Middle and high elevation coniferous forest communities of the North Rim region of Grand Canyon National Park,Arizona, USA

We examined the composition and structure of forest communities in a 3700 ha watershed in relation to environmental gradients and changes in land management practices. We identified four mixed-conifer forest types dominated by different combinations of Abies concolor, Picea pungens, Pinus ponderosa, Populus tremuloides, and Pseudotsuga menziesii, and a spruce-fir type dominated by Picea engelmannii and Abies lasiocarpa. The forest types occur in a complex pattern related to elevation and topographicmoisture gradients and variations in past fire regimes. However, widespread regeneration of A. concolor following possible changes in the fire regime in the late 19th century and continuing with institution of a fire suppression policy early in the 20th century is producing a more homogenous mixed-conifer forest with greater horizontal and vertical continuity of fuel. This shift toward landscape homogeneity not only may adversely affect biodiversity, but also may be perpetuated as the probability of large, high-severity fires increases with continued fire suppression.     

Understory vegetation indicates historic fire regimes in ponderosa pine‐dominated ecosystems in the Colorado Front Range

Question: Can current understory vegetation composition across an elevation gradient of Pinus ponderosa‐dominated forests be used to identify areas that, prior to 20th century fire suppression, were characterized by different fire frequencies and severities (i.e., historic fire regimes)? Location: P. ponderosa‐dominated forests in the montane zone of the northern Colorado Front Range, Boulder and Larimer Counties, Colorado, USA. Methods: Understory species composition and stand characteristics were sampled at 43 sites with previously determined fire histories. Indicator species analyses and indirect ordination were used to determine: (1) if stands within a particular historic fire regime had similar understory compositions, and (2) if understory vegetation was associated with the same environmental gradients that influence fire regime. Classification and regression tree analysis was used to ascertain which species could predict fire regimes. Results: Indicator species analysis identified 34 understory species as significant indicators of three distinct historic fire regimes along an elevation gradient from low‐ to high‐elevation P. ponderosa forests. A predictive model derived from a classification tree identified five species as reliable predictors of fire regime. Conclusions: P. ponderosa‐dominated forests shaped by three distinct historic fire regimes have significantly different floristic composition, and current understory compositions can be used as reliable indicators of historical differences in past fire frequency and severity. The feasibility demonstrated in the current study using current understory vegetation properties to detect different historic fire regimes, should be examined in other fire‐prone forest ecosystems.     

Removal of Encroaching Conifers to Regenerate Degraded Aspen Stands in the Sierra Nevada

Aspen is considered a keystone species, and aspen communities are critical for maintaining biodiversity in western landscapes. Inventories of aspen stand health across the Eagle Lake Ranger District (ELRD), Lassen National Forest, California, U.S.A., indicate that 77% of stands are in decline and at risk of loss as defined by almost complete loss of mature aspen with little or no regeneration. This decline is due to competition from conifers establishing within aspen stands as a result of modification of natural fire regimes coupled with excessive browsing by livestock. Restoration treatments were implemented in four aspen stands in 1999 using mechanical equipment to remove competing conifers to enhance the growth environment for aspen. Recruitment and establishment of aspen stems were measured in treated stands (removal of competing conifers) and non‐treated stands (control) immediately prior to treatment and 2 and 4 years post‐treatment. There was a significant increase in total aspen stem density and in two of three aspen regeneration size classes for treated stands compared to controls. Pre‐treatment total aspen density was positively associated with total aspen density and density in all size classes of aspen (p < 0.001). The results demonstrate that mechanical removal of conifers is an effective treatment for restoring aspen.     

Spatial and temporal variation in historic fire regimes in subalpine forests across the Colorado Front Range in Rocky Mountain National Park, Colorado, USA

