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We tested the hypothesis that recurrent fires may limit juniper expansion into southwestern semiarid grasslands and savannas.Seven large fires at Wupatki National Monument, Arizona, burned 38% of the grasslands and dynamic savannas between 1995 and 2017.Mortality of one-seed junipers (Juniperus monosperma) was >50% among smaller trees ( 2 m tall), and 0% in trees >4 m tall.Herbaceous vegetation recovered within 2 to 3 years postfire.This suggests that fires may have limited juniper expansion historically and that fire may be an option for controlling juniper expansion and restoring grassland/savanna ecosystems today.

The influence of tree age on climate sensitivity is of central importance in dendrochronology. Recent research has highlighted the disparate nature of age-dependent growth responses across species and geographic locations. We compared growth sensitivity and the influence of climate in Pinus edulis (Pinon) of varying ages at Dinosaur National Monument (DINO, northwestern Colorado, USA. Pinon is a particularly good species for this study because of its long life-span and climate sensitivity, and the DINO site is at the northern extreme of the current distribution. We evaluated changes in climate-growth relationships in pinon using total ring-width measurements and running averages of chronology statistics, mean sensitivity (MS) and coefficient of variation (CV), and we investigated growth response to climate variability as trees age. These measures indicated initial low growth sensitivity, increasing as trees reached mid-life stages, approximately 200–250 years, then relatively constant sensitivity from 250-800+ years. First order partial auto correlation (PAC1) declined throughout the life stages of pinon at DINO. The trend in declining autocorrelation leads to higher MS values in the older age classes. Greater year to year variation indicates less persistence in the study population, hence lower autocorrelation. We investigated the degree to which this relationship could be explained by the summer Palmer Drought Severity Index (PDSI) and whether this relationship varied with tree age. The strength of the tree-ring growth response to PDSI was at a maximum during the first two centuries of growth (R2 = 0.54). between two and six centuries (R2 = 0.48), after which we detected a decline in the sensitivity of tree growth to PDSI with increasing age (R2 = 0.41). This study adds to the literature on age-related climate sensitivity in trees; our findings indicate that age-related changes in climate-tree-ring growth responses should be considered when climate variables are reconstructed from tree-ring width chronologies, and specifically from Pinus edulis.

Twentieth-century fire exclusion has produced unnatural and undesirable changes in vegetation structure and dynamics of many rangelands of western North America, but not all kinds of ecosystems have been so affected. A comparison of the historical and modern fire regimes, especially in peripheral populations that can be particularly vulnerable to climatic change, can help guide fire management planning with information on the degree to which a local area has been altered by past fire exclusion. Historical fire rotations in pinon-juniper (Pinus edlis Engelm.-Juniperus spp. L.) woodlands vary widely across woodland types, hence management applications should be specific to local historical and modern fire characteristics. We asked if the modern fire rotation is similar to or longer than the historical fire rotation before arrival of Euro-American settlers on the northern woodland boundary in northwestern Colorado and northeastern Utah. This study was initiated by managers from Dinosaur National Monument (DINO) concerned that lack of 20th-century fire may have allowed unnatural expansion of pinon-juniper woodlands into grasslands and shrublands. Fire history analysis using dendrochronology methods suggests a historical (pre-1900) fire rotation of ca. 550 yr, comparable with or longer than many other woodlands on the Colorado Plateau. In contrast, analysis of digital fire records reveals that the fire rotation between 1981 and 2010 was substantially shorter than historical; if only natural fires are considered, the pinon-juniper fire rotation was 364 yr, and if anthropogenic fires were included, the fire rotation was 233 yr. This shorter fire rotation supports a previously documented contraction in woodland extent in DINO during the past 90 yr. Our data support reducing the amount of fire in the landscape to preserve the integrity of the natural vegetation of this and other pinon-juniper woodlands, especially under projections of warmer and drier future climates.

The greatest extent of Afromontane environments in the world is found in Ethiopia. These areas support exceptional biodiversity, but forest cover and ecological integrity have declined sharply in recent decades. Conservation and management efforts are hampered in part by an inadequate understanding of the basic ecology of major tree species. We investigated population structure and inferred population dynamics from size frequency distributions of 22 forest tree species encountered in montane forests of Ethiopia. We collected new empirical data from four sites in the Bale Mountains, where some of the country's most extensive and least disturbed forests remain, and conducted a systematic review and analysis of all such studies that reported population structure for one or more of these species in Ethiopia. Thirteen widespread montane tree species showed a reverse-J size distribution, indicating a relatively stable population structure. Six other species had size-frequency distributions that indicate episodic recruitment and/or removal of certain size classes. Specific causes of these patterns are uncertain: they may involve timber harvesting, herbivory, fire, or natural disturbances, but patterns were inconsistent and locality dependent. For three other tree species, existing data are inadequate for any interpretation of population structure and dynamics. A species of particular conservation concern that emerged from this analysis was Hagenia abyssinica, which was found in all areas to consist only of larger individuals with no recent recruitment. For management and conservation purposes, the species in most urgent need of new research are those with inadequate or inconsistent data, and H. abyssinica.

Young, recently burned forests are increasingly widespread throughout western North America, but forest development after large wildfires is not fully understood, especially regarding effects of variable burn severity, environmental heterogeneity, and changes in drivers over time. We followed development of subalpine forests after the 1988 Yellowstone fires by periodically resampling permanent plots established soon after the fires. We asked two questions about patterns and processes over the past 25 years: (1) Are plant species richness and community composition converging or diverging across variation in elevation, soils, burn severity, and post-fire lodgepole pine (Pinus contorta var. latifolia) density? (2) What are the major controls on post-fire species composition, and has the relative importance of controls changed over time? For question 1, we sampled 10-m2 plots (n = 552) distributed among three geographic areas that differ in elevation and substrate; plots spanned the spectrum of fire severities and were resampled periodically from 1991 to 2013. For question 2, we sampled 0.25-ha plots (n = 72), broadly distributed across areas that burned as stand-replacing fire, in 1999 and 2012. Richness and species composition diverged early on between infertile low-elevation areas (lower richness) and more fertile high-elevation areas (greater richness). Richness increased rapidly for the first 5 yr post-fire, then leveled off or increased only slowly thereafter. Only 6% of 227 recorded species were nonnative. Some annuals and species with heat-stimulated soil seed banks were associated with severely burned sites. However, most post-fire species had been present before the fire; many survived as roots or rhizomes and regenerated rapidly by sprouting. Among the 72 plots, substrate, temperature, and precipitation (the abiotic template) were consistently important drivers of community composition in 1999 and 2012. Post-fire lodgepole pine abundance was not significant in 1999 but was the most important driving variable by 2012, with a negative effect on presence of most understory species, especially annuals and shade-intolerant herbs. Burn severity was significant in 1999 but not in 2012, and distance to unburned forest had no influence in either year. The 1988 fires did not fundamentally alter subalpine forest community assemblages in Yellowstone, and ecological memory conferred resilience to high-severity fire.

