Exploring the complexities in the life of trees and their importance.
In a previous article, we covered the basics of trees, such as what they are, how they grow, how they make energy, and what they need to survive. Now we'll introduce some of the lesser-known dynamics of some of Earth's most important life forms and how, when they come together, they support life on the whole planet.
The Symbiotic Relationships of Trees
Trees form symbiotic relationships with other trees and even other types of organisms. Blue-green algae colonizes the moss on trees, capturing nitrogen from the air and processing it into a form the tree can use. Rain then washes this natural fertilizer down the trunks, making it available to the roots. Young trees don't have their own moss because it grows very slowly and takes decades to get established. So old trees fertilize the forest and help the young ones get a better start in life.
Trees form symbiotic relationships with mycorrhizal fungi through their roots that aid the growth of both the fungi and the tree. The fungi colonize the tree roots and help the tree by increasing water and nutrient absorption. In return, trees provide the fungi with carbohydrates produced through photosynthesis, called photosynthate. This partnership not only enhances the tree's nutrient uptake but also increases its resilience against pathogens and environmental stressors.
Many trees form symbioses only with ectomycorrhizal fungi, which grasp the outside of the root tips. Grasses and some tropical trees form relationships only with arbuscular mycorrhizal fungi, which penetrate the cortical cells of their roots.
Communication and Defense Mechanisms
Auxins are a class of plant hormones that play a crucial role in coordinating various growth and behavioral processes in trees. They regulate cell elongation and help direct tree growth toward light (phototropism) and against gravity (gravitropism).
Normally, fruit development requires prior fertilization of the flowers to produce seeds. It's the auxin from these seeds that prompts the fruit to become fleshy. Removing a strawberry’s seeds halts its juiciness. However, applying synthetic auxin can induce some plants to yield seedless fruits, which is how we produce seedless varieties.
Different auxins have varying effects on plant parts; some encourage the retention of leaves and fruits on trees, while others can trigger fruit shedding. Fruit producers use these different auxins to manage the timing of fruit harvests.
Trees utilize auxins to stimulate the growth of new shoots and leaves, influence leaf formation and shedding, and assist in root development. Auxins are even involved in wood formation.
Trees communicate with each other using chemical signals, electrical impulses, and scent. Research reveals that through a vast underground mycorrhizal fungi network, known as the “wood wide web,” trees can alert each other to dangers like insect attacks or drought. By emitting chemical messages, they enable their neighbors to mount defenses.
This communication extends to electrical signals, allowing trees to share information over distances at a slow but effective pace. Moreover, trees use scent to issue warnings or call for help. When under threat from herbivores or insects, they release gases that signal danger to surrounding trees, prompting preemptive defensive actions. For instance, an African acacia tree under a giraffe attack will increase tannin production to deter feeding, simultaneously emitting ethylene gas to alert nearby trees. Upon detecting this ethylene gas, nearby acacia trees pre-emptively increase their own tannin production, even before the giraffes reach them.
Trees also emit scents to attract beneficial insects for pollination and pest control. When pests attack, trees can release pheromones to attract their natural enemies, such as parasitic wasps.
Additionally, research indicates trees might communicate via sound, producing a crackling noise at 220 Hz, possibly for cell communication or to sense their surroundings. Experiments show that roots can grow towards these sound sources, suggesting trees use sound for communication or environmental sensing.
The mycorrhizal network of tree roots and fungi serves more than just communication purposes. Suzanne Simard's research and experimentation show that trees share resources such as water and carbon in the form of sugars through their roots. She experimented to explore how Douglas fir and birch trees interact at different times of the year.
Douglass firs don't drain birch of carbon, but instead give it back in the shoulder seasons. The two species are in an alternating feedback system that depend on their size differentials and changes in source/sink status. In this way they coexist in harmony. By being in place together in a network of fungi and bacteria, birch and fir share resources, even as they outgrow each other and cast shade. Through this reciprocal alchemy, they remain healthy and productive.
In the summer birches were sending more carbon to the fir trees, especially when the firs were put under shade and could not photosynthesize. But in the winter, the firs were sending more carbon to the birches since birches are deciduous and are leafless in winter, and therefore unable to photosynthesize.
It turns out many trees share resources in this way. Not only do they share carbon, they share water, nitrogen, phosphorous, defense chemicals, hormones, minerals, and information to help each other survive.
Large older trees, which Simard calls "Mother Trees," serve as central hubs connected to hundreds of other trees. They play a crucial role in supporting and nurturing other trees and plants in the forest. Mother trees can even recognize their own offspring. They preferentially send nutrients to their own seedlings, which increases their chances of survival fourfold.
Bears, Salmon, and Trees
Bears hunting spawning salmon carry up to 150 fish per day into the forest, where tree roots absorb the decaying proteins and nutrients from the uneaten parts of the fish. This process supplies these trees with over 75% of their required nitrogen. The nitrogen found in the tree rings, originating from the salmon, is identifiable due to the enrichment of salmon flesh with the isotope nitrogen-15 while in the ocean. This isotope acts as a distinct marker of salmon-derived nitrogen in the wood. Consequently, an ancient cedar tree can preserve a 1000-year history of salmon migrations.
