Mass Area, Epicuticular Wax, Modulus of Rupture, Modulus of Elasticity, Microfibril Angle

Abstract

While humans have only very recently been able to construct long-lasting buildings of impressive heights, down in the southern hemisphere, trees have been doing this for millions of years. Among such is the Araucaria genus from the Araucariaceae family. As one of the oldest trees dating back 200 million years, they have developed specific physical properties to their crowns among the tallest in the south.  

To overcome the challenge of maintaining hydration as tall trees in an arid climate, Araucaria species utilize structural adaptations ranging from highly optimized, efficient xylem architectures to leaves with thick epicuticular wax and high leaf mass area. Further still, with seeds shaped aerodynamically for proper dispersal and a customized strength-to-weight ratio, Araucaria trees can both extend their lifespan and ensure the success of future generations. 

An old quote says, “Experience is the best teacher” and this is clearly seen with Araucaria’s advancements. There is a lot to be learned from its years spent evolving and adapting to life in the southern hemisphere.

Introduction

As a member of the Araucariaceae family, the Araucaria genus has persistently evolved to embody biomechanical refinement and thrive in changing environments. Originating from the prehistoric super continent Gondwana, these trees have lived alongside the dinosaurs more than 200 million years ago, continuously adapting to poor soils and varied climates. In the modern era, species such as A. araucana (monkey puzzle tree), A. bidwillii (bunya pine), and A. hunsteinii (klinki pine) span parts of South America, Australia, and the Pacific, dominating their ecosystems as tall emergent trees that can exceed 70 meters in height (Xie et al., 2024). In Fig. 1, a klinki pine can be observed towering over neighboring vegetation. With the towering advantage of height in mind, these specimens are required to overcome various challenges in transport optimization, strength, reproduction, and survival under challenging conditions. Their success can be observed through the ancient lineage of the genus’s existence and the physical strategies it has employed throughout time. 

Fig. 1. Picture of an Araucaria husteinii (klinki pine) on New Guinea island towering above all other vegetation (Utteridge & Jennings, 2022).

Hydraulic optimization is a core evolutionary trait for all tall organisms to be able to lift water to great heights without catastrophic failure. Reducing embolism risk in its xylem, Araucaria can sustain having small, yet productive crowns that tower far above other vegetation. This allows them to achieve optimal stability and hydraulic safety (Zimmer et al., 2015). Moreover, the tall genus has managed to reach a superior strength-to-weight ratio with their often fine-grained and uniformly textured trunks, prompting extended lifespan and enhanced structural balance in both stiffness and elasticity. In other words, the trees can resist buckling and absorb ambiguous wind stress, allowing them to act as mast-like columns that can survive weather extremes (Vanclay, 1994). Reflecting evolutionary trade-offs between stability and metabolic cost, the product supports long-term vertical growth and longevity without compromising resilience. As another highlight to the trees' resilience, Araucaria leaves and needles have made foliar adaptations to water scarcity and environmental stress. With their special traits, they reduce water loss, resist physical damage, and provide enhanced mechanical support under drought-induced tension (Mastroberti & Mariath, 2003). Finally, Araucaria have evolved their reproductive seed dispersal technique away from other known conifers. With their ingenious cones that degrade at maturity, the large resistant seeds fall independently from the tree, adapting strong aerodynamics for increased descent stability (Fauli, Rabault, & Carlson, 2019). Taken together, the study of the physical properties of these biomechanical perspectives present Araucaria not only as relics of deep time but as living models of incredible design.

The Hydraulic Challenges of the Tall Tree

With many species exceeding 60 meters in height, such as A. Husteinii and A. Araucana, the Araucaria genus stands as a model for pushing the limits of passive water transport – making it one of nature’s most intriguing hydraulic experiments. Given the characteristic crown structure throughout the genus, water must be lifted from the roots up to the treetop without embolism. Considering gravitational potential, path length, capillary properties, amongst many other factors, there is an elevated risk of water cavitation if there is no fine regulation or system reinforcement. With air bubbles that can cause blockages, Araucaria trees have evolved their water-lifting strategy to withstand high tension. When crunching the numbers, P50 values (the water potential resulting in a 50 percent loss of shoot hydraulic conductance) in Araucaria performs very well (Zimmer et al., 2015). Especially considering that most of their foliage is found near the top of the trunk.

