Western United States forest wildfire activity is widely thought to have increased in recent decades, yet neither the extent of recent changes nor the degree to which climate may be driving regional changes in wildfire has been systematically documented. Much of the public and scientific discussion of changes in western United States wildfire has focused instead on the effects of 19th- and 20th-century land-use history. We compiled a comprehensive database of large wildfires in western United States forests since 1970 and compared it with hydroclimatic and land-surface data. Here, we show that large wildfire activity increased suddenly and markedly in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons. The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.

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Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity

There is a special reason for reviewing this book at this time: it is the 50th edition of a compendium that is known and used frequently in most chemical and physical laboratories in many parts of the world. Surely, a publication that has been published for 56 years, withstanding the vagaries of science in this century, must have had something to offer. There is another reason: while the book is a standard fixture in most chemical and physical laboratories, including those in medical centers, it is not as frequently seen in the laboratories of physician's offices (those either in solo or group practice). I believe that the Handbook can be useful in those laboratories. One of the reasons, among others, is that the various basic items of information it offers may be helpful in new tests, either physical or chemical, which are continuously being published. The basic information may relate

Handbook of Chemistry and Physics

Fire is a worldwide phenomenon that appears in the geological record soon after the appearance of terrestrial plants. Fire influences global ecosystem patterns and processes, including vegetation distribution and structure, the carbon cycle, and climate. Although humans and fire have always coexisted, our capacity to manage fire remains imperfect and may become more difficult in the future as climate change alters fire regimes. This risk is difficult to assess, however, because fires are still poorly represented in global models. Here, we discuss some of the most important issues involved in developing a better understanding of the role of fire in the Earth system.

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Fire in the Earth System

Many physical, chemical, mineralogical, and biological soil properties can be affected by forest fires. The effects are chiefly a result of burn severity, which consists of peak temperatures and duration of the fire. Climate, vegetation, and topography of the burnt area control the resilience of the soil system; some fire-induced changes can even be permanent. Low to moderate severity fires, such as most of those prescribed in forest management, promote renovation of the dominant vegetation through elimination of undesired species and transient increase of pH and available nutrients. No irreversible ecosystem change occurs, but the enhancement of hydrophobicity can render the soil less able to soak up water and more prone to erosion. Severe fires, such as wildfires, generally have several negative effects on soil. They cause significant removal of organic matter, deterioration of both structure and porosity, considerable loss of nutrients through volatilisation, ash entrapment in smoke columns, leaching and erosion, and marked alteration of both quantity and specific composition of microbial and soil-dwelling invertebrate communities. However, despite common perceptions, if plants succeed in promptly recolonising the burnt area, the pre-fire level of most properties can be recovered and even enhanced. This work is a review of the up-to-date literature dealing with changes imposed by fires on properties of forest soils. Ecological implications of these changes are described.

/pdf/effects-of-fire-on-properties-of-forest-soils-a-review-1ymllzm5e7.pdf

Effects of fire on properties of forest soils: a review

https://bio.uib.no/te/papers/Grimm_2010_The_ODD_protocol_.pdf

The ODD protocol: A review and first update

Heavy livestock grazing, logging, and fire exclusion associated with Euro-American settlement has brought about substantial changes in forest conditions in western forests. Thus, old-growth definitions based on current forest conditions may not be compatible with the natural conditions prevalent throughout the evolutionary history of western forest types. Detailed analysis of data from two study areas in the southwestern ponderosa pine type suggests that average tree densities have increased from as few as 23 trees per acre in presettlement times to as many as 851 trees per acre today. Associated with these increases in tree density are increases in canopy closure, vertical fuel continuity, and surface fuel loadings resulting in fire hazards over large areas never reached before settlement. In addition, fire exclusion and increased tree density has likely decreased tree vigor (increasing mortality from disease, insect, drought, etc.), herbaceous and shrub production, aesthetic values, water availability a...