Aim The historical variability of fire regimes must be understood in the context of drivers of the occurrence of fire operating at a range of spatial scales from local site conditions to broad‐scale climatic variation. In the present study we examine fire history and variations in the fire regime at multiple spatial and temporal scales for subalpine forests of Engelmann spruce–subalpine fir (Picea engelmannii, Abies lasiocarpa) and lodgepole pine (Pinus contorta) of the southern Rocky Mountains. Location The study area is the subalpine zone of spruce–fir and lodgepole pine forests in the southern sector of Rocky Mountain National Park (ROMO), Colorado, USA, which straddles the continental divide of the northern Colorado Front Range (40°20′ N and 105°40′ W). Methods We used a combination of dendroecological and Geographic Information System methods to reconstruct fire history, including fire year, severity and extent at the forest patch level, for c. 30,000 ha of subalpine forest. We aggregated fire history information at appropriate spatial scales to test for drivers of the fire regime at local, meso, and regional scales. Results The fire histories covered c. 30,000 ha of forest and were based on a total of 676 partial cross‐sections of fire‐scarred trees and 6152 tree‐core age samples. The subalpine forest fire regime of ROMO is dominated by infrequent, extensive, stand‐replacing fire events, whereas surface fires affected only 1–3% of the forested area. Main conclusions Local‐scale influences on fire regimes are reflected by differences in the relative proportions of stands of different ages between the lodgepole pine and spruce–fir forest types. Lodgepole pine stands all originated following fires in the last 400 years; in contrast, large areas of spruce–fir forests consisted of stands not affected by fire in the past 400 years. Meso‐scale influences on fire regimes are reflected by fewer but larger fires on the west vs. east side of the continental divide. These differences appear to be explained by less frequent and severe drought on the west side, and by the spread of fires from lower‐elevation mixed‐conifer montane forests on the east side. Regional‐scale climatic variation is the primary driver of infrequent, large fire events, but its effects are modulated by local‐ and meso‐scale abiotic and biotic factors. The low incidence of fire during the period of fire‐suppression policy in the twentieth century is not unique in comparison with the previous 300 years of fire history. There is no evidence that fire suppression has resulted in either the fire regime or current forest conditions being outside their historic ranges of variability during the past 400 years. Furthermore, in the context of fuel treatments to reduce fire hazard, regardless of restoration goals, the association of extremely large and severe fires with infrequent and exceptional drought calls into question the future effectiveness of tree thinning to mitigate fire hazard in the subalpine zone.     

Inter-hemispheric comparison of fire history: The Colorado Front Range,U.S.A., and the Northern Patagonian Andes,Argentina

Fire history was compared between the Colorado Front Range (U.S.A.) and northern Patagonia (Argentina) by dating fire-scars on 525 Pinus ponderosa and 418 Austrocedrus chilensis, respectively, and determining fire weather on the basis of instrumental and tree-ring proxy records of climatic variation. Years of above average moisture availability preceding fire years, rather than drought alone, is conducive to years of widespread fire in the Colorado Front Range and the northern Patagonian study areas. Above-average precipitation promotes fire by enhancing the growth of herbaceous plants which increases the quantity of fine fuels during the fire season a few years later. The short-term variability in moisture availability that is conducive to widespread burning is strongly related to El Niño Southern Oscillation (ENSO) activity. The warm (El Niño) phase of ENSO is associated with greater moisture availability during the spring in both regions which leads to peaks in fire occurrence several years after El Niño events. The warmer and drier springs associated with la Niña events exacerbate the drying of fuels so that fire years commonly coincide with La Niña events. In both regions, there was a dramatic decline in fire occurrence after the early 1900s due to a decline in intentionally set fires by Native Americans and European settlers, fuel reduction by livestock grazing, and increasingly effective organized fire suppression activities after the 1920s. In both regions there was a marked increase in fire frequency during the mid-and late-19th centuries which coincides with increased ignitions by Native Americans and/or European settlers. However, year-to-year variability in ring widths of Pinus ponderosa and Austrocedrus chilensis also increased from relatively low values in the late 1700s and early 1800s to peaks in the 1850s and 1860s. This implies frequent alternation of years of above and below average moisture availability during the mid-19th century when the frequencies of major fire years rise. The high correlation of tree-growth variability betweem the two regions implies a strong inter-hemispheric variation in climatic variability at a centennial time scale which closely parallels a variety of proxy records of ENSO activity. Based on the relationship of fire and ENSO events documented in the current study, this long-term trend in ENSO activity probably contributed to the mid- and late-19th century increase in fire spread in both regions. These similar trends in fire occurrence have contributed to similar patterns of forest structures, forest health, and current hazard of catastrophic wildfire in the Colorado Front Range and northern Patagonia.     

