Sharing land with trees: Global climatic implications
of forest-human coexistence
Anastassia Makarieva, Andrei Nefiodov and Ugo Bardi
I know: to the trees, but not to us,
Perfection of the life is given, whole.
And on the Earth – the sister of the stars –
We live in exile, while they do at home.
Nikolai Gumilev
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. 2020).
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. 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
(Makarieva et al. 2020, 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 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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