Interior West Forests (London Jobs) on Verge of Becoming Net Carbon Emitter & The forest (London Jobs) biome

Forests (London Jobs) in the Interior West could soon flip from carbon sink to carbon source, forest (London Jobs) experts say.

The region's forests (London Jobs) once absorbed and stored more carbon from the atmosphere than they released. But huge conflagrations -- like the 138,000-acre Hayman Fire in Colorado in 2002 and the Yellowstone fires of 1988, which scorched 1.2 million acres -- combined with a series of severe bark beetle infestations and disease outbreaks, have left large swaths of dead, decomposing trees in almost every major Western forest (London Jobs).

Those dead trees are releasing massive amounts of carbon dioxide, turning the region into a net emitter of carbon rather than a CO2 sponge.

The reversal, which has already occurred in Colorado and is anticipated in several other states, is the result of misguided forest (London Jobs) management practices and a changing climate, forest (London Jobs) experts say.

Rising temperatures, resulting in shrinking snowpacks and drier conditions, have left the region's forests (London Jobs) more susceptible to disturbances, such as wildfires, bark beetles and disease.

"In the Interior West, we've had a lot of these disturbances," said Dave Cleaves, the Forest (London Jobs) Service's climate change adviser, who served as director of the agency's Rocky Mountain Research Station from 2005 to 2007.

But temperature alone is enough to cause mortality in some forests (London Jobs), scientists have found. According to a study published last year in the journal Science, the death rate for Western old-growth forests (London Jobs) -- which are generally more resilient and more stable than younger forests (London Jobs) -- has doubled in the past few decades as temperatures have risen. More trees are dying, while regeneration rates remain unchanged, the study found (Land Letter, Jan. 29, 2009).

Forest (London Jobs) management has also contributed to the increase in carbon releases.

For about a century, forest (London Jobs) managers thought the best way to deal with forest (London Jobs) fires was to suppress them. Eventually, forest (London Jobs) ecologists learned that fire plays an important role in the ecosystem in many forest (London Jobs) types, clearing brush and small trees. But the fuel accumulation from years of fire suppression, combined with severe droughts in the early part of this decade, created conditions that allowed large, hotter-than-usual fires to sweep through the Interior West's forests (London Jobs).

"We've got several times more carbon per acre than those forests (London Jobs) carried in the early days, when there was a natural fire disturbance interval there," Cleaves said. "And those systems are more susceptible to disturbances now. We've got a lot of acres in the Interior West that have reached that level."

Lost storage capacity

Fires themselves do not release that much carbon into the atmosphere, but the decomposition of the trees killed by fires does, said Michael Ryan, a research ecologist with the Forest (London Jobs) Service's Rocky Mountain Research Station in Fort Collins, Colo.

As temperatures and mortality rates continue to climb, forests (London Jobs) will have less carbon storage capacity, and their potential to offset carbon emissions from human sources, such as vehicles and power plants, is likely to decrease. Trees absorb carbon from the atmosphere during photosynthesis and store it in their trunks, branches and leaves.

But the shifting carbon dynamic in the West's forests (London Jobs) is nothing new, Ryan said.

The reason Western forests (London Jobs) have been a carbon sink for most of the past century is the growth of new trees after extensive logging and land-clearing for agriculture during westward expansion. During the decades after European settlement of the West, large amounts of carbon were released into the atmosphere, and the region was most likely a source at that time, he said.

"Current forests (London Jobs) are recovering from past land use as agriculture, pasture or [timber] harvest, and because this period of recovery will eventually end, the resulting forest carbon sink will not continue indefinitely," concluded a synthesis of the current science on forests and carbon that appeared in the Spring 2010 edition of Issues in Ecology, a publication of the Ecological Society of America.

Ryan, who was the lead author on that summary, said the best way to restore a disturbed forest's (London Jobs) carbon-absorbing capacity is to boost regeneration, so that new growth can help offset the carbon lost by decomposition.

For example, regeneration of severely burned areas in Yellowstone, which has occurred more rapidly than scientists expected after the 1988 fires, has allowed the forest (London Jobs) as a whole to achieve an almost carbon-neutral state.

