Fire Exclusion Research Articles

Examining Historical and Current Mixed-Severity Fire Regimes in Ponderosa Pine and Mixed-Conifer Forests of Western North America

There is widespread concern that fire exclusion has led to an unprecedented threat of uncharacteristically severe fires in ponderosa pine (Pinus ponderosa Dougl. ex. Laws) and mixed-conifer forests of western North America. These extensive montane forests are considered to be adapted to a low/moderate-severity fire regime that maintained stands of relatively old trees. However, there is increasing recognition from landscape-scale assessments that, prior to any significant effects of fire exclusion, fires and forest structure were more variable in these forests. Biota in these forests are also dependent on the resources made available by higher-severity fire. A better understanding of historical fire regimes in the ponderosa pine and mixed-conifer forests of western North America is therefore needed to define reference conditions and help maintain characteristic ecological diversity of these systems. We compiled landscape-scale evidence of historical fire severity patterns in the ponderosa pine and mixed-conifer forests from published literature sources and stand ages available from the Forest Inventory and Analysis program in the USA. The consensus from this evidence is that the traditional reference conditions of low-severity fire regimes are inaccurate for most forests of western North America. Instead, most forests appear to have been characterized by mixed-severity fire that included ecologically significant amounts of weather-driven, high-severity fire. Diverse forests in different stages of succession, with a high proportion in relatively young stages, occurred prior to fire exclusion. Over the past century, successional diversity created by fire decreased. Our findings suggest that ecological management goals that incorporate successional diversity created by fire may support characteristic biodiversity, whereas current attempts to “restore” forests to open, low-severity fire conditions may not align with historical reference conditions in most ponderosa pine and mixed-conifer forests of western North America.

Temperate and boreal forest mega‐fires: characteristics and challenges

Mega‐fires are often defined according to their size and intensity but are more accurately described by their socioeconomic impacts. Three factors – climate change, fire exclusion, and antecedent disturbance, collectively referred to as the “mega‐fire triangle” – likely contribute to today's mega‐fires. Some characteristics of mega‐fires may emulate historical fire regimes and can therefore sustain healthy fire‐prone ecosystems, but other attributes decrease ecosystem resiliency. A good example of a program that seeks to mitigate mega‐fires is located in Western Australia, where prescribed burning reduces wildfire intensity while conserving ecosystems. Crown‐fire‐adapted ecosystems are likely at higher risk of frequent mega‐fires as a result of climate change, as compared with other ecosystems once subject to frequent less severe fires. Fire and forest managers should recognize that mega‐fires will be a part of future wildland fire regimes and should develop strategies to reduce their undesired impacts.

Fire exclusion and the changing landscape of Queensland’s Wet Tropics Bioregion 2. The dynamics of transition forests and implications for management

SummaryMeeting all four natural criteria for United Nations World Heritage listing, the Wet Tropics World Heritage Area that covers 61% of the remaining vegetation of the bioregion conserves, on a biodiversity basis, habitats ‘containing threatened species of outstanding universal value from the point of view of science or conservation’. As discussed in an accompanying paper by the same authors, the Wet Tropics Bioregion faces a level of habitat change unprecedented in threat and scale and manifested in the transition of sclerophyll forests and woodlands to closed forest habitats, a change attributable to the absence of fire or reduction in its frequency.The implications of habitat change for certain threatened species and ecosystems present a challenge to conserving the ark of biodiversity the Wet Tropics represents. Evidence indicates that the dominant reason for these changes in vegetation patterns is the reduction in fire or its exclusion across a bioregion with the highest rainfall of the nation. It is clear that the bioregion is suffering and with present trends, will continue to suffer, a major loss of habitat diversity.