Xylem Architecture and Conduit System

Zooming in on the xylem, compared to most angiosperms, who use both vessels and tracheids to move water, Araucaria rely exclusively on the latter. Notice in Fig. 2, these narrow, elongated cells have small diameters and thick walls, reducing the likelihood of air seeding across pit membranes (Carlquist, 2017).  As seen in Fig. 3, the cell walls of Araucaria tracheids are extremely thick. Pittermann describes them as being “overbuilt” for the tree’s realistic safety needs against embolism (Pittermann et al., 2006). These conduits are very narrow, which limits the maximum flow rate compared to vessel-bearing plants such as angiosperms. For Araucaria, this trade-off provides exceptional safety for water transport under extreme tensions in tall stems. Looking at the numbers, Araucaria trees typically operate with a safety margin of around 0.6 MPa (Zimmer et al., 2015) – representing the difference between the minimum and maximum hydraulic tensile operation of the tree free of embolism. As an additional point, tracheids in Araucaria are often double in length (10,000 µm) compared to other conifers (Carlquist, 2017). This evolutionary choice limits the number of tracheid junctions, reducing air seedings, and supplying a high level of structural integrity and mechanical strength. 

Fig. 2. Anatomical structure of softwood xylem tracheids in gymnosperms (Silvester, 1967).

Fig. 3. Transverse section of latewood in Araucaria angustifolia. Cells represented are latewood tracheids which have very thick cell walls (Carlquist, 2017).

A robust, cavitation-resilient trunk enables efficient hydraulic control to be maintained across other sections of the tree. Considering foliage in Araucaria, a specialized network of transfusion tracheids interspersed with parenchyma cells can be found in the vascular bundles of leaves and peripheral branch tissues. As seen in Fig. 4, transfusion tracheids (marked as tf) form a thin but crucial buffer between the axial xylem conduits and the mesophyll. These tissues, found across most gymnosperms, drive the evaporation of transpiration streams, providing lateral water redistribution within the leaves. This allows mesophyll cells to maintain hydration even as transpiration rates fluctuate rapidly in response to sunflecks or wind (Mastroberti & Mariath, 2003). As for parenchyma cells within this zone, they act as water and carbohydrate storage, facilitating storage management, ensuring smooth pressure fluctuations, and regulating sap content and secretion (Słupianek et al., 2021). Combined, although the Araucaria are quite conservative and “inefficient” with their water transport, the different xylem tracheids give Araucaria an elevated level of control on biomechanical safety and hydraulic regulation.

Fig. 4. Diagram of the transverse section of a vascular bundle taken from the leaf of an Araucaria araucana (syn. Araucaria imbricata), showing xylem (x¹ for primary and x² for secondary/later formed), phloem (ph), surrounding transfusion tissue (tf), and protoxylem (px) (Worsdell, 1897).

Whorled Branching as a Hydraulic Strategy

It is relevant to consider and understand the Araucaria phyllotactic branching strategy and its impact on the hydraulic front. With most species limiting their branching to the mid to high end of the tree (excluding certain variants: A. heterophylla), and some exclusively relying on branching near the crown such as A. araucana, and A. angustifolia, it becomes intriguing to understand how the trees handle the high levels of pressure at the nodes and the strategy to regulating the hydraulic system up the rest of the trunk. As for the phyllotactic (whorled) branching type, multiple branches emerge around the stem at specific nodes acting as a distribution for water as seen in Fig. 5. Here, trees can evenly distribute water radially by concentrating lateral outflows at discrete levels (Sperry et al., 2007). This allows the trunk to maintain a relatively uniform vertical flow between whorls, reducing turbulence and asymmetric pressure losses that can destabilize the water column (Ellmore et al., 2006). This is compared to alternate or opposite branching patterns as seen in Fig. 6, where the whorled architecture minimizes side-tapping along the stem, equalizing xylem tension, lowering the risk of localized cavitation (Schönauer et al., 2023). Moreover, Araucaria show reinforced densely packed tracheids and parenchyma at the nodes providing enhanced hydraulic regulation, where small pressure resets can easily be performed to ensure smooth passthroughs at nodal junction (Słupianek et al., 2021).