Quaking aspen (Populus tremuloides Michx.) at treeline: a century of change in the San Juan Mountains, Colorado, USA

During a repeat photography study quaking aspen (Populus tremuloides Michx.) was observed invading conifer stands at treeline in the San Juan Mountains of south‐western Colorado. Aspen tree core samples were collected from nine plots ranging in elevation from 3192 to 3547 m, and estimated dates of establishment of aspen were grouped into 10‐year intervals for analysis. Estimated periods of establishment were compared with century‐long climate data records to derive any correlations with aspen invasion. Other disturbance agents, such as fire and livestock grazing were also considered. Quantitative analysis of climate variables suggests that decreased mean spring precipitation and increased mean summer maximum temperature provide optimal conditions for aspen establishment. Episodes of invasion were non‐synchronous, but all occurred after 1900, and are likely from seed germination, considered unusual in aspen. Different climate variables explain stand initiation from seed and subsequent peak establishment from vegetative reproduction. Long‐term climate records indicate a general warming since the beginning of the 20th century and explain the continued invasion and persistence of aspen at treeline, resulting from asexual reproduction. Short‐term climate records identify anomalously cool, moist years that explain rarely observed sexual reproduction in aspen.     

A Historical Perspective on Pitch Pine–Scrub Oak Communities in the Connecticut Valley of Massachusetts

We present a regional–historical approach to the interpretation, conservation, and management of pitch pine–scrub oak (PPSO) communities in the Connecticut Valley of Massachusetts. Historical studies, aerial photographs, GIS analyses, and extensive field sampling are used to (a) document changes in the historical distribution, composition, and dynamics of these communities, and (b) evaluate the importance of regional–historical approaches to understanding, conserving, and managing uncommon communities. At the time of European settlement, pine plains dominated by both pitch and white pine were widespread, occurring on 9000 ha or more of the extensive (approximately 32,000 ha) xeric outwash deposits in the Connecticut Valley. Pine plains were harvested for diverse forest products from the 17th to the early 19th centuries. After 1830, most sites were cleared and plowed for agriculture and then abandoned in the late 19th and early 20th centuries, resulting in widespread natural reforestation. Modern PPSO communities differ from historical communities with respect to landscape distribution, composition, and structure. Nearly all modern pitch pine stands in the Connecticut Valley became established on former agricultural fields. Current vegetation on these former fields differs substantially from those few sites that were never plowed. In particular, several species (for example, Gaultheria procumbens, Gaylussacia baccata, Quercus ilicifolia, and Q. prinoides) that are characteristic of unplowed sites have not successfully colonized former fields in the 50 to more than 100 years since agricultural abandonment. Urban, commercial, and residential development have been widespread in the 20th century. By 1985, only 38.6% of the outwash deposits remained forested, and only 1094 ha of pitch pine stands and 74 ha of scrub oak stands occurred, primarily in numerous small patches. Several stands have been destroyed since 1985, and development threatens all remaining sites. The trend towards rapid urban development in the 20th century makes it increasingly urgent that the few, relatively large, undeveloped sites be protected. Our results suggest that (a) land protection efforts should prioritize large, undeveloped sand plains, areas that were not plowed historically, and reestablishment of contiguity between isolated sites to facilitate colonization of former agricultural lands by sand plain species; (b) management of PPSO communities should not be restricted to maintenance of open barrens; “old-growth” pitch and white pine stands occurred historically, and some PPSO communities should be allowed to mature without frequent disturbance; (c) the exclusive use of prescribed fires during the spring months is unlikely to maintain communities similar to modern ones or to restore communities similar to historical ones. Establishment or maintenance of open barrens species and communities may require more varied disturbance regimes, perhaps including mechanical treatment in combination with prescribed fire to simulate severe summer fires; (d) regional–historical perspectives are critical for understanding modern community dynamics and for evaluating conservation objectives and management strategies for uncommon plant communities.     

