Do forests play a role in the continental hydrological cycle in tropical regions?

20 March 2024

Do forests grow where it rains, or does it rain where forests grow? This was the title given by Antonio Nobre, a climate researcher at Brazil’s National Institute for Space Research, to one of his texts on the role of the Amazon rainforest in climate regulation (Instituto Socioambiental (Brazil), 2008). Nobre often emphasizes, in his texts and interviews, the importance of conserving the forest and the phenomena controlled by it, such as the biotic pump of atmospheric moisture (Makarieva and Gorshkov, 2007), the formation of flying rivers (Arraut et al., 2012), and the emission of aerosols capable of generating cloud condensation nuclei and ice nuclei (Pöschl et al., 2010). Such phenomena are essential for maintaining regional rainfall patterns and the climatic comfort that we experience in some regions of the planet, which was established in the last thousands of years of Earth’s history.

The biotic pump theory explains that, through transpiration and condensation, forests create regions of low pressure, which absorb moist air from the oceans, generating winds capable of transporting moisture and sustaining rain on continents (Makarieva et al., 2013). As landmasses are elevated above the sea level, all liquid water accumulated in the soil and underground reservoirs inevitably flows down to the ocean, in the direction of the maximum slope of continental surfaces. Therefore, to accumulate and maintain optimal moisture stores on land, it is necessary to compensate for the gravitational runoff of water from land to the ocean, through a reverse flow of moisture, from the ocean to the land. According to this theory, this reverse flow is driven and maintained by large continuous areas of forest. This means that, if the forest is removed, the continent will have much less evaporation than the adjacent ocean – with a consequent reduction in condensation –, which will determine a reversal in moisture flows, which will start to go mostly from land to sea, creating a desert where there was once forest. In turn, forest restoration actions can increase local precipitation and contribute to strengthening total moisture transport from the ocean to the continental land, increasing the magnitude and reliability of precipitation.

The Amazon rainforest maintains a higher air moisture content and exports atmospheric rivers of vapor, which contribute to the formation of abundant rain and to irrigate distant regions in the southern hemisphere summer (Nobre, 2015). Its importance was early claimed in 1979 by a Brazilian agronomist, Eneas Salati, who reported studies on the isotopic composition of rainwater sampled from the Amazon Basin, showing that half of the rainfall over the Amazon came from the transpiration of the forest itself (Pearce, 2020). While meteorologists were investigating the South American Low-Level Jet (i.e., speed winds at a height of about 1.5 kilometers above forests, which blows East to West across the Amazon, before the Andes Mountains divert them to the South), Salati and others surmised the jet carried much of the transpired moisture and dubbed it a ‘flying river’. The term ‘atmospheric river’ was proposed later, in reference to filamentary structures in the vertically integrated moisture flow field, which are responsible for very intense transport, as described in Newell et al. (1992), Newell and Zhu (1994), and Zhu and Newell (1998). Today, we known that this phenomenon is not limited to the Amazon rainforest and southeastern South America. Other regions in the world, such as cities in the U.S., China, Pakistan, India, and the region of African Sahel are highly dependent on distant forests and their flying rivers for much of their water supply (Pearce, 2020).

The process of evapotranspiration is the loss of water by plants in the form of vapor. This process begins with the influence of various factors, particularly changes in atmospheric temperature, which affect the movement of water circulating through the plant body and reaching the surface of its leaves. When clouds precipitate, much of the water passes through the canopy and infiltrates the forest. Part of this water is stored in the ground, or further down, in aquifers. Water stored in the soil returns to the atmosphere when it is absorbed by the roots and then released through evapotranspiration from trees. This water absorbed by the roots ascends to the leaves through the xylem, a tissue that conducts water and mineral salts in vascular plants.

Although evapotranspiration occurs in any part of the plant above the ground, the largest proportion takes place in the leaves, which account for more than 90% of the process. This is associated with leaf anatomy. On the leaf surface, there exists a layer of wax interrupted by pores, known as stomata. These stomata enclose clusters of cells responsible for emitting water vapor into the surrounding space, which includes an opening to the atmosphere. This facilitates the release of vapor, i.e., evapotranspiration. As water is lost to the atmosphere, its movement through the plant body continues to ensure the hydration of internal tissues and the maintenance of the evapotranspiration cycle itself. This elevation of the water column through the plant conductive tissue is due to the cohesion of the water molecules and their subjection to a tension force that overcomes gravity. The dynamic transport of water from the soil to the roots and then to the stem and leaves depends on positive root pressure and capillarity. Positive root pressure arises from the continual loss of water from within the plant, creating a force that draws soil water towards the roots, subsequently elevating the water column through the xylem. As noted, this ascent is limited by the opposing force of gravity, but capillarity further contributes to the rise in the water column. This physical phenomenon occurs when liquids move within the confines of extremely thin tubes, such as those of the xylem, relying on the properties of cohesion and adhesion. Adhesion force is contingent on the affinity between the liquid and the tube’s solid surface, while cohesion force operates in the opposite direction, adhering the liquid molecules to each other. As water is lost through evapotranspiration, the leaves function as a suction pump that sustains this process.

