CLIMATE, FORESTS, INSECTS, FIRE

University of Wisconsin-Madison
Public release date: 31-Oct-2007

Contact: S. Tom Gower
stgower@wisc.edu
608-262-0532

Wildfire drives carbon levels in northern forests

MADISON – Far removed from streams of gas-thirsty cars and pollution-belching factories lies another key player in global climate change. Circling the northern hemisphere, the conifer-dominated boreal forests-one of the largest ecosystems on earth-act as a vast natural regulator of atmospheric carbon levels.

Forest ecologists at the University of Wisconsin-Madison are studying how environmental factors such as forest fires and climate influence carbon levels in this forest system. Their most recent findings, reported in the Nov. 1 issue of the journal Nature, offer insight into the balance of carbon uptake and release that contribute to atmospheric carbon dioxide levels worldwide.

Second in size among forests only to the tropical rainforests, the boreal forests form a massive green band spanning the higher latitudes of Canada, Alaska, Siberia, China, and Scandinavia. Their sheer size, coupled with the fact that they are expected to experience the greatest warming of any forest biome as global temperatures rise, means that climate-related changes here are likely to resonate well beyond the forest boundaries, says S. Tom Gower, UW-Madison professor of forest ecology and management and primary investigator of the project.

In the new study, Gower and his colleagues used a computer model to simulate the carbon balance of one million square kilometers of the Canadian forest over the past 60 years, to determine the relative impacts of climate and disturbance by wildfire.

The group found that the effects of carbon dioxide and climate-temperature and precipitation – varied from year to year but generally balanced out over time and area. Instead, forest fires during the 60-year period had the greatest direct impact on carbon emissions from the system.

However, “because fire frequency and fire intensity are directly controlled by climate change, it doesn’t mean that we shouldn’t be focusing on climate change,” Gower says. “Climate change is what’s causing the fire changes. They’re very tightly coupled systems.”

The researchers believe that fires shift the carbon balance in multiple ways. Burning organic matter quickly releases large amounts of carbon dioxide. After a fire, loss of the forest canopy can allow more sun to reach and warm the ground, which may speed decomposition and carbon dioxide emission from the soil. If the soil warms enough to melt underlying permafrost, even more stored carbon may be unleashed.

A trend toward hotter and drier conditions is likely to exacerbate the effects of fire by increasing the frequency, intensity, and size of burns. “All it takes is a low snowpack year and a dry summer,” Gower says. “With a few lightning strikes, it’s a tinderbox.”

Historically, scientists believe the boreal forest has acted as a carbon sink, absorbing more atmospheric carbon dioxide than it releases, Gower says. Their model now suggests that, over recent decades, the forest has become a smaller sink and may actually be shifting toward becoming a carbon source.

“The soil is the major source, the plants are the major sink, and how those two interplay over the life of a stand really determines whether the boreal forest is a sink or a source of carbon,” he says.

Though the model is not currently designed to forecast future conditions, Gower says, “Based on our current understanding, fire was a more important driver (of the carbon balance) than climate was in the last 50 years. But if carbon dioxide concentration really doubles in the next 50 years and the temperature increases 4 to 8 degrees Celsius, all bets may be off.”

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Other scientists involved in the research are Ben Bond-Lamberty,
Scott Peckham, and Douglas Ahl. Funding for the work was provided by
the National Science Foundation and the National Aeronautics and
Space Administration.

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SCIENCE
VOL 318
26 OCTOBER 2007

Thousand-year records of animal population patterns and climate yield insights into the impacts of environmental change.

Thinking Long Term
Robert A. Cheke

The author is at the Natural Resources Institute, University of Greenwich at Medway, Chatham Maritime, Kent ME4 4TB, UK. E-mail: r.a.cheke@greenwich.ac.uk

Ecologists seeking patterns in populations and environmental correlations dream of coming to grips with lengthy data sets. Usually, animal numbers are determined both by density- independent environmental factors and by density-dependent population processes involving time lags. Disentangling these different factors requires painstaking fieldwork and mathematical skills from the scientists and the patience of Job among funding agencies. Two new analyses of 1000-year-long data series illustrate how long series can reveal insights and improve predictions of pest outbreaks (1, 2).