Fire history of mixed conifer ecosystems in the Great Basin/Mojave Deserts transition zone, Nevada, USA

Vegetation processes in terrestrial ecosystems are closely linked with wildfire regime, but fire histories at the boundary between the Great Basin and Mojave Deserts of North America are relatively sparse. We investigated wildfire regime and its driving factors before and after Euro-American settlement in high-elevation mixed-conifer ecosystems that are found as “mountain islands” in south-eastern Nevada, USA. Field-based results obtained at the Clover Mountains were compared with those already published for Mt. Irish, less than 100 km away, and also to remotely sensed information provided by the LANDFIRE project, which is commonly used for natural resource management. Annually resolved wildfire history at the Clover Mountains was derived back to year 1500 from fire scar samples taken from 139 ponderosa pines (Pinus ponderosa) located in six stands. During the 1785–2007 period, when at least 20 recorder trees (and a total of 241 fire scars) were available, the Clover Mountains were characterized by frequent (mean fire interval <10 years) low-severity fires, half of which scarred more than 10 % of recorder trees. The 1877 and 1946 fires scarred 50 % or more of recorder trees and spread to four out of six sampled stands. After the 1946 event, the site has experienced a 61-year fire-free period tied to fire suppression activity starting in the mid-1900s. In comparison with Mt. Irish, the Clover Mountains showed a longer mean fire return interval, larger fires, and some patchy high-severity events, even before Euro-American settlement. Variations in ecosystem composition and associated fire regime in these high-elevation mixed-conifer woodlands were not adequately captured by remotely sensed data used for vegetation management, revealing a need for additional field-based assessments of fire regime characteristics in this region.     

Unsupported inferences of high‐severity fire in historical dry forests of the western United States: response to Williams and Baker

Reconstructions of dry western US forests in the late 19th century in Arizona, Colorado and Oregon based on General Land Office records were used by Williams & Baker (2012; Global Ecology and Biogeography, 21 , 1042–1052; hereafter W&B) to infer past fire regimes with substantial moderate and high‐severity burning. The authors concluded that present‐day large, high‐severity fires are not distinguishable from historical patterns. We present evidence of important errors in their study. First, the use of tree size distributions to reconstruct past fire severity and extent is not supported by empirical age–size relationships nor by studies that directly quantified disturbance history in these forests. Second, the fire severity classification of W&B is qualitatively different from most modern classification schemes, and is based on different types of data, leading to an inappropriate comparison. Third, we note that while W&B asserted ‘surprising’ heterogeneity in their reconstructions of stand density and species composition, their data are not substantially different from many previous studies which reached very different conclusions about subsequent forest and fire behaviour changes. Contrary to the conclusions of W&B, the preponderance of scientific evidence indicates that conservation of dry forest ecosystems in the western United States and their ecological, social and economic value is not consistent with a present‐day disturbance regime of large, high‐severity fires, especially under changing climate.     

Changes in resilience to fire disturbance in lowland pine forest ecosystems

The effect of major disturbances on the structure of pine forests in the interfluve of the Chernaya and Pugai rivers has been retrospectively analyzed back to the late 19th century. The results show that intensive timber harvesting and fires produced a synergistic effect so that the natural fire regime in these forests has changed; in particular, the severity of fires and the frequency of stand-replacement fires have increased. The ecosystem of lowland pine forests, adapted to periodic fires, largely loses its resilience to fire disturbance if the age structure of pine populations becomes relatively homogeneous and communities at early succession stages gain significant prevalence in the vegetation. The main cause of the largest fire that occurred in pine forests of the Nizhni Novgorod Transvolga region (Nizhegorodskoe Zavolzhie) in 1972 was human activity rather than natural and climatic features of this territory.     

Competition and regeneration in quaking aspen–white fir (Populus tremuloides–Abies concolor) forests in the Northern Sierra Nevada,USA

Question: How does competition between quaking aspen (Populus tremuloides) and white fir (Abies concolor) affect growth and spatial pattern of each species? Location: The northern Sierra Nevada, California, USA. Methods: In paired plots in mixed aspen‐ (n=3) or white fir‐dominated (n=2) stands, we mapped trees and saplings and recorded DBH, height, species, and condition and took increment cores. We tallied seedlings by species. Tree ring widths were used as a measure of basal area change over the last decade, and canopy openness was identified using hemispherical photographs. Linear mixed models were used to relate neighborhood indices of competition, stand, and tree‐level variables to diameter increment. Spatial patterns of stems were identified using the Neighborhood Density Function. Results: White fir radial growth was higher in aspen‐ than white fir‐dominated plots. Individual‐level variables were more important for white fir than for aspen growth, while variables representing competitive neighborhood were important only for aspen. The forest canopy was more open in aspen‐ than white fir‐dominated stands, but ample aspen seedlings were observed in all stands. Canopy stems of aspen and white fir were randomly distributed, but saplings and small trees were clumped. Aspen saplings were repelled by canopy aspen stems. Conclusions: Variation in canopy openness explained more stand–stand variation in white fir than aspen growth, but high light levels were correlated with recruitment of aspen seedlings to the sapling class. Radial growth of aspen was predicted by indices of neighborhood competition but not radial growth of white fir, indicating that spacing and stem arrangement was more important for aspen than white fir growth. Fire suppression has removed a major disturbance mechanism that promoted aspen persistence and reduced competition from encroaching conifers, and current forests favor species that regenerate best by advance regeneration (white fir).     