In his 2015 report titled ‘The Climate Future of the Amazon Rainforest,’ Nobre revealed that, by using evaporation data collected from flow towers as part of a large-scale project, it was possible to estimate the total daily volume of water transpired from the soil to the atmosphere through the trees in the Amazon basin. The estimated value, covering an area of 5.5 million square kilometers, reached a staggering total of 20 billion tons of water per day, equivalent to 20 trillion liters. If all the forests in the equatorial portion of South America were taken into account, this number would rise to 22 billion tons. Considering the forests that existed in 1500, the estimate would exceed 25 billion tons. For reference, the Amazon River discharges approximately 17 billion tons daily into the Atlantic Ocean, which is at least 3 billion tons less than the estimated contribution of the aerial river.

Another phenomenon controlled by the forest, also capable of influencing the climate, involves aerosol emissions modulated by trees (Pöschl et al., 2010). These aerosols play a vital role in the climate system as they can alter rainfall patterns in the Amazon region, due to the redistribution of energy and the formation of cloud condensation nuclei and ice nuclei (Andreae et al., 2004; Mohler et al., 2007). Aerosols influence cloud formation and precipitation by affecting nuclei where water droplets condense or ice forms (Barth et al., 2005). Clouds consist of suspended droplets in the air. At low temperatures, these droplets condense from vapor. However, to form condensation nuclei, there must be a solid or liquid surface that acts as a ‘seed’ for the deposition and condensation of vapor molecules. These seeds are created by aerosols present in the atmosphere. Depending on their composition and abundance, aerosols can scatter or absorb radiation and either enhance or suppress precipitation.

Aerosols are classified into primary particles, which are deliberately produced by flora (e.g., the release of pollen and fungal spores), as well as incidentally (e.g., as leaf and soil debris or suspended microorganisms). There are also secondary particles produced in the atmosphere through the oxidation of trace gases into low-volatility compounds (Martin et al., 2010). In addition to aerosol, particles from other sources, such as mineral dust, sea salt, biomass burning, and pollution particles, can influence cloud formation and precipitation processes through several mechanisms, which are crucial for maintaining the hydrological cycle. Precipitation induced by primary and secondary particles, whether emitted by the forests or formed in the atmosphere and acting as condensation nuclei or ice nuclei, supports the reproduction of plants and microorganisms in the ecosystem from which the precursors of these very particles originate (see Figure 1). This circular causality is illustrated by the question posed by Nobre in his 2008 text: ‘Do forests grow where it rains, or does it rain where forests grow?’ This can be understood as a chicken-or-egg dilemma that leads to the following conclusion: ‘Where there is a forest, there is rain’ (Instituto Socioambiental (Brazil), 2008, p. 369).

There are different ways of interpreting the circular causality between forests and rainfall. One of them is a recent proposal that has been discussed in the philosophy of biology: the organizational theory of ecological functions. In the next section, we will talk about it and its underlying approach, the theory of biological autonomy.

Figure 1: Main mechanisms associated with water control by the forest in the continental hydrological cycle. The water present in the soil enters the plant body and is then carried to the leaves, where much of it is evapotranspired. Trees also contribute volatile organic compounds, which oxidize upon contact with the atmosphere and are largely responsible for the formation of cloud condensation nuclei. Primary particles, such as fungal spores and pollen grains, contribute to the formation of ice nuclei, but they can also act as ‘giant’ condensation nuclei, generating large droplets and inducing warm rain without ice formation. Figure by Jeferson Coutinho.

The relations between forests and rainfall according to the organizational theory of ecological functions

One way to interpret the causal circularity between organisms and abiotic components in an ecosystem has been explored through the development of an organizational theory of ecological functions. According to this theory, as well as other approaches like niche construction theory (Laland et al., 2016) or the Gaia theory (Lovelock and Margulis, 1974), life influences the physicochemical conditions of the environment in a manner that ultimately contributes to its own self-maintenance. This theory also shares with other organizational approaches the idea of attributing functions to ecosystem components based on the proposition that biological systems exhibit a specific type of causal regime. In this regime, the actions of a set of parts are essential for the persistence of the entire organization over time.

