Our focus on forests and complex life bridges the gap between the approach taken by the NASA Astrobiology Institute, which is largely inspired by Earth’s robust microbes and the SETI Institute, which is scanning the skies for radio signals from other civilisations.

But why forests? Forests played a central role in our origins and evolution and in sustaining our civilisation. It was as they clambered amongst the branches of the trees that our primate ancestors developed the ability to see in colour and 3D. The same skills in climbing and grasping that enabled them to forage in the tree tops enable members of our own species today to fashion rockets and space suits and to  construct the International Space Station as they float hundreds of kilometres above the surface of the Earth.

Trees continue to provide us with food and wood is a strong, but relatively lightweight material that is ideal for building, for tools, for manufacturing a huge range of essential items and for making wheels and wagons. The fact that wood floats was probably first exploited for primitive rafts. Later, the sailing ships that explored the world during Europe’s great age of discovery, and which set the stage for a truly global society, were constructed of wood. The first machines were made of wood and without them there would have been no industrial revolution. At the same time, wood burns, and so, it provides us with light and heat. nature has provided us with an astonishingly versatile material and we remain dependent upon it today.

Dr. Martin Heath, who  came to the study of habitable planets with a wide background in the natural sciences, has asked, “If there are other civilisations in our Galaxy, we must ask what has served them in the role of forests.”  

The same chemical elements that make up the Earth, its atmosphere, ocean and its life forms occur throughout the cosmos. We mustn’t be parochial, however. Off course, we want to investigate the ability of other planets to support forests of the kind that we find down here on Earth, but we mustn’t stifle our imaginations and rule out the possibility that forests elsewhere may take fantastic forms unknown to us.

Dr. Martin Heath explains in a bit more detail

Many theoretical investigations of the prevalence of basically Earth-like planets (described classically as “habitable planets”) have concerned themselves with the most basic conditions tolerable in principle by the most rudimentary and robust life-forms of which we know. For this reason, it has become a convenient and common practice to cite the ability of a planet to maintain bodies of liquid water on its surface as a surrogate for habitability (for example, Kasting et al., 1993). Certain other analyses, such as the classic work of Dole (1964), have considered the more stringent requirements that would be needed for a planet to be human-habitable.

My own research, conducted with L. R. Doyle, has focussed principally on forests.

“I believe that the hallmark of a truly Earth-like planet is that it would be forested.”

It is not simply that no list of the features found at the Earth’s surface would be complete without forests.

Tree-dominated ecosystems are actually of immense significance in planetary habitability studies because:

1. On a global scale they are by far the greatest component of the Earth’s surface ecosystem in terms of biomass;
2. They impact on the hydrological cycle and biogeochemical cycles;

Forests are an integral part of what we, as human beings understand as habitability, and arboreal environments  have been essential in supplying the material needs of our primate ancestors, pre-civilised Homo sapiens, and those of civilisations, up to the present day.

“Forests will become a key research topic in the study of habitable planets.”

My colleagues and I believe that forests are destined to become a key research topic in the search for life-bearing Earth-sized planets, and in SETI (Search for Extra-Terrestrial Intelligence) science. At the beginning of the 1990s, I introduced a forest-orientated approach to planetary habitability studies and promoted it through scientific and public outreach events.

The concept became the basis of collaboration with Laurance R. Doyle following the 1994 First International Conference on Circumstellar Habitable Zones, staged by LRD with funding from NASA, at the NASA Ames Research Center. That meeting, the process of preparing papers for the proceedings, and subsequent seminars, enabled the concept to involve through peer review amongst workers with a broad range of competence.

Forests are not inert passengers on spaceship Earth. They are integrated dynamically into biogeochemical and climatic systems and exert a moderating influence on those systems, so as to enhance the habitable condition of the Earth’s surface environment. They create their own distinctive microclimates in terms of the intensity and spectral quality of light, temperature, wind speed, atmospheric CO2 concentration, water availability, element cycling, and these are different from those that would be found on a barren world.

