R. Scott Anderson

Long-term perspective on wildfires in the western USA

Understanding the causes and consequences of wildfires in forests of the western United States requires integrated information about fire, climate changes, and human activity on multiple temporal scales. We use sedimentary charcoal accumulation rates to construct long-term variations in fire during the past 3,000 y in the American West and compare this record to independent fire-history data from historical records and fire scars. There has been a slight decline in burning over the past 3,000 y, with the lowest levels attained during the 20th century and during the Little Ice Age (LIA, ca. 1400–1700 CE [Common Era]). Prominent peaks in forest fires occurred during the Medieval Climate Anomaly (ca. 950–1250 CE) and during the 1800s. Analysis of climate reconstructions beginning from 500 CE and population data show that temperature and drought predict changes in biomass burning up to the late 1800s CE. Since the late 1800s , human activities and the ecological effects of recent high fire activity caused a large, abrupt decline in burning similar to the LIA fire decline. Consequently, there is now a forest “fire deficit” in the western United States attributable to the combined effects of human activities, ecological, and climate changes. Large fires in the late 20th and 21st century fires have begun to address the fire deficit, but it is continuing to grow.

Extended megadroughts in the southwestern United States during Pleistocene interglacials

The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern. Multi-year droughts during the instrumental period and decadal-length droughts of the past two millennia were shorter and climatically different from the future permanent, ‘dust-bowl-like’ megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C4 plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ∼2 °C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4–6 °C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase.

Middle- and late-Wisconsin paleobotanic and paleoclimatic records from the southern Colorado Plateau, USA

The Colorado Plateau is a distinct physiographic province in western North America, which presently straddles the transition between summer-wet and summer-dry climatic regimes to the south and northwest, respectively. In addition to climate, the diversity of environments and plant communities on the Colorado Plateau has resulted from extreme topographic diversity. Desert lowlands as low as 360 m elevation are surrounded by forested plateaus, and even higher peaks greater than 3800 m elevation. This environmental diversity provides a unique opportunity to study the history of biotic communities in an arid region of North America. Although the Colorado Plateau harbours numerous potential sites, the paleoecological record of the Plateau is poorly known. Potential deposits for analysis include packrat middens, alluvial and cave sites at lower elevations, and lake, bog and wetland sites at higher elevations. Forty-six sites have been analysed across the nearly 337,000 km2 region, of which 27 contain records that span Marine Oxygen Isotope Stage (IS) 2 data, with IS 3 information coming from only 12 sites. Most IS 2 and 3 sites are clustered along the lowland regions of the Colorado River corridor and the uplands of the Mogollon Rim area. We compiled selected data from long paleoecological records to examine patterns of vegetation and climate change across the southern Colorado Plateau for the middle and late Wisconsin. During the middle Wisconsin, mixed conifers covered middle-elevations presently dominated by ponderosa pine (Pinus ponderosa), and juniper (Juniperus) woodland grew at elevations today covered by blackbrush (Coleogyne) and sagebrush (Artemisia) desert. During the late Wisconsin, boreal conifers, primarily Engelmann spruce (Picea engelmannii), replaced the mixed conifer association. Estimates of mean annual temperatures (MAT) during IS 3 were at least 3–4°C cooler than today, whereas IS 2 MAT estimates are at least 5°C colder. Our investigation of millennial-scale climatic variability within the region provided equivocal results. The packrat midden sequence could not distinguish vegetation changes that might be associated with Heinrich events in the North Atlantic. From the lake records, however, many Heinrich events were associated with generally drier intervals, often with elevated sagebrush pollen concentrations. Future paleoecological investigations should concentrate on the northern Colorado Plateau, as well as the eastern and western margins. Additional sites, along with closer-spaced sampling in regions already studied, will be important in determining the history of important climatic phenomena such as the timing of the Arizona monsoon.

Paleoclimatic interpretations of meadow sediment and pollen stratigraphies from California

A comparison of sediment stratigraphies from nine meadow cores in the Sierra Nevada, California, with fossil pollen reconstructions of paleoclimates shows excellent correlation between these parameters and confirms the results of recent paleoclimatic models. Colluvium and alluvium dominate early Holocene sedimentary deposits, and undoubtedly originated from greater erosion on dry basin side slopes. Fossil pollen suggests that species characteristic of dry microclimates flourished. The transition to peats after ∼4.5 ka is concurrent with increases in pollen of plants that require abundant soil moisture during the dry summer. These results confirm the importance of analysis of the physical stratigraphy of Holocene deposits in pollen-based reconstructions.

Late Wisconsin paleoecologic record from Swamp Lake, Yosemite National Park, California

Abstract A 7.86-m sediment core from Swamp Lake in Yosemite National Park, California, provides a continuous record of environmental change over the last ca. 16,000 yr, as inferred from pollen, macrofossil, and microscopic charcoal analyses. The core stratigraphy documents late Wisconsin (Tioga stage) deglaciation between >16,000 and 13,700 yr B.P., approximately 6000-3500 yr earlier than higher-elevation Sierra Nevada records. The core includes five volcanic ash layers, chemically identified as four Mono Craters ashes and the Tsoyawata ash (Mt. Mazama, Oregon). The fossil record shows that herbs and sagebrush dominated the glacial environment at Swamp Lake. By 12,000 yr B.P., a mixed conifer forest composed of high- and mid-elevation conifers grew around the lake, suggesting a cool, wet late-glacial environment. The modern Sierra montane forest did not become established until ca. 10,400 yr B.P., when maximum charcoal concentrations and minimum fir pollen percentages indicate an early Holocene xeric period. The record suggests that a cooling trend began ca. 6500 yr B.P. and persisted until ca. 3700 yr B.P. when the modern climatic regime was established.