Fire exclusion and the changing landscape of Queensland’s Wet Tropics Bioregion 1. The extent and pattern of transition

SummaryThe vegetation and geology of the Wet Tropics Bioregion of North Queensland, covering 1 998 150 ha, were mapped at a scale of 1:50 000. The resulting geographic information system (GIS) data base provided an unprecedented opportunity to examine vegetation condition across the entire bioregion. Mapping used colour aerial photography at 1:25 000, informed by ground truthing. Vegetation type, nature of the understory and ground cover, degree and type of disturbance, and the presence of secondary vegetation were described by a coding system, with codes marked directly on the aerial photos.Analysis of these data has confirmed a picture, which emerged from ground truthing, of large areas of sclerophyll woodland and forest being invaded by a rainforest understory that prevents regeneration of the sclerophyll canopy. Fifty-three per cent of the native vegetation of the bioregion consists of non-rainforest vegetation types, dominated in both area and number by sclerophyll woodlands and forests. Seventeen per cent of the 735 713 ha of sclerophyll woodland and forest types were assessed as having suffered irreversible change. Between 25% and 79% of individual forest vegetation types were judged to have been affected by irreversible change. No climatic changes, or changes in the environment, apart from those related to changing fire regimes, were identified as causative factors. Changed fire regimes, predominantly fire exclusion, are considered to be the most likely cause.

Is fire exclusion in mountain big sagebrush communities prudent? Soil nutrient, plant diversity and arthropod response to burning

Fire has largely been excluded from many mountain big sagebrush communities. Managers are reluctant to reintroduce fire, especially in communities without significant conifer encroachment, because of the decline in sagebrush-associated wildlife. Given this management direction, a better understanding of fire exclusion and burning effects is needed. We compared burned to unburned plots at six sites in Oregon. Soil nutrient availability generally increased with burning. Plant diversity increased with burning in the first post-burn year, but decreased by the third post-burn year. Burning altered the arthropod community, which included doubling the density of arthropods in the first post-burn year. Some arthropod Orders increased and others decreased with burning. For example, Araneae were 1.7- and 1.8-fold less and Hemiptera were 6.6- and 2.1-fold greater in the burn compared with the control in 2008 and 2009. Our results provide evidence that burning can create spatial and temporal heterogeneity in sagebrush communities and thus, it is an important component of the ecosystem. We suggest that management plans for many mountain big sagebrush communities may need to include infrequent burning. At the very least managers should be aware that fire exclusion has some potentially negative effects other than the encroachment of conifers in these communities.

Tree recruitment in relation to climate and fire in northern Mexico

Extensive changes in montane forest structure have occurred throughout the U.S. Southwest following Euro-American settlement. These changes are a product of confounding effects of disturbance, climate variability, species competition, and modern land use changes. Pronounced forest reproduction events in the Southwest have generally occurred in climatically wet periods but have also followed widespread fire exclusion. Understanding the ecological processes driving such events has important implications for forest restoration, although these efforts remain difficult due to confounding factors. Separation of these interacting factors was possible in the Sierra San Luis of northern Mexico where we investigated climate, fire, and tree recruitment in areas with continued frequent fires or where fire exclusion came relatively late (1940s). Fires were strongly tied to interannual wet-dry cycles of climate, whereas recruitment peaks were more closely tied to local processes, namely, fire-free periods, than to broad-scale climatically wet conditions. The greatest pulse of tree recruitment coincided with a pronounced mid-century drought (1942-1957) and a period of reduced fire frequency. The second largest pulse of recruitment (ca. 1900) preceded a well-documented period of recruitment (and an anomalously wet period) elsewhere across the Southwest in the 1910s-1920s, and also coincided with specific fire-free periods during below-average precipitation. We also found greater spatial dependence and clustering in older age classes of trees. This spatial pattern indicates a legacy of fire-induced mortality in shaping stand structure, underscoring the importance of frequent fire effects on spatial variability in forests.

Fire regime shift linked to increased forest density in a piñon–juniper savanna landscape

Piñon–juniper (PJ) fire regimes are generally characterised as infrequent high-severity. However, PJ ecosystems vary across a large geographic and bio-climatic range and little is known about one of the principal PJ functional types, PJ savannas. It is logical that (1) grass in PJ savannas could support frequent, low-severity fire and (2) exclusion of frequent fire could explain increased tree density in PJ savannas. To assess these hypotheses I used dendroecological methods to reconstruct fire history and forest structure in a PJ-dominated savanna. Evidence of high-severity fire was not observed. From 112 fire-scarred trees I reconstructed 87 fire years (1547–1899). Mean fire interval was 7.8 years for fires recorded at ≥2 sites. Tree establishment was negatively correlated with fire frequency (r=–0.74) and peak PJ establishment was synchronous with dry (unfavourable) conditions and a regime shift (decline) in fire frequency in the late 1800s. The collapse of the grass-fuelled, frequent, surface fire regime in this PJ savanna was likely the primary driver of current high tree density (mean=881treesha–1) that is >600% of the historical estimate. Variability in bio-climatic conditions likely drive variability in fire regimes across the wide range of PJ ecosystems.