On another topic, hydraulic segmentation provides an additional layer of safety for these towering trees. Specifically, under various stresses due to drought, nutrient deficiency, or infection, Araucaria are able to segment which parts of the tree are important for survival and which are not. In hydraulics, the tree can regulate which branches get water in tense situations, for which different parts of plants have different thresholds of vulnerability to cavitation (Scholz et al., 2014). 

Fig. 5. Drawing of the water distribution in trees that utilize phyllotactic branching with darker colored arrows indicating higher water pressure and straight arrows representing limited tension and stability across the conduit system. Note that the water of emerging branches at the same node is very similar (Courtesy of Benjamin Albers, 2025).

Fig. 6. Cartoon diagrams of different types of branching arrangements (Admin, 2022).

Vulnerability of a plant's organ to cavitation is often expressed as P50, representing the water potential resulting in a 50 percent loss of hydraulic conductance. In other words, a tree's ability to sustain greater negative pressure corresponds to a more negative P50 value. Coming off the topic of hydraulic segmentation, segments can often be distinguished by their P50 at a given point. For instance, A. bidwillii, A. cunninghamii, and A. heterophylla (Australian species) express P50 values of around -3 MPa in the stem (trunk) and around -1.75 MPa in the shoots (branches) (Zimmer et al., 2015). Naturally, even lower pressure values can be found in leaves or needles. From this, it can be deduced that branchlets and leaves are designed to fail first during a severe drought or infection. This sacrificial strategy protects the main stem and root system by localizing embolism to distal organs where water loss is less catastrophic. Once conditions improve, peripheral tissues can recover or be replaced (Piraino et al., 2022). Such segmentation is critical in species that cannot rapidly replace large volumes of embolized xylem.

Drought Resilience and Seasonal Strategies

Although many Araucaria species inhabit humid montane regions such as A. araucana in humid Andean forests, or A. cunninghamii in the tropical rainforests of Australia and Papua New Guinea, others suffice to different environments such as A. labaunfelsii found on ultramafic soil (nutrient-poor soil derived from ultramafic rocks, characterized by high magnesium, low calcium, and elevated heavy metal concentrations) off the coast of New Caledonia, or A. angustifolia found in southern Brazil where winters can be very dry (Molina et al., 2016, Richards, 1967, Perry & Enright, 2002, Arenhardt et al., 2024). For those which have adapted to the harsher conditions, their hydraulic systems have become complemented by a suite of drought-resilient leaf traits (Mastroberti & Mariath, 2003). High leaf mass per area and a thick waxy cuticle limit transpiration and protect against desiccation. Additionally, stomatal control is conservative, closing early to prevent excessive water loss even at the cost of reduced carbon gain. This will be developed in greater detail in the section about foliage. Though together with cavitation-resistant xylem, these foliar adaptations ensure that negative pressures remain within safe limits (Zimmer et al., 2015). On the front of evolution, the ability to maintain hydraulic function under fluctuating water availability is likely to have been a main contributor to the genus’ survival through past climatic upheaval dating back to the Mesozoic era.

The Ingenious Architecture of Seeds

The seeds of Araucaria plants are integrated into a single cone (Nimsch, 2011) and after a period of time, the mature cone disintegrates, releasing individual seeds (Fig. 7). Seeds possess thin, papery double wings at their ends, giving them a kite-like appearance, which strongly influences their terminal velocity.