Determining how ecological filters (e.g., climate, soils, biotic interactions) influence where species succeed in heterogeneous landscapes is challenging for long-lived species (e.g., trees), because filters can vary over space and change slowly through time. Stand-replacing wildfires create opportunities for establishment of tree-species cohorts and can catalyze rapid shifts in where species occur, facilitating unique opportunities for long-term study. We quantified effects of multiple ecological filters on a colonizing cohort of aspen (Populus tremuloides) that established from seed throughout burned lodgepole pine (Pinus contorta var. latifolia) forests after the 1988 fires in Yellowstone National Park (Wyoming, USA) to ask: (1) How have aspen presence, density, and size varied across the postfire landscape, and what filters explain these spatial and temporal patterns? (2) How does aspen above-and belowground biomass vary with postfire lodgepole-pine density? Aspen persisted to postfire year 25 in 58% of the plots in which aspen were present in postfire year 11 (n = 45), and mean stem density declined from 522 to 310 stems ha−1. Mean aspen height doubled (from 29 to 59 cm) over this period. Ecological filters related to climate, competition, herbivory, and soils all differentially affected aspen presence, persistence, and size. Growing season temperature, inter-specific competition, and herbivory also changed through time, altering their effects on the colonizing cohort, and shifting where on the landscape aspen persistence and growth were ultimately favored. Eleven years postfire, aspen were favored at warmer, low elevations; ungulate browsing strongly constrained aspen heights; and competition was unimportant. By 25-years postfire, temperatures warmed nearly 1 °C, and aspen were more likely to persist at cooler, high elevations. Browsing pressure declined, as ungulate populations decreased during this time, but aspen height and basal diameters were constrained by dense, rapidly growing postfire conifers. Landscape mosaics of ecological filters shift over space and time and can facilitate or constrain the persistence and growth of colonizing species. Long-term study of post-disturbance colonizing cohorts uniquely reveal how species are responding to real-time environmental change in heterogeneous landscapes, which will help us better anticipate 21st century species distributions and abundances.

Projected increases in fire occurrence and severity in the western United States have created heightened interest in postfire ecological research in Pacific Northwest forest communities. A 2017 mixed-severity fire event at the northern range margin of Sequoia sempervirens (coast redwood) provided a rare opportunity to analyze early postfire data on the interacting effects of burn severity, stem size, and species biology on survivorship and recovery of coast redwood and three forest community associates (Pseudotsuga menziesii var. menziesii, “Douglas-fir”; Notholithocarpus densiflorus, “tanoak”; and Rhododendron macrophyllum, “coast rhododendron”). We employed recursive partitioning to build decision trees that accurately classified (80–88% prediction accuracy) the postfire response of the four primary woody species present in this community. Species specific postfire responses were variable across stem diameters and burn severities, but the ability of three of the four primary woody species to resprout resulted in a resilient postfire community that quickly initiated recovery. Large coast redwood parent stems most typically survived and resprouted from their bole and base, regardless of fire severity, and even the smallest redwood individuals resprouted from their base. Basal sprouting characteristics of coast redwood including sprout presence, growth, and abundance were found to be positively correlated with increasing burn severity. Douglas-fir showed considerable fire resistance at low to moderate burn severities, but high severity fire killed trees of all sizes. Tanoak and coast rhododendron parent stems typically died and resprouted from the base except where burn severity was very low. We conclude by using our findings to discuss the utility and role of fire in forest management, and consider future research avenues that could help to solidify our understanding of the impact and role of fire in these and similar communities.

Two decades of drought and rising temperatures have triggered bark beetle outbreaks and extensive mortality of Colorado pinon (Pinus edulis) and Utah juniper (Juniperus osteosperma) across the southwest US. For example from 2002 to 2005, over one-third of the pinons in Mesa Verde National Park’s old-growth woodlands were killed by the bark beetle Ips confusus and other drought-related factors. Extensive wildfires have also burned through the Park over the last two decades, reducing the extent of old-growth woodlands by nearly one-half. We compared historical (1970–1990s) and recent (2005–2011) data to gauge the effects of the 2002–2005 mortality event on canopy structure, woody surface fuels, and tree recruitment. Large, mature pinons were the most susceptible to mortality; we measured significant reductions in pinon cover and diameter. Due to the loss of large trees, woodlands that were subjected to substantial mortality had reduced structural complexity relative to nearby unaffected stands. In a longitudinal fuels comparison, 2010–2011 fuels were compared to historical 1993 values in the same plots; we could not detect a change in woody fuel load. However when the 2010–2011 fuels are compared across mortality severity classes, there was a trend toward slightly greater 1–100 h, greater 1000 h fuel loads, and more litter in stands with higher levels of tree mortality. However, modeled surface fire behavior did not differ across mortality severity classes. Modeled crown fires needed higher wind speeds to spread from tree to tree in stands without tree mortality, but this is likely due to the inherently more open canopy structure in stands that resisted mortality rather than due to tree mortality per se. Post-mortality pinon reproduction and recruitment were recorded from 2005 to 2011 and compared with NPS monitoring data from 1975 to 1995. Cone production in 2005 was comparable to historical trends, but recent seedling and sapling densities were significantly below historical values. Yearly seedling and sapling density in 1975–2011 was positively correlated with precipitation during the sampling year and two years prior. The largest proportion of seedlings and saplings occurred under pinon or juniper canopies, and a smaller proportion under native shrubs. Given the loss of a substantial component of the adult trees, reduced stand structural complexity, and demonstrated need for moist conditions and nurse plants for effective recruitment, these old-growth woodlands may be vulnerable to decline under future warm and dry climate projections. Management strategies focusing on maintaining structural features of both the overstory and understory could help provide conditions for sustainable recruitment and contribute to the goal of conservation of rare old-growth pinon–juniper woodlands.

Understanding how disturbances interact to shape ecosystems is a key challenge in ecology. In forests of western North America, the degree to which recent bark beetle outbreaks and subsequent fires may be linked (e.g., outbreak severity affects fire severity) and/ or whether these two disturbances produce compound effects on postfire succession is of widespread interest. These interactions remain unresolved, largely because field data from actual wildfires following beetle outbreaks are lacking. We studied the 2008 Gunbarrel Fire, which burned 27 200 ha in Douglas-fir (Pseudotsuga menziesii ) forests that experienced a bark beetle outbreak 4-13 years prefire (''gray stage,'' after trees have died and needles have dropped), to determine whether outbreak severity influenced subsequent fire severity and postfire tree regeneration. In 85 sample plots we recorded prefire stand structure and outbreak severity; multiple measures of canopy and forest-floor fire severity; and postfire tree seedling density. Prefire outbreak severity was not related to any measure of fire severity except for mean bole scorch, which declined slightly with increasing outbreak severity. Instead, fire severity varied with topography and burning conditions (proxy for weather at time of fire). Postfire Douglas-fir regeneration was low, with tree seedlings absent in 65% of plots. Tree seedlings were abundant in plots of low fire severity that also had experienced low outbreak severity (mean ¼ 1690 seedlings/ha), suggesting a dual filter on tree regeneration. Although bark beetles and fire collectively reduced live basal area to ,5% and increased snag density to .2000% of pre-outbreak levels, the lack of relationship between beetle outbreak and fire severity suggests that these disturbances were not linked. Nonetheless, effects on postfire tree regeneration suggest compound disturbance interactions that contribute to the structural heterogeneity characteristic of mid/lower montane forests.