“Trees are  essential to habitability as we know it because they are an integral part of the regional and global hydrological cycles.”

Not only do trees modify the environment immediately beneath their canopies, but because trees take up water through their roots and lose it through their leaves in the process of transpiration, they are an integral part of the regional and global hydrological cycle. Researchers often refer to combined evaporation and transpiration as “evapotranspiration”.

Shukla & Mintz (1982, p. 1500): “Vegetation and clouds play complementary roles: the clouds convert atmospheric water vapor into liquid water, which is transferred to the soil; the vegetation converts soil water into water vapor, which is transferred to the atmosphere.”

Shukla et al. (1990, p. 1322): “The distribution of global vegetation was traditionally thought to be determined by local climate factors, especially precipitation and radiation. This view has been modified because controlled numerical experiments with complex models of atmosphere showed that the presence or absence of vegetation can influence the regional climate.”

Transpiration is no mere detail of the Earth’s water cycle. Jasechko et al. (2013) investigated the isotopic ratios ¹⁸O/¹⁶O) and ²H/¹H in waters, knowing that the purely physical process of evaporation leaves residual waters slightly enriched in the heavier isotopes ¹⁸O and ²H, whilst transpiration leaves no such signature. They estimated that of 111,000 km³ of water falling on the land every year, some 62,000 ± 8,000 km³ per year is returned to the atmosphere via transpiration, and that this process requires about half of the solar energy absorbed at the land surface. Looked at in this way, plant life is making much more effective use of the energy arriving in sunlight than would appear if we just looked at photosynthesis by itself.  

“Don”t overlook soil – it’s a key part of the picture.”

Another major way in which forests have modified the Earth’s environment is through the creation of soil. It is easy to take soil for granted, but it is a very different substance, not least when it contains rich humus and packed with organisms (notably microbes, invertebrates, and sometimes burrowing vertebrates) from the sterile, impact-pulverised regolith that cloaks the surface of our companion world, the Moon.

Biology plays an integral role in the production of soil through rock decay, and trees aid the breakdown of bedrock not only by root penetration, but also because plants produce acidic compounds that can accelerate weathering rates. Because they have the potential to modify the rate of weathering, photosynthetic organisms must have impacted on the natural equilibrium atmospheric partial pressure of the greenhouse gas CO₂ over geological time (see, for example, the arguments in Volk, 1989; Schwartzman & Volk; 1989; Schwartzman, 1999). Higher plants have been considered as accelerating weathering and (Williams et al., 1998, p. 19):

“ecosystems dominated by angiosperm deciduous plants regrow leaves and flowers annually, taking fresh nutrients from the soil, and hence are associated with higher weathering rates than are conifer/evergreen plant communities.” 

Howbeit,

“It is also possible that by binding weathering products, and by isolating these from meteoric water, plants may inhibit chemical weathering (Drever 1994). Much depends on the particular vegetation cover and on the nature of the regolith mantling the land surface.”

 

If a planet supports any kind of forests, it would seem essentially inescapable that, as on our planet, that they must have a significant impact  on the global environment, and that interactions which moderate the environment will have an evolutionary advantage.

Earth’s forests are a phenomenon of the land. It is the challenge of the land that enabled forests to evolve. The surface of the land, where forests meet the sky, is an exciting place.

It is where the rocks of the crust, rising and falling in response to tectonism (which is driven by the Earth’s geothermal energy), interact with the oceans, glaciers and the atmosphere, and it is where solar energy, having been transmitted through the atmosphere, its clouds and airborne dust (with some back-scattering to space), arrives to power climate and biology.

The Earth’s present level of geothermal flux (global average = 0.0614 W m^-2; Sass, 1972) would be adequate to prevent the deepest oceans from freezing right down to their floor even if the solar output were to be reduced by tens of percent (see Bada et al., 1994). The implication is that some kind of microbial habitability might potentially be sustained even if the Earth lay at a greater distance from the Sun, beyond the classic HZ, and it presented an icy surface to the universe.