Trees and The Water Cycle
Trees affect the water cycle by absorbing groundwater through their roots and releasing water vapor into the atmosphere through transpiration. This process contributes to cloud formation and subsequent precipitation. Furthermore, trees intercept rainwater on their leaves and bark, which can then evaporate or be absorbed into the soil, replenishing groundwater and reducing runoff. By influencing precipitation patterns and enhancing groundwater recharge, trees significantly impact the distribution and quantity of the world's freshwater resources.
Hydraulic redistribution is a process where trees move water from wetter to drier parts of the soil using their root systems. During periods of soil moisture imbalance, trees absorb water from deeper, moist soil layers through their roots and transport it to the drier, upper layers. This movement of water helps to maintain the moisture balance within the soil, supporting not only the tree itself but also the surrounding plant life and microorganisms during dry conditions. Essentially, trees act as natural irrigation systems, redistributing water within the soil to sustain the ecosystem.
For every square yard of a forest, 27 square yards of leaves and needles blanket the crown. Part of every rainfall is intercepted in the canopy and immediately evaporates again. In addition, each summer trees use up to 8,500 cubic yards of water per square mile, which they release into the air through transpiration. This water vapor creates new clouds that travel farther inland to release their rain. As the cycle continues, water reaches even the most remote areas. This water pump works so well that the downpours thousands of miles inland are almost as heavy as they are on the coast.
For the pump to work, there are a few requirements. Forests must exist from the ocean to the furthest corner. Most importantly, coastal forests are the foundation for this system. If they do not exist, the system falls apart.
Terpenes, organic compounds produced by trees, have a significant impact on rainfall patterns. When trees release terpenes into the atmosphere, they can react with other particulates in the air to form aerosols. These aerosols act as cloud condensation nuclei, which are crucial for cloud formation. The presence of these nuclei in the atmosphere increases cloud density, which can enhance rainfall. Thus, the terpenes emitted by trees not only contribute to the forest's aromatic ambiance but also play a vital role in the hydrological cycle by influencing the formation of clouds and promoting rain.
Senses and Adaptive Strategies
All creatures, including trees, can produce more offspring than the environment can support. Therefore, the survivors are the ones most adapted to the conditions. This suggests that trees in a forest have specialized adaptations for their particular location, climate, pests, and neighbors.
In a forest, the genetic makeup of each individual tree belonging to the same species is very different. This is in contrast to people, who are genetically very similar. Members of the same species of many trees have genetics that are as far apart as those of different animal species. This means each tree has different characteristics. Some deal better with drought than cold. Others have powerful defenses against insects. And yet others are particularly impervious to wet roots.
To minimize the risk of parasite infestation, it's beneficial for a tree to grow far from its relatives. Often young trees that grow near their parents are killed off by their parents' parasites. Survival tends to favor those trees that establish themselves at greater distances from their direct kin, leading to their proliferation. As a result, the spaces between these distant relatives are often occupied by trees of different species, each seeking to maximize distance from its own kind to avoid similar fates. This pattern contributes to the vast diversity and spacing seen in tropical forests, suggesting that parasites play a key role in fostering ecological variety.
Sensing Time
Deciduous trees gauge the arrival of spring not just by the warmth of days but through a combination of temperature and daylight duration. They seemingly "count" the number of warm days, waiting for a certain amount before deeming it safe to begin new growth, indicating an innate timing mechanism. For instance, beech trees require at least 13 hours of daylight to start growing, suggesting they possess a way to perceive light. While leaves, acting as solar cells, absorb light in summer, it's likely the buds, with their light-permitting brown caps, and the tree trunk itself, playing a role in light detection during early spring. Influenced by a mix of light exposure and temperature changes, this process implies trees have a form of memory that allows them to distinguish between the seasons. This is important for knowing when they should grow new leaves in the Spring and drop them in the Fall.
After trees have dropped their leaves, the trunk and branches of trees are shaped such that their combined wind resistance is somewhat less than that of a modern car. Moreover, the whole construction is so flexible that the forces of a strong gust of wind are distributed throughout the tree. These measures all work together to ensure that hardly anything happens to deciduous trees over the winter.
Faced with strong winds, individual trees are much safer being in a forest among others. Every trunk is different, and each has its own individual pattern of woody fibers, a testament to its unique history. This means that after the first wind gust, which bends all the trees in the same direction at the same time, each tree springs back at a different speed, and usually it is the subsequent gusts that break a tree because they catch the tree while it's still severely bowed and bend it over again - even farther this time. But in an intact forest, every tree gets help. As the crowns spring back up, they hit each other because each of them is straightening up at a different pace. While some of them are still moving backward, others are already swinging forward again. The result is a gentle impact that slows both trees down. This way they dissipate the energy in the wind so they don't snap. By the time the next gust of wind comes along, the trees have almost stopped moving altogether, and the struggle begins all over again.