Climate conditions and forest structure interact to determine the extent and severity of bark beetle outbreaks, yet the relative importance of each may vary though the course of an outbreak. In 2008, we conducted field surveys and reconstructed forest conditions at multiple stages within a recent mountain pine beetle (MPB) outbreak in Rocky Mountain National Park, Colorado. At each stage in the outbreak, we examined changes in (1) lodgepole pine mortality and surviving stand structure, (2) the influence of topographic versus stand structure variables on mortality rates, and (3) stand complexity and landscape heterogeneity. Lodgepole pine mortality reduced basal area by 71 %, but only 47 % of stems were killed. Relative to pre-outbreak stands, surviving stands had lower mean dbh (11.0 vs. 17.4 cm), lower basal area (8.5 vs. 29.3 m2 ha−1), lower density (915 vs. 1,393 stems ha−1), and higher proportions of non-host species (23.1 vs. 10.6 % m2 ha−1). Factors predicting mortality rates changed through the course of the outbreak. Tree mortality during the early stage of the outbreak was associated with warm, dry sites and abundant large trees. During the middle and late stages, mortality was associated with stand structure alone. Stand complexity increased, as defined by stand-scale variability in density, basal area, and the proportion of susceptible trees. Landscape heterogeneity decreased according to semi-variograms of tree diameter and basal area. Increased stand complexity may inhibit future MPB population development, but decreased landscape heterogeneity may facilitate outbreak spread across the landscape if a future outbreak were to irrupt.

Large-scale bark beetle (Curculionidae: Scolytinae) outbreaks across western North America have prompted widespread concerns over changes to forest wildfire potentials. Management actions following outbreaks often include the harvest of beetle-killed trees and subsequent fuel treatments to mitigate expected changes to fuel profiles, but few data exist to inform these actions. In both lodgepole pine (Pinus contorta var. latifolia) and Douglas-fir (Pseudotsuga menziesii var. glauca) forests of the Greater Yellowstone Ecosystem, Wyoming, USA, we used the Forest Vegetation Simulator to evaluate how fuel profiles, stand structure, and biomass carbon storage are influenced by various post-outbreak fuel treatments (removal of beetle-killed trees [‘salvage’] followed by either no treatment, prescribed burning, pileand-burn, or whole-tree-removal). The model was initialized with field data from five unmanaged gray-stage stands in each forest type and projected over 50 years of post-treatment time. Across all treatment methods, the strongest projected effects relative to unharvested stands were reductions in coarse woody surface fuels (after 10–20 yr), fewer well-decayed standing snags (after 40 yr), and reduced biomass carbon storage (throughout all 50 years). The reduction in coarse woody surface fuels suggests reduced heat release and resistance to control in future fires. Projected effects on fine fuels, both in the canopy and surface layers, were surprisingly minor or short-lived; natural fall and decay of fine material in unharvested stands led to the convergence of most fuel variables between treated and untreated stands within about a decade, especially in Douglas-fir forests. Most follow-up treatment methods – whether unmerchantable tree parts were left in place, burned, piled, or removed entirely – had similar impacts on most aspects of fuel and stand structure in both lodgepole pine and Douglas-fir forests. However, the prescribed burning treatment was distinct and generally had the strongest effects, owing to greater consumption of forest floor mass and mortality of small trees, which had persistent influences on both the canopy and surface fuel layers. Treatment effectiveness in reducing fuels was mirrored by reductions in biomass carbon storage and recruitment of well-decayed snags, illustrating common tradeoffs involved in fuel treatments. Harvest of beetle-killed trees and subsequent treatments altered the fuel profile and structure of outbreak-impacted stands, but overall effects were similar among treatments, suggesting flexibility in management options in post-outbreak forests.

A warming climate may increase the frequency and severity of stand-replacing wildfires, reducing carbon (C) storage in forest ecosystems. Understanding the variability of postfire C cycling on heterogeneous landscapes is critical for predicting changes in C storage with more frequent disturbance. We measured C pools and fluxes for 77 lodgepole pine (Pinus contorta Dougl. ex Loud var. latifolia Engelm.) stands in and around Yellowstone National Park (YNP) along a 300-year chronosequence to examine how quickly forest C pools recover after a stand-replacing fire, their variability through time across a complex landscape, and the role of stand structure in this variability. Carbon accumulation after fire was rapid relative to the historical mean fire interval of 150–300 years, recovering nearly 80% of prefire C in 50 years and 90% within 100 years. Net ecosystem carbon balance (NECB) declined monotonically, from 160 g C·m−2·yr−1 at age 12 to 5 g C·m−2·yr−1 at age 250, but was never negative after disturbance....

A major environmental problem in semi-arid landscapes of western North America is the invasion of native vegetation by cheatgrass (Bromus tectorum L.), an annual Eurasian grass that covers >40 million ha of range and woodland in the western US. Cheatgrass can be especially problematic after fire—either prescribed fire or wildfire. Although cheatgrass is known to generally thrive in regions of moderate temperatures, dry summers, and reliable winter precipitation, the spatial patterns of postfire cheatgrass invasion are not well characterized at finer spatial scales (e.g., within most individual landscapes). We used boosted regression trees to develop a spatial model of cheatgrass abundance 0 yr to 19 yr postfire in an 8000 km2 semiarid landscape centered on Dinosaur National Monument (Colorado and Utah, USA). Elevation, a deterministic variable, was the strongest single predictor, with higher cheatgrass cover occurring below 1600 meters. Two other contingent variables, fire severity and climatic conditions in the year after the fire, increased the model’s predictive power. The influence of fire severity differed with the scale of analysis. Across the landscape as a whole (including extensive areas at moderate to high elevation), a greater likelihood of high postfire cheatgrass cover (≥10 %) was associated with lower fire severity. Focusing only on low-elevation areas (

Changes in key drivers (e.g., climate, disturbance regimes and land use) may affect the sustainability of forest landscapes and set the stage for increased tension among competing ecosystem services. We addressed two questions about a suite of supporting, regulating and provisioning ecosystem services in each of two well-studied forest landscapes in the western US: (1) How might the provision of ecosystem services change in the future given anticipated trajectories of climate, disturbance regimes, and land use? (2) What is the role of spatial heterogeneity in sustaining future ecosystem services? We determined that future changes in each region are likely to be distinct, but spatial heterogeneity (e.g., the amount and arrangement of surviving forest patches or legacy trees after disturbance) will be important in both landscapes for sustaining forest regeneration, primary production, carbon storage, natural hazard regulation, insect and pathogen regulation, timber production and wildlife habitat. The paper closes by highlighting five general priorities for future research. The science of landscape ecology has much to contribute toward understanding ecosystem services and how land management can enhance—or threaten—the sustainability of ecosystem services in changing landscapes.

Pinon-juniper ( Pinus spp. -Juniperus spp.) vegetation covers a large area of western North America and exhibits enormous diversity in environmental conditions, species composition, historical disturbance regimes, and changes during the past century. Many stands have degraded canopies, understories, or both, and are candidates for ecological restoration. Development of an effective restoration strategy entails understanding the historical conditions and dynamics of the specific area and distinguishing between passive vs. active restoration, structural vs. functional restoration, and restoration of canopy vs. understory. Restoration objectives and methods must be tailored to suit a particular woodland area; consequently, a “one-size-fits-all” approach is unlikely to produce satisfactory results. This paper presents 5 case studies to illustrate a spectrum of historical and current ecological conditions and appropriate restoration strategies that managers may encounter in pinon-juniper woodlands of the southwestern U.S. The case studies include passive restoration of a generally pristine old-growth woodland and active restoration of 4 habitats that are degraded in different ways. We close by suggesting a general approach to assessing restoration needs and methods. Although the examples are from the Four Corners region of the U.S., the general principles apply to other pinon-juniper vegetation across the West.