Caterpillars of the larch budmoth can reach densities of 30,000 per tree when they defoliate larch trees and inhibit tree growth, effects detectable as narrow growth rings. Esper et al. recently examined larch wood from the European Alps dating back 1173 years (1). The results show that budmoth outbreaks have occurred every 9.3 years on average since 844 C.E.; the authors attribute their absence since 1981 to contemporary warming, which stimulates early egg development and premature hatching. This may be good news for the trees, but is it yet another sign of the effects of anthropogenic climate change?

Thinking of insects’ activities more than a thousand years ago recalls biblical accounts of plagues of desert locusts, but there is no continuous historical record of such plagues before the 20th century. However, a Chinese Emperor instigated the sporadic collection of data on Chinese migratory locusts as early as 707 B.C.E., and his successors maintained a continuous series of annual records from 957 C.E. (3-5). Stige et al have now reanalyzed these data in the context of rainfall and temperature changes (2). As in time series of desert locusts (6), brown locusts (7), and Australian plague locusts (8), the data are not insect numbers but proxies based on numbers of administrative areas infested. Significant relationships with rainfall can be found in all of these locusts, but how rainfall affects the insects’ survival may vary according to species, depending on whether they have eggs that can remain dormant for a year or longer and so survive droughts, and on the spatiotemporal distribution of the rain.

For the Chinese locusts, Stige et al. show that both floods and droughts are important, with temperature and rainfall interacting to set the scene (2). The study also emphasizes the importance of low-frequency phenomena, which involve effects discernible at time scales longer than a year. These are known in many ecosystems and were detected in desert and brown locusts as unexplained 16- and 17-year cycles, respectively (6,7). Previous studies of the Chinese locust focused on interannual rather than longer-term variations, with one notable exception showing that population variability  increased at longer time-scales (9). Stige et al have now re-examined the data at lower frequencies than annual. In a kind of ecological archaeology, they used mean  decadal temperature (derived from ice cores, tree ring data, lake  sediments, and contemporary records) and mean decadal rainfall (based  on samples of juniper that tally with precipitation indices) to show  that there were more locusts when the climate was cold and wet and  fewer when it was warm and dry.

The authors find that these climatic effects accounted for locust  variability for periodicities of 30 years or more. Decadal frequencies of droughts and floods have  a multiplicative effect on the locusts. Both droughts and floods are more common in cold, wet periods, conditions associated with high locust numbers because droughts allow the insects to lay eggs on river- banks and lakesides; retreating floods also provide ideal breeding  conditions. These responses detected at decadal scales have important  practical implications: A projected warming Chinese climate would be  expected to lead to fewer locusts as a result of a reduced breeding  habitat, despite a positive association between locusts and  temperature at the annual scale (3).

Frequency-dependent effects of this kind may need to be taken into
account to correctly interpret other phenomena liable to disruption by global warming,  such as wind systems that affect locust migrations and the mixing of swarms originating  from different sources. Examinations at finer scales than the whole of China and further  understanding of the interactions between subpopulations are needed.  Desert locusts, for instance, have regional populations whose dynamics are cross-correlated (10).

Further insights from China are likely after the compilation of  meteorological and ecological records from the past 3000 years (11). Science needs such long  data sets and the financial commitments to provide them. Some series could be  reconstructed, as in the larch budmoth case, but finding biological data sets on a par with  those for the budmoths and locusts will need imagination and help from historians. The  Chinese Emperors thought long term, and so should we, by maintaining  current data collection programs essential for the understanding of  contemporary phenomena in the short, medium, and very long term,  perhaps 1000 years hence.

References
1. J. Esper et al., Proc. R. Soc. London Ser. B274, 671 (2007).
2. L. C. Stige et al., Proc. Natl. Acad. Sci. U.S.A. 104, 16188 (2007).
3. S.-C. Ma, Y.-C. Ting, D. M. Li, Acta Entomol. Sinica14, 319 (1965).
4. S.-C. Ma, Acta Entomol. Sinica8, 1 (1958).
5. C. Tsao, Chinese J. Agric. Res. 1, 57 (1950).
6. R. A. Cheke, J. Holt, Ecol. Entomol.18, 109 (1993).
7. M. C. Todd et al., J. Appl. Ecol. 39, 31 (2002).
8. D. E. Wright, Aust. J. Ecol.12, 423 (1987).
9. G. Sugihara, Nature378, 559 (1995).
10. R. A. Cheke, J. A. Tratalos, BioScience 57, 145 (2007).
11. D. E. Zhang, Ed., A Compendium of Chinese Meteorological Records
of the Last 3000 Years (Jiangsu Education Publishing House, Nanjing,
China, 2004).

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