Mortality of native grasses after a summer fire in natural temperate grassland suggests ecosystem instability

SummaryThe exclusion of regular fire and the introduction of livestock grazing have altered native grassland composition on Victoria's volcanic plains, commonly resulting in spear‐grass and wallaby‐grass pastures replacing Kangaroo Grass grasslands. The effect of reintroducing fire to these pastures is currently unknown, although it may be an important part of restoring this ecosystem. We measured the changes in basal area of the dominant grasses in a mixed Spear‐grass/Wallaby‐grass pastures after a summer wildfire, which we assume burnt a relatively homogenous grass sward. We found a 90–95% reduction in the basal area of live spear‐grass tussocks in burnt plots compared with unburned controls, due to the mortality of tussocks. This suggests that caution and structured experimentation should be applied when using fire to manage spear‐grass‐dominated grasslands.

Long‐term effects of fire severity on oak–conifer dynamics in the southern Cascades

We studied vegetation composition and structure in a mixed conifer-oak ecosystem across a range of fire severity 10 years following wildfire. Sample plots centered on focal California black oaks (Quercus kelloggii) were established to evaluate oak and neighboring tree and shrub recovery across a gradient of fire severity in the southern Cascade Range, USA. Shrub and oak resprouting was strongest around focal oaks where conifer mortality was greatest. Linear modeling revealed negative relationships between California black oak sprout height or basal area and residual overstory tree survival, primarily white fir (Abies concolor). The two dominant competing species, California black oak and white fir, showed opposite responses to fire severity. Sprouting California black oak and associated shrubs dominated in severely burned areas, while surviving, non-sprouting white fir maintained dominance by its height advantage and shading effects in areas that burned with low fire severity. Our results indicate that high-severity fire promotes persistence and restoration of ecosystems containing resprouting species, such as California black oak, that are increasingly rare due to widespread fire exclusion in landscapes that historically experienced more frequent fire. We present a conceptual model based on our results and supported by a synthesis of postfire resprouting dynamics literature. Our results and conceptual model help illuminate long-term postfire vegetation responses and the potential ability of fire to catalyze formation of alternate vegetation community structures that may not be apparent in studies that evaluate postfire effects at shorter time-since-fire intervals or at coarser scales.

Initial Ecosystem Restoration in the Highly Erodible Kisatchie Sandstone Hills

Restoration of the unique and diverse habitats of the Kisatchie Sandstone Hills requires the re-introduction of fire to reduce fuel accumulation and promote herbaceous vegetation, but some soils in the area are extremely erodible, and past fires have resulted in high erosion rates. Overstory and understory vegetation, downed woody fuels, and other stand attributes were measured on sites that received either no management or two prescribed burns after >20 years of fire exclusion. The two burns (one dormant season and one growing season) reduced the live fuel-load (understory biomass) and forest floor (litter and duff mass) by 90 and 71%, respectively, but did not change the downed woody fuel load. Understory plant diversity was not affected by burning, but burning stimulated both colonization and sprouting for most plant species. Habitat for Picoides borealis (Red-cockaded Woodpecker) was improved; understory plant height was reduced by 2 m, and herbaceous vegetation was found in 40% of the areas sampled in the burned sites but it was found in only 6.7% of the reference (unburned) sites. Erosion risk was still elevated due to the sparse vegetative cover on the forest floor. Future management should consider erosion prevention, and plan the timing and intensity of additional burns to maximize plant cover on the forest floor and to improve the habitat by converting the woody understory to an herbaceous understory.