The 1988 Yellowstone fires were among the first in what has proven to be an upsurge in large severe fires in the western USA during the past 20 years. At the time of the fires, little was known about the impacts of such a large severe disturbance because scientists had had few previous opportunities to study such an event. Ecologists predicted short- and long-term effects of the 1988 fires on vegetation, biogeochemistry, primary productivity, wildlife, and aquatic ecosystems based on scientific understanding of the time. Twenty-plus years of subsequent study allow these early predictions to be evaluated. Most of the original predictions were at least partially supported, but some predictions were refuted, others nuanced, and a few postfire phenomena were entirely unexpected. Post-1988 Yellowstone studies catalyzed advances in ecology focused on the importance of spatial and temporal heterogeneity, contingent influences, and multiple interacting drivers. Post-1988 research in Yellowstone also has changed public perceptions of fire as an ecological process and attitudes towards fire management. Looking ahead to projected climate change and more frequent large fires, the well-documented ecological responses to the 1988 Yellowstone fires provide a foundation for detecting and evaluating potential changes in fire regimes of temperate mountainous regions.

Climate change is likely to alter wildfire regimes, but the magnitude and timing of potential climate-driven changes in regional fire regimes are not well understood. We considered how the occurrence, size, and spatial location of large fires might respond to climate projections in the Greater Yellowstone ecosystem (GYE) (Wyoming), a large wildland ecosystem dominated by conifer forests and characterized by infrequent, high-severity fire. We developed a suite of statistical models that related monthly climate data (1972–1999) to the occurrence and size of fires >200 ha in the northern Rocky Mountains; these models were cross-validated and then used with downscaled (∼12 km × 12 km) climate projections from three global climate models to predict fire occurrence and area burned in the GYE through 2099. All models predicted substantial increases in fire by midcentury, with fire rotation (the time to burn an area equal to the landscape area) reduced to

Following fire, fine-scale variation in early successional vegetation and soil nutrients may influence development of ecosystem structure and function. We studied conifer forests burned by stand-replacing wildfire in Greater Yellowstone (Wyoming, USA) to address two questions: (1) How do the variability and spatial structure of aboveground cover and soil nitrogen availability change during the first 4 years following stand-replacing fire? (2) At fine scales (2–20 m), are postfire soil inorganic N pools and fluxes related to aboveground cover? Aboveground cover, soil N pools, and annual net N transformations were measured from 2001 to 2004 using a spatially explicit sampling design in four 0.25-ha plots that burned during summer 2000. Within-stand variability (coefficient of variation) in postfire live vegetative cover declined with time since fire, whereas variability in bare mineral soil, charred litter and fresh litter was greatest 2-3 years postfire. The soil nitrate pool was more variable than the soil ammonium pool, but annual net nitrification was less variable than annual net N mineralization. Spatial structure (quantified by semivariograms) was observed in some aboveground cover variables (for example, graminoids and fresh litter), but there was little spatial structure in soil N variables and no obvious congruence in spatial scales of autocorrelation for soil N and aboveground cover. Significant Spearman correlations (at the sample point) indicated that aboveground cover and soil N were coupled following severe fire, and the dominant influence was from aboveground cover to soil N, rather than from soil N to vegetation. Initial patterns of fire severity and re-vegetation contributed to fine-scale heterogeneity in soil N availability for at least 4 years after severe fire.

A mountain pine beetle (Dendroctonus ponderosae Hopkins) epidemic has caused widespread mortality of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) trees across western North America,. We characterized the initial effects of beetle-induced mortality on forest structure and composition in Rocky Mountain National Park, Colorado. In 2008, we surveyed stand structure and tree species composition across lodgepole pine dominated forests in the western portion of the Park. We defined five lodgepole pine forest types to describe variability in pre-epidemic forest conditions. This forested landscape appears to be resilient to the effects of the beetle. Surviving trees, including both canopy trees and saplings, were plentiful in most of the post-epidemic forests, even after accounting for anticipated future mortality. Subalpine fir (Abies lasiocarpa (Hook.) Nutt.), Engelmann spruce (Picea engelmannii Parry ex Engelm.), and aspen (Populus tremuloides Michx.) had modestly higher relative abundances after the epidemic. Lodgepole pine remained the dominant species on approximately 85% of the landscape. The impact of the outbreak on forest structure and composition varied considerably among the five forest types, suggesting that post-epidemic forest developmental trajectories will vary according to pre-outbreak stand characteristics. Active management efforts to regenerate lodgepole pine forests, e.g., tree planting, will likely not be necessary on this landscape.

Cones of lodgepole pine (Pinus contorta var. latifolia) are often serotinous, releasing their seeds from closed cones under heat from fire. Stand-replacing fires in predominantly serotinous stands can thus be expected to result in a strong regeneration response. After large-scale mortality caused by mountain pine beetle (Dendroctonus ponderosae), however, the seeds in serotinous cones may remain on the dead trees for a number of years, impacting germination and viability. We tested seeds collected from living and beetle-killed serotinous stands to determine whether they remain viable after tree death, and whether germination rates were affected by cone age. There was no significant difference in percent germination from the living stand vs. the dead stand. While there was a significant relationship between cone age and percent germination, cones that were 21–25 y still had >30% germination. We conclude that post-beetle regeneration likely will not be limited by viable seed availability in stands ...

Recent increases in insect and fire activity throughout the western US have presented forest managers with formidable challenges. The extent and severity of bark beetle (Curculionidae: Scolytinae) epidemics have reached unprecedented levels, and the number of large, severe fires continues to increase. These trends are expected to continue because climate change is implicated for both disturbances. Insects and fire have tremendous ecological and economic effects in western forests, yet surprisingly little is known about how fire hazard may change following bark beetle epidemics, and the efficacy of alternative forest management practices (e.g., removal of beetle-killed trees or remaining small trees) designed to reduce future fire hazard is largely unknown. We are employing a combination of field studies, remote sensing and simulation modeling to understand how bark beetle infestation affects fire hazard in two widespread but contrasting forest types, lodgepole pine (Pinus contorta) and Douglas-fir (Pseudotsuga menziesii). Lodgepole pine and Douglas-fir forests are key components of Rocky Mountain landscapes, and both are experiencing extensive and severe bark beetle outbreaks. Published research on beetle effects on fire in lodgepole pine forests is inconclusive, and almost no studies have examined Douglas-fir. We hypothesize that differences in fire regime, stand structure, regeneration potential and decomposition of woody fuels lead to important differences in fuel profiles, fire hazard and, in turn, the effectiveness of alternative mitigation strategies in lodgepole pine and Douglas-fir. We also anticipate that ecosystem responses, especially nitrogen cycling, to beetle attack will differ between these two forest types. Our studies are being conducted in Grand Teton and Yellowstone National Parks, and the Bridger-Teton and Shoshone National Forests within the Greater Yellowstone Ecosystem (GYE), where we build on >20 years of research and our recent studies of bark beetles and fire in lodgepole pine forests. During the summer of 2010, we conducted a significant portion of the field component of the project, measuring stand structure and fuel profiles in a chronosequence of Douglas-fir forests of differing time since beetle attack (TSB), and also measuring burn severity and forest regeneration following a 2008 fire that burned a recently beetle-attacked forest. Data analyses are ongoing and results will be forthcoming.

