Sharing Land With Trees

An essay on global climatic implications of forest-human coexistence

This essay was first published in Italian in Terapia Forestale 2 (A new volume on Forest Therapy in Italy) in co-authorship with Andrei Nefiodov and Ugo Bardi. With the world on fire, is there a chance for a new strategic ecological thinking to emerge?

Introduction: The Great Water Problem of life on land, and forests as its solution

Several hundred million years ago, when the young terrestrial life was about to colonize land, it faced two serious challenges unheard of in the ocean. One was the shortage of water, which, under gravity, tends to leave land for the ocean. Another was the flammability of life’s stuff in the oxygen atmosphere. Exposed to such vagaries, prone to desiccation and burning, it looked like terrestrial life stood few chances to ever become as mighty and prominent as her primordial oceanic kin.

Fortunately for all inhabiting land today, the laws of nature provided life with an opportunity of solving both problems at one fell swoop. Water quenches fire. If life could evolve a mechanism to keep land wet, this would simultaneously minimize the probability of ecosystems burning alive. But how then could life bring moisture inland?

Earth is a blue planet: two thirds of its surface is covered with water. The most energetic among the water molecules break out from the liquid to become gas. The atmosphere of Earth bears an appreciable amount of water vapor that travels freely above the planetary surface with winds. If only life could find ways to (1) direct the winds inland and (2) extract moisture from the incoming air, the Great Water Problem of the terrestrial life would be solved.

Here comes the trick: water vapor is a condensable gas. The lower the temperature, the less water vapor the atmosphere can hold. When the temperature drops, the colliding vapor molecules no longer have energy enough to overcome the intermolecular attraction forces, so they stick together forming droplets: the water vapor condenses. As there is less gas, the air pressure drops. As the air pressure drops, the higher pressure elsewhere starts pushing the air towards the low-pressure area where condensation occurs. In other words, moisture extraction can itself drive air motion!

As birds and flying insects evolved a perfect “knowledge” of aerodynamics that enabled them to fly, life, when it raised from the ocean, “learnt” the above physical laws and evolved a mechanism, the biotic pump of atmospheric moisture, that allowed land to be moistened and life on land thriving.

The key process of the biotic pump is plant transpiration: terrestrial plants emit about three hundred water molecules per each molecule of carbon dioxide fixed by photosynthesis (Cramer 2009). Such “wastefulness” has been conventionally considered as an “inevitable evil” caused by biochemical and environmental limitations. However, plants are known to differ substantially in their water use efficiencies depending on their metabolic pathways. For example, the so-called C₄ plants may have water use efficiencies several times higher than their C₃ relatives (Vogan and Sage 2011). While it is apparently possible to spend less water, plants, and especially trees -- among which, remarkably, there are practically no “water-efficient” C₄ plants (Sage 2001; Osborne and Sack 2012) -- have evolved high transpiration rates.

Transpiration unlashes water vapor keeping the atmosphere close to the dew point when condensation can commence. The cooling necessary for condensation is provided by the Earth’s gravitational field. As an air parcel ascends, its potential energy grows, while the internal energy (and, hence, temperature) is accordingly reduced. To switch on the condensation over a large area, the forest enhances transpiration to drive the atmospheric humidity beyond the dew point. Upon condensation, the air pressure drops in the lower atmosphere facilitating inflow of moist air from the adjacent ocean.

The mighty Amazon rainforest illustrates this majestic process (Wright et al. 2017). While a grassland ecosystem, incapable of efficiently controlling its water cycle, meets the end of the dry season in the state of maximum desiccation, the Amazon forest, in sharp contrast, begins to photosynthesize and transpire most actively (Saleska et al. 2016). New, vigorous leaves sprout under the full sunshine of the dry season clear skies using the water carefully stored from the wet season. As transpiration grows, so does the atmospheric moisture. Condensation intensifies accordingly, modifying the land-ocean air pressure contrasts. Finally, moist air rushes inland from the Atlantic Ocean bringing the much-needed moisture to the forest. The wet season promoted by forest transpiration sets in two months earlier than it arrives to unforested regions at the same latitude around the globe.

Another biotic pump example is the Eurasian forest belt that spreads across the continent over seven thousand kilometers and, during the vegetation season, draws moisture in from the three oceans: the Atlantic, the Arctic and the Pacific (Fig. 1).

Fig. 1. Land-ocean precipitation ratio (LOPR) in the Eurasian forest belt and in the unforested Australia (after Makarieva et al. 2013). In Australia, precipitation over land is smaller than over the ocean at the same latitude in both wet and dry season. In the boreal forest in summer, when it is biochemically active in summer, precipitation is higher than over the ocean and uniform over several thousand kilometers across the continent. In winter, the forest is dormant and the biotic pump does not work. Both the forest and unforested Australia during the wet season are about 5 degrees Kelvin warmer than the ocean.