Fine-scale factors influence fire regimes in mixed-conifer forests on three high mountains in Mexico

We investigated the influence of broad- v. fine-scale factors on fire in an unusual landscape suitable for distinguishing the drivers of fire synchrony. Our study was conducted in the Sierra Madre Oriental mountain range, in north-eastern Mexico. We worked in nine sites on three parallel mountains that receive nearly identical broad-scale climatic influence, but between which fires are unlikely to spread. We collected and cross dated samples from 357 fire-scarred trees in nine sites in high-elevation mixed-conifer forests and identified fire dates. We used Jaccard similarity analysis to evaluate synchrony among sites and quantified relationships between climate and fire occurrence. Fires were historically frequent (mean fire interval ranged from 8 to 16 years in all sites) and dates of fire exclusion ranged from 1887 to 1962. We found low fire synchrony among the three mountains, indicating a strong influence of fine-scale factors on fire occurrence. Fire regime attributes were similar across mountains despite the independence of fire dates. La Niña events were associated with fire over time, although not significantly since the 1830s. Our results highlight the importance of scale in describing fire regimes and suggest that we can use fire history to understand controls on complex ecosystem processes and patterns.

Small mammals exhibit limited spatiotemporal structure in Sierra Nevada forests

Forests in the Sierra Nevada, similar to those across the continent, have been substantially altered by logging, fire exclusion, and other human activities. Current forest management emphasizes maintenance or restoration of resiliency in the face of contemporary disturbance factors that include wildfire, climate change, continued urbanization, and invasive species. We evaluated responses of small mammals to forest management by monitoring a series of 12 replicate trapping grids in compositionally homogeneous forest over 8 years, and implemented 2 levels of canopy thinning. Livetrapping efforts (119,712 trap-nights) yielded 15,613 captures of 2,305 individuals of 13 species, and although forest structure was significantly influenced by canopy treatments, small mammal numbers and assemblage composition were not. To better understand this we assessed habitat associations of small mammals at 599 census points on 75 transects established in a stratified random manner throughout Plumas National Forest. We analyzed these data with 2 complementary forms of constrained ordination (canonical correspondence and canonical correlation) that extract major gradients in 1 data set (e.g., distribution of small mammal captures) and explain these in terms of measured variables from a 2nd data set (e.g., habitat and environmental measurements). Over 3 years and 57,504 trap-nights of effort we captured 1,367 individuals of 11 species. Both forms of ordination exposed significant associations between small mammals and underlying habitat metrics, but they explained remarkably little variation in these data, suggesting that small mammals are responding only modestly to habitat variation as expressed by the available environmental variables measured at each plot. We followed this with stepwise multiple Poisson regression to build models of habitat associations of these species. We applied model-averaging and employed Akaike's information criterion corrected for small sample size (AICC) to evaluate candidate models. Reflecting ordination results, competitive models (e.g., those with Akaike differences [ΔAICC] < 2.0) cumulatively explained little variation (12–36%) and regression coefficients were very low. Hence, both ordination and Poisson multiple regression suggest that the limited response by small mammals to canopy thinning primarily reflects the generalist habits of the common species in this forest. We propose that anthropogenic influences have led to structural homogenization of these forests, even across > 800 m of elevation, such that habitat specialists (e.g., old-forest-dependent species such as Myodes [Clethrionomys] californicus and Glaucomys sabrinus) have become less common due to the lack of suitable habitat. Further efforts should target nonforested habitats (meadows and riparian corridors), but results presented here suggest that managers should strive to increase heterogeneity at large spatial scales and especially to promote the development of mature forest structure.

Comparison of plant communities on the Ibity and Itremo massifs, Madagascar, with contrasting conservation histories and current status

Background: Deforestation processes and species extinction on islands have made them the focus of substantial efforts to create and strengthen local environmental organisations, establish new protected areas and improve natural resource management. In Madagascar, despite promising new policies, the flora and fauna are under serious threat. More than 80% of the island shows a significant loss of natural plant cover. Current levels of diversity reflect natural disturbance regimes, with fire representing one of the more important factors.Aims: We compared two new protected areas (Ibity and Itremo) with different environmental and management contexts to identify the roles and impacts of the environmental conditions and the different types of management applied by their respective local communities on vegetation. Both areas have extensive stands of tapia (Uapaca bojeri) woodland.Methods: Herbaceous vegetation surveys were carried out enumerating 16 1-m² quadrats in each of 10 sites on Ibity and seven on Itremo to characterise plant community composition and structure, complemented by tree and shrub surveys within three 40 × 40 m quadrats at each site.Results: Floristic richness was higher at Ibity. Populations of U. bojeri were better conserved at Itremo.Conclusions: While the total exclusion of fire is impracticable, careful management should enable the maintenance of natural conditions and at least lead to a reduction in the impacts on the structure of tapia woodland vegetation.