Recent increases in insect and fire activity throughout the western US have presented forest managers with formidable challenges. The extent and severity of bark beetle (Curculionidae: Scolytinae) epidemics have reached unprecedented levels, and the frequency of large, severe fires continues to increase. These trends are expected to continue because climate change is implicated for both disturbances. Insects and fire have tremendous ecological and economic effects in western forests, yet surprisingly little is known about how fire hazard may change following bark beetle epidemics, and how changing fire regimes may potentially alter forests of Greater Yellowstone. We are employing a combination of field studies, remote sensing and simulation modeling to understand how bark beetle infestation affects fire hazard in Douglas-fir (Pseudotsuga menziesii) forests. The Douglas-fir type is a key component of Rocky Mountain landscapes, and is experiencing extensive and severe bark beetle outbreaks. However, almost no studies have examined Douglas-fir. We hypothesized that differences in fire regime, stand structure, regeneration potential and decomposition of woody fuels lead to important differences in fuel profiles, fire hazard and, in turn, the effectiveness of alternative mitigation strategies in Douglas-fir. Our studies are being conducted in Grand Teton and Yellowstone National Parks, and the Bridger-Teton and Shoshone National Forests within the Greater Yellowstone Ecosystem (GYE), where we build on >20 years of research and our recent studies of bark beetles and fire in lodgepole pine forests. During the summer of 2011, we conducted a significant portion of the field component of the project, collecting ancillary data in our previously measured chronosequence of Douglas-fir forests of differing time since beetle attack (TSB), and measuring burn severity and forest regeneration following a 2008 fire that burned a recently beetle-attacked Douglas-fir forest on the Shoshone National Forest. We also sampled forest regeneration and dead wood biomass following a short (28-year) interval ‘reburn’ in lodgepole pine forests to test whether reduced seed sources associated with younger trees at the time of burning might reduce postfire regeneration potential. Data analyses are ongoing and results will be forthcoming.

Understanding nutrient dynamics of young postfire forests may yield important insights about how stands develop following stand-replacing wildfires. We studied 15-year-old lodgepole pine stands that regenerated naturally following the 1988 Yellowstone fires to address two questions: (1) How do foliar nitrogen (N) concentration and total foliar N vary with lodgepole pine density and aboveground net primary production? (2) Is foliar N related to litter production and to rates of gross production, consumption, and net production of soil NH4+ and NO3–? Foliar N concentration of new lodgepole pine needles averaged 1.38%; only stands at very high density (>80 000 trees·ha–1) approached moderate N limitation. Foliar N concentration in composite (all-age) needles averaged 1.08%, varied among stands (0.87%–1.39%), and declined with increasing tree density. The foliar N pool averaged 48.3 kg N·ha–1, varied among stands (3.6–218.4 kg N·ha–1), and increased with aboveground net primary production. Total foliar N was not related to laboratory estimates of net production of NH4+ or NO3– in soils. Lodgepole pine foliage is a strong N sink, and N does not appear to be limiting at this early successional state. The initial spatial patterns of postfire tree density strongly influence landscape patterns of N storage.

Pinon–juniper is a major vegetation type in western North America. Effective management of these ecosystems has been hindered by inadequate understanding of 1) the variability in ecosystem structure and ecological processes that exists among the diverse combinations of pinons, junipers, and associated shrubs, herbs, and soil organisms; 2) the prehistoric and historic disturbance regimes; and 3) the mechanisms driving changes in vegetation structure and composition during the past 150 yr. This article summarizes what we know (and don't know) about three fundamentally different kinds of pinon–juniper vegetation. Persistent woodlands are found where local soils, climate, and disturbance regimes are favorable for pinon, juniper, or a mix of both; fires have always been infrequent in these woodlands. Pinon–juniper savannas are found where local soils and climate are suitable for both trees and grasses; it is logical that low-severity fires may have maintained low tree densities before disruption of fire regimes following Euro-American settlement, but information is insufficient to support any confident statements about historical disturbance regimes in these savannas. Wooded shrublands are found where local soils and climate support a shrub community, but trees can increase during moist climatic conditions and periods without disturbance and decrease during droughts and following disturbance. Dramatic increases in tree density have occurred in portions of all three types of pinon–juniper vegetation, although equally dramatic mortality events have also occurred in some areas. The potential mechanisms driving increases in tree density—such as recovery from past disturbance, natural range expansion, livestock grazing, fire exclusion, climatic variability, and CO2 fertilization—generally have not received enough empirical or experimental investigation to predict which is most important in any given location. The intent of this synthesis is 1) to provide a source of information for managers and policy makers; and 2) to stimulate researchers to address the most important unanswered questions.

The interaction between disturbance and climate change and resultant effects on ecosystem carbon (C) and nitrogen (N) fluxes are poorly understood. Here, we model (using CENTURY version 4.5) how climate change may affect C and N fluxes among mature and regenerating lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Wats.) stands that vary in postfire tree density following stand-replacing fire. Both young (postfire) and mature stands had elevated forest production and net N mineralization under future climate scenarios relative to current climate. Forest production increased 25% [Hadley (HAD)] to 36% [Canadian Climate Center (CCC)], compared with 2% under current climate, among stands that varied in stand age and postfire density. Net N mineralization increased under both climate scenarios, e.g., + 19% to 37% (HAD) and + 11% to 23% (CCC), with greatest increases for young stands with sparse tree regeneration. By 2100, total ecosystem carbon (live + dead + soils) in mature stands was higher than prefire levels, e.g., + 16% to 19% (HAD) and + 24% to 28% (CCC). For stands regenerating following fire in 1988, total C storage was 0-9% higher under the CCC climate model, but 5-6% lower under the HAD model and 20-37% lower under the Control. These patterns, which reflect variation in stand age, postfire tree density, and climate model, suggest that although there were strong positive responses of lodgepole pine productivity to future changes in climate, C flux over the next century will reflect complex relationships between climate, age structure, and disturbance-recovery patterns of the landscape.

Nitrogen (N) is the major nutrient limiting plant growth and production in terrestrial ecosystems around the world. However, nutrient limitation is spatially variable, and different species within the same ecosystem may be limited by different nutrients. N constraints on plant growth have been investigated via fertilization experiments in a variety of ecosystems; however, recently burned coniferous forests are notably under-studied. Given the recent increase in fire activity across western North America, it is important to understand what limits plant growth and carbon sequestration in coniferous forests recovering from wildfire. We evaluated potential inorganic N limitation in four common native plant species, 3–5 years after stand-replacing wildfire in a lodgepole pine forest (Pinus contorta var. latifolia )i n Wyoming, USA. Granular reagent grade ammonium nitrate was added around individual plants at a rate equal to the natural background rate of net N mineralization and at 10 this rate. The grass Calamagrostis rubescens exhibited clear evidence of inorganic N limitation: above-ground biomass and shoot:root ratio increased with the high-fertilizer treatment. Nitrogen:phosphorus (N:P) ratio in un-fertilized C. rubescens plants was 16 in the high-fertilizer treatment, suggesting the onset of P limitation. The upland sedge Carex rossii and seedlings of lodgepole pine were not limited by inorganic N: neither species showed any growth response to N fertilization; N:P ratios were only slightly