Fig. 7. Diagram of a single cone holding multiple individual seeds inside (Nimsch, 2011).

In a double-winged diaspore model, the fall velocity was reported to be minimized when the dimensionless curvature parameter (KL) ranged from approximately 1.7 to 3.0 (Fauli et al., 2019). Among the optimal KL range (1.7–3.0) suggested by (Fauli et al., 2019), Araucaria seeds are estimated to fall within the range of approximately 2.0–2.5, considering wing length and mass distribution (Fig. 8). This is favorable for wind dispersal by reducing fall speed and increasing aerial retention time. 

Fig. 8. Diagram shows that double-wing seed falls varies with KL: too low or high KL speeds up descent, while an intermediate KL (~2.3) slows it, showing wing geometry strongly affects aerodynamics (Fauli et al., 2019).

Thus, the architecture of double wings increases the surface area to weight ratio that increases air resistance and sensitivity to wind turbulence (der Weduwen & Ruxton, 2019; Hou et al., 2024). Moreover, the wings are symmetrical, reducing rotational instability and creating a torque to ensure seeds to fall “point first,” in order to maintain a stable orientation in airflow and minimize turbulence during landing (Wright et al., 2008; Fang, Zhang & Liu, 2017). The cones are generally ovoid in shape and vary in size depending on species. For example, cones of Araucaria species such as A. angustifolia have large cones, whereas others are smaller. As the cone turns brown and dries, the seeds within the cone mature and eventually fall out of the cone or are scattered by mechanical decomposition. Seed mass, wing thickness, and the degree of endosperm development vary among different species and the environment. The seeds with thinner and lighter wings benefit from greater wind dispersion – being aerodynamically stable - while heavier wings possess greater energy storage, leading to deposition closer to favorable microsites even under inadequate dispersal conditions (Fauli et al., 2018; Hou et al., 2024). One way to build on this trade-off is as of "design solution." For instance, it is beneficial to produce heavy seeds to encourage germination at close range and preserve the environmental advantage if the mother tree is already established in a favorable habitat. Producing light seeds, on the other hand, increases the possibility of spreading farther and taking up residence in a more favorable environment if the habitat is unfavorable. Ultimately, these structural characteristics of Araucaria seeds are not simply a reproductive strategy, but rather a natural mechanism designed to represent both aerodynamic stability and reproduction efficiency.

The Aerodynamics behind Seed Dispersal

Gravity and air resistance (drag) are the most fundamental and important physical forces that affect the falling of Araucaria seeds. Gravity always acts downward, while air resistance opposes the gravity that slows the seed's fall. When the seed begins to fall, gravity accelerates it; however, the air resistance increases with the velocity of seed rotation, eventually reaching a speed where the net force becomes zero; this equilibrium speed is called the terminal velocity. Terminal velocity is determined by the seed's mass (m), the acceleration due to gravity (g), the air density (ρ), the seed's projected area (A), and the drag coefficient (C_d) (1). 

In general, lower air density, greater mass, and smaller area lead to a higher terminal velocity in order to ensure a faster landing. (Nathan et al., 2011; Lee et al., 2025). 

If the seed lacks directionality or exhibits significant rotation, it will experience significant turbulence, making its descent unpredictable and increasing the likelihood of predation by birds or landing failure. To prevent instability and ensure that the seed is partially embedded in the ground upon landing, the center of mass is located on the blunt side, and the center of pressure is located on the wider edge (Fig. 9). This creates a drag force that creates a restoring moment, forcing the pointed end of the seed toward the ground.

Fig. 9. Diagram showing that the center of mass/gravity – the yellow dot – is located towards the blunt side (Benson, 2003).

As seen in (2), Reynolds number (Re) is a dimensionless number representing the ratio of inertial and viscous forces, which affects the properties of the flow around the seed (laminar vs turbulence), the magnitude of the drag coefficient, and the formation of the vortex structure (Vogel, 1994).