Is the Longleaf Type a Climax?

What is a climax? The commonly accepted definition is, a type of vegetation which has adapted itself over a long period to a set of environmental conditions, so completely that it is stabilized as long as those conditions remain unchanged. In a narrower sense, a climax type assumes adaptation to a given set of conditions excluding other conditions. The conditions excluded are those which would usually result in disturbance of this stability or equilibrium. These are; fire, under the conception that fire acts to destroy the stand; insects, when epidemic; destructive winds. The effect of imported diseases and insects, or of man's interference in lumbering, destroys the balance and the type may completely change. Climax types may therefore yield at any time to new conditions to which they are not adapted. Excluding the above factors, there would remain only the climatic and soil factors of site. These factors, isolated from the effects of the so-called destructive group, would finally establish a forest type in which the only surviving species would be those capable of enduring the shade of a forest cover during their establishment. This condition necessitates the elimination of fire, wind, insect epidemics and disease for periods longer than the life span of species which cannot endure shade. When, as with Douglas fir, this span is from 300 to 500 years, and the environmental conditions are such as to create extreme conflagration hazard at intervals of from 3 to io years, the establishment of climax types of more shade-enduring species, which are usually inferior in utility, is a pure accident. In the life span of the species, any factor, such as fire, which can be depended on to occur even once within that period, and create conditions favorable to its reproduction, may become the determining factor in its perpetuation as against shadeenduring or so-called climax types. The characteristics of such a relationship are that while fire is required (or its man-made substitute, lumbering) for the removal of the overwood shade and establishment of reproduction, the total exclusion of subsequent fires is equally necessary for survival of this reproduction. The distribution and composition of the forest types in such a region is, therefore, very irregular, much young growth is destroyed prematurely, and here and there a patch may finally attain the climax or shade-enduring type. If for any reason the normal or natural frequency of fire is increased so that its chance of occurrence on any given area becomes once in IO to i5 years, the only hope for survival of the tree species is ability to develop a fire resistant bark within such a period and the fact that the fires when they do occur will

Toward a mechanism for eastern North American forest mesophication: differential litter drying across 17 species

Long-term fire exclusion has altered ecological function in many forested ecosystems in North America. The invasion of fire-sensitive tree species into formerly pyrogenic upland forests in the southeastern United States has resulted in dramatic shifts in surface fuels that have been hypothesized to cause reductions in plant community flammability. The mechanism for the reduced flammability or "mesophication" has lacked empirical study. Here we evaluate a potential mechanism of reduced flammability by quantifying moisture retention (response time and initial moisture capacity) of foliar litter beds from 17 southeastern tree species spanning a wide range of fire tolerance. A k-means cluster analysis resulted in four species groups: a rapidly drying cluster of eight species; a five-species group that absorbed little water but desorbed slowly; a two-species group that absorbed substantial moisture but desorbed rapidly; and a two-species cluster that absorbed substantial moisture and dried slowly. Fire-sensitive species were segregated into the slow moisture loss clusters while fire-tolerant species tended to cluster in the rapid drying groups. Principal-components analysis indicated that several leaf characteristics correlated with absorption capacity and drying rates. Thin-leaved species with high surface area : volume absorbed the greatest moisture content, while those with large, curling leaves had the fastest drying rates. The dramatic shifts in litter fuels as a result of invasion by fire-sensitive species generate a positive feedback that reduce the windows of ignition, thereby facilitating the survival, persistence, and continued invasion of fire-sensitive species in the uplands of the southeastern United States.

The effect of fire exclusion on the structure and tree mortality patterns of a caldén (Prosopis caldenia Burkart) woodland in semi-arid Central Argentina

We studied the effects of long-term fire exclusion on the structure and mortality of a caldén (Prosopis caldenia Burkart) woodland, a rare vegetation type remaining only in fragments in semi-arid Central Argentina. We tested the hypothesis that differences in caldén stand structure are consequence of differences in fire history by quantifying DBH and tree height in 30 plots set in caldén woodlands with fire occurring at least every 10 y, with fire excluded for 20–30 y and without fire for at least 60 y. Stem height, density and basal area increased after 20–30 years of fire exclusion. The frequently burned woodland had high mortality in all but the largest (>60 cm in DBH and >8 m in height) size class, whereas the fire-excluded plots exhibited high mortality only in the smallest stems (<15 cm in DBH and <3 m in height). Our results showed that the fire exclusion increased stem basal area and density while decreasing mortality of larger size classes and reducing resprouting of stems of any size; the probable explanation for these changes is a shift in the prevalent thinning mechanisms from fire-induced to competition-induced mortality.