Nitrogen (N) limits productivity in many coniferous forests of the western US, but the influence of postfire structure on N cycling rates in early successional stands is not well understood. We asked if the heterogeneity created by downed wood and regenerating pine saplings affected N mineralization and microbial community composition in 15-yr old lodgepole pine (Pinus contorta var. latifolia) stands established after the 1988 fires in Yellowstone National Park (Wyoming, USA). In three 0.25-ha plots, we measured annual in situ net N mineralization in mineral soil using resin cores (n = 100 per plot) under pine saplings, downed wood (legacy logs that survived the fire, and fire-killed trees that had fallen and were contacting or elevated above the ground), and in bare mineral soil. Annual in situ net N mineralization and net nitrification rates were both greater in bare mineral soil (8.4 � 0.6 and

Biome-scaledisturbancesbyeruptiveherbivoresprovidevaluableinsightsintospeciesinteractions,ecosystemfunction,andimpactsof globalchange. We present a conceptual framework using one system as a model, emphasizing interactions across levels of biological hierarchy and spatiotemporal scales. Bark beetles are major natural disturbance agents in western North American forests. However, recent bark beetle population eruptions have exceeded the frequencies, impacts, and ranges documented during the previous 125 years. Extensive host abundance and susceptibility, concentrated beetle density, favorable weather, optimal symbiotic associations, and escape from natural enemies must occur jointly for beetles to surpass a series of thresholds and exert widespread disturbance. Opposing feedbacks determine qualitatively distinct outcomes at junctures at the biochemical through landscape levels. Eruptions occur when key thresholds are surpassed, prior constraints cease to exert influence, and positive feedbacks amplify across scales. These dynamics are bidirectional, as landscape features influence how lower-scale processes are amplified or buffered. Climate change and reduced habitat heterogeneity increase the likelihood that key thresholds will be exceeded, and may cause fundamental regime shifts. Systems in which endogenous feedbacks can dominate after external forces foster the initial breach of thresholds appear particularly sensitive to anthropogenic perturbations.

Navajo Point, on the southeast tip of the Kaiparowits Plateau, supports Pinus edullis Engelm. var edulis-Juniperus osteosperma (Torrey) Little (pinon-juniper) woodlands undisturbed by large wildfires in the recent past. We developed a fire history and characterized the current fuel structure and plant biodiversity in the pinon-juniper woodlands on Navajo Point. Using a combination of 18 stand ages, stand structural characteristics classified from satellite imagery, and line intercept sampling, we determined that it would take 400–600 years to burn a cumulative area equal in extent to Navajo Point. Despite a long history of livestock grazing and fire suppression policies, the woodlands on Navajo Point still retain most of their primeval character. Specifically, the landscape patch mosaic on Navajo Point has not been fundamentally altered by 20th century fire exclusion. We conclude that the old-growth woodlands that cover at least half of Navajo Point are a natural and ecologically significant comp...

For at least two decades, expansion of low-density residential development at the wildland–urban interface has been widely recognized as a primary factor influencing the management of US national forests. We estimate the location, extent, and trends in expansion of the wildland–urban interface (WUI) in the continental United States. We mapped the WUI by determining the intersection of housing density classes computed from refined US Census data with a map of wildfire hazards based on broad forest types using definitions of WUI from the Federal Register. Our methods allowed us to provide a more spatially precise estimation of the WUI that better reflects development patterns of interest to forest land managers. We defined three wildfire hazard classes based on vegetation type. “High” severity applies to vegetation types in which stand-replacing fires dominate both historical and recent fire regimes, e.g., lodgepole pine forest. “Low” severity applies where fuels and climate foster mostly low-intensity fires, e.g., aspen-birch forest. “High (historically low or variable)” applies to vegetation types in which fires historically were of low or variable intensity, but recently have often burned at high intensity because of a century of fire exclusion, e.g., southwestern ponderosa pine forest. In 2000, the WUI that includes a 3.2 km community protection zone occupied 465,614 km 2 , and contained over 12.5 million housing units. This is an expansion of over 52% from 1970, and by 2030 the WUI is likely to expand to at least 513,670 km 2 with the greatest expansion occurring in the intermountain west states. Roughly 89% of the WUI is privately owned land and about 65% of the WUI occurs in high or high (historically low or variable) severity fire regime classes.

Spatial and temporal variability in cone production may influence post-disturbance succession, yet it is not well understood. We sampled 15year old lodgepole pine (Pinus contortavar. latifolia) stands (n = 16) that regenerated naturally after the 1988 Yellowstone fires and varied in stand density (566‐545,200 stems ha 1 ) and elevation to quantify and explain variation in cone production. Among stands, the percentage of trees bearing cones ranged from 10 to 80% and was greater in stands with larger trees and at lower elevations. Mean cone abundance was 3.7 cones tree 1 (range of 0.10‐17.85 cones tree 1 for the 16 stands) and increased with mean basal diameter and decreasing elevation. The earliest onset of cone production was 1997, and early onset was associated with lower elevations (

Understanding ecosystem processes as they relate to wildfire and vegetation dynamics is of growing importance as fire frequency and extent increase throughout the western United States. However, the effects of severe, stand-replacing wildfires are poorly understood. We studied inorganic nitrogen pools and mineralization rates after stand-replacing wildfires in the Greater Yellowstone Ecosystem, Wyoming. After fires that burned in summer 2000, soil ammonium concentration peaked in 2001 (33 mg NH 4 -N· kg soil −1 ); soil nitrate increased subsequently (2.7 mg NO 3 -N·kg soil −1 in 2003) but was still low. However, annual net ammonification rates were largely negative from 2001 to 2004, indicating ammonium depletion. Thus, although net nitrification rates were positive, annual net nitrogen mineralization (net ammonification plus net nitrification) remained low. Aboveground net primary production (ANPP) increased from 0.25 to 1.6 Mg·ha −1 ·yr −1 from 2001 to 2004, but variation in ANPP among stands was not related to net nitrogen mineralization rates. Across a broader temporal gradient (stand age zero to >250 yr), negative rates of net annual ammonification were especially pronounced in the first postfire year. Laboratory incubations using 15 N isotope pool dilution revealed that gross production of ammonium was reduced and ammonium consumption greatly exceeded gross production during the initial postfire years. Our results suggest a microbial nitrogen sink for several years after severe, stand-replacing fire, confirming earlier hypotheses about postdisturbance succession and nutrient cycling in cold, fire-dominated coniferous forests. Postfire forests in Yellowstone seem to be highly conservative for nitrogen, and microbial immobilization of ammonium plays a key role during early succession.

In response to the needs of local fire managers, we developed a map of wildfire hazard for La Plata County in southwestern Colorado, USA. Our measure of fire hazard had two components: (i) the probability, should fire occur under dry weather conditions, that fire behavior will be extreme, and (ii) the human values that may be lost or damaged if extreme fire behavior occurs. Using a classification approach in a GIS environment, we developed quantitative indices of potential heat release, flame length, and rate of spread for each vegetation type in the County. This is based on output of the Behave fire behavior system and adjusted for effects of slope and aspect. We then overlaid a map of residential developments to identify locations where homes are most vulnerable to wildfire damage. Results revealed a zone in the central part of the county where extensive exurban development is occurring within pine, juniper, and oak vegetation, leading to a high potential for extreme fire behavior. This assessment is notable in that (1) it treats a wide range of land ownership types including public lands, private lands, and American Indian reservations; (2) it encompasses substantial variability in vegetation/fuel types, from low-elevation semi-arid grasslands and woodlands to alpine forests and meadows to cultivated and residential lands; and (3) it maps wildfire hazard at relatively fine resolution (1 ha) for a large area (ca 4,500 km2) based on quantitative indices of wildfire behavior. Formal validation of this kind of map is nearly impossible, but we tested the final product by asking local experts to evaluate our interpretations for specific locations with which they were familiar. Feedback from the experts led us to revise some of our initial fire behavior indices, by developing custom fuel models. The same experts concluded that the final product was very accurate. By developing a relatively simple mapping algorithm, and drawing upon spatial data sources readily available to local land managers, our wildfire hazard map for La Plata County provides a template for more extensive fire hazard mapping throughout southwestern Colorado and portions of adjacent states, for use in prioritization of fire mitigation treatments and public education.

