In order to survive, plant life relies 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 Reviews of Geophysics presents recent developments in our understanding of forest-atmosphere exchange. We asked the authors about advances in our understanding of forest-atmosphere exchange and what is still unknown.
What is “forest-atmosphere exchange”?
Forests are landscapes dominated by trees. “Forest-atmosphere exchange” is a catch-all phrase referring to the exchange of “stuff” (specifically, mass, momentum, and energy) between the atmosphere and the trees, other plants, and soil 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 huge quantities of heat with the atmosphere, both directly through the sensible heating and cooling of leaves and indirectly as latent heat passing to the atmosphere when water vapor is released.
The microbes, fungi, and animals living in forests further add to the quantity and variety of exchanges.
Forest-atmosphere exchanges are vital to forests’ physiology and ecology. The exchanges also have important effects on our weather, climate, and how the whole Earth works as an interconnected system.
What tools do researchers use to understand forest-atmosphere exchange?
The three traditional pillars of research for forest-atmosphere exchange are field observations, mathematical theory, and physical scale-models. Many of these techniques were developed to answer applied questions in forestry and agriculture or are extensions of more general fluid flow theory.
By the 1970s and 1980s the development of better sensors and a robust theoretical framework allowed researchers to get at more fundamental aspects of forest-atmosphere exchanges, such as their role in cycling essential nutrients.
With the subsequent advent of high-performance computers, numerical modeling now provides a fourth pillar in the investigation of forest-atmosphere exchange.
At larger time and space scales, remote sensing (from satellites, drones, or tall towers) can infer some aspects of forest–atmosphere exchange, typically through indirect measures such as canopy greenness or changes in atmospheric composition.
How does forest-atmosphere exchange vary between different types of forests and on different scales?
Forest-atmosphere exchanges span time and space scales from milliseconds and millimeters, such as when gases diffuse through a leaf’s stomata, to scales measured in decades and continents, such as when considering the impact of deforestation on the Earth’s atmosphere. There is no single correct scale at which to view these exchanges.
Our study focuses on exchanges happening over tens of centimeters up to one kilometer, and over times from one second to tens of minutes. Our key message is that the real-world structure of forests really matters for forest-atmosphere exchange. Real forests are patchy and that greatly affects their aerodynamics. For example, the aerodynamics of forest edges dominate patches as large as 120 hectares (roughly equivalent to the size of 170 soccer pitches!). Real forests edges are also filled with low branches and tall woody shrubs, presenting a green wall to the wind, rather than the set of lollipop-like trees seen in many models. Lastly, we argue 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 a patchy forest canopy when it is in leaf.
How do different human activities affect forest-atmosphere exchange?
Almost everything human beings can do in or to a forest can affect forest-atmosphere exchange. Even simply walking through a forest can affect microbial activity and therefore the amount of CO2 exchanged. Of course, some activities have larger potential impacts than others.
Globally, over the past few decades, forests have become increasingly fragmented. Only about half of the world’s remaining forest area lies more than 500 meters from the nearest edge. This fragmentation affects forest-atmosphere exchange because edges differ from the forest interior in their local climates and the habitats they provide.
We are only beginning to unravel the large-scale impact of many human activities on forest-atmosphere exchange. For example, what is the net effect of increasing atmospheric CO2 on the world’s forests? Research on immature trees shows some evidence of a “greening” effect, with more vigorous growth under elevated CO2. The response of mature ecosystems is less well understood— elevated CO2 increases carbon uptake by photosynthesis but does it increase the long-term carbon storage, or do other factors, such as nutrient availability, limit biomass growth and therefore carbon capture?
To what extent do current computer models capture the realism of forest-atmosphere exchange?
There is a perpetual tradeoff in computer modeling between scale and resolution. Generally, we can make models more realistic when considering smaller distances and shorter times, but we are forced to sacrifice detail at larger scales. This tradeoff is particularly relevant to weather and climate simulations because of the extreme computational expense of simulating air movement. Despite the prevalence and importance of forests, most forest-atmosphere interactions must be reduced to coarse approximations to be included in these models, if they are included at all.
At the scales of time and space we discuss in our review, advances in non-destructive scanning techniques, computing power, and theory are poised to allow researchers to investigate forest-atmosphere exchange at real sites, using models capable of resolving turbulence. These models are potentially powerful tools to study smaller-scale processes, and to improve the realism of the parametrizations in the larger-scale models that inform policy and commerce.
What are some of the broader scientific and societal applications of a better understanding of forest-atmosphere exchange?
A better understanding of forest-atmosphere exchange could improve weather forecasts and climate simulations, and therefore generate more informed policy and commercial decisions based on those models. For example, the most recent versions of Earth-system models allow researchers to simulate how vegetation responds to environmental change, such as damage from tropospheric ozone.
Other applications include the risk management industry. Large insurers and specialist agencies constantly update their probabilistic models to help manage the impact of catastrophic weather events on forestry and agriculture, whose resulting annual losses in Europe alone amount to billions of US dollars. A better understanding of forest-atmosphere exchange could help us use money and resources more effectively in large-scale tree planting schemes, and so reduce the impact of human activity on our climate and the living world.
What are some of the unresolved questions where additional research, data, or modeling are needed?
Many questions remain when considering different scales of time and space. Taking just the ecosystem scale that we discuss, outstanding research areas include:
- improving our understanding of how water vapor and CO2 move through the forest
- targeted observations of the actual exchange of gases and particles, particularly around forest edges
- how best to allocate computing resources for weather, climate, and Earth-system studies
- improving our understanding of exchange at night and during low winds
- developing statistical approximations of forest-atmosphere exchange that can be used in larger models.
Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.