Fire Interval Effects on Successional Trajectory in Boreal Forests of Northwest Canada

Although succession may follow multiple pathways in a given environment, the causes of such variation are often elusive. This paper describes how changes in fire interval mediate successional trajectory in conifer-dominated boreal forests of northwestern Canada. Tree densities were measured 5 and 19 years after fire in permanent plots and related to pre-fire vegetation, site and fire characteristics. In stands that were greater than 75 years of age when they burned, recruitment density of conifers was significantly correlated with pre-fire species basal area, supporting the expectation of stand self-replacement as the most common successional pathway in these forests. In contrast, stands that were under 25 years of age at the time of burning had significantly reduced conifer recruitment, but showed no change in recruitment of trembling aspen (Populus tremuloides). As a result, young-burned stands had a much higher probability of regenerating to deciduous dominance than mature-burned stands, despite the dominance of both groups by spruce (Picea mariana and Picea glauca) and pine (Pinus contorta) before the fire. Once initiated, deciduous-dominated stands may be maintained across subsequent fire cycles through mechanisms such as low on-site availability of conifer seed, competition with the aspen canopy, and rapid asexual regeneration of aspen after fire. We suggest that climate-related increases in fire frequency could trigger more frequent shifts from conifer to deciduous-dominated successional trajectories in the future, with consequent effects on multiple ecosystem processes.     

Effects of fire and spruce beetle outbreak legacies on the disturbance regime of a subalpine forest in Colorado

Aim There is increasing research attention being given to the role of interactions among natural disturbances in ecosystem processes. We studied the interactions between fire and spruce beetle (Dendroctonus rufipennis Kirkby) disturbances in a Colorado subalpine forest. The central questions of this research were: (1) How does fire history influence stand susceptibility to beetle outbreak? And conversely, (2) How does prior occurrence of a beetle outbreak influence stand susceptibility to subsequent fire? Methods We reconstructed the spatial disturbance history in a c. 4600 ha area by first identifying distinct patches in the landscape on aerial photographs. Then, in the field we determined the disturbance history of each patch by dating stand origin, fire scars, dates of mortality of dead trees, and releases on remnant trees. A geographical information system (GIS) was used to overlay disturbance by fire and spruce beetle. Results and main conclusions The majority of stands in the study area arose following large, infrequent, severe fires occurring in c. 1700, 1796 and 1880. The study area was also affected by a severe spruce beetle outbreak in the 1940s and a subsequent low‐severity fire. Stands that originated following stand‐replacing fire in the late nineteenth century were less affected by the beetle outbreak than older stands. Following the beetle outbreak, stands less affected by the outbreak were more affected by low‐severity fire than stands more severely affected by the outbreak. The reduced susceptibility to low‐severity fire possibly resulted from increased moisture on the forest floor following beetle outbreak. The landscape mosaic of this subalpine forest was strongly influenced by the interactions between fire and insect disturbances.     

Fire-induced shifts in overstory tree species composition and associated understory plant composition in Glacier National Park,Montana

In Rocky Mountain forests, fire can act as a mechanism of change in plant community composition if postfire conditions favor establishment of species other than those that dominated prefire tree communities. We sampled pre and postfire overstory and postfire understory species following recent (1988–2006) stand-replacing fires in Glacier National Park (GNP), Montana. We identified changes in relative density of tree species and groups of species (xerophytes vs. mesophytes and reseeders vs. resprouters) in early succession. Postfire tree seedling densities were adequate to maintain prefire forest structure, but relative densities among species were variously changed. Changes were directly related to individual species’ response to severe fires. Most notably, relative density of the mesophytic resprouter quaking aspen (Populus tremuloides) and the xerophytic reseeder lodgepole pine (Pinus contorta) increased substantially following fire, with a concomitant decline in proportional abundance of other tree species that, in some cases, dominated stands before fire. Trends identified in our study suggest that forest community shifts toward those dominated by lodgepole pine and quaking aspen are occurring in GNP. Cover of understory species was not affected by tree species composition or density. These forest communities will likely change throughout succession with the addition of shade-intolerant species in early seral stages and shade-tolerant species later in succession. However, with increased fire frequency, the lodgepole pine-dominated postfire communities observed in our study may become more common throughout time.     

