Peter M. Brown

Contingent Pacific–Atlantic Ocean influence on multicentury wildfire synchrony over western North America

Widespread synchronous wildfires driven by climatic variation, such as those that swept western North America during 1996, 2000, and 2002, can result in major environmental and societal impacts. Understanding relationships between continental-scale patterns of drought and modes of sea surface temperatures (SSTs) such as El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) may explain how interannual to multidecadal variability in SSTs drives fire at continental scales. We used local wildfire chronologies reconstructed from fire scars on tree rings across western North America and independent reconstructions of SST developed from tree-ring widths at other sites to examine the relationships of multicentury patterns of climate and fire synchrony. From 33,039 annually resolved fire-scar dates at 238 sites (the largest paleofire record yet assembled), we examined forest fires at regional and subcontinental scales. Since 1550 CE, drought and forest fires covaried across the West, but in a manner contingent on SST modes. During certain phases of ENSO and PDO, fire was synchronous within broad subregions and sometimes asynchronous among those regions. In contrast, fires were most commonly synchronous across the West during warm phases of the AMO. ENSO and PDO were the main drivers of high-frequency variation in fire (interannual to decadal), whereas the AMO conditionally changed the strength and spatial influence of ENSO and PDO on wildfire occurrence at multidecadal scales. A current warming trend in AMO suggests that we may expect an increase in widespread, synchronous fires across the western U.S. in coming decades.

Drought frequency in central California since 101 B.C. recorded in giant sequoia tree rings

Well replicated tree-ring width index chronologies have been developed for giant sequoia at three sites in the Sierra Nevada, California. Extreme low-growth events in these chronologies correspond with regional drought events in the twentieth century in the San Joaquin drainage, in which the giant sequoia sites are located. This relationship is based upon comparison of tree-ring indices with August Palmer Drought Severity Indices for California Climate Division 5. Ring-width indices in the lowest decile from each site were compared. The frequency of low-growth events which occurred at all three sites in the same year is reconstructed from 101 B.C. to A.D. 1988. The inferred frequency of severe drought events changes through time, sometimes suddenly. The period from roughly 1850 to 1950 had one of the lowest frequencies of drought of any one hundred year period in the 2089 year record. The twentieth century so far has had a below-average frequency of extreme droughts.

Was Aldo Leopold Right about the Kaibab Deer Herd

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.

Climatic Inferences from Dendroecological Reconstructions

Tree rings have long been employed by ecologists to study the local-scale dynamics of forest stands and woodlands, but only recently have network approaches been applied to evaluate regional and broader-scale processes. As with dendroclimatic data (e.g., ring-width and ring-density chronologies), climatic drivers become much more evident in dendroecological data aggregated at broad spatial scales (relative to local-scale data). Study of dendroecology networks has led to new insights on climatic variability and change and their impacts on ecosystems. In addition to the power of network approaches, dendroecology has advanced in recent decades because of the ready availability of, and comparison with, high quality, independent dendroclimatic reconstructions of various hydro-climatic parameters (e.g., drought indices, precipitation and temperature) and ocean-atmosphere indices (e.g., ENSO, PDO, and AMO). Dendroecological reconstructions that have been most commonly employed in climate-related analyses are disturbance histories (e.g., fire and insect outbreaks). We review examples of these applications from our studies in the Southwestern United States. We also compile and describe here, for the first time, a regional network of ponderosa pine (Pinus ponderosa) establishment dates from the Southwest, and we show that episodic natality patterns are probably associated with decadal wet periods. Using another example of decadal variability in forest fire histories—specifically a hiatus in fire occurrence in the circa 1780–1840 time period—we make a case that regional to continental-scale dendroecological reconstructions can provide useful insights about ‘ecologically-effective climate change’. We define this type of climatic variability as the patterns of climate at interannual, decadal and centennial scales that are most distinctly reflected in synchronous ecological responses at regional and broader scales. In the context of dendroclimatology, primary values of the investigation of specific climatic patterns that elicit regional and broader ecological responses is that these studies provide insights about climate variability that is relevant to ecosystems, and in turn, human concerns about future climate change impacts on ecosystems.