Why forests?

Not just forests (in their generic sense, meaning big tall plants forming a canopy) but also grasslands are able to enrich the atmosphere with moisture via evapotranspiration. Could the biotic pump be driven by grasses? Not really. The main obstacle is how to control condensation preventing extremes. Evaporation that replenishes atmospheric moisture is a slow process driven by solar power. In contrast, condensation can occur at an arbitrarily high rate. Once there is ascending air motion, condensation rate is proportional to the vertical velocity: the more rapidly the air ascends and cools, the more water vapor condenses releasing energy that drives the air motion. This process can self-accelerate to produce wind speeds common to hurricanes and tornadoes. Such rapid outbursts would deplete atmospheric moisture and cause prolonged rain absence. Meanwhile the remaining soil moisture would partly leak as runoff, partly evaporate into the dry atmosphere producing desiccation in plant life.

Tall tree canopy puts a break on these uncontrolled processes. First, it ensures turbulent friction that decreases wind speeds. Second, a reverse vertical temperature gradient is caused during daytime under the canopy, with the ground being the coldest, and the tree tops the warmest. To take moisture away from the ground layer, a rising air parcel would have to work against a strong buoyancy force, as it would be colder than the surrounding air. This prevents loss of soil moisture by uncontrolled evaporation. Short grasses do not develop such a gradient.

Grasses and herbs have always been an essential part of the forest ecosystem. When a large tree dies and falls, a big space (“gap”) opens up for succession to start that will ultimately culminate in another big tree occupying the spot. Succession is process of ecosystem recovery from a disturbance (e.g., big tree death or fire). Early stages of succession are dominated by non-tree species including grasses. They rapidly cover the disturbed soil with a green carpet preventing leakage of nutrients. While unable to do biotic pumping efficiently, forest grasses did not actually need it – they were provided with moisture by the surrounding trees. As long as the gaps occupied a small relative area in the forest, the biotic pump of the forest as a whole was not impaired.

The problem of large herbivores

So, to keep land moistened, life invented forest. Forests have huge biomass. This is the main distinction between terrestrial and oceanic life. They have comparable primary productivities, of about 50 GtC/year. But, amazingly, if we look through the oceanic surface, there is apparently nobody there to be seen! Primary producers, the phytoplankton, are invisibly small microscopic creatures. Their total mass is only about one gigaton of carbon compared to several hundred gigatons of wood biomass! Even the biomass of green leaves, at about 10 GtC, is an order of magnitude larger (Bar-On et al. 2018).

So, big trees brought with themselves an unprecedented abundance of plant biomass. This surplus of energy resources opened an opportunity for large mammals to evolve in the forest. As hunters, humans are genetically tuned to be pleased when seeing a big animal from a safe place. But pause to think that an elephant, and any big mammal, locally consumes energy at a rate hundreds of times exceeding what the biosphere can locally photosynthesize (~100 W/m² versus 0.5 W/m²). This makes big mammals potential destroyers of the entire ecosystem, if their numbers go unchecked. In a stable natural forest, big animals should consume no more than 1% of total productivity (Makarieva et al. 2020a).

As big mammals evolved in the Eocene and began to destroy the canopy exacerbating natural disturbances and creating big openings, the early successional species of grasses and herbs found themselves in progressively more favorable conditions (Sage 2001). As such grass species have normally existed benefiting from the rainfall-generating capacity of the surrounding forest, they did not possess the skills necessary to run the biotic pump. As these grass species begin to spread, a pronounced aridification of the global climate followed. The climate became more harsh and unpredictable.

So, when discussing the retreat of humid forests and the spread of grasslands at the Eocene-Oligocene transition, as well as the more recent spread of the “water-efficient” C₄ plants that transpire relatively little, increased aridity is mentioned as a possible cause favoring such expansions (e.g., Sage 2001; Osborne and Sack 2012). However, if we take into account the biotic pump mechanism, we can conclude that aridity was a consequence rather than the cause of the grasslands extension. The ultimate cause was the inherent instability of an ecosystem with high biomass, the forest, in the presence of big herbivores (Makarieva and Gorshkov 2010). Grassland dominance could be triggered by big mammals exterminating closed canopies. The biotic pump processes globally dwindled causing a drier climate.

We note in passing that the great extinction that happened in the end of the Eocene in the ocean affecting microscopic species (Prothero 1994a) might also have to do with the evolutionary appearance of the first oceanic mammals (cetaceans and others). As a modern counterpart, humans depleted a major part of macroscopic life in the ocean (Perissi and Bardi 2021).

