Peter T. Soulé

Western juniper expansion on adjacent disturbed and near- relict sites

We determined rates of western juniper (Juniperus occidentalis spp. occidentalis Hook.) density and cover change during the period 1951 to 1994 at 3 adjacent sites with nearly identical elevation, slope, aspect, soils, plant communities, and climate, but different land-use histories. The 3 sites are located in central Oregon at the confluence of the Deschutes and Crooked Rivers. Two of the sites are typical of central Oregon rangelands in that they have a history of anthropogenic disturbance including active fire suppression and domestic livestock grazing. The third site is a relict mesa that is a protected Research Natural Area and has experienced minimal anthropogenic impacts. We used large scale aerial photography to determine cover and density of western juniper in 1951, 1956, 1961, 1972, 1982, and 1994. We found that western juniper density and cover during the last 4 decades increased at all sites, with changes on the relict site similar to those on one of the disturbed sites. We suggest that even though 2 of the traditionally cited causes of western juniper expansion since the late 1800s (altered fire regimes, domestic livestock grazing) may have contributed to expansion on our disturbed sites, these mechanisms can not explain expansion on the near-relict mesa. Further, we examined climatic changes since 1900 in the region and concluded that the data did not fully support a climate-driven mechanism for the expansion. In seeking to explain western juniper expansion on semiarid rangelands, we suggest that all potential causal mechanisms (e.g., fire history, biological inertia, climate, domestic grazing, atmospheric CO2 enrichment) be considered.

HUMAN AGENCY, ENVIRONMENTAL DRIVERS, AND WESTERN JUNIPER ESTABLISHMENT DURING THE LATE HOLOCENE

Western juniper (Juniperus occidentalis var. occidentalis) is undergoing rapid rates of expansion, and human activities (domestic livestock grazing, fire suppression) are typically presented as the primary agents of change. To determine the importance of active disturbance as a vehicle for these major ecosystem changes (e.g., a near doubling of western juniper cover at some locales over a recent 30-yr period), we conducted a comparative study of western juniper establishment at matched disturbed and less-disturbed sites in Oregon (USA). We used dendroecological techniques to randomly sample and cross-date 160–200 trees per site from five Research Natural Areas (RNAs) or proposed RNAs, and from areas adjacent to these RNAs that are actively disturbed. For each location we determined the density of adult and juvenile western juniper and created a timeline of tree establishment. We discuss the probable causes of these establishment pattern changes. Recent increases in establishment could best be described as geometric on the disturbed sites. While trees on the less disturbed sites are generally older and have a more consistent establishment history, they also are experiencing increasing rates of establishment. Disturbance does appear to accelerate rates of establishment of western juniper, especially with domestic livestock grazing on sites that are downslope from established woodlands. Climatic changes, a lack of high-severity fires, an increasing seed rain, and atmospheric carbon dioxide enrichment are more recent causal mechanisms contributing to establishment. The driving forces proffered to explain the late 1800s to early 1900s pulse of establishment for western juniper (e.g., favorable climatic conditions, domestic livestock grazing) appear to be operative at the majority of our study sites. Conversely, a second pulse of establishment (post 1940s) occurred during a period that was not characterized by climatic conditions favorable for above-average radial growth of western juniper. Because our results show that rates of establishment are generally accelerating regardless of the disturbance regime, we suggest that active human disturbance is not a required element for these ecosystem changes to occur, and that other agents of change have either appeared or become more dominant in recent decades.

Does I = PAT Work in Local Places?

An aspect of global change currently not well understood is how processes operating on spatial scales finer than those used in recent global circulation models (GCMs) contribute to changes in atmospheric composition and the subsequent changes in climate. We use the ‘IPAT’ formulation as a framework to test relationships among social driving forces and user group greenhouse gas (GHG) emissions in northwestern North Carolina. Using regression, correlation, and bivariate mapping to examine relationships between a suite of socioeconomic variables and GHG emissions for the residential, commercial/industrial, and agricultural end-user categories, we find that various measures of population and affluence serve equally well as explanatory variables.

Ocean-Atmosphere Influences on Low-Frequency Warm-Season Drought Variability in the Gulf Coast and Southeastern United States

AbstractFrom the 344 state climate divisions in the conterminous United States, nine distinct regions of warm-season drought variability are identified using principal component analysis. The drought metric used is the Palmer hydrological drought index for the period 1895–2008. The focus of this paper is multidecadal drought variability in the Southeast (SEUS) and eastern Gulf South (EGS) regions of the United States, areas in which the low-frequency forcing mechanisms of warm-season drought are still poorly understood. Low-frequency drought variability in the SEUS and EGS is associated with smoothed indexed time series of major ocean–atmosphere circulation features, including two indices of spatiotemporal variability in the North Atlantic subtropical anticyclone (Bermuda high). Long-term warm-season drought conditions are significantly out of phase between the two regions. Multidecadal regimes of above- and below-average moisture in the SEUS and EGS are closely associated with slow variability in sea sur...