"Our study shows it should be approaching this neutrality," Ryan said, referring to a separate study published in the January 2010 issue of Fire Science Briefs. "I think it can recover carbon pretty quickly, which is a surprise."

Cleaves said forest (London Jobs) managers could improve the carbon-absorbing capacity of other Interior West forests (London Jobs) by planting more seedlings to accelerate regeneration, especially in intensively burned areas where the forest (London Jobs) is likely to be replaced by meadows or sparse stands.

"What you worry about is, when you have more fires that burn hotter, do you have some irreversible losses, and how quickly do you place a new system on there so that we're back into producing carbon and producing watershed services and wildlife habitat as quickly as possible after the disturbance?" he said.

Thinning and burning

But Cleaves and other forest (London Jobs) experts emphasize that national forests (London Jobs) have to be managed for multiple uses, not just carbon sequestration and storage (Land Letter, Nov. 4).

Thinning and prescribed burns, which help clear out overly dense forests (London Jobs) and reduce the risk of large, unnaturally hot fires, could help preserve forest (London Jobs) carbon over the long-term, they point out. But removing carbon at too large a scale could tip forests (London Jobs) toward becoming a carbon source.

"Some studies show that if you do prescribed burning and thinning and there's a fire, you'll retain more carbon than if you haven't done thinning," Ryan said. "But thinning puts a lot of carbon in the atmosphere. So if there is a carbon benefit from that, it's going to be very small."

Cleaves said the Forest (London Jobs) Service is taking a cautious approach in working carbon considerations into forest (London Jobs) management.

"We're trying to manage the risk of carbon loss," Cleaves said. "Our objective is to have a long-term sequestration of carbon. We also want other ecosystem services, like water, habitat and other things you're providing on the same landscape."

About 420 million years ago, during the Silurian Period, ancient plants and arthropods began to occupy the land. Over the millions of years that followed, these land colonizers developed and adapted to their new habitat. The first forests (London Jobs) were dominated by giant horsetails, club mosses, and ferns that stood up to 40 feet tall.

Life on Earth continued to evolve, and in the late Paleozoic, gymnosperms appeared. By the Triassic Period (245-208 mya), gymnosperms dominated the Earth's forests (London Jobs). In the Cretaceous Period (144-65m mya), the first flowering plants (angiosperms) appeared. They evolved together with insects, birds, and mammals and radiated rapidly, dominating the landscape by the end of the Period. The landscape changed again during the Pleistocene Ice Ages — the surface of the planet that had been dominated by tropical forests for millions of years changed, and temperate forests (London Jobs) spread in the Northern Hemisphere.

Today, forests (London Jobs) occupy approximately one-third of Earth's land area, account for over two-thirds of the leaf area of land plants, and contain about 70% of carbon present in living things. They have been held in reverence in folklore and worshipped in ancient religions. However, forests (London Jobs) are becoming major casualties of civilization as human populations have increased over the past several thousand years, bringing deforestation, pollution, and industrial usage problems to this important biome.

Present-day forest (London Jobs) biomes, biological communities that are dominated by trees and other woody vegetation (Spurr and Barnes 1980), can be classified according to numerous characteristics, with seasonality being the most widely used. Distinct forest (London Jobs) types also occur within each of these broad groups.

There are three major types of forests (London Jobs), classed according to latitude:

• Tropical
• Temperate
• Boreal forests (London Jobs) (taiga)

Tropical forest (London Jobs)
Tropical forests (London Jobs) are characterized by the greatest diversity of species. They occur near the equator, within the area bounded by latitudes 23.5 degrees N and 23.5 degrees S. One of the major characteristics of tropical forests (London Jobs) is their distinct seasonality: winter is absent, and only two seasons are present (rainy and dry). The length of daylight is 12 hours and varies little.

