The Gravitational Beauty of Trees

Gravity is one of the four fundamental interactions in nature — the three others being electromagnetism, the strong nuclear interactions, and the weak nuclear interactions — and it pervades space like an invisible field until infinity. Albert Einstein taught us that the gravitational field is spacetime itself, and that gravity is experienced as a result of large masses bending spacetime. That is, you are constantly falling towards the centre of the Earth due to its large mass curving spacetime, which is why you are able to walk on its surface and not float away.

In a certain way, gravity connects us all, since it affects all particles. Moreover, it is responsible for keeping large-scale structures in the Universe together, given that it always acts in an attractive manner (it will never repel like in the case when the south poles of two magnets are put in close vicinity).

Aside from bearing upon the behaviour of inanimate matter, gravity indirectly enables life. Trees, for instance, communicate with the assistance of underground fungal networks partly because gravity ensures that hydrogen atoms merge inside the Sun.

Let us see how all of this links together.

Under Pressure

Roughly 4.57 billion years ago, a swirling hydrogen gas and dust cloud collapsed under gravitational pressure into a dense object, a protostar. With increasing pressure came rising temperatures and eventually nuclear fusion kicked in when the temperature amounted to about 15 million degrees Celsius. That moment marks the birth of our Sun.

Nuclear fusion is the subatomic process whereby two low-mass atoms consolidate into one heavier atom. Up until the chemical element iron (Fe), the net outcome of fusing two atoms is the release of energy. For atoms heavier than iron, the reaction demands more energy than that it is able to give off. That is why high-mass atoms will only release energy when split apart — this severing procedure is called nuclear fission.

The one place in our Solar System where nuclear fusion occurs spontaneously is inside the core of our Sun. The process of creating heavier chemical elements by virtue of nuclear fusion in the interior of stars is referred to as stellar nucleosynthesis.

Notwithstanding the net outflow of energy for the joining of two low-mass atoms, nuclear fusion still needs a considerable amount of energy input to make it happen in the first place. The reason is the fact that the strong nuclear interactions, which hold the positively charged protons inside an atom’s nucleus together, must first overcome the repelling electromagnetic interactions that arise between the protons of two different atoms, when bringing them closer together.

Though it would usually take 5 billion degrees Celsius on average to unite two protons, the Sun pulls it off with ‘merely’ 15 million degrees Celsius. Quantum tunneling is why nuclear fusion is possible nonetheless: This quantum mechanical effect allows (sub)atomic particles to tunnel through an energetic barrier at lower energy levels compared to the scenario under classical physics.

Nucleosynthesis in the Sun involves combining hydrogen atoms into helium atoms, and there are two ways to achieve this transition. The prevalent nuclear reaction, which accounts for 99% of the Sun’s energy, is the proton-proton chain (Fig.1). The less common one consists of the carbon-nitrogen-oxygen (CNO) cycle.

Fig. 1. The proton-proton cycle of nuclear fusion in the Sun. (Source: adapted from arizona.edu).

In the case of the proton-proton chain, most of the Sun’s energy (60.5%) comes from converting “hydrogen into other forms of hydrogen, or helium into other forms of helium”, rather than the nuclear reaction of coalescing hydrogen atoms into helium (39.5%) — the second step in Fig. 1.

The released energy from nuclear fusion inside the core finds its way to the outer layers of the Sun by means of the transportation methods of radiation and convection, and finally radiates out into the Universe when reaching the surface.

Fig. 2. Outward motion of energy inside the Sun, conveyed by photons, i.e. fundamental units of electromagnetic waves, or light particles. (Source: Professor Chris Mihos).

The Sun how we see her today is a stable star in dynamic equilibrium: radiation pressure from the inside pushes the material outwards, while gravity from the outside pushes it inwards, and both pressure forces cancel each other out.

As long as hydrogen atoms continue to fuel nuclear fusion under gravitational pressure, the Sun will be actively supplying its surroundings with energy.

The Long Journey Home

The energy that radiates from the Sun propagates through space at the speed of light in the form of electromagnetic (EM) waves. Solar radiation covers almost the entire EM spectrum: radio waves, microwaves, infrared (IR) light, visible light, ultraviolet (UV) light, and X-rays.

What the Sun does not emit are gamma rays. Even though the Sun’s inner core produces gamma radiation, it is transformed into other types of EM radiation before arriving at the surface.

Radio waves have the longest wavelength and the lowest frequency, whereas gamma rays are high-frequency waves with the shortest wavelength. The former carries the lowest amount of energy, while the latter the highest amount.