Trees and Environmental Impacts
The mass of a tree comes from carbon dioxide in the air. The older the tree, the more quickly it grows. Trees with trunks three feet in diameter generate three times as much biomass as trees that are only half as wide. Older trees are far more productive at sequestering carbon than young ones, and this is true on every continent. If we want to use trees and forests to sequester carbon, we must allow them to grow old.
Side note on where some carbon goes:
Rainwater always contains gasses dissolved from the atmosphere, and among the most soluble is carbon dioxide. Thus rain is a weak solution of carbon dioxide, otherwise known as carbonic acid. As the carbonic acid falls in some regions, it reacts with calcium and magnesium in the rock to form a weak solution of bicarbonates. This is washed away in rivers and into the ocean, where the bicarbonate salts become incorporated into the ocean bed and, eventually, thanks to plate tectonics, are thrust down into the magma beneath. The net result is that carbon dioxide is steadily leached out of the atmosphere.
Air Quality
Forest air is the epitome of healthy air. The air truly is considerably cleaner under the trees because the trees act as huge air filters. Their leaves and needles hang in a steady breeze, catching large and small particles as they float by. Per year and square mile, this can amount to 20,000 tons of material. Trees trap so much because their canopies present such a large surface area. In comparison to a meadow of a similar size, the surface area of the forest is hundreds of times larger. The filtered particles contain not only pollutants, such as soot but also pollen and dust blown up from the ground. It is the filtered particles from human activity, however, that are particularly harmful. Acids, toxic hydrocarbons, and nitrogen compounds accumulate in the trees like fat in the filter of an exhaust fan above a kitchen stove.
Not only do trees filter material out of the air, but they also pump substances into it. Phytoncides are antimicrobial organic compounds that trees produce, which can reduce airborne bacteria and boost human immune system function when inhaled during forest visits. Terpenes released into the air serve various functions, including deterring herbivores, attracting pollinators, and protecting the tree from heat stress. These terpenes give forests their characteristic aromas, which have been found to lower blood pressure and stress in humans.
Human Impact on Trees and Forests
Just like humans, trees need their rest at night. Sleep deprivation is as detrimental to trees as it is to us. Trees subjected to light every night can die, whether it's light from an indoor light kept on or a bright streetlamp outside.
In the late 1930s it became clear that plants do not measure the length of the day, but of the night. If the light is turned on even briefly during the night, short-day plants will not flower. In contrast, a long-day plant that flowers in 16 hours of light and 8 hours of dark will also flower with 8 hours of light and 16 hours of dark if the darkness is interrupted by a brief light. Plants sense the light with a pigment called phytochrome.
Forestry
Some forestry operations disturb forest ecosystems by opening up large clearings. Some people believe the open spaces increase species diversity and they miss the fact that this is traumatic for the forest. In exchange for a few species adapted to open areas, hundreds of microscopic organisms die out locally.
Logging and clearing allow sunlight to penetrate the forest canopy, warming the forest floor. This warmth boosts microbial activity in the soil, accelerating the decomposition of organic matter and releasing stored carbon into the atmosphere as greenhouse gases at a faster rate. The volume of these emissions closely matches the amount of wood harvested. Consequently, for every log you burn in your fireplace or woodstove, a similar amount of carbon dioxide is released from the forest floor. Interestingly, a compelling case can be made that burning biomass for energy is much more environmentally damaging than fossil hydrocarbons.
Tree plantations are not forests. Although they may look similar at a glance, the differences are vast. Large monocultures planted with the same tree species desirable for timber have very little biodiversity of animals, plants, fungi, and microorganisms. These plantations are also much more susceptible to disease, pests, fire, and wind damage than forests with an enormous variety of species and old trees.
Conservation and the Future of Forests
We've seen that trees are not just simple static organisms. They are one of the primary energy capturers on the planet, converting solar energy to biomass. They form dynamic relationships with each other and other organisms, communicate, share resources, fight pests, and help each other survive in times of stress. They affect climate by moving water across the land, balancing soil moisture content, influencing rainfall patterns, sequestering carbon, and cleaning the air.
Trees form the backbone of forests, which provide habitat for countless species of animals, plants, fungi, and microorganisms. Forests not only create ecosystem biodiversity and support life for other species, they support life for all humans.
Forests provide humans with non-timber products such as medicinal plants and fungi. They regulate the climate and hydrologic cycles, control pests and diseases, clean the air, refill aquifers, and enrich the soil with nutrients – all of which support human food production. No farms, no food. No forests, no farms, no food, no clean water.
Forests also support tourism, pharmaceuticals, mental health and wellbeing, scientific knowledge, and cultural and aesthetic value.
The older the forest, the more valuable it is for the planet. Old-growth forests sequester more carbon, support more life, and provide more of the great things that we all need – even if we personally don't observe these things from the city or suburbs.
The value of a forest is orders of magnitude greater than the economic value of its timber.
Preserving trees and forests is imperative for the long-term survival of civilization. Any innovation that preserves established forests has far-reaching benefits for all of humanity and life on Earth.
Questions for you:
What is one fascinating fact about trees or forests you've learned and think more people should know?
What sustainable forestry practices are you aware of, and how do they differ from conventional practices?
What are some ways we can minimize deforestation?
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