/pdf/postsettlement-changes-in-natural-fire-regimes-and-forest-5dr56newur.pdf

Postsettlement Changes in Natural Fire Regimes and Forest Structure: Ecological Restoration of Old-Growth Ponderosa Pine Forests

The age structure in 1876, the last year of the natural frequent-fire regime, of an unharvested ponderosa pine forest in northern Arizona was reconstructed from living and dead dendrochronological samples. Approximately 20% of the trees were .200 yr old in 1876 with ages ranging to 540 yr. If dead trees had not been included in the reconstruction, the distribution would have been biased toward younger trees and a 40% shorter age range. The presettlement age distribution was multimodal with broad peaks of establishment, consistent with the model of regeneration in ''safe sites'' where herbaceous competition and fire thinning are reduced. Although fire disturbance regimes and climatic conditions varied over the centuries before 1876, a clear relationship between these variations and tree establishment was not observed. Due to fire exclusion, reduced grass competition, and favorable climatic events, high levels of regeneration in the 20th century raised forest density from 60 trees/ha in 1876 to .3000 trees/ha in 1992. An ecological restoration experiment initiated in 1993 conserved all living presettlement trees and reduced the density of young trees to near-presettlement levels. Two important components for evaluating the restoration treatment effects are monitoring of old-tree persistence and patterns of future regeneration in the context of the presettlement reference age structure.

Restoration of presettlement age structure of an arizona ponderosa pine forest

Euro-American settlement of the Inland West has altered forest and woodland landscapes, species composition, disturbance regimes, and resource conditions. Public concern over the loss of selected species and unique habitats (e.g., old-growth) has caused us to neglect the more pervasive problem of declining ecosystem health. Population explosions of trees, exotic weed species, insects, diseases, and humans are stressing natural systems. In particular, fire exclusion, grazing, and timber harvest have created anomalous ecosystem structures, landscape patterns, and disturbance regimes that are not consistent with the evolutionary history of the indigenous biota. Continuation of historical trends of climate change, modified atmospheric chemistry, tree density increases, and catastrophic disturbances seems certain. However, ecosystem management strategies including the initiation of management experiments can facilitate the adaptation of both social and ecological systems to these anticipated changes. A fairly ...

Historical and Anticipated Changes in Forest Ecosystems of the Inland West of the United States

https://www7.nau.edu/mpcer/direnet/publications/publications_c/files/Covington_WW_Sackett_SS_Soil_mineral_nitrogen_changes_following.pdf

Soil mineral nitrogen changes following prescribed burning in ponderosa pine

We compared historical (1909-1913) and contemporary (1997-1999) forest struc- ture and composition on 15 permanent plots in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests of Arizona and New Mexico. We used the same sampling methods as in the early 1900s and compared stand density, diameter distributions, species composition, and broad age classes from the two periods. Stand density (trees 9.14 cm dbh) significantly (P 0.001) increased on plots from an average of 77.4 trees per plot (s 49.9) at plot establishment in 1909-1913 to 519.1 trees per plot (s 252.3) at remeasurement in 1997-1999. Basal area significantly (P 0.001) increased from 8.0 m 2 per plot (s 3.5) to 28.5 m 2 per plot (s 10.1). Contemporary tree diameter distribution shifted toward smaller size classes as demonstrated by a significant (P 0.001) decrease in quadratic mean diameter from 38.5 cm (s 7.5) in 1909-1913 to 28.6 cm (s 7.1) in 1997-1999. Broad age classes yielded an average of 61.5 (s 49.5) residual live trees classified as "blackjack" ponderosa pine (P. ponderosa 150 years) and 13.3 (s 11.9) "yellow pine" (P. ponderosa 150 years) in 1909-1913. In 1997-1999, 416 live trees (s 229.6) were "blackjack" and 57.2 (s 28.5) trees on average were "yellow pine." Twelve of the 15 plots were not invaded by other tree species (remained pure ponderosa pine type), while composition shifted slightly on three plots toward more shade-tolerant and fire-in- tolerant species. Ninety-one percent of the historically (1909-1913 or older) mapped tree struc- tures (live trees, snags, logs, stumps, etc.) were relocated, which suggested that the forest reconstruction field techniques are reliable within 10%. Dramatic increases in tree densities may represent an increased potential for bark beetle epidemics and stand replacing wildfire over large areas in the Southwest. FOR .S CI. 50(2):162-176.

W. Wallace Covington

Papers

Postsettlement Changes in Natural Fire Regimes and Forest Structure: Ecological Restoration of Old-Growth Ponderosa Pine Forests

Restoration of presettlement age structure of an arizona ponderosa pine forest

Historical and Anticipated Changes in Forest Ecosystems of the Inland West of the United States

Soil mineral nitrogen changes following prescribed burning in ponderosa pine

Comparison of Historical and Contemporary Forest Structure and Composition on Permanent Plots in Southwestern Ponderosa Pine Forests