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On the Cato Institute's website is this claim:

The heightened danger to national forests is, ironically, the result of longstanding federal efforts to suppress fires going back many decades. Those efforts have produced an enormous buildup of small trees, under-brush, and deadwood that provide “excess fuels” to feed flames. Before the policy of fire suppression, small fires in many forests cleared away excess fuels and thinned competing plant life, leaving these forests less susceptible to devastating fires like the ones of 2000.

Is that true?

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    According to the National Wildlife Federation, it's certainly one of the causes, but it doesn't stand alone. I imagine it would be hard to isolate how much suppression efforts have contributed to the increase compared to other causes, but someone may be able to find something in one of their many listed references. – Is Begot Jul 25 '14 at 2:16
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    Somewhat anecdotal, but that was a reasonably unanimous statement from most tourguides I spoke to in National Parks, as well as park employees/forest service. – user5341 Aug 8 '14 at 16:24
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In short, yes.

Reference #1:

Recent history has seen an increasing trend of record-breaking wildfires on public forests and grasslands... Why so many large fires? “The most extensive and serious problem related to health of national forests in the interior West is the over-accumulation of vegetation, which has caused an increasing number of large, intense, uncontrollable and catastrophically destructive wildfires,” according to the General Accounting Office (1999). During the past 10,000 – 15,000 years, North American forests have evolved under the influence of humans and natural fire.(USDA Forest Service 2003) Indigenous people harvested timber and used fire for thinning and land clearing to meet their needs for shelter, hunting, gathering, and protecting their communities. In the arid West, where moisture is too scarce to support fungal decay, fire is the primary mechanism for removing dead trees and limbs from the forest floor. Climate factors and widespread wildfire suppression efforts, which became effective after World War II, have contributed to overgrown conditions over the past 75 years. Many forests now require hands-on active management to restore fire-adapted ecosystems (Sebellius and Rosen, 2003)

(source: Fire and Fuels Buildup (U.S. Forest Service publication))

Reference #2:

One hundred years of fire suppression is partly to blame. To protect homes, local fire managers frequently set California's chaparral-covered hills ablaze. But the decades spent earnestly "masticating" (mechanically removing potential fire fuel) devil-red manzanitas with trunks as thick as thighs, and the repeated prescribed burns, replaced native chaparral with incendiary invasive species like cheatgrass, according to the USGS. (source: We've Been Fighting Forest Fires Wrong For 100 Years)

The decades of effort were for naught, finds a series of recent studies from the USGS and colleagues at the Conservation Biology Institute and several research universities.

Frankly, the whole Business Insider article is filled with references and confirmations of the theory, I'm not sure citing more would further improve the asnwer but I would recommend reading the whole article.

Reference #3:

Our understanding of fire’s role in the ecosystem continues to grow. With the success of fire prevention and its exclusion from the forest, natural fire cycles have been disrupted in eco-systems dependent upon or adaptive to fire disturbances. Increased tree densities, understory growth, and accumulation of flammable forest fuel have led to larger and more severe wildfires. The Yellowstone fires of 1988 are considered by some as a high profile example of such a wild-fire. Since the 1980s, large wildfires in dead and dying forests have accelerated the rate of forest mortality.

Susceptibility of trees to insects and disease has increased and many fire dependent species are threatened with extinction due to the exclusion of fire in their ecosystem. Many forest managers have come to recognize how essential fire is in certain forest ecosystems.

Long-leaf pine once covered nearly 70 million acres of the southeastern coastal plain of the United States. Today, less than 5 million acres of this southern pine type remain Long-leaf pine forests are among the most fire dependent forest types in the world. Most original long-leaf forests were regularly burned on a natural frequency of 2-7 years ignited by lightning. The suppression of wildland fires in long-leaf stands is thought by many to have seriously affected the integrity and health of this ecosystem. During the past several decades, prescribed burning has become an accepted forest management practice for southeastern pine woods....

... Fire is essential for the longleaf-bluestem forest ecosystem occurring from Mississippi to east Texas and for the longleafslash, pine-wire, grass-saw palmetto ecosystems found insouth Georgia and much of Florida. Without fire in these ecosystems, shade tolerant species invade the sites, displace the grasses and surpass the growth of pines. Plants, such as orchids and pitcher plants, are benefited by fire. ...

(source: Fire Management. Study Unit. Prepared by Rachel G. Schneider, USDA Forest Service (U.S. Forest Service publication))

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