Paired charcoal and tree-ring records of high-frequency Holocene fire from two New Mexico bog sites

Two primary methods for reconstructing paleofire occurrence include dendrochronological dating of fire scars and stand ages from live or dead trees (extending back centuries into the past) and sedimentary records of charcoal particles from lakes and bogs, providing perspectives on fire history that can extend back for many thousands of years. Studies using both proxies have become more common in regions where lakes are present and fire frequencies are low, but are rare where high-frequency surface fires dominate and sedimentary deposits are primarily bogs and wetlands. Here we investigate sedimentary and fire-scar records of fire in two small watersheds in northern New Mexico, in settings recently characterised by relatively high-frequency fire where bogs and wetlands (Chihuahuenos Bog and Alamo Bog) are more common than lakes. Our research demonstrates that: (1) essential features of the sedimentary charcoal record can be reproduced between multiple cores within a bog deposit; (2) evidence from both fire-scarred trees and charcoal deposits documents an anomalous lack of fire since ~1900, compared with the remainder of the Holocene; (3) sedimentary charcoal records probably underestimate the recurrence of fire events at these high-frequency fire sites; and (4) the sedimentary records from these bogs are complicated by factors such as burning and oxidation of these organic deposits, diversity of vegetation patterns within watersheds, and potential bioturbation by ungulates. We consider a suite of particular challenges in developing and interpreting fire histories from bog and wetland settings in the Southwest. The identification of these issues and constraints with interpretation of sedimentary charcoal fire records does not diminish their essential utility in assessing millennial-scale patterns of fire activity in this dry part of North America.

Anthropogenic impact and lead pollution throughout the Holocene in Southern Iberia.

Present day lead pollution is an environmental hazard of global proportions. A correct determination of natural lead levels is very important in order to evaluate anthropogenic lead contributions. In this paper, the anthropogenic signature of early metallurgy in Southern Iberia during the Holocene, more specifically during the Late Prehistory, was assessed by mean of a multiproxy approach: comparison of atmospheric lead pollution, fire regimes, deforestation, mass sediment transport, and archeological data. Although the onset of metallurgy in Southern Iberia is a matter of controversy, here we show the oldest lead pollution record from Western Europe in a continuous paleoenvironmental sequence, which suggests clear lead pollution caused by metallurgical activities since ~3900 cal BP (Early Bronze Age). This lead pollution was especially important during Late Bronze and Early Iron ages. At the same time, since ~4000 cal BP, an increase in fire activity is observed in this area, which is also coupled with deforestation and increased erosion rates. This study also shows that the lead pollution record locally reached near present-day values many times in the past, suggesting intensive use and manipulation of lead during those periods in this area.

Comparison of pollen and macrofossils in packrat (Neotama) middens: A chronological sequence from the Waterman mountains of southern Arizona, U.S.A.

Abstract Pollen percentages in moss polsters and soil samples were compared with percent plant cover along 30-m transects in desertscrub vegetation, Waterman Mountains, Arizona, in the northeastern Sonoran Desert. These data were used to interpret pollen assemblages and associated plant macrofossils from 14 packrat (Neotoma sp.) middens radiocarbon dated from 1320 to 22,450 yr B.P. A macrofossil: pollen index (MPI) was used to compare the presence and relative abundances of pollen and macrofossils in midden samples. In general, pollen and macrofossils from Sonoran Desert middens reflect similar vegetation signals. Calculation of Sorensens Similarity Index (SI) comparing the presence of midden pollen and macrofossils indicates greater similarities for Sonoran desertscrub than pinyon-juniper woodland. SI values from the Waterman series are greater for all age samples, compared to samples from other desert locations, perhaps reflecting greater species richness and summer rainfall in the northeastern Sonoran Desert. The pollen and macrofossil assemblages reflect a middle and late Wisconsin (22,450-ca. 11,500 yr B.P.) pinyon-juniper woodland with sagebrush and Joshua tree on limestone slopes. A transitional early Holocene juniper woodland/chaparral was replaced by 8900 yr B.P. by a mesic Sonoran desertscrub of saguaro, catclaw acacia, blue paloverde and velvet mesquite. The presence of the latter three on exposed rocky locations, now restricted mostly to riparian washes, indicates moister conditions during the middle Holocene than today. Essentially modern vegetation developed by ca. 4000 yr B.P. Changes over the last ca. 1300 years suggest that modern climate is the most xeric in the entire record.

Fire Climatology in the western United States: Introduction to special issue

Advances in fire climatology have derived from recent studies of modern and paleoecological records. We convened a series of workshops and a conference session to report and review regional-scale findings, and these meetings led to the 10 papers in this special issue. Two papers focus on fire and climate patterns in the modern era using documentary records, four papers utilise tree rings to evaluate recent centuries of change, and four papers evaluate charcoal and pollen in lake, bog, and alluvial sediments over the Holocene. Here we summarise some of the key findings from these papers in the context of other recent fire climatology literature. These studies illustrate the value of long-term perspectives and spatial networks of fire and climate data in discovering the patterns and modes of past fire regime and climate variations.

Vegetation history and paleoclimates of the coastal lowlands of Sonora, Mexico - pollen records from packrat middens

Fossil pollen from packrat (Neotoma sp.) middens indicate that elements of the modern Sonoran Desert, such as Bursera microphylla (elephant tree), have been established within the Sierra Bacha of Sonora, Mexico, for the last 10,000 years. However, Fouquieria columnaris (boojum tree) grew on xeric east-facing slopes prior to c. 9000 years ago, indicating locally stronger upwelling or greater effective precipitation over modern conditions. Nearmodern environments were established sometime prior to c. 5400 years ago. The lack of correspondence between pollen-based vegetation reconstructions from middens and nearby marine sediment cores probably reflects different source areas for the pollen.