Re = Reynolds number

ρ = Density

V = Velocity

L = Length

μ = Bulk Viscosity

v = Kinematic viscosity

In common, small 'fur' (plume) seeds (e.g., dandelion) have low Re, so viscous effects and permeable structures show dominant flow behavior (Cummins et al., 2018), whereas for relatively large and heavy Araucaria seeds, Re is estimated to be around hundreds to thousands (order 10³) due to their characteristic length (seed/wing size) and drop velocity (i.e., vortex and wake structures and inertial drag are important; Nathan et al., 2011; Zhu et al., 2022). However, fluctuations in atmospheric pressure, temperature, and humidity across different altitudes can alter the Reynolds number by tens to hundreds of percent; consequently, shifting from sea level to elevations of 1,200–1,500 m modifies drag coefficients and flow structures, substantially impacting seed flight time and dispersal distances (Schippers & Jones, 2005). Laminar flows typically occur when Re < ~2,300, transitional flows between Re ≈ 2,300–4,000, and fully turbulent regimes when Re > 4,000. Re is also used in fluid dynamics to distinguish flow regimes (Vogel, 1994).

These factors (gravity, air resistance, terminal velocity, stability, and Reynolds number) interact with each other to optimize the seed dispersal (Fig. 10).

Fig. 10. The diagram represents the relationship between the air temperature (T, black), relative humidity (RH, gray) and the change in vapor pressure deficit (VPD), under Araucaria angustifolia canopy (•), (Marcón et al., 2014).

For example, low air density and small area can increase terminal velocity but decrease stability and hence the seed shape, wings and mass distribution moderate this trade-off. In fact, in Araucaria angustifolia forests at high altitudes in southern Brazil (1,200–1,500 m above sea level), the air pressure is about 85 kPa compared to the plains (about 101 kPa), which accelerates seed drop and tends to concentrate seeds near the forest edge rather than being carried far by the wind (Marcón et al., 2021). Conversely, in lowland Queensland, Australia (<200 m above sea level), Araucaria bidwillii colonies have been reported to have relatively high atmospheric pressure and average humidity (>70%), resulting in low speed and extended average wind dispersal distances above ten meters (Korndörfer et al., 2015). Therefore, whether Araucaria seeds benefit from rapid landing or longer aerial retention is not simply a matter of “environmental conditions,” but rather depends on regional climate data such as altitude, atmospheric pressure, humidity, and wind speed.

The Role of Leaves in Drought and Wind Resilience  

The Araucaria genus is composed of almost 20 species across the southern hemisphere (Armstrong, 2024). They are a collection of ‘siblings’ much like a human family, sharing most of their genetic information, yet they look very distinct from one to another. 

As seen in Fig. 11, the most obvious visual difference between Araucaria species is their leaves. The Araucaria are classified as coniferous plants because they produce cones to hold their seeds, but apart from that, these trees look almost nothing like a classic conifer. Araucaria leaves range from thin and needle-like to broad and spiky. In this section we’ll look specifically at one end of the spectrum with Araucaria araucana and its needle-like leaves. It is commonly said in biology that “form follows function” and by looking at the geographic location of this species of Araucaria and the various functions its leaves perform, we can better understand why this shape and its properties were nature’s choice.

Fig. 11. Image of branchlets from five species of Araucaria, from the top: Cook pine (Araucaria columnaris), Norfolk Island pine (A. Heterophylla), hoop pine (A. cunninghamii), bunya-bunya (A. bidwillii), and paranà pine (A. angustiifolia) (Armstrong, 2024).

The Araucaria araucana are the oldest Araucariaeceae trees in South America. After a long time of evolution, they have developed optimized leaf structures for their location. A. araucana are xerophytic trees, specialized to live in places with low water supply (Rivera et al., 2022). Their leaves have several distinct physical properties that collectively contribute to the entire system’s ability to maximize the use of limited water.