However, the energy input that keeps most of the ocean and the atmosphere from freezing, and which manifests in weather systems, and in the weathering and erosion, of rocks is solar radiation. On the average, the amount leaving the Earth emerging through the surface of the Earth per unit area from its interior is just ~ 4.5 x 10^-5 the amount of energy arriving per unit area at the top of the Earth’s atmosphere from the Sun (on a surface perpendicular to the incoming radiation).

“Will photosynthesis always develop on habitable planets where there’s ample sunlight? There’s no disputing that photosynthesis is an effective method for importing energy into ecosystems.”

Sunlight (the visible portion of the Sun’s electromagnetic spectrum) also directly powers the life processes of plants, which are the primary producers at the base of the food chain, and indirectly all those organisms that rely on plants. Trees, the spatially dominant organisms of forests, are dramatic examples of primary producers. They spread out arrays of special organs – their leaves – to harvest the radiant energy emitted by our parent star. This energy is used, in the process of photosynthesis, to energise the metabolic processes whereby plants manufacture their own foodstuffs, make structural substances for growth and repair, and create energy storage compounds. The part of the solar spectrum that is used to drive the types of photosynthesis used by higher plants lies in the approximate range 4,000 Å to 7,000 Å, and is designated as Photosynthetically Active Radiation (PAR).

Any other habitable planet must have some means of importing energy into its biological systems, and trees demonstrate that photosynthesis is a very effective mechanism for doing so (even though it is nominally inefficient in terms of the total PAR that penetrates through the atmosphere to the Earth’s surface; Hall & Rao, 1999 estimated that production of organic matter represents an overall efficiency of 0.27 %).

The complex process of photosynthesis may be summarised simply:

                CO₂   +    H₂O    →    CH₂O (carbohydrate)  +    O₂                                                                                                                                                           

Energy is produced also through the process of respiration, which effectively reverses photosynthesis, when carbohydrate foodstuffs react with oxygen to give water and carbon dioxide as products:

            O₂     +    CH₂O (carbohydrate)    →    CO₂    +   H₂O                                                                                     

Respiration powers the active metabolism of higher plants and animals, and so a significant atmospheric partial pressure of oxygen (pO₂) is essential for both trees and a metabiota. PO₂ has been able to achieve high values because a small proportion of the carbon-rich products of plants suffers burial in geological basins, before it can be re-oxidised (notably through the actions of decomposer organisms).

Although trees must absorb nutrients from the soil, in which they take root, the bulk of the raw matter processed by a tree consists simply of carbon dioxide and water, absorbed from the immediate environment; CO₂ from the air, and water through the roots. A wide range of macro-nutrients and micro-nutrients is essential for plant metabolism, of course, but it is from the cosmically and geochemically abundant elements hydrogen, carbon and oxygen that higher plant biochemistry synthesises foodstuffs and structural materials.

The domain on the surface of the Earth that is available in principle to trees is substantial. The surface area of the Earth is 509.7 x 10⁶ km², and emergent land presently comprises 148.4 x 10⁶ km² (Schönwiese 1994), some 29.12 % of that area. Trees are the dominant organisms in habitats that prior to the advent of civilisations, would have covered > 40 % of our planet’s land area. Ball (2001), using data from the Food and Agriculture Organization of the United Nations (FAO) Forest Resource Assessment Programme report “State of the World’s Forests 1997” (FAO, 1997), reported (p. 6) that:

“In 1995 forests were estimated to cover 3454 million ha, or 26.6 % of thetotal land area of the world (Greenland and Antarctica excepted).”

