The structure of a tropical rainforest is what many people would consider a bit upside down. While the deep-rooted trees of temperate forests are free-standing, and often ancient, behemoths, tropical rainforest trees are not as massive or old. They have roots only at the surface and they are supported mainly by their crowns.
A healthy tropical rainforest possesses a highly ordered and organised architecture.
Enough trees have reached a sufficient height to form a “canopy” of interlocking tree crowns bound together by vines.
The canopy forms a skin over the rainforest ecosystem and it regulates many of the conditions within the forest.
The canopy provides almost complete shade to the forest floor. In the absence of light, plants don’t do much growing and this subdues the plants below the canopy. Thus, the understory is dominated by a relatively open forest floor, studded with an even spread of younger plants and trees of all ages which are not so much growing as much as waiting for an opportunity to grow.
When rainforest reaches this mature and stable state it is called “Primary Rainforest”.
“Virgin Rainforest” has never been logged or otherwise modified by humans and will be dominated by primary rainforest with patches of thicker vegetation dotted throughout.
Walking through primary rainforest is easy. It is not the impenetrable “jungle” that many of us expect. In fact the term “jungle” comes from the Hindi word for messy or chaotic. It entered our language during the Raj when British colonial administrators in India heard the locals referring to the thick vegetation around their facilities as “jungle”. When they returned to England and regaled their society friends over dinner with their “Adventures in the Jungles of India”, and the name just stuck.
Had more of the colonial thugs from Old Blighty taken more breaks from counting loot on the cushy cots on the verandas of their pukka bungalows, they may have learned something. If they’d gotten out of their pyjamas and into their khaki, then jumped in a dinghy to venture into the primary forest further away from the base, they would not have heard the term “jungle” again until they got back to the thickets on the edges of their clearings. Hindi words italicised.
If the canopy is breached, the undergrowth rapidly thickens in the extra light. This thickened understory is referred to as “Secondary Growth”. Secondary growth is found anywhere the canopy is breached, in gaps both man-made and natural. Healthy primary forest will be studded with secondary growth thickets in places where canopy gaps have formed naturally. These natural thickets are islands of important habitat for many plants and animals.
A rainforest that is dominated by secondary growth is not a mature forest. Densely vegetated rainforests are often that way due to logging or other forest damaging practices. Many tours claim to be accessing “Virgin Rainforest” while taking people to impenetrable secondary growth areas which have been heavily logged. This is obvious to anyone who knows these facts about rainforest architecture.
In most rainforests, there has been so much voracious vegetation growing in the soil for so long, that every last molecule of nutrient has been stripped from the dirt and turned into living tissue. In the absence of nutrients deeper down, the roots of the rainforest compete for resources at the surface as the leaf litter decomposes.For the past many millions of years, the forest has recycled, in the leaf litter and in the first few feet of soil, the nutrients it originally acquired from much deeper down.
Rainforest root systems form a dense mat of interlocking roots just below the surface. The root mat of the rainforest is exceptionally shallow, generally reaching no more than a meter below the surface, even on truly massive trees.
The roots of the rainforest plants and trees are highly interconnected both directly and via a vast fungal network which penetrates the root cells and physically connects almost every plant in the rainforest. This network is capable of sending nutrients and chemical signals throughout the forest.
The signals transacted by this network are known to include information about pests, leading to leaf defense activation in local plant communities prior to pests arriving on most community members.
They include information about disease threats, sent by infected plants, which lead to the ramping up of appropriate immune responses to the identified pathogens in neighbor plants, before they themselves become infected.
Information about resource depletion, prompting as yet unaffected plants to create stores of the resource that other plants report a local shortage of.
The science that is revealing this capacity for information networks to arise in ecosystems is very convincing. Under conditions of predation or infection and for many reasons beyond defense, plants have been found to communicate through the soil and through the emission of chemical signals into the air which other plants receive, act upon and re-emit, passing the message down the line
In terms of defense against predation, upon exposure to substances which identify a threat, like caterpillar spit, plants begin synthesizing chemicals which waft away to be detected by neighboring plants. The leaves and roots emit and receive signals that activate plant defenses in parts of the population potentially distant from the point of the original transmission.
These chemical messages are specific to particular threats and elicit appropriate, threat specific responses in the plants that receive the messages.
The agents of the outbreak encounter much more resistance in subsequent plants because they are now actively anticipating a threat and have bolstered their defenses.
The interconnection of myriad trees and other plants from multiple species, via an intricately networked web of fungal signaling architecture and broadcast chemicals, which enables the system as a whole to respond dynamically to the changing world, is unignorably analogous to a nervous system.
The canopy is an intricately interconnected fabric of living tree crowns bound together by vines.
It is highly homeostatic. The canopy is a sensitive regulator of the internal balance of temperature and humidity within the forest.
Under varying conditions of temperature and humidity the canopy trees may adjust leaf aspect, alter transpiration rates or even emit substances into the air to modify the weather.
The canopy is literally the “skin” of the forest.
The canopy provides physical stability to the trees in the same way that a very crowded bus is easier to ride than if you were standing alone. If you are jammed in tight on all sides then there’s no direction in which you could lean far enough to lose balance and fall over. As long as a tree is part of the canopy structure it will be able to survive cyclones despite the fact of its insubstantial roots.
This function is as architectural as is your own skeleton's function.
By the time a tree reaches three or four hundred years old, it will be a very well established member of the canopy community. Any further growth, begins to remove the tree from the canopy, at which point the tree becomes known as an “emergent”.
The heartwood is invaded by termites and beetles who inoculate it with fungus and bacteria.
The resultant wood-rot creates hollows and vital habitat for many animals.
These animals are in many known cases, the exclusive vectors for the distribution of certain seeds or the pollination of certain flowers. Therefore the loss of the big old emergents could cause the loss of many other vital components of the rainforest's complex architecture.
If a rainforest tree gets to 700 years old it is very lucky. Emergents eventually get so big that they lose the support of the canopy and will ultimately be toppled by a tropical storms, ripping a gaping hole in the skin of the rainforest.
Throughout the rainforest there are many plants which maintain a low background presence or which remain dormant as seeds in the soil "seed bank".
If the canopy is breached, these plants explode in size and number. Many are rapid growing climbers which quickly seal the open forest edges. This has the effect of insulating the forest’s delicate internal climatic balance from the atmospheric conditions outside the canopy. This is highly analogous to the function provided by scar tissue.
In the clearing, surrounded by the thickening walls of vines and young rainforest trees, plants which have been waiting as dormant saplings for many years, explode in growth in response to the sunlight and huge compost dump caused by the fallen tree. Everything in the gap will grow at a prodigious rate until the canopy closes over again. Small gaps will be quickly closed by established trees spreading into the space available. Big gaps are slowly closed by the vegetation within them replacing the fallen trees.
Until the gap closes, the thicket will host a large array of thicket specialists who rely on these dense patches of forest for their existence. The high activity around thickets attracts animals who deposit seeds and nutrients from the intact forest, reminiscent of stem cells migrating into badly damaged body parts to restore the full range of function to the regenerating tissue.
The analogies between a rainforest and an organism are not trivial. In many ways a rainforest ecosystem behaves as an organism in its own right. This reality gets very close to home when you consider the reality that the absolute number of genetically "Human" cells in your body is outnumbered by genetically diverse "Other" cells by more than an order of magnitude. Many of the components of this internal ecosystem are critical to your very function as an organism, determining even aspects of your very personality...
The features we see as being in common between systems on different scales are reflections of deeper realities in nature that connect us in ways we're only beginning to fathom.