Ecological impacts of fire and postfire restoration in Korea

Fire is a major driving force shaping and maintaining the integrity of various forest ecosystems. In particular, pine forests often depend on fire for sustainability, because fire removes the organic layer of the forest floor to prepare a desirable seedbed and helps eliminate competitors. The majority of Korean forests are dominated by pine species, because pine forests were historically protected for various purposes (e.g., source for building material), and pine species were widely planted in the 1960s and 1970s to restore the barren soil that resulted from the Korean War and excessive natural resource extraction since 1907. Fires, however, have been excluded from the modern Korean landscape for an extended time mainly due to an aggressive fire exclusion policy and low fuel loading to sustain crown fires. Consequently, it is not clearly known how dominant native species (e.g., Pinus densiflora, P. thunbergii, and Quercus mongolica) interact with fire in Korea and how fire influences interspecific competition among them. Naturally, the general public did not consider fire an integral and essential part of forest maintenance in Korea and believed it could only yield harmful results. Moreover, some people even believed that Korean forests could be maintained without fire. For the first time in modern history, Korea experienced a wildland fire [1,000 ha in 1996 (3,762 ha), which was followed by several large fires, including the Eastern Coastal Fire in 2000 that burned 23,794 ha (or approximately 0.4 % of Korea’s total forest) in 9 days. These series of fires concerned the general public for two reasons: (1) people became aware that large-scale fires ([1,000 ha) can occur in Korea, and (2) forest fire can threaten daily life because many people live very close to the forest (two thirds of Korea is forest). Climate change is also believed to increase wildfire intensity and size, and high fuel loading that is a result of a fire exclusion policy exacerbates it. These circumstances emphasize the need to understand fire regime and behavior in Korea. Various studies were also conducted pertaining to fire prevention and prediction and forest restoration and ecology over the last 15 years. Synthesizing and sharing current knowledge regarding fire science in Korea with the community will generate better synergism. This special issue contains five contributions intended to describe the effects of forest fires on forest ecosystems in eastern coastal areas of Korea and provide an overview of restoration projects. The first contribution, by Ahn et al. (2013) provides a review of studies in Korea over the last two decades and compares findings with other such studies worldwide. Their review covers various fire ecology topics (i.e., postfire stand dynamics, soil property, water quality, and fire effects on insects and soil fauna), as well as postfire restoration planning issues (i.e., pine restoration, fuel breaks, and soil erosion). This work summarizes up-to-date fire science studies in Korea and help readers understand the dynamics of fires in Korea. Moreover, it helps researchers identify future study needs. Korea has approximately 1,300 mm of annual precipitation, approximately half of which falls in July and August; forest fires generally occur in April and May. Generally, the forest canopy intercepts more than half of the precipitation, and removal of the forest canopy increases soil erosion owing to increased water volume and S. R. Ryu (&) Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2H1, Canada e-mail: soung.ryu@ualberta.ca

Long-term overstory and understory change following logging and fire exclusion in a Sierra Nevada mixed-conifer forest