The organizational theory of ecological functions is derived from the theory of biological autonomy, which posits that living systems are not reducible to physicochemical systems, because they exhibit qualitatively distinct properties due to their specific organization, described as a closure. This closure involves a causal circularity that differs from that found in physicochemical systems, as we will explain below.

The theory of biological autonomy considers that living systems are organizationally closed and thermodynamically open (Montévil and Mossio, 2015; Moreno and Mossio, 2015). When we say that a living system is organizationally closed, we are referring to what we called ‘causal circularity’ above. In more precise terms, this means that the biological organization in question (e.g., of an organism or an ecosystem) exhibits ‘closure’, that is, its components and operations depend on each other for their own production and maintenance, and collectively determine the conditions for the system itself to exist and continue to exist. According to the theory of biological autonomy, the characteristic closure of living systems is a ‘closure of constraints’. To understand this idea one needs, of course, to know what constraints are.

Constraints are local and contingent causes that reduce the degrees of freedom of the dynamics or processes on which they act (Pattee, 1972), but remain conserved at the time scale relevant to describe their causal action with respect to that process or dynamics (Mossio et al., 2013). As constraints reduce the degrees of freedom of processes internal to the living system, they contribute to their coordination, which in turn generates for the system new possibilities of behavior and adaptation to the environment. A variety of entities or material structures can play the role of constraints on an organism, for example, macromolecules (say, enzymes), organelles (say, ribosomes), organs (say, the liver), and specific material configurations (such as the blood vessel system or the neural circuits within the brain). Reducing degrees of freedom of a process means that, under the action of a constraint, a process has a smaller universe of possible trajectories, compared to what it would have in the absence of a constraint. It is this reduction in degrees of freedom that makes processes, under the influence of constraints, more coordinated, in such a way that the maintenance of life is possible. Therefore, when we refer to a closure of constraints, we are considering a network of mutual dependencies between constituent parts of a system that act as constraints, contributing not only to the maintenance and existence of other parts of the system, but also of the system itself as a whole and, consequently, of themselves.

Circular chains of processes can also occur under the influence of external constraints, no produced by the system in which they take place. This behavior is typically observed in physical or chemical systems and is characterized by ordered sequences of occurrences or dynamic states that are systematically linked to each other, often in a causal manner. A closure of processes, as it is called, occurs when these states or occurrences form a closed cycle: a process A causes a process B, which leads to a process C, which subsequently causes A. This phenomenon is exemplified, for instance, by the circular flow of water under the influence of solar radiation in a sealed glass bottle filled halfway (Figure 2). This phenomenon is qualitatively different from the closure observed in living systems, as it does not involve constraints that are produced by the system itself.

Figure 2: Closure of processes in the circular flow of water under the influence of solar radiation in a sealed glass bottle filled halfway: (1) solar radiation passes through the walls of the bottle and heats the water; (2) upon reaching a certain temperature, water begins to evaporate; (3) the water vapor, after rising, condenses at the top of the bottle and falls as liquid water; (4) which is again subject to evaporation. Figure by Clarissa Leite.

The cycling of water molecules inside the bottle is a thermodynamic, physicochemical circular flow, limited only by external entities, such as the glass bottle. The glass bottle acts as an external constraint, which is not regenerated by the cyclic thermodynamic flow of water. To sum up, in the case of closure of processes constraints are only external, not depending on the dynamics on which they act. Rather than just a circular chain of processes influenced by external constraints, biological systems produce internal constraints, which act on their own processes, and thus exhibit two distinct albeit interdependent causal regimes: an open thermodynamic regime of processes and reactions and a closed dependency between components that act as constraints. This is what it means to say that these systems are thermodynamically open and organizationally closed.

By considering the continental hydrological cycle from the perspective of the organizational theory of ecological functions, we can see how the activities carried out by the vegetation, as illustrated in Figure 1, promote its own conditions of existence, by controlling the hydrological cycle that is so fundamental for its self-maintenance. Living systems play a vital role, in this manner, in the water supply within tropical forests and in other biomes directly or indirectly influenced by this cycle. The suppression of vegetation can disrupt the operations of this cycle, leading to a system reconfiguration in which the continental hydrological cycle is not sustained as before, thereby compromising the existence of the phenomenon modeled here. Based on the arguments presented, can we indeed assert that forests play a role in maintaining the continental hydrological cycle in tropical regions and, in these terms, are involved in their own self-maintenance? This topic will be further explored in a subsequent post!

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