Geology topics

Peter M. Brown

Publications and source records attributed to Peter M. Brown.

5 recordsLinked to original sources

USGS2020

Fire history across forest types in the southern Beartooth Mountains, Wyoming

Fire is a critical ecosystem process that has played a key role in shaping forests throughout the Beartooth Mountains in northwestern Wyoming. The highly variable topography of the area provides ideal conditions to compare fire regimes across contiguous forest types, yet pyro-dendrochronological research in this area is limited. We reconstructed fire frequency, tree age structure, and post-fire tree growth response in the Clarks Fork Ranger District of the Shoshone National Forest to infer variations in historical fire behavior and stand effects. We collected fire-scarred trees and plot-based tree ages on plots ranging 0.5-5 km 2 in size across two forest types separated by 2 km: a lower-elevation forest of mixed Douglas-fir and lodgepole pine and a higher elevation treeline forest dominated by whitebark pine. Fires occurred in the lower-elevation forest in 1664, 1706, 1785, 1804, 1846, and 1900 with a mean fire return interval of 47 years. The fires in 1804 and 1900 were also recorded in the higher elevation forest, with significant tree mortality at high elevation in the 1900 fire. Both forests were multi-aged with little evidence of tree cohorts in response to severe, stand-replacing events. On average, tree growth increased after fires, with mean ring widths after fire 39% wider in Douglas-fir and 40% wider in lodgepole pine than pre-fire averages, suggesting that some tree mortality likely occurred in association with lower-elevation forest fires. Burns were more frequent in the lower-elevation forest and were occasionally able to spread into the upper-elevation whitebark stand. Although we suspect the transition of fires from low-to high-elevation occurred during drier years, we did not find any relationship between fire years and available climatic reconstructions via superposed epoch analysis. Regeneration during the 20 th Century in the whitebark forest documents recovery of this forest after the 1900 moderate-severity fire event. Finally, especially in the lower-elevation Douglas-fir forest, the period since the last recorded fire (1900) appears to be longer than any fire-free period in the historical record, suggesting that fire exclusion may be creating changes in landscape and patch-scale stand structures, which will likely impact future fire behavior, especially the extent of crown-replacing fire, in these forests.

Wyoming↗

USGS2018

Advancing dendrochronological studies of fire in the United States

Dendroecology is the science that dates tree rings to their exact calendar year of formation to study processes that influence forest ecology (e.g., Speer 2010 , Amoroso et al., 2017 ). Reconstruction of past fire regimes is a core application of dendroecology, linking fire history to population dynamics and climate effects on tree growth and survivorship. Since the early 20th century when dendrochronologists recognized that tree rings retained fire scars (e.g., Figure 1) , and hence a record of past fires, they have conducted studies worldwide to reconstruct the historical range and variability of fire regimes (e.g., frequency, severity, seasonality, spatial extent), the influence of fire regimes on forest structure and ecosystem dynamics, and the top-down (e.g., climate) and bottom-up (e.g., fuels, topography) drivers of fire that operate at a range of temporal and spatial scales. As in other scientific fields, continued application of dendrochronological techniques to study fires has shaped new trajectories for the science. Here we highlight some important current directions in the United States (US) and call on our international colleagues to continue the conversation with perspectives from other countries.

Fire↗

USGS2017

A high-elevation, multi-proxy biotic and environmental record of MIS 6-4 from the Ziegler Reservoir fossil site, Snowmass Village, Colorado, USA

In North America, terrestrial records of biodiversity and climate change that span Marine Oxygen Isotope Stage (MIS) 5 are rare. Where found, they provide insight into how the coupling of the ocean–atmosphere system is manifested in biotic and environmental records and how the biosphere responds to climate change. In 2010–2011, construction at Ziegler Reservoir near Snowmass Village, Colorado (USA) revealed a nearly continuous, lacustrine/wetland sedimentary sequence that preserved evidence of past plant communities between ~ 140 and 55 ka, including all of MIS 5. At an elevation of 2705 m, the Ziegler Reservoir fossil site also contained thousands of well-preserved bones of late Pleistocene megafauna, including mastodons, mammoths, ground sloths, horses, camels, deer, bison, black bear, coyotes, and bighorn sheep. In addition, the site contained more than 26,000 bones from at least 30 species of small animals including salamanders, otters, muskrats, minks, rabbits, beavers, frogs, lizards, snakes, fish, and birds. The combination of macro- and micro-vertebrates, invertebrates, terrestrial and aquatic plant macrofossils, a detailed pollen record, and a robust, directly dated stratigraphic framework shows that high-elevation ecosystems in the Rocky Mountains of Colorado are climatically sensitive and varied dramatically throughout MIS 5.

Colorado↗

USGS2013

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

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

Arizona, Colorado, Oregon↗

USGS2009

Historical and modern disturbance regimes, stand structures, and landscape dynamics in piñon-juniper vegetation of the western United States

Piñon–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 piñons, 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 piñon–juniper vegetation. Persistent woodlands are found where local soils, climate, and disturbance regimes are favorable for piñon, juniper, or a mix of both; fires have always been infrequent in these woodlands. Piñon–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 piñon–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 CO 2 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.

Rangeland Ecology and Management↗