For our purposes, namely assessing the ability of a planet to support forests, we cannot, of course, ignore Greenland and Antarctica, simply because, being mostly smothered by ice at the present geological epoch, they are not available for colonisation by forests. They are an essential part of the equation. Together, these two glacial lands comprise some 11.9 % of the Earth’s total land area. Estimates in FAO (2001) indicated that the area classified as forest covered 3,869 x 10⁶ ha. This is 26.07 % of the total land area, including areas beneath ice caps (the melting of the ice caps would alter the land/sea ratio not only through a rise in sea level, but also due to isostatic rebound of land following removal of glacial overburden; precise estimates are not necessary here). Some 3,682 x 10⁶ ha was covered by natural forest. Of total forest over 95.2 % was natural and 4.83 % plantation. Some 56 % of forest was tropical or subtropical, whilst 44 % was temperate and boreal.

“If we ignored all other reasons why they’re important, forests account for most of the biomass on the surface of planet Earth.”

Forest ecosystems account for ~ 60 % of the Net Primary Productivity (= Gross Primary Productivity – respiration losses) of the land. Moreover, around 90 % of the carbon in standing biomass resides in tree-dominated habitats on the land (see data after P. Duvigneaud in Bolin et al., 1979). This figure remains high, around 70 %, even when soil carbon is taken into account (Solomon & Kirilenko, 1997).

Estimates of the carbon biomass in plants and animals in land ecosystems, and their productivity have varied somewhat. Whitaker & Likens (1973) posited a global standing plant biomass 829 x 10⁹ tonnes C, with a global NPP of 73.2 x 10⁹ tonnes C, just 48.3 x 10⁹ tonnes of which is continental. A major difference between the biomass on land and that in the sea is that in the sea, there is little standing biomass at any time because it is consumed rapidly by animal life, which, contrary to the arrangement on land, dominates the biomass pyramid.

Holland (1978), Duvigneaud (1979) and Olson et al. (1983) accepted a standing biomass on land of ~ 560 x 10⁹ tonnes C, and a NPP of 62.7 x 10⁹ tonnes C yr^-1. Berner (1989) adopted a value of 560 x 10⁹ tonnes C for the terrestrial biomass (following Solomon et al., 1985), and a mere 7 x 10⁹ tonnes C for the standing biomass in the oceans (after Holland, 1978). The latter source adopted a value for terrestrial NPP of 48 x 10⁹ tonnes yr^-1, and for the marine NPP of 35 x 10⁹ tonnes yr^-1. Woodwell (1984) and Walker et al. (1999) edited useful discussions of biomass investigation, including remote sensing surveys, and modelling. Trees play a major role in productivity; for example, a study of European mixed oak (Quercus) forest with beech (Fagus) and hornbeam (Carpinus), Duvigneaud & Denaeyer-De Smet (1970) derived a total productivity of 14.58 tonnes dry weight ha^-1 yr^-1, of which the productivity of trees comprised 11.490 tonnes (~ 79 %). As observed by Schwartzman (1999, p. 19): “most of the terrestrial biomass is in the form of dead woodin trees.” Notwithstanding, a substantial proportion of the carbon in ecosystems is stored below ground, rather than in trees above ground. Using the methodology of Prentice et al. (1993), Solomon & Kirilenko (1997) estimated that forest ecosystems retain a carbon inventory of 1,461 x 10⁹ tonnes, as against a total of 2,056 x 10⁹ tonnes of carbon for all terrestrial ecosystems. Remote sensing using satellites, combined with ground studies, has the potential to greatly improve our ability to estimate biomass in forest biomes.

Given the substantial biomass of forests on Earth and their role in environmental modification, it is evident that in order to characterise a truly Earth-like planet it would be necessary to include forest ecosystems. Forests cannot be disentangled meaningfully from discussions about the possibility of other habitable planets.

Of course, such forests as might exist elsewhere need not be precise analogues of any of the types of forests present here on Earth, and for all that we yet know, there may well be very unEarth-like forests on very unEarth-like planets.

“One way or the other, however, the search for other Earths must be a search for other forested worlds”.

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