Reconstructing fire history in central Mongolia from tree-rings

Rising temperatures are expected to increase wildfire activity in many regions of the world. Over the last 60 years in Mongolia, mean annual temperatures have increased ,28C and the recorded frequency and spatial extent of forest and steppe fires have increased. Few long records of fire history exist to place these recent changes in a historical perspective. The purpose of this paper is to report on fire history research from three sites in central Mongolia and to highlightthepotentialofthisregionasatestcaseforunderstandingtherelationshipsbetweenclimatechange,fireandland use.Wecollectedpartialcross-sectionsfromfire-scarredtreesandstumpsateachsiteusingatargetedsamplingapproach. All three sites had long histories of fire ranging from 280 to 450 years. Mean Weibull fire return intervals varied from 7 to 16 years. Fire scars at one protected-area site were nearly absent after 1760, likely owing to changes in land use. There is limited synchrony in fire occurrence across sites, suggesting that fire occurrence, at least at annual time scales, might be influenced by local processes (grazing, human ignitions, other land-use factors) as well as regional processes like climate. Additional data are being collected to further test hypotheses regarding climate change, land use and fire.

Holocene and historical vegetation change and fire history on the north-central coast of California, USA

Pollen, non-pollen palynomorphs (NPPs), and charcoal particle stratigraphies are used to determine environmental change at Glenmire, Point Reyes Peninsula, northcentral coastal California, over the last c. 6200 years. Pollen was not preserved in early Holocene sediments when climate was drier than present. However, groundwater tables rose after c. 6200 cal. BP, allowing for greater subsequent preservation of organic matter. Middle and late Holocene environments were a mosaic of vegetation types, including mixed conifer forest with coastal scrub grassland prior to c. 4000 cal. BP. Subsequently, hardwoods such as alder (Alnus) and coastal scrub (e.g. Artemisia, Baccharis) expanded until c. 2200 cal. BP, followed by tanoak (Lithocarpus densiflorus), Douglas fir (Pseudotsuga menziesii), and coast redwood (Sequoia sempervirens). With increasing amounts of oak (Quercus), this mosaic of vegetation types continued to dominate until the arrival of Euro-Americans in the early to mid-1800s. The fire history is probably tied closely to human settlement, since natural ignitions are rare. Elevated charcoal amounts coincide with increased sedentism of the native populations by about 3500 cal. BP. Increased sedentism may have caused a more intense and constant use of the coastal environment around Glenmire. For the most recent centuries, we compared historical records of explorations, Spanish Mission establishment, consolidation of the native Coast Miwok population, ranching by Mexican nationals, and dairying by Americans at the height of California’s gold rush with the paleoecological record. The Glenmire record thus documents changing fire use following the ad 1793 fire suppression proclamation; declines in native forest species; introductions of non-native species, including those associated with livestock grazing and land disturbance; and an increase in coprophilous fungi (NPPs) associated with the presence of large numbers of sheep and cattle, among other changes. During the historical period, the sedimentary record of historical fires closely matches the nearby fire-scar tree-ring record.

Multicentury fire and forest histories at 19 sites in Utah and eastern Nevada

Our objective is to provide site-specific fire and forest histories from Utah and eastern Nevada that can be used for land management or additional research. We systematically sampled fire scars and tree-recruitment dates across broad gradients in elevation and forest type at 13 sites in Utah and 1 in eastern Nevada to characterize spatial and temporal variation in historical fire regimes as well as forest structure and composition. We collected similar data non-systematically at five additional sites in Utah. These 19 sites include a broad range of forest types (from pinyon-juniper woodlands to spruce-fir forests) and fire regime types. In this report, we summarize local-scale spatial and temporal variation with site-specific details of historical fire regimes and forests that will be useful for local natural resource and fire management of the individual sites. For each site, we report topography, chronologies of fire and tree recruitment, and properties derived from those chronologies such as time-averaged fire regime parameters (mean fire interval and fire severity) and changes in forest composition and structure that have occurred since the late 1800s.