Little previous work has been conducted on effects of natural, high-severity wildfires on nitrogen (N) dynamics. We measured aboveground plant biomass, foliar N, and net N mineralization 2 years after stand-replacing fires in lodgepole pine (Pinus contorta var. latifolia) forests in Grand Teton National Park, Wyoming, USA. We detected a five-fold difference in foliar N (% dry weight) among 14 species, from 0.77% in the native grass Calamagrostis rubescens, to 3.4% in the native N-fixer Lupinus argenteus and the non-native forb Lactuca serriola. We also observed higher foliar N in the burned stands for four of six species that occurred in both burned and unburned areas. Mean net N mineralization ranged from −23 to +27 mg-N kg soil−1 year−1, and was predominantly NO3−. However, total biomass and foliar N, for all species combined, showed no relationships with site, fire severity, or net N mineralization—possibly because of (i) lack of inorganic N limitation, (ii) methodological shortcomings, (iii) spatial structure existing at different scales than we measured, or (iv) insufficient plant biomass at this early stage of post-fire development to respond to local variation in N availability.

In ecology textbooks prior to the 1970s, Aldo Leopold’s classic story of predator control, overpopulation of deer, and habitat degradation on the Kaibab Plateau during the 1920s epitomized predator regulation of herbivore populations. However, the story disappeared from texts in the late 20th century after several papers noted uncertainties in estimations of the deer population and provided alternative explanations. We re-examined the case study by determining the age structure of aspen (Populus tremuloides Michx.) on the plateau. Aspen comprises the majority of deer browse in the summer, and the absence of a normal cohort of aspen from the 1920s would indicate deer overpopulation. The number of aspen (at 1.4 m) dating to the 1920s was an order of magnitude lower than the null expectation. Other periods of unusual numbers of aspen included high numbers of aspen dating to the 1880s and 1890s (when regular surface fires ceased), few aspen dating from 1953 to 1962 (after a second irruption of the deer population), and very high numbers from 1968 to 1992 (coincident with widespread logging). These convergent lines of evidence support the idea of extreme deer herbivory in the 1920s, consistent with food limitation of deer at high populations (bottom–up control) and predation limitation at low deer populations (top–down control). Some uncertainty remains within the overall story, and this level of ambiguity is common in case studies that involve population ecology, land management, and people at the scale of 1,000 km2 and 100 years. A complete version of the Kaibab deer story and its history would be a valuable, realistic case study for ecology texts.

Wildfire and bark beetle epidemics are two ecologically important natural disturbances in the Intermountain West, yet we know very little about how these two phenomena interact. It is widely believed that beetle-killed trees increase the risk of severe fires; and trees that are weakened, but not killed by fire, are thought to be more susceptible to beetle invasion. However, few studies have rigorously tested these hypotheses. The GYE is currently experiencing an outbreak of unprecedented intensity and complexity, involving several species of bark beetles, including the mountain pine beetle. The outbreak is affecting multiple species of coniferous trees in and near recently burned areas, providing a timely opportunity to investigate these interactions at multiple scales.

Understanding the effect of variation in climate on large-fire occurrence across broad geographic areas is central to effective fire hazard assessment. The El Nino- Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) affect winter tem- perature and precipitation regimes in western North America through mid-latitude tele- connections. This study examines relationships of ENSO and the PDO to drought-induced fire occurrence in subalpine forests of three study areas across the Rocky Mountains: Jasper National Park (JNP, northern Rockies), Yellowstone National Park (YNP, central Rockies) and Rocky Mountain National Park (RMNP, southern Rockies) over the 1700-1975 period. Large-scale climatic anomalies captured by ENSO (NINO3) and PDO indices had differ- ential effects on large-fire occurrence across the study areas. Superposed epoch analysis (SEA) showed that large fires in RMNP occurred during extreme La Nina years, while the PDO, although predominantly negative during fire years, did not depart significantly from the mean. In YNP and JNP, neither ENSO nor PDO indices were significantly different from the mean during large-fire years, although fires tended to occur during El Nino and positive PDO years. Constructive phases (years of combined warm (positive) or cool (neg- ative) phases) of ENSO and the PDO were significantly associated with large-fire occurrence across the Rockies, even though these large-scale climatic anomalies were not significant when considered singly in SEAs. Combined warm phases (positive PDO during El Nino) co-occurred with large fires in the central and northern Rockies, while the combined cool phases (negative PDO during La Nina) appeared to promote large fires in the southern Rockies. Almost 70% of large fires in RMNP burned during La Nina events that coincided with a negative PDO, although these phases co-occurred during only 29% of the 1700- 1975 period. Spatial teleconnection patterns between drought, PDO and ENSO across west- ern North America independently support the sign and strength of relationships between these climatic anomalies and subalpine fire occurrence along a broad north-south gradient of the Rockies. Forecasts of ENSO that are dependent on the expected PDO phase suggest promise for fire hazard prediction across the West.

The 1988 Yellowstone fires resulted in a complex mosaic within which post- fire lodgepole pine seedling densities varied by over five orders of magnitude. Investigators have speculated that such postfire mosaics of vegetation structure may persist until the next large disturbance, but the fate of the initial structural variability of postfire communities is currently poorly understood. We studied lodgepole pine (Pinus contorta var. latifolia Englem. ex Wats.) stands in Yellowstone National Park (Wyoming, USA) unburned by the 1988 fires to determine how variation in stand structure changes with increasing stand age. The coefficient of variation in stand density decreased from 231% in 12-yr-old stands to 91% in stands aged 50-100 yr, and to 37% for stands ages 200-250 yr. Substantial variability in age distributions both within and among age classes suggested that both gradual infilling of initially sparse stands and self-thinning of initially dense stands are important processes affecting variation in stand density. Variation in stand density was explained primarily by stand age (P < 0.0001) and by geographic location (P < 0.01). Field estimates and re- constructions of stand density trajectories suggest the importance of biotic processes and the contingent effects that produce initial patterns of stand density. Variation in stand density is substantially reduced 125 yr following fire and remains relatively unchanged beyond approximately 200 yr. These results suggest that large, infrequent fires impose a pattern of stand structural variability that may persist for centuries, but stand density likely converges within the fire-free interval in this landscape.