Multi-Functional Epicuticular Wax

Certain fruits in grocery stores, such as apples, tend to have a greasy, wax-like feel. Far from a sensory deception, all plants are covered in a layer of wax called ‘Epicuticular Wax’ (Pradeep et al., 2022). As seen in Fig. 12, the external layer of plants is called the cuticle of the cuticle is to protect the plant from environmental stresses (Yeats & Rose, 2013) and one of the ways that it accomplishes this task is through producing and secreting epicuticular wax.

Fig. 12. Illustration of the epicuticle of the plant, seen in cross-section. The lines dividing the layers above the epidermal cells indicate regions of major change in the construction of components rather than sharp boundaries. Indi-vidual plant species may depart greatly from this general arrangement (Eglinton & Hamilton, 1967).

One of A. araucana’s xerophytic traits is a high production of epicuticular wax on both sides of its leaves - adaxial and abaxial (Rivera et al., 2022) (See Fig. 13). The wax gives plants greater control over how it uses its water. The wax increases the leaf reflectivity and decreases its conductance. By decreasing conductance, the rate at which water vapor moves from the leaf through the stomata and back into the atmosphere is also reduced, conserving water (Yeo, 1998). This is especially useful for plants such as A. araucana, living in the southern hemisphere, as they can use the epicuticular wax to limit the amount of time spent in a water deficit when rainfall is scarce.

Fig. 13. Image of low-temperature field emission scanning electron micrographs of the surface of Araucaria araucana leaves. Comparison between (A) adaxial and (D) abaxial leaf side (Adapted from Dragota & Riederer, 2008).

In addition to regulating transpiration, epicuticular wax also plays a role in minimizing mechanical damage to leaves (Eglinton & Hamilton, 1967). In the southern hemisphere, winds are a lot stronger than in the northern hemisphere (White, 2023), and the rubbing of leaves together due to such winds could cause abrasions to the epidermal cells in leaves. A. araucana’s abundance of  epicuticular wax limits such damage. In addition, the wax also protects the leaves from insect and fungal attacks. The rough texture of the epicuticular wax as seen in Fig. 6 also plays a role in light scattering and when combined with light- absorbing polyphenolics, they work together shielding the plant from excessive ultraviolet radiation (Eglinton & Hamilton, 1967).

Flexible Growth enabled by Collenchyma

The presence of collenchyma in young A. araucana is another physical property that has aided its survival in the southern hemisphere. Collenchyma is a specific type of tissue that is most often found in herbaceous plants – plants without a woody stem, but A. araucana has evolved to develop collenchyma in its growing organs. Made up of elongated cells, collenchyma is an ideal tissue for areas in the plant where there is active cell division (Leroux, 2012). There is some variation between its structure in different parts of the plant and this can be seen in Fig. 14. In A. araucana, annular collenchyma is present, being best suited for growing. This specific shape of collenchyma provides “support, structure, mechanical strength, and flexibility to the petiole, leaf veins and stem of young plants, allowing for easy bending without breakage.” (Carrillo-López & Yahia, 2019)

Fig. 14. Schematic drawings of the most common types of collenchyma: Angular collenchyma (A), Tangential collenchyma (B), Annular collenchyma (C), and Lacunar collenchyma (D) (Leroux, 2012).

Mechanical Strength through High Leaf Mass Area

Another adaptation that A. araucana has made due to environmental location is its high leaf mass area. While collenchyma provides the leaves flexibility during their youth, as the tree ages, the properties shift towards mechanical stiffness to increase the life span of their leaves and make them more resistant to water shortages (Nardini, 2022). The process of creating what is known as “hard leaves” requires a heavy carbon investment during the active growth of the plant, but this quickly becomes worth it over decades and centuries spent in a hot and dry climate.

The leaf mass area of a plant is related to several sub-properties which all play a role in its drought tolerance. A visual representation of this can be seen in Fig. 16. Essentially, to have a higher leaf mass area, plants build up depth by enhancing three different components - the cell wall, major vein structures, and the xylem (see Fig. 15). A thicker cell wall is mechanically stiffer, which allows the leaf to maintain negative turgor pressure in the absence of water or when its amounts are limited (Nardini, 2022). This pressure is what keeps plants such as  A. araucana’s leaves from wilting and able to maintain turgor pressure within drought conditions. 

