William H. Romme

Bio: William H. Romme is an academic researcher from Colorado State University. The author has contributed to research in topics: Pinus contorta & National park. The author has an hindex of 56, co-authored 136 publications receiving 15651 citations. Previous affiliations of William H. Romme include Prescott College & Fort Lewis College.

Regional vegetation die-off in response to global-change-type drought

Abstract: Future drought is projected to occur under warmer temperature conditions as climate change progresses, referred to here as global-change-type drought, yet quantitative assessments of the triggers and potential extent of drought-induced vegetation die-off remain pivotal uncertainties in assessing climate-change impacts. Of particular concern is regional-scale mortality of overstory trees, which rapidly alters ecosystem type, associated ecosystem properties, and land surface conditions for decades. Here, we quantify regional-scale vegetation die-off across southwestern North American woodlands in 2002-2003 in response to drought and associated bark beetle infestations. At an intensively studied site within the region, we quantified that after 15 months of depleted soil water content, >90% of the dominant, overstory tree species (Pinus edulis, a pinon) died. The die-off was reflected in changes in a remotely sensed index of vegetation greenness (Normalized Difference Vegetation Index), not only at the intensively studied site but also across the region, extending over 12,000 km2 or more; aerial and field surveys confirmed the general extent of the die-off. Notably, the recent drought was warmer than the previous subcontinental drought of the 1950s. The limited, available observations suggest that die-off from the recent drought was more extensive than that from the previous drought, extending into wetter sites within the tree species' distribution. Our results quantify a trigger leading to rapid, drought-induced die-off of overstory woody plants at subcontinental scale and highlight the potential for such die-off to be more severe and extensive for future global-change-type drought under warmer conditions.

Cross-scale Drivers of Natural Disturbances Prone to Anthropogenic Amplification: The Dynamics of Bark Beetle Eruptions

Abstract: 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.

The Interaction of Fire, Fuels, and Climate across Rocky Mountain Forests

Abstract: Understanding the relative influence of fuels and climate on wildfires across the Rocky Mountains is necessary to predict how fires may respond to a changing climate and to define effective fuel management approaches to controlling wildfire in this increasingly populated region. The idea that decades of fire suppression have promoted unnatural fuel accumulation and subsequent unprecedentedly large, severe wildfires across western forests has been developed primarily from studies of dry ponderosa pine forests. However, this model is being applied uncritically across Rocky Mountain forests (e.g., in the Healthy Forests Restoration Act). We synthesize current research and summarize lessons learned from recent large wildfires (the Yellowstone, Rodeo-Chediski, and Hayman fires), which represent case studies of the potential effectiveness of fuel reduction across a range of major forest types. A “one size fits all” approach to reducing wildfire hazards in the Rocky Mountain region is unlikely to be effective and may produce collateral damage in some places.

Fire and landscape diversity in subalpine forests of yellowstone national park

Abstract: Fire history was determined by fire scar analysis in a 73—km2 subalpine watershed in Yellowstone National Park, Wyoming, USA. Evidence was found for 15 fires since 1600, of which 7 were major fires that burned >4 ha, destroyed the existing forest, and initiated secondary succession. Most of the upland forest area was burned by large, destructive fires in the middle and late 1700's. Fires since then have been small and have occurred at long intervals. Fire frequency in this area is partly controlled by changes in the fuel complex during succession. Fuels capable of supporting a crown fire usually do not develop until a stand is 300—400 yr old, and ignitions prior to that time usually extinguish naturally before covering more than a few hectares. Thereafter a destructive crown fire is likely whenever lightning ignites small fuels during warm, dry, windy weather. On the extensive subalpine plateaus of Yellowstone National Park there appears to be a natural fire cycle of 300—400 yr in which large areas burn during a short period, followed by a long, relatively fire—free period during which a highly flammable fuel complex again develops. The 73—km2 study area appears to be about midway between major fire events in this cycle. This, rather than human fire suppression, apparently is the major reason for the small number and size of fires in the area during the last 180 yr. On the basis of the fire history data, the sequence of vegetation mosaics during the last 200 yr was reconstructed for the watershed. Indices of landscape diversity were computed for each reconstruction, treating forest types and successional stages as taxa and incorporating components of richness, evenness, and patchiness. Landscape diversity was highest in the early 1800's following the large fires in the 1700's, then declined in the late 1800's during a 70—yr period when no major fires occurred and the landscape was dominated by even—aged forests developing on the areas burned in the 1700's. Landscape diversity has increased somewhat during the last half—century as a result of two small fires and the effects of the mountain pine beetle. These landscape reconstructions for the last 200 yr suggest that the Yellowstone subalpine ecosystem is a nonsteady—state system characterized by long—term, cyclic changes in landscape composition and diversity. Such cyclic patterns may significantly influence wildlife habitat, streamflow, nutrient cycling, and other ecological processes and characteristics within the Park, and they may be an important consideration in judging whether recent ecological changes are natural or man induced. The landscape reconstructions were also made using a simulation model based on hypothetical fire management policies of total fire exclusion and selective fire control (permitting only small fires to burn). These hypothetical management policies generally reduced the richness and patchiness of the landscape compared to the natural fire regime, but they increased the evenness and reduced the magnitude of periodic fluctuations in overall landscape diversity. At times, overall landscape diversity may actually be higher with a fire control policy than with a natural fire regime. At other times, fire significantly increases landscape diversity, as would be expected.

Continued warming could transform Greater Yellowstone fire regimes by mid-21st century

Abstract: 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 <30 y from the historical 100-300 y for most of the GYE. Years without large fires were common historically but are expected to become rare as annual area burned and the frequency of regionally synchronous fires increase. Our findings suggest a shift to novel fire-climate-vegetation relationships in Greater Yellowstone by midcentury because fire frequency and extent would be inconsistent with persistence of the current suite of conifer species. The predicted new fire regime would transform the flora, fauna, and ecosystem processes in this landscape and may indicate similar changes for other subalpine forests.

The Theory of Island Biogeography

Abstract: Preface to the Princeton Landmarks in Biology Edition vii Preface xi Symbols Used xiii 1. The Importance of Islands 3 2. Area and Number of Speicies 8 3. Further Explanations of the Area-Diversity Pattern 19 4. The Strategy of Colonization 68 5. Invasibility and the Variable Niche 94 6. Stepping Stones and Biotic Exchange 123 7. Evolutionary Changes Following Colonization 145 8. Prospect 181 Glossary 185 References 193 Index 201

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A Large and Persistent Carbon Sink in the World’s Forests

Abstract: The terrestrial carbon sink has been large in recent decades, but its size and location remain uncertain. Using forest inventory data and long-term ecosystem carbon studies, we estimate a total forest sink of 2.4 ± 0.4 petagrams of carbon per year (Pg C year–1) globally for 1990 to 2007. We also estimate a source of 1.3 ± 0.7 Pg C year–1 from tropical land-use change, consisting of a gross tropical deforestation emission of 2.9 ± 0.5 Pg C year–1 partially compensated by a carbon sink in tropical forest regrowth of 1.6 ± 0.5 Pg C year–1. Together, the fluxes comprise a net global forest sink of 1.1 ± 0.8 Pg C year–1, with tropical estimates having the largest uncertainties. Our total forest sink estimate is equivalent in magnitude to the terrestrial sink deduced from fossil fuel emissions and land-use change sources minus ocean and atmospheric sinks.