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To survive, plant life depends on exchanges with the atmosphere. These exchanges are particularly complex in forests where they are affected by a variety of microbes and animals and are further complicated by human interaction with the environment. A new article published in Geophysics Opinion presents recent developments in our understanding of the forest-atmosphere exchange. We asked the authors about advances in our understanding of forest-atmosphere exchange and what is still unknown.
What is the “forest-atmosphere exchange”?
Forests are tree-dominated landscapes. “Forest-atmosphere exchange” is a catch-all term referring to the exchange of “things” (specifically, mass, momentum and energy) between the atmosphere and trees, other plants and the ground in these landscapes. For example, trees draw carbon dioxide from the air and release water vapor, oxygen, pollen and a variety of organic compounds.
Forests also exchange enormous amounts of heat with the atmosphere, both directly through the sensible heating and cooling of leaves and indirectly as latent heat passing into the atmosphere when water vapor is released.
Forest-atmosphere exchanges have significant effects on our weather, our climate, and the way the entire Earth functions as an interconnected system.
The microbes, fungi and animals living in the forests further add to the quantity and variety of the exchanges.
Forest-atmosphere exchanges are vital for the physiology and ecology of forests. Trade also has significant effects on our weather, climate, and how the entire Earth functions as an interconnected system.
What tools do researchers use to understand forest-atmosphere exchanges?
The three traditional pillars of forest-atmosphere exchange research are field observations, mathematical theory and physical scale models. Many of these techniques have been developed to answer applied questions in forestry and agriculture or are extensions of the more general theory of fluid flow.
In the 1970s and 1980s, the development of better sensors and a solid theoretical framework allowed researchers to address more fundamental aspects of forest-atmosphere exchanges, such as their role in the cycling of essential nutrients.
With the subsequent advent of high-performance computers, numerical modeling now provides a fourth pillar in the study of forest-atmosphere exchanges.
At larger time and space scales, remote sensing (from satellites, drones, or tall towers) can infer aspects of forest-atmosphere exchange, usually through proxy measures such as greenness canopy or changes in atmospheric composition.
How do forest-atmosphere exchanges vary between different forest types and at different scales?
Forest-atmosphere exchanges span time and space scales ranging from milliseconds and millimeters, as when gases diffuse through the stomata of a leaf, to scales measured in decades and continents, as during the examining the impact of deforestation on the earth’s atmosphere. There is no single correct scale to visualize these exchanges.
Our study focuses on exchanges taking place over tens of centimeters up to one kilometer, and over durations ranging from one second to tens of minutes. Our key message is that the actual structure of forests really matters for the forest-atmosphere exchange. Real forests are uneven and this greatly affects their aerodynamics. For example, the aerodynamics of forest edges dominate plots as large as 120 hectares (about the size of 170 football pitches!). True forest edges are also filled with low branches and tall, woody shrubs, presenting a vegetated wall to the wind, rather than the cluster of lollipop-like trees seen in many models. Finally, we argue that measurements and models should encompass a wider range of forest densities. This is important because the flow around forests can be counter-intuitive; for example, the wind penetrates deeper into uneven tree canopy when it is in leaf.
How do different human activities affect forest-atmosphere exchanges?
Almost anything that humans do in or to a forest can affect the forest-atmosphere exchange. The simple act of walking in a forest can affect microbial activity and therefore the amount of CO2 exchange. Of course, some activities have greater potential impacts than others.
We are only beginning to understand the large-scale impact of many human activities on forest-atmosphere exchanges.
Globally, in recent decades, forests have become increasingly fragmented. Only about half of the world’s remaining forest area is more than 500 meters from the nearest edge. This fragmentation affects forest-atmosphere exchanges because the edges differ from the interior of the forest in their local climates and the habitats they provide.
We are only beginning to understand the large-scale impact of many human activities on forest-atmosphere exchanges. For example, what is the net effect of increased atmospheric CO2 on the world’s forests? Research on immature trees shows evidence of a “greening” effect, with more vigorous growth under high levels of CO2. The response of mature ecosystems is less well understood – high CO2 increases carbon uptake by photosynthesis, but does it increase long-term carbon storage, or do other factors, such as nutrient availability, limit biomass growth and therefore carbon capture? carbon?
To what extent do current computer models account for the realism of forest-atmosphere exchanges?
There is a perpetual trade-off in computer modeling between scale and resolution. Generally, we can make models more realistic by considering smaller distances and shorter times, but we are forced to sacrifice detail at larger scales. This trade-off is particularly relevant for weather and climate simulations due to the extreme computational expense of simulating air motion. Despite the prevalence and importance of forests, most forest-atmosphere interactions must be reduced to rough approximations to be included in these models, if they are included at all.
At the time and space scales we discuss in our review, advances in nondestructive analysis techniques, computing power, and theory are poised to allow researchers to study forest- atmosphere on real sites, using models capable of resolving turbulence. These models are potentially powerful tools for studying smaller-scale processes and for improving the realism of parameterizations in larger-scale models that inform policy and trade.
What are some of the wider scientific and societal applications of better understanding the forest-atmosphere exchange?
A better understanding of forest-atmosphere exchanges could improve weather forecasts and climate simulations, and therefore generate more informed policy and business decisions based on these models.
A better understanding of forest-atmosphere exchanges could improve weather forecasts and climate simulations, and therefore generate more informed policy and business decisions based on these models. For example, the most recent versions of Earth system models allow researchers to simulate the response of vegetation to environmental changes, such as damage from ground-level ozone.
Other applications include the risk management industry. Major insurers and specialist agencies are constantly updating their probabilistic models to help manage the impact of catastrophic weather events on forestry and agriculture, whose annual losses in Europe alone amount to billions of dollars. Americans. A better understanding of forest-atmosphere exchanges could help us use money and resources more efficiently in large-scale tree-planting projects, and thus reduce the impact of human activity on our climate and the world. living.
What are some of the unresolved questions where additional research, data or modeling is needed?
Many questions remain when considering different scales of time and space. Taking just the scale of the ecosystem we are discussing, outstanding research areas include:
- improve our understanding of how water vapor and CO2 move in the forest
- targeted observations of actual gas and particle exchange, especially around forest edges
- how to best allocate computing resources for weather, climate and earth system studies
- improve our understanding of exchanges at night and in light winds
- develop statistical approximations of the forest-atmosphere exchange that can be used in larger models.
-A. Robert MacKenzie (armackenzie@bham.ac.uk; 
Editor’s note: It is the policy of AGU Publications to invite authors of articles published in Reviews of Geophysics to write an abstract for Eos Editors’ Vox.