Aspen Increase Soil Moisture,Nutrients, Organic Matter and Respiration in Rocky Mountain Forest Communities

Development and change in forest communities are strongly influenced by plant-soil interactions. The primary objective of this paper was to identify how forest soil characteristics vary along gradients of forest community composition in aspen-conifer forests to better understand the relationship between forest vegetation characteristics and soil processes. The study was conducted on the Fishlake National Forest, Utah, USA. Soil measurements were collected in adjacent forest stands that were characterized as aspen dominated, mixed, conifer dominated or open meadow, which includes the range of vegetation conditions that exist in seral aspen forests. Soil chemistry, moisture content, respiration, and temperature were measured. There was a consistent trend in which aspen stands demonstrated higher mean soil nutrient concentrations than mixed and conifer dominated stands and meadows. Specifically, total N, NO₃ and NH₄ were nearly two-fold higher in soil underneath aspen dominated stands. Soil moisture was significantly higher in aspen stands and meadows in early summer but converged to similar levels as those found in mixed and conifer dominated stands in late summer. Soil respiration was significantly higher in aspen stands than conifer stands or meadows throughout the summer. These results suggest that changes in disturbance regimes or climate scenarios that favor conifer expansion or loss of aspen will decrease soil resource availability, which is likely to have important feedbacks on plant community development.     

Fire and the relative roles of weather,climate and landscape characteristics in the Great Lakes‐St. Lawrence forest of Canada

Question: In deciduous‐dominated forest landscapes, what are the relative roles of fire weather, climate, human and biophysical landscape characteristics for explaining variation in large fire occurrence and area burned? Location: The Great Lakes‐St. Lawrence forest of Canada. Methods: We characterized the recent (1959–1999) regime of large (≥ 200 ha) fires in 26 deciduous‐dominated landscapes and analysed these data in an information‐theoretic framework to compare six hypotheses that related fire occurrence and area burned to fire weather severity, climate normals, population and road densities, and enduring landscape characteristics such as surficial deposits and large lakes. Results: 392 large fires burned 833 698 ha during the study period, annually burning on average 0.07%± 0.42% of forested area in each landscape. Fire activity was strongly seasonal, with most fires and area burned occurring in May and June. A combination of antecedent‐winter precipitation, fire season precipitation deficit/surplus and percent of landscape covered by well‐drained surficial deposits best explained fire occurrence and area burned. Fire occurrence varied only as a function of fire weather and climate variables, whereas area burned was also explained by percent cover of aspen and pine stands, human population density and two enduring characteristics: percent cover of large water bodies and glaciofluvial deposits. Conclusion: Understanding the relative role of these variables may help design adaptation strategies for forecasted increases in fire weather severity by allowing (1) prioritization of landscapes according to enduring characteristics and (2) management of their composition so that substantially increased fire activity would be necessary to transform landscape structure and composition.     

Long-term potential for fires in estimates of the occurrence of savannas in the tropics

Aim   This study aims to improve the formulation and results of the Brazilian Center for Weather Forecasting and Climate Studies Potential Vegetation Model (CPTEC-PVM) by developing a new parameterization for the long-term occurrence of fire in regions of potential savannas in the tropics. Compared with the relatively slow processes of carbon uptake and growth in vegetation, fast mortality and biomass consumption by fires may favour grasses and reduce tree coverage.
Location   The tropics.
Methods   For finding large-scale relationships between fires and other environmental factors, we made two main simplifying assumptions. First, lightning is the most important source of ignition for natural fires. Second, over continental areas in the tropics, lightning is mainly related to the zonal flux of moisture transport.
Results   The parameterization of fire occurrence was built based on a simple empirical relationship, combining information on mean and intra-annual variance of the zonal wind.
Main conclusions   The implementation of this new relationship improved the formulation and the results of the CPTEC-PVM. As a result of this new parameter, the accuracy of the model in allocating the correct vegetation (seasonal forests) instead of savannas for large regions in India and Southeast Asia is now substantially higher than in previous studies.     