It must be underscored that solar radiation is not uniformly distributed across the EM spectrum: although the Sun indeed issues IR, X-ray, UV light, and some radio waves, the main chunk of solar radiation is reserved for visible light.

Fig. 3. The distribution of EM energy emitted from the Sun. (Source: windows2universe.org).

Upon entering into the Earth’s atmosphere, most of the high-energy solar radiation (X-rays and UV light) is blocked by the upper atmospheric layers. Ozone (O₃) in the stratosphere absorbs these EM waves to form molecular oxygen (O₂) and atomic oxygen (O), because they transport sufficient energy to interact with these atmospheric molecules.

A portion of the microwaves and IR light will be equally fended off in the lower atmospheric regions. On the one hand, carbon dioxide (CO₂), water vapour (H₂0), and methane (CH₄) tend to temporarily take in IR radiation, playing a key role in facilitating the greenhouse effect. On the other hand, low-energy microwaves mostly interact with moisture (clouds). For higher frequency bands within the microwave continuum, however, atmospheric gases are also capable of absorbing microwaves.

Fig. 4. Schematic overview of the Earth’s atmosphere’s impact on incoming EM radiation. Note that in this image there are multiple sources of radiation, not just the Sun, given that gamma rays are included (which are not emitted by the Sun). (Source: lumenlearning).

In contrast, visible light, taking up the largest fraction of solar radiation, is granted full access to the Earth’s surface — this event is known as an atmospheric window. The same privilege is bestowed upon the majority of radio waves.

Fuel for Trees

Of the spectrum of visible light that reaches Earth, it is blue, violet, and red light in which trees typically take an interest. The role of green light is less well understood, but nevertheless contributes to the efficiency of photosynthesis, according to Ichiro Terashima et al.

During photosynthesis, trees switch solar energy into chemical energy, which is deposited in carbohydrates, such as glucose. More concretely, with the help of visible light, trees turn CO₂ and H₂0 into glucose (C₆H₁₂O₆), yielding O₂ as a by-product.

Despite the fact that over 50% of global photosynthetic oxygen production stems from phytoplankton swimming around in our oceans, the remaining quota provided by land vegetation is no less vital to our survival.

For a tree to survive and grow, it furthermore needs supplementary nutrients, like phosphorous and nitrogen from the soil. For instance, combining nitrogen with glucose leads to the assembly of amino acids, which link up to build proteins. Trees deploy proteins to, among other things, monitor cell wall growth, regulate extra- and intra-cell communication, manage stress response and disease resistance, and set up nitrogen reserves.

Gravity Makes Trees Talk

In order to secure both phosphorous and nitrogen, trees have made a pact with an underground network of fungi. The trade-off exists of the following: trees donate about 30% of their photosynthesized glucose (some accounts go as high as 80%) in exchange for the nutrients hoarded by fungal filaments, which are entangled with the trees’ hair-like root tips.

The fungi come with extra benefits: they enhance the soil structure, they shield the tree roots from pathogens, and they enlarge the trees’ belowground access area to soil resources.

What is more, this interlocking subterranean web of tree roots and fungi — technically described as the mycorrhizal network — ties trees with one another, a grid through which they pass on nutrients. Not just are resources, such as carbon, shared, but they are allocated in such a way to protect trees, even of different species, that are younger, that stand in the shade during summer, or that are damaged.

Fig. 5. An illustration of the mycorrhizal network among trees. (Source: nzgeo).

Apart from these resource-based incentives, the concealed fungal connections additionally possess an explicit cooperative function: trees rely on them to send early warning signals. For example, Yuan Yuan Song et al. demonstrate that, if a tree is under attack either by humans or insects, it predominately depends on the mycorrhizal network to transmit chemical signals, i.e. defensive enzymes, to other connected trees so that they can hedge themselves against possible future attacks.

No Gravity, No Life

As an indirect consequence of the gravitational pressure exerted upon the hydrogen atoms inside the Sun’s core, trees have been able to survive on Earth not only by harvesting light, but also by using the photosynthesized end products to establish mutually beneficial partnerships among them.

And without trees, our entire ecosystem would rapidly degrade. It is then not too far of a stretch to remark that, if it were not for gravity, life as we know it today could not have been sustained.

We may take gravity for granted during our day-to-day lives, but, as with everything that we take for granted, it is good to once in a while pause and celebrate that which has had our backs for as long as we can remember.

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