Large disturbances such as the 1988 Yellowstone fires produce considerable spatial heterogeneity in ecosystem processes across landscapes, in part by affecting vegetation structure. However, the persistence of this heterogeneity with time since disturbance, and thus the role of large disturbances in shaping the heterogeneity of ecosystem processes over large spatial and temporal scales, remains unclear. Such an inquiry requires that variability as well as mean conditions of forest structure and growth be examined if changes are to be projected for heterogeneous postdisturbance landscapes. We studied a chronosequence of unburned, mature lodgepole pine stands (stand ages ranging from 50 to 300 or more years) to examine the variability in stand density, leaf-area index (LAI), and stem growth [basal area increment (BAI), a surrogate for aboveground net primary productivity (ANPP)] with stand age, the relationships between these factors, and how these factors were related to stand and site characteristics. Variation in LAI and BAI was explained primarily by differences in stand density and age (r 2 = 0.51 for both LAI and BAI), and both LAI and BAI were most variable in the youngest age class [coefficient of variation (CV), 38% and 41% for LAI and BAI]. The relationship between LAI or BAI and stand density was significantly weaker (r 2 < 0.20) at stand ages characterized by canopy closure (50–175 years), suggesting that stand structure and production are closely linked. Thus, the spatial variability of stand production, which is initially very high following large fires in this landscape, is detectable for over a century before successional changes in forest structure greatly affect the initial postdisturbance landscape pattern of stand production. Given the recent focus on spatial heterogeneity of ecosystem processes across large landscapes, projecting changes in postdisturbance patterns of stand production has very strong significance for ecosystem science.

Understanding succession following severe wildfire is increasingly important for forest managers in western North America and critical for anticipating the resilience of forested landscapes to changing environmental conditions. Successional trajectories set the stage for future carbon storage, abundance and distribution of fuels, and habitat for many species. Early successional forests are increasing throughout the West in response to greater fire activity, but few long-term studies have considered succession following stand-replacing wildfires over large areas. The size and heterogeneity of the 1988 Yellowstone fires created novel opportunities to study succession at an unprecedented scale following severe fire, and we have studied the consequences of these fires for >20 years. In 2012, we began a re-sampling effort in long-term vegetation plots within the area burned by the 1988 fires to answer three overarching questions: (1) Are stand structure and function beginning to converge twenty-five years after the Yellowstone Fires, and what mechanisms may contribute to convergence or divergence? Heterogeneity in forest structure was the rule after the 1988 fires, and postfire lodgepole pine (Pinus contorta var. latifolia) densities ranged from zero to >500,000 trees/ha. The post-1988 cohort of lodgepole pine is reaching a time of critical transitions in structure and function. (2) Are plant community composition and species richness converging or diverging across gradients in local fire severity, post-fire lodgepole pine density, elevation and soil type a quarter-century after the 1988 fires? A central objective in our research has been to understand the relative influence of contingent factors (e.g., local fire severity) vs. deterministic factors (e.g., elevation, soils) on postfire ecosystem development, and how these influences may change through time. (3) How do canopy and surface fuels vary across the postfire landscape, and how will the variation in fuels influence potential fire behavior a quarter century post-fire? Field sampling was conducted for this third question during summer 2012, and data analyses and interpretation are in progress. Overall, results from the proposed study will enhance understanding of succession after one of the most notorious fires of the 20th century. Yellowstone’s postfire forests may serve as benchmarks for forests throughout the region and effective sentinels of change for the Rockies.

Causes and implications of spatial variability in postfire tree density and understory plant cover for patterns of aboveground net primary production (ANPP) and leaf area index (LAI) were examined in ninety 11-year-old lodgepole pine (Pinus contorta var. latifolia Engelm.) stands across the landscape of Yellowstone National Park (YNP), Wyoming, USA. Field studies and aerial photography were used to address three questions: (1) What is the range and spatial pattern of lodgepole pine sapling density across the burned Yellowstone landscape and what factors best explain this variability? (2) How do ANPP and LAI vary across the landscape and is their variation explained by abiotic factors, sapling density, or both? (3) What is the predicted spatial pattern of ANPP and LAI across the burned Yellowstone landscape? Stand density spanned six orders of magnitude, ranging from zero to 535,000 saplings ha )1 , and it decreased with increasing elevation and with increasing distance from unburned forest (r 2 = 0.37). Postfire densities mapped

The fire history of Pinon–Juniper ( Pinus edulis – Juniperus osteosperma ) woodlands in much of the southwestern United States is poorly understood, and as a result, fire management decisions are being made without a rigorous ecological underpinning. We investigated the historic fire regimes in Pinon–Juniper woodlands on the Mesa Verde cuesta utilizing stand and age structures. All Pinon trees in eight stands were aged and stand age was extrapolated to the surrounding landscapes using digital imagery, creating a time-since-fire map of the 1995 landscape. Six sampled stands were over 400 years, while two were between 200 and 300 years. Stand-replacing fire with a rotation of 400 years or longer characterized this Pinon–Juniper landscape before 1995; low-severity surface fires apparently have never been an important component of the fire regime in Mesa Verde. Superposed epoch analysis revealed that large fires occur following significantly low precipitation in May or in the winter (October–March) preceding a summer fire season. Since the mid-1990’s, a severe drought has characterized climate in this region, but the recent drought is similar to previous drought periods since 1950. A combination of canopy fuel build-up during two wet decades before 1995 and the current drought conditions has resulted in unprecedented fire activity (six large wildfires between 1996 and 2003) when compared with the reference period 1700–1900. We may be witnessing a unique period in the ecological history of the southwest, a period when vegetation patterns are being altered over extensive areas within a very short time. If the current drought continues, we ultimately may lose much of the old-growth Pinon–Juniper woodland in the southwest. We emphasize however, that these changes are not due to fire suppression or other direct human intervention but rather result from natural ecological responses to climatic variability. Therefore, our data provide no ecological justification for aggressive management activities such as mechanical fuel reduction or prescribed burning, except in the immediate vicinity of vulnerable cultural resources.

Fire severity, frequency, and extent are expected to change dramatically in coming decades in response to changing climatic conditions, superimposed on the adverse cumulative effects of various human-related disturbances on ecosystems during the past 100 years or more. To better gauge these expected changes, knowledge of climatic and human influences on past fire regimes is essential. We characterized the temporal and spatial properties of fire regimes in ponderosa pine forests of the southern San Juan Mountains of southwestern Colorado by collecting 175 fire-scarred tree samples from nine sites across a wide range of topographic settings. All tree rings and fire scars were dated using standard dendrochronological techniques. Fire-free intervals were statistically modeled using the Weibull distribution to provide quantitative measures that characterized the historical range of variation in pre-EuroAmerican fire regimes. Fires during our reference period were more frequent in the low elevation ponderosa pine forests (6-10 yr) than in the high elevation, mixed conifer forests (18-28 yr). Fires at lower elevations were predominantly low-severity, isolated fires. Fires during some years (e.g., 1748) were spatially extensive throughout the entire mountain range. Intervals that delimited significantly long fire-free periods ranged from 10-19 yr (low elevation) to 27- 50 yr (high elevation). Fire histories were similar between the eastern and western portions of the mountain range, although we found significant evidence of topographic isolation on fire regimes at one site. Pre-1880 fires primarily occurred in the dormant season, and we found no temporal changes in past fire seasonality. We found no compelling evidence that Native Americans influenced fire regimes in our study sites. We found a hiatus in fire occurrence between 1750 and 1770 that we believe was likely related to weakened El Nino-Southern Oscillation activity, an extended series of cool-phase Pacific Decadal Oscillation events, and weakened monsoonal moisture, all possibly en- trained in an invasive air mass typical of locations that are more northerly. In addition, pre-1880 fires occurred during years of severe drought, conditioned by above average moisture conditions in preceding years. The 20th century is characterized by a near complete absence of fires (fire-free interval of .100 yr), suggesting future wildfires may be more widespread and ecologically severe compared to pre-1880 fires.