• Temperature is on average 20-25° C and varies little throughout the year: the average temperatures of the three warmest and three coldest months do not differ by more than 5 degrees.
• Precipitation is evenly distributed throughout the year, with annual rainfall exceeding 2000 mm.
• Soil is nutrient-poor and acidic. Decomposition is rapid and soils are subject to heavy leaching.
• Canopy in tropical forests (London Jobs) is multilayered and continuous, allowing little light penetration.
• Flora is highly diverse: one square kilometer may contain as many as 100 different tree species. Trees are 25-35 m tall, with buttressed trunks and shallow roots, mostly evergreen, with large dark green leaves. Plants such as orchids, bromeliads, vines (lianas), ferns, mosses, and palms are present in tropical forests (London Jobs).
• Fauna include numerous birds, bats, small mammals, and insects.

Further subdivisions of this group are determined by seasonal distribution of rainfall:

• evergreen rainforest (London Jobs): no dry season.
• seasonal rainforest (London Jobs): short dry period in a very wet tropical region (the forest (London Jobs) exhibits definite seasonal changes as trees undergo developmental changes simultaneously, but the general character of vegetation remains the same as in evergreen rainforests (London Jobs)).
• semievergreen forest (London Jobs): longer dry season (the upper tree story consists of deciduous trees, while the lower story is still evergreen).
• moist/dry deciduous forest (London Jobs) (monsoon): the length of the dry season increases further as rainfall decreases (all trees are deciduous).

More than one half of tropical forests (London Jobs) have already been destroyed.

Temperate forest (London Jobs)
Temperate forests (London Jobs) occur in eastern North America, northeastern Asia, and western and central Europe. Well-defined seasons with a distinct winter characterize this forest (London Jobs) biome. Moderate climate and a growing season of 140-200 days during 4-6 frost-free months distinguish temperate forests (London Jobs).

• Temperature varies from -30° C to 30° C.
• Precipitation (75-150 cm) is distributed evenly throughout the year.
• Soil is fertile, enriched with decaying litter.
• Canopy is moderately dense and allows light to penetrate, resulting in well-developed and richly diversified understory vegetation and stratification of animals.
• Flora is characterized by 3-4 tree species per square kilometer. Trees are distinguished by broad leaves that are lost annually and include such species as oak, hickory, beech, hemlock, maple, basswood, cottonwood, elm, willow, and spring-flowering herbs.
• Fauna is represented by squirrels, rabbits, skunks, birds, deer, mountain lion, bobcat, timber wolf, fox, and black bear.

Further subdivisions of this group are determined by seasonal distribution of rainfall:

• moist conifer and evergreen broad-leaved forests (London Jobs): wet winters and dry summers (rainfall is concentrated in the winter months and winters are relatively mild).
• dry conifer forests (London Jobs): dominate higher elevation zones; low precipitation.
• mediterranean forests (London Jobs): precipitation is concentrated in winter, less than 1000 mm per year.
• temperate coniferous: mild winters, high annual precipitation (greater than 2000 mm).
• temperate broad-leaved rainforests (London Jobs): mild, frost-free winters, high precipitation (more than 1500 mm) evenly distributed throughout the year.

Only scattered remnants of original temperate forests (London Jobs) remain.

Boreal forest (London Jobs) (taiga)

Boreal forests (London Jobs), or taiga, represent the largest terrestial biome. Occuring between 50 and 60 degrees north latitudes, boreal forests (London Jobs) can be found in the broad belt of Eurasia and North America: two-thirds in Siberia with the rest in Scandinavia, Alaska, and Canada. Seasons are divided into short, moist, and moderately warm summers and long, cold, and dry winters. The length of the growing season in boreal forests (London Jobs) is 130 days.

• Temperatures are very low.
• Precipitation is primarily in the form of snow, 40-100 cm annually.
• Soil is thin, nutrient-poor, and acidic.
• Canopy permits low light penetration, and as a result, understory is limited.
• Flora consist mostly of cold-tolerant evergreen conifers with needle-like leaves, such as pine, fir, and spruce.
• Fauna include woodpeckers, hawks, moose, bear, weasel, lynx, fox, wolf, deer, hares, chipmunks, shrews, and bats.

Current extensive logging in boreal forests (London Jobs) may soon cause their disappearance.