Fig. 15. Cross-section of a leaf midvein. The midvein is in the center of the image, with xylem (water-conducting tissue) and phloem (food-conducting tissue) labelled. Two smaller veins can be seen to the left and right of the midvein (Paleontological Research Institution, 2021).

When a plant is dehydrated, there is a reduction in the amount of pressure normally found in the xylem. This can cause the conduits to collapse, preventing the flow of water. By having thicker cell walls, A. araucana is able to support the xylem, limiting their risk of collapsing. In addition, the presence of more major vein structures creates multiple pathways for water if an existing passage collapses due to a reduction in pressure (Nardini, 2022).  

These three physical features create a high leaf mass area, which allow for physiological traits such as turgor loss point, vascular and extravascular hydraulic vulnerability (Nardini, 2022), which then make A. araucana more drought resistant.

Fig. 16.  Functional basis of coordination between leaf mass per area and drought tolerance, as mediated by mechanistic correlations between anatomical features (written in green) and physiological traits (written in blue) (Nardini, 2022).

Beyond drought resistance, having a higher leaf mass area also supports A. araucana’s growth with the high carbon dioxide levels found in the southern hemisphere. Specifically, it boosts photosynthesis by providing more space for chloroplasts and creating more surface area within plant cells for gas exchange (Zheng et al., 2019).

Looking just at one end of the spectrum with narrow needle-like leaves, we can see that the Araucaria has evolved very precise features to excel within its climate zone. From its wax coating controlling the transpiration of water, the presence of collenchyma during its youth for flexibility, to its matured high leaf mass area establishing and protecting water channels, A. araucana is overflowing with advanced, cohesive drought-resilient physical properties.

The Structural Challenges of Height

At first glance, the Araucaria tree is riddled with defects that would hinder its survival capacities greatly. Trees of this genus grow to heights upwards of 60 meters, towering over most other trees in their ecosystems, despite the small diameter of their trunks. Their crowns are substantially big and often grow very high on the tree, making it top-heavy (Xie et al., 2024). Combined, these aspects make it difficult to live in environments prone to rough weather, as strong wind could easily topple it over or cause immense stress on the tree’s foundation. To make matters worse, the Araucaria has a significantly low resistance to stress in comparison to other trees. This resistance is measured by the modulus of rupture (MOR), which is the maximum load that a material can take in bending without breaking (Kretschmann, 1970). Take the Araucaria Angustifolia for example, which has a modulus of rupture of 49.6 MPa. It ranks among the lowest in MOR, with the great majority of the trees having an MOR around the 60.0 MPA to 150 MPa range (Kretschmann, 1970). This means that Araucaria Angustifolia is very weak. Araucarias also do not have the ability to naturally replace a leader branch if it breaks, an ability shared by many species facing the same problems as the Araucaria (Sharma & Altaner, 2014). The leader branch guides the growth of the tree’s upright trunk, so not being able to replace it is a disadvantage. With all of this in mind, it is a wonder how Araucaria trees have stood tall for millions of years despite their weakness, but it was possible due to some of the Araucaria’s attributes.

Strong Root Anchorage

In order to deal with the possible imbalance that strong winds or other forces can cause on Araucaria trees, they grow particularly deep and strong roots that hold them in place despite their height. In fact, Araucaria roots were shown to adapt their depth and strength to optimally support the tree when faced with growth restraints. Araucaria Angustifolia trees were grown in soil of varying depths from 10 to 65 cm, and surprisingly, the ones that grew with less available rooting space reached the same heights as the trees with more space. Furthermore, the trees in shallow soil had more mass, a thicker trunk, thicker branches and thicker roots (Korndörfer, Mósena, & Dillenburg, 2008). In the case where the tree does not have enough rooting space, it will be thicker and more massive to withstand greater forces. Inversely, when given a greater rooting depth of 65 cm, the Araucaria will reduce its thickness to minimize its mass. This begs the question: why would the tree opt for a weaker trunk when given more available resources?