Changing growth response to wildfire in old‐growth ponderosa pine trees in montane forests of north central Idaho

North American fire‐adapted forests are experiencing changes in fire frequency and climate. These novel conditions may alter postwildfire responses of fire‐adapted trees that survive fires, a topic that has received little attention. Historical, frequent, low‐intensity wildfire in many fire‐adapted forests is generally thought to have a positive effect on the growth and vigor of trees that survive fires. Whether such positive effects can persist under current and future climate conditions is not known. Here, we evaluate long‐term responses to recurrent 20th‐century fires in ponderosa pine, a fire‐adapted tree species, in unlogged forests in north central Idaho. We also examine short‐term responses to individual 20th‐century fires and evaluate whether these responses have changed over time and whether potential variability relates to climate variables and time since last fire. Growth responses were assessed by comparing tree‐ring measurements from trees in stands burned repeatedly during the 20th century at roughly the historical fire frequency with trees in paired control stands that had not burned for at least 70 years. Contrary to expectations, only one site showed significant increases in long‐term growth responses in burned stands compared with control stands. Short‐term responses showed a trend of increasing negative effects of wildfire (reduced diameter growth in the burned stand compared with the control stand) in recent years that had drier winters and springs. There was no effect of time since the previous fire on growth responses to fire. The possible relationships of novel climate conditions with negative tree growth responses in trees that survive fire are discussed. A trend of negative growth responses to wildfire in old‐growth forests could have important ramifications for forest productivity and carbon balance under future climate scenarios.     

Aspen lichens in agricultural and forest landscapes: the importance of habitat quality

The amount of aspen Populus tremula , has declined in the boreal forest landscape. This decline is especially marked in young and intermediate stands due to the lack of regeneration. Aspen regeneration is nowadays mainly restricted to abandoned agricultural land. The decrease of aspen is of particular concern as it has more host-specific species than any other boreal tree species. The main question addressed is whether regenerating aspen stands in agricultural habitats can compensate for the deficiency of young stands in the forest. Data on epiphytic macrolichens show that cyanolichens increased, in number and frequency, with stand age in the forest landscape, and that there was a striking difference in species composition between stands in the two landscapes. Lichens with cyanobacterial and green-algal photobionts dominated in the forest and agricultural stands, respectively. Notably, cyanolichens were not found in stands younger than 50 yr in the forest, and stands younger than 100 yr in the agricultural landscape. This difference between the landscapes cannot be explained by stand age, stand size or isolation. Instead, differences in habitat quality, due to differences in the physical environment associated with the presence of conifers in the older forest stands, appear to be involved. We suggest that in order to conserve cyanolichens that are confined to aspen, active management practices have to be adopted that promote the regeneration of aspen in the forest landscape, and the establishment of conifers in areas where aspen regeneration is confined to the agricultural landscape. In addition, until new aspen stands with appropriate physical environments have been established, these measures must be combined with the preservation of existing old-growth stands, which can provide appropriate source populations.     

Interactions among fire, insects and pathogens in coniferous forests of the interior western United States and Canada

1 Natural and recurring disturbances caused by fire, native forest insects and pathogens have interacted for millennia to create and maintain forests dominated by seral or pioneering species of conifers in the interior regions of the western United States and Canada. 2 Changes in fire suppression and other factors in the last century have altered the species composition and increased the density of trees in many western forests, leading to concomitant changes in how these three disturbance agents interact. 3 Two‐ and three‐way interactions are reviewed that involve fire, insects and pathogens in these forests, including fire‐induced pathogen infection and insect attack, the effects of tree mortality from insects and diseases on fuel accumulation, and efforts to model these interactions. 4 The emerging concern is highlighted regarding how the amount and distribution of bark beetle‐caused tree mortality will be affected by large‐scale restoration of these fire‐adapted forest ecosystems via prescribed fire. 5 The effects of fire on soil insects and pathogens, and on biodiversity of ground‐dwelling arthropods, are examined. 6 The effects of fire suppression on forest susceptibility to insects and pathogens, are discussed, as is the use of prescribed fire to control forest pests.