Paul A. Knapp

Drought-Busting Tropical Cyclones in the Southeastern Atlantic United States: 1950-2008

Droughts and tropical cyclones (TCs) are climatologically common events in the southeastern United States, yet little research has examined the potential for TCs to ameliorate drought impacts. Here, we identify the frequency of TCs that abruptly end drought conditions (i.e., drought busters, or DB) and determine possible influences of coupled ocean–atmosphere teleconnections on the likelihood of a TC-induced DB (TCDB). Using the HURDAT database and Palmer Drought Severity Indexes from 1950 through 2008, we identified every TCDB for thirty-one climate divisions in the southeastern Atlantic United States. We present the spatial patterns of the total number of TCDBs and the percentage of all droughts ended by TCs using choropleth maps. To determine what teleconnections influenced TCDBs, we used logistic regression analysis and included multiple synoptic-scale circulation indexes as predictor variables. In addition, we used a Fishers exact test to examine the association between the North Atlantic Oscillation (NAO) and TCDBs. We found that up to 41 percent of all droughts and at least 20 percent of droughts in three fourths of the climate divisions were ended by TCDBs. NAO was a significant predictor (p = 0.005) in the logistic regression model (χ2 = 10.91, p = 0.001), and the Fishers exact test showed a significant association between NAO and TCDBs (p = 0.003). An odds-ratio calculation showed that TCDBs are 5.8 times more likely to occur during a negative NAO phase than a positive NAO phase.

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.

Tropical Cyclones and Drought Amelioration in the Gulf and Southeastern Coastal United States

Precipitation from land-falling tropical cyclones (TCs) has a significant hydroclimatic influence in the southeastern United States, particularly during drought years. The frequency with which TCs ended drought conditions was examined for southeasterncoastal states from Texas to North Carolina during 1895–2011. The region was divided into the Gulf Coast states (GCS) and the southeastern Atlantic coast states (ACS). The spatiotemporal patterns of tropical cyclone drought busters (TCDBs) were analyzed. Larger-scale ocean– atmosphere influences on TCDBs were examined using chi-squared analysis. The ACS experienced TCDBs more frequently and farther inland compared to the GCS. The number of TCDBs has significantly increased withtime in theACS.TCDBsnumbersin the GCSdidnot exhibitsignificant increases, butthe areaalleviated ofdroughtconditionsincreased significantlyin the last 117 years. The dominantlarger-scaleocean–atmosphere forcingofTCDBswasacombinationofawarmAtlanticOcean[positiveAtlanticmultidecadaloscillationindex (AMO1)] and weak westerlies [negative North Atlantic Oscillation index (NAO2)]. AMO1 leads to an increase in the number of TCs throughout the North Atlantic basin, and NAO2 increases the likelihood of TC landfall by controlling the steering of TCs toward the southeastern United States.

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...

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.

Use of atmospheric CO2-sensitive trees may influence dendroclimatic reconstructions

We examined recent radial growth increases in western juniper trees using an 11-site chronology dating from AD 1000–2006. By various measures, radial growth during the late 20th/early 21st centuries was exceptional, with increases occurring absent of regional climatic change. We found that 54% of annual radial growth variability was explained by June Palmer Drought Severity Index (PDSI) values, but the inclusion of atmospheric CO2 values accounted for a 14% increase in explanatory power. We reconstructed June PDSI both including and excluding CO2, and found that PDSI values were overestimated at the end of the record with CO2 omitted from the model. We conclude that: 1) western juniper radial growth was associated with rising CO2 during the late 20th/early 21st centuries; and, the use of CO2-sensitive trees such as western juniper for dendroclimatic reconstructions may influence the results if the impacts of CO2 fertilization are omitted.

Lewis and Clarks’ Tempest: The ‘perfect storm’ of November 1805, Oregon, USA

Three weeks after arriving near the Pacific Northwest (PNW) coast in November 1805, Lewis and Clarks’ Corps of Discovery experienced a two-day windstorm that may have rivaled the strongest historically documented storms of the nineteenth and twentieth centuries. Based on the Corps’ detailed historical accounts describing the event as the perfect storm, we characterized the severity of the 1805 windstorm using tree-ring growth anomalies from windsnapped Sitka spruce collected at three sites along the northern Oregon Coast. We compared the 1805 gale to eight other documented events with comparable storm tracks and exceptional magnitude including the 1880 and 1951 events that each caused more than a billion board feet (c. 2.4 million m3) of windthrow. Statistical comparison of tree-growth responses revealed that the 1805 windstorm was the only event to differ significantly (χ2; test, p < 0.05, d.f. = 1) from all other storms. Our findings demonstrate the potential application of tree-ring data and historical documents to understand previously obscure climatic events similar to the extreme droughts that led to the demise of the Roanoke Colony during the sixteenth century and adversity experienced by the Jamestown Colony during the seventeenth century. Specifically, we identify the Lewis and Clark tempest of 1805 as being among the most severe PNW windstorms during the past two centuries, and may have contributed to the Corps’ dismal view of coastal Pacific Northwest weather.

Observations on a Rare Old-Growth Montane Longleaf Pine Forest in Central North Carolina, USA

ABSTRACT: Montane longleaf pine (Pinus palustris) forests are rare and no detailed inventory exists documenting stands in North Carolina. We inventoried all longleaf pine trees (n = 403) growing in a 24-ha remnant montane longleaf pine forest in the Uwharrie Mountains of central North Carolina, USA, in autumn 2014 to (1) map their location, (2) document age/height/diameter characteristics, and (3) determine special ecological features of this rare montane population. All longleaf pine were geographically referenced via GPS, measured for height and diameter, and a subsample of trees was cored to determine age. All longleaf pine were mapped based on growth-stage categories—grass, juvenile, young adult, and mature—to determine spatial patterning of stand-age characteristics. The longleaf pine stand contains a variety of growth-stage categories, but is dominated (63%) by mature-stage trees growing on south- and southwestern-facing slopes, while nearly all regeneration-stage trees (i.e., grass and juvenile) are growing on northwest-facing slopes, suggesting environmental conditions conducive to establishment have changed. Median (maximum) tree height and trunk diameter for young adult and mature were 17 (25) m and 38 (72) cm, respectively. Median (maximum) tree age at 0.3 m height was 116 years (272 years), and at least seven trees were greater than 150 years old, with four trees establishing in the 18th century. We conclude that the stands characteristics—400+ trees of various ages including old-growth, occurring principally on steep, southerly slopes with a total relief of 85 m, and extending over 24 ha—warrant “montane” longleaf pine forest status in North Carolina.