Prioritizing Stress Reduction

The Araucaria works around its low modulus of rupture by avoiding any potential causes of stress. As mentioned earlier, its height and large crown could bring about enormous bending loads on its trunk. It grows with lower mass and density to make its crown lighter and supportable and reduce loads that would lead to failure (Sharma & Altaner, 2014). This also helps the Araucaria withstand stress by lowering its modulus of elasticity (MOE). The MOE measures a material’s capacity to deform elastically without breaking when put under loads. Lower densities tend to reduce the MOE, which means a greater capacity to deform and greater absorption of stress. The Araucaria Angustifolia for example, has a low MOE of 9300 MPa, meaning it has low stiffness and high energy absorption (Kretschmann, 1970). The stress reduction that comes from the tree’s low modulus of elasticity absorbs enough energy from strong stress to keep the loads under the modulus of rupture. However, density alone is not enough to ensure a low MOE.

The Microfibril Angle and Flexibility

The key component to lowering the modulus of elasticity in the Araucaria is the microfibril angle (MFA). The MFA describes the angle at which cellulose microfibrils twist around the trunk axis (see Fig. 17). 

Fig. 17. Diagram of the direction of the microfibrils and the angle formed between the fibre axis (parallel to the trunk axis). The cellulose microfibrils are structural components of wood which form a loop around the axis (Tabet, & Aziz, 2013).

The microfibril angle has a strong correlation with the modulus of elasticity. A higher MFA improves the distribution of stress on the tree, so it lowers its MOE by allowing more deformation (Donaldson, 2008). Araucaria trees tend to make use of high MFA angles to have a coiled trunk as a design solution to counteract strong forces. Araucaria heterophylla, for example, can reach an MFA of about 44.2° (Sharma & Altaner, 2014). The twisting in the composition of the wood is crucial for the Araucaria as it dissipates energy through the trunk which reduces the load on a specific point of the tree, which allows it to bend greatly without much stress and helps stay under the tree’s low modulus of rupture (Donaldson, 2008). The Araucaria is therefore able to overcome the challenges of its height and weak material by coupling its low weight with its flexibility to build immense elastic strength.

Conclusion

As living fossils, Araucaria demonstrate that structural persistence and functional efficiency can be achieved by the gradual refinement of ancient design through long-evolved biological optimizations. Success is not derived from novelty but rather from the strong optimization of form and function. Narrower, stronger tracheids with smaller diameters minimize air seeding across pit membranes as much as possible, resulting in hydraulic continuity even with extreme negative pressure. Through the possession of conservative xylem architecture, such tissues facilitate lateral redistribution of water and allow distal structures such as branchlets and leaves to die back first under drought or infection to preserve integrity of the main hydraulic axis.

At the cellular level, a higher microfibril angle allows for more uniform stress distribution within the stem to enable more elastic deformation and lower the effective modulus of elasticity. This modification enhances wind loading and drought stress flexibility, reducing the chances of catastrophic failure. At the same time, epicuticular wax coat also provides optical and chemical protection. The embossed surface reflects excess light, and polyphenolic compounds absorb destructive ultraviolet radiation and deter fungal or insect penetration. Multifunctional surface advancements like these illustrate the integration of mechanical, optical, and protective properties into a single design element.

Even reproductive structures are regulated by this law of conservative efficiency, for which, larger energy-storing seeds have a higher inertial stability, depositing more locally at favourable microsites. This ensures establishment even with adverse dispersal conditions.

These traits cumulatively illustrate how Araucaria survival is an outcome of constraint-based design: compartmentalization, redundancy, and failure control. Their structure is a balance of strength and flexibility, protection and openness, danger and repair. With such integrated methods, Araucaria have survived more than 200 million years, the very embodiment of the principle that robust design is not the result of complexity, but of disciplined optimization of function over a long period of generations.