In many forests of the western US, increased potential for fires of uncharacteristic intensity and severity is frequently attributed to structural changes brought about by fire exclusion, past land management practices, and climate. Extent of forest change and effect on understory vegetation over time are not well understood, but such information is useful to forest management focused on restoring biodiversity and resilience to these ecosystems. We re-measured three large (4ha) historical “Methods of Cutting” (MC) plots in mixed-conifer forest of the central Sierra Nevada installed in 1929 to evaluate the effects of different logging methods. Trees ⩾10cm were surveyed across the entire plots and understory vegetation (tree seedlings, shrubs, and herbaceous species) was quantified within quadrats in the old-growth condition in 1929 prior to logging, later in 1929 after logging, and again in 2007 or 2008. We also compared forest structure in the MC plots with an adjacent unlogged “control” area and collected fire scar samples from nearby stumps to evaluate the historical fire regime. The contemporary tree density in the MC plots (739treesha−1) was 2.4times greater than the 1929 pre-logging density (314treesha−1). Trees in the small and intermediate size classes (10–75cmdbh) were significantly over-represented, and trees in the larger size classes (>90cmdbh) generally significantly under-represented, compared with historical conditions. The proportion of pine dropped from 37% of tree basal area in 1929 to 21% in 2007/08. Density of small to intermediate sized trees was similar in the contemporary logged and unlogged control plots, suggesting that over the long term, ingrowth may have been influenced more by lack of fire than historical logging. Change to non-tree vegetation was most pronounced for shrub cover, which averaged 28.6% in 1929 but only 2.5% in 2008. CART analysis indicated that the highest shrub cover in 1929 was in areas having four or fewer trees growing within 15m to the south of the quadrat, suggesting that reduced light was the most likely explanation for the decline over time. Herbaceous species richness in 2008 was significantly lower than in 1931, two years after logging, but did not differ significantly from 1929, prior to logging. Understory vegetation should benefit from thinning or prescribed fire treatments that lead to a greater abundance of higher light environments within stands.

Natural forest floor clearings around trees in Jeffrey pine forests reduce tree damage and mortality following wildfire

In the western United States, trees in pine-dominated forests historically characterized by frequent, low severity wildfires often support deep litter and duff accumulations due to the fire exclusion policies of the last century. These accumulations at the bases of trees can smolder for long periods of time after the passage of fire, which can result in high temperatures, cambial injury, and subsequent tree mortality. As a result, prescribed fire prescriptions often call for the manual removal of litter and duff from the bases of large trees by raking or other means before fire is applied. Here we report on the presence of naturally formed, circular clearings in the soil surface litter layer that occur around the bases of tree boles in seasonally dry forests dominated by Pinus jeffreyi (Jeffrey pine) in eastern California and Baja California (along the eastern edge of the North American Mediterranean-climate zone). We show that such clearings significantly reduce fire severity and tree mortality in surface fires. In the Lake Tahoe Basin (LTB) study area, Jeffrey pine individuals surrounded by clearings experienced much lower bole char heights, and were 1/3 as likely to support crown torching as individuals lacking the clearings; mortality of Jeffrey pine lacking clearings was more than five times higher than individuals surrounded by clearings. Clearings also reduced mortality in incense cedar and white fir. Data from both the LTB and Inyo National Forest (INF) study areas show that clearings are larger and much more common in forests that have recently burned at low severity. In both locations, natural clearings around trees were about four times more common, and existing clearings about three times wider, in recently burned versus long unburned (>60years) sites. Logistic regression models show that clearing size, specifically the percentage of tree bole surrounded by a clearing, is an important predictor of Jeffrey pine survivorship in low to moderate severity fires, but clearing measures have never been included in previously published assessments of tree survival. We suggest that these previously unreported clearings are important features of Jeffrey pine and related semiarid forests and we recommend that more in-depth studies be made of their occurrence, formation, and ecological relationships.

Spatio-temporal log-Gaussian Cox processes for modelling wildfire occurrence: the case of Catalonia, 1994–2008

Wildfires have become one of the principal environmental problems in the Mediterranean basin. While fire plays an important role in most terrestrial plant ecosystems, the potential hazard that it represents for human lives and property has led to the application of fire exclusion policies that, in the long term, have caused severe damage, mainly due to the increase of fuel loadings in forested areas, in some forest systems. The lack of an easy solution to forest fire management highlights the importance of preventive tasks. The observed spatio-temporal pattern of wildfire occurrences may be idealized as a realization of some stochastic process. In particular, we may use a space–time point pattern approach for the analysis and inference process. We studied wildfires in Catalonia, a region in the north-east of the Iberian Peninsula, and we analyzed the spatio-temporal patterns produced by those wildfire incidences by considering the influence of covariates on trends in the intensity of wildfire locations. A total of 3,166 wildfires from 1994–2008 have been recorded. We specified spatio-temporal log-Gaussian Cox process models. Models were estimated using Bayesian inference for Gaussian Markov Random Field through the integrated nested Laplace approximation algorithm. The results of our analysis have provided statistical evidence that areas closer to humans have more human induced wildfires, areas farther have more naturally occurring wildfires. We believe the methods presented in this paper may contribute to the prevention and management of those wildfires which are not random in space or time.