Forests in a Changing Climate

Post by Andria Dawson, University of Notre Dame/University of California, Berkeley Postdoc

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Forests play an important role in the global carbon cycle by storing and releasing carbon through processes such as establishment, growth, mortality, and disturbance. Forests can be carbon sources if they release more carbon than they absorb, or carbon sinks if they absorb more than they release. Knowing that forests affect the carbon budget, it is natural to ask about the interactions between forests and the changing climate. Do forests mitigate climate change? The answer to this question is seemingly complex. Here are a few of the many reasons why…

Albedo

Some of the solar radiation that reaches the Earth is absorbed, while some is reflected. The reflectivity of a surface, or albedo, is some measure of the whiteness of a surface. Snow has a high albedo, while open ocean has a low albedo. Forests typically have a low albedo. To keep the earth cool, all we need to do is absorb less of this incoming radiation. Researchers at Dartmouth college found that in regions where snow is common and forest productivity is low, it is beneficial to the economy and the climate to clear those forests, which modulates the temperature by increasing albedo [1].

Carbon dioxide

Trees use carbon dioxide to photosynthesize. As atmospheric CO2 increases, trees are expected to experience increased growth, at least up to a point. CO2 is absorbed through the stomata, but while these stomata are open and readily absorbing CO2, they are also allowing the tree to lose moisture. When CO2 is more readily available, trees don’t have to open their stomata as wide to absord it. This leads to less moisture loss through the stomata, leaving the tree with additional resources for other processes, such as growth [2]. And in turn, increased growth leads to increased CO2 consumption. But only up to a point.

Terpenes

Forests interact with the atmosphere by releasing biological aerosols as well as compounds known as terpenes. Terpenes react and form aerosols, forming clouds, which in part determines how much light is reflected back to space. Spracklen et al. found that terpenes from a simulated pine forest increased cloud thickness, causing an additional 5% of solar radiation to be reflected back to space [3].

Changes in natural disturbance regimes

Climate change has the potential to affect disturbance regimes. Dale et al. succinctly wrote that ‘climate change can affect forests by altering the frequency, intensity, duration, and timing of fire, drought, introduced species, insect and pathogen outbreaks, hurricanes, windstorms, ice storms, or landslides’ [4]. These disturbance events affect forests in different ways, from causing widespread mortality to causing changes in structure, composition, and function.

How to make sense of all of this?

The take home message is that an important relationship exists between forests and climate. The cumulative effect of these feedback mechanisms are difficult to disentangle, and further collaborative research based on ecosystem and atmospheric models confronted with data are key as we move forward. PalEON is one such collaborative effort, drawing from talents to work towards a better understanding of forest systems.

References
[1] Updated citation as of 4-17-14:Lutz, David A., Howarth, Richard B., “Valuing albedo as an ecosystem service: implications for forest management. Climatic Change (2014): doi:10.1007/s10584-014-1109-0
Original citation: Dartmouth College. “Can cutting trees help fight global warming? More logging, deforestation may better serve climate in some areas, study finds.” ScienceDaily. ScienceDaily, 5 December 2013.

[2] Keenan, Trevor F., et al. “Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise.” Nature 499.7458 (2013): 324-327.

[3] Spracklen, Dominick V., Boris Bonn, and Kenneth S. Carslaw. “Boreal forests, aerosols and the impacts on clouds and climate.” Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 366.1885 (2008): 4613-4626.

[4] Dale, Virginia H., et al. “Climate Change and Forest Disturbances Climate change can affect forests by altering the frequency, intensity, duration, and timing of fire, drought, introduced species, insect and pathogen outbreaks, hurricanes, windstorms, ice storms, or landslides.” BioScience 51.9 (2001): 723-734.

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