One can say that with the advent of big mammals, the entire terrestrial life, except for the remaining forests that were cluttered to regions with more favorable geophysical conditions, fell into the “browse trap” (Staver et al. 2014). An ecological trap (landscape trap, fire trap etc.) is a term coined a decade ago to describe how repeated disturbances of the early successional vegetation by new disturbances (burning, grazing or, in the industrial context, cutting) prevents the ecosystem from recovery and puts it on the degradation trajectory (Lindenmayer et al. 2022). This degradation can be slow or rapid, depending on the disturbance regime. And here we come to our species.

Implications: Let’s overcome the big animal’s instincts before it’s too late

As a big mammal originated in savannah, the human species is genetically predisposed to at best appreciate individual trees rather than (closed canopy) forests. Not surprisingly, quite a lot of humans perceive a mowed lawn as etalon of natural (savannah indeed) beauty. The overwhelming majority of humans have never been in a natural forest. Except for the scale, what our species has been doing to forests – exterminating them – is unoriginal. Human population growth continued the devastation of forests that began with the appearance of the first big mammalian herbivores forty million years ago in the Eocene. Had humans been an arboreal primate, we would have perceived forests, and behaved, differently.

Besides being crucial for continental moisture transport that currently sustains the world’s major agricultural regions, natural forests (and natural oceanic ecosystems) stabilize climate by keeping it moist. The contemporary climate change narrative emphasizes the dynamics of the mean temperature (warming/cooling). However, major climate-related sufferings of today are linked to extremes like droughts, floods, heat waves rather than to the long-term mean changes of precipitation, wind and temperature. “Paradise lost” – that is how Prothero (1994a,b) characterized the Eocene-Oligocene transition from the warm, humid and stable climate of the forest-dominated Earth to the more modern-like colder, drier and severely fluctuating climate with a greater proportion of Earth covered by grasslands. [Having got rid of another clade of giants, the dinosaurs, the forests had been keeping the Earth stable for over twenty million years before the big mammals arrived.] Today, the remaining large-scale forests, the frontiers of climate stability, are still buffering against climate extremes (O’Connor et al. 2021) possibly preventing a tipping point towards a completely inhospitable state of the planet (Gorshkov et al. 2000).

Lacking the genetic program to genuinely respect forests, we could nevertheless appreciate their importance, and prevent their destruction, based on rational scientific arguments. For a long time, forests have been valued in terms of the market cost of the wood they produced. In recent decades, attempts have been made to apply the economic term of “services” to forest ecosystems and to assess the economic value to such “natural services” that people are apparently receiving “for free”. The next step in deepening our understanding of how forests matter for the Earth’s well-being should be the recognition of the drastically different climate impacts of disturbed versus undisturbed natural ecosystems. Currently, no such distinction is clearly made; in the result, pristine forests continue to be rapidly destroyed.

The concept of biotic regulation unambiguously highlights the unique feature of natural (in particular, forest) ecosystems (Gorshkov et al. 2000). It is these ecosystems that have a climate-regulating function and are able to keep the environment in a favorable state, at least during the life of humankind as a biological species. The time of natural restoration of the forest after disturbances that do not go beyond the sustainability threshold to a stationary (climax) state with maximum climate-regulating competence, is at least several hundred years. Heavily disturbed forests (artificial plantations, equal-aged forest stands, early successional forest species) do not have such a climate-regulating function. Taking into account the fact that further destruction of natural ecosystems will lead to irreversible degradation of the global climate and make it impossible for our civilization to live on Earth, the cost of natural ecosystems is reduced to the cost of human life itself as a unique phenomenon. Such a cost goes beyond the applicability of traditional economic theory and tends to an infinite value.

Since human civilization cannot exist without the transformation (destruction) of the natural biota (we are big animals genetically encoded to destroy plant life), the resolution of the contradiction consists in limiting the total consumption, including our population number. In the meantime, the economic and ecological functions of forests must be spatially delineated (Moomaw et al. 2019, Makarieva et al. 2020b, Cary et al. 2021, Betts et al. 2021). The exploitation of the forest should be allowed only in strictly prescribed areas, where it is followed by replanting after felling in the form of plantations. Intact and naturally recovering forest ecosystems should be protected from industrial-scale felling and restored over large areas in order to fulfill their climate-regulating functions. Such territories must not be privately owned (having an infinite price, they cannot be bought) or rented. In the context of progressive changes in the global climate, the nations must assume obligations to revise the legal framework for the economic regulation of the forest fund, taking into account these restrictions. Since it is difficult to carry out such significant reforms quickly due to the natural inertia of thinking, it is necessary to introduce an urgent moratorium on industrial felling of intact forest areas. Any violation of such a moratorium should be elevated to the rank of crimes against humanity.

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