Post‐drought growth responses of western Juniper (Juniperus occidentals var. occidentalis) in central Oregon

Increased atmospheric CO2 may affect the physiological response of natural trees to drought. We examined radial growth rates developed from five western juniper chronologies to determine if post-drought growth responses have changed. Using prior year October to current year June precipitation from 1896-1998, we identified drought recovery years as having standardized scores (z-scores) > 0 and preceded by a year with a z-score <-0.6. We defined our analysis by an early period, 1896-1930, when atmospheric CO2 concentrations were close to preindustrial levels, and a late period, 1964-1998, when concentrations were substantially higher. Mean growth index values of recovery years between early and late periods were significantly greater (p<0.05) for four of the five sites, and for all sites combined. These results are consistent with the drought-ameliorating effects of CO2 shown by controlled laboratory studies and suggest that rising levels of atmospheric CO2 may impact western juniper growth rates.

A Comparison of 30-yr Climatic Temperature Normals for the Southeastern United States

Thirty-year climatic normals are an integral part of climate and climate assessment, but they are typically not used to address issues of climatic change. For 104 stations within the southeastern United States, I analyze spatial parameters of the two most recent 30-yr temperature normals (1961-1990, 1971-2000) to illustrate the utility of 30-yr normals for an assessment of climatic change. My comparison of the two normal periods shows that the Southeast as a whole has experienced a small (0.10°C) but significant increase in average temperature. However, of the seven physiographic provinces examined, only the lower Coastal Plain has experienced a significant increase in temperature. My analysis of urban versus rural sites produced mixed results on the potential impacts of urbanization and the associated heat island effects on the observed changes in temperature. While some long-term analyses of the thermal climate of the Southeast have shown the region to be cooling, my results suggest that the thermal climate of the southeastern United States since 1961 is stable or slightly warming.

Some Spatial Aspects of Southeastern United States Climatology

Abstract This article focuses on the climatology of an eight-state region in the southeastern United States. General controls of climate are discussed and spatial patterns of various climatic averages and extremes are mapped and examined as a means of fostering increased awareness of the variability that exists for climatic conditions in the region.

Mountain pine beetle selectivity in old-growth ponderosa pine forests, Montana, USA.

A historically unprecedented mountain pine beetle (MPB) outbreak affected western Montana during the past decade. We examined radial growth rates (AD 1860–2007/8) of co-occurring mature healthy and MPB-infected ponderosa pine trees collected at two sites (Cabin Gulch and Kitchen Gulch) in western Montana and: (1) compared basal area increment (BAI) values within populations and between sites; (2) used carbon isotope analysis to calculate intrinsic water-use efficiency (iWUE) at Cabin Gulch; and (3) compared climate-growth responses using a suite of monthly climatic variables. BAI values within populations and between sites were similar until the last 20–30 years, at which point the visually healthy populations had consistently higher BAI values (22–34%) than the MPB-infected trees. These results suggest that growth rates two–three decades prior to the current outbreak diverged between our selected populations, with the slower-growing trees being more vulnerable to beetle infestation. Both samples from Cabin Gulch experienced upward trends in iWUE, with significant regime shifts toward higher iWUE beginning in 1955–59 for the visually healthy trees and 1960–64 for the MPB-infected trees. Drought tolerance also varied between the two populations with the visually healthy trees having higher growth rates than MPB-infected trees prior to infection during a multi-decadal period of drying summertime conditions. Intrinsic water-use efficiency significantly increased for both populations during the past 150 years, but there were no significant differences between the visually healthy and MPB-infected chronologies.

Trends in midlatitude cyclone frequency and occurrence during fire season in the Northern Rockies: 1900–2004

[1] We examined changes in the timing and frequency of major midlatitude cyclones (MLCs) during August through October for eight climate stations in the Northern Rockies from 1900–2004. As MLCs can effectively diminish fire activity through both cooler temperatures and higher humidity/precipitation, we also determined if area burned by wildfires from 1940–2004 was correlated with the timing and frequency of these events. Our results indicate that: (1) significant long-term trends in MLCs exist, as the timing of the first MLC has occurred later in the year during the past century, with a marked upward shift post-mid1980s; (2) MLC frequency has significantly declined since 1900, with a pronounced decrease also beginning in the mid-1980s; (3) the relationships between the timing of the first MLC and frequency of MLCs with forest area burned are significant; and (4) mid-tropospheric ridging upstream from the Northern Rockies that blocks MLCs has become more pronounced. Citation: Knapp, P. A., and P. T. Soule ´ (2007), Trends in midlatitude cyclone frequency and occurrence during fire season in the Northern Rockies: 1900 – 2004, Geophys. Res. Lett., 34, L20707, doi:10.1029/2007GL031216.

Radial Growth and Increased Water-Use Efficiency for Ponderosa Pine Trees in Three Regions in the Western United States

We examined changes in and relationships between radial growth and intrinsic water-use efficiency (iWUE) of ponderosa pine (Pinus ponderosa) trees, climate, and atmospheric CO2 in the western United States since the mid-nineteenth century. We developed tree-ring chronologies for eight sites in three climate regions and used carbon isotope data to calculate pentadal values of iWUE. We examined relationships among radial growth, climate, iWUE, and CO2 via correlation and regression analyses. Significant upward trends in iWUE occurred at all sites, and despite an absence of climate changes that would favor growth, upward radial growth trends occurred at five sites. Our findings suggest that increased iWUE associated with rising CO2 can positively impact tree growth rates in the western United States and are thus an evolving component of forest ecosystem processes.