Justin T. Maxwell

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.

A climatic deconstruction of recent drought trends in the United States

We present high spatial-resolution trends of the Palmer drought severity index (PDSI), potential evapotranspiration (PET), and selected climate variables from 1979?2013 for the contiguous United States in order to gain an understanding of recent drought trends and their climatic forcings. Based on a spatial grouping analysis, four regions of increasing (upper Midwest, Louisiana, southeastern United States (US), and western US) and decreasing (New England, Pacific Northwest, upper Great Plains, and Ohio River Valley) drought trends based on Mann?Kendall Z values were found. Within these regions, partial correlation and multiple regression for trends in climate variables and PDSI were performed to examine potential climatic controls on these droughts. As expected, there was a US-wide concurrence on drought forcing by precipitation. However, there was correspondence of recent PET trends with recent drought trends in many regions. For regions with an increase in recent droughts, average air temperature was generally the second most important variable after precipitation in determining recent drought trends. Across the regions where recent drought trends are decreasing, there was no clear ranking of climate-variable importance, where trends in average temperature, specific humidity and net radiation all played significant regional roles in determining recent drought trends. Deconstructing the trends in drought show that, while there are regions in the US showing positive and negative trends in drought conditions, the climate forcings for these drought trends are regionally specific. The results of this study allow for the interpretation of the role of the changing hydroclimatic cycle in recent drought trends, which also have implications for the current and impending results of climate change.

Hydrological shifts and tree growth responses to river modification along the Apalachicola River, Florida

Shifts in the hydrologic regime of Florida’s Apalachicola River have been attributed to anthropogenic changes throughout its watershed, including local dam construction. To assess impacts of those shifts on floodplain forests, we reconstructed tree growth using dendrochronology and compared these trends with hydrological and climatic variables. Comparisons of stream-gage data before and after dam construction on the Apalachicola River revealed statistically significant mean declines in annual average stage. Mean minimum annual stages, rise rates, and fall rates also decreased, while hydrograph reversals increased. Growth in four tree species correlated strongly with site-specific inundation parameters. A wetter climate in the two decades following dam construction and fine-scale fluctuation of the hydrograph may have set the stage for positive growth releases. Logging and hurricane wind throw events may have also contributed. However, drier conditions in the last two decades are now exacerbated by stage-discharge declines that had been masked previously. Tree growth rates and recruitment have decreased and, in the absence of a major disturbance, the forest canopy is composed of an older cohort of individuals. Our findings highlight how hydrograph variability, climate change, and vegetation disturbance are all relevant for gaging and anticipating the range of impacts of river modification on floodplain forests.

On the declining relationship between tree growth and climate in the Midwest United States: the fading drought signal

Tree rings are widely considered to be a reliable proxy record of variations in climate and soil moisture. Here, using data from the Midwest United States (US), we provide documentation of a deteriorating relationship between radial tree growth and drought that is consistent across multiple species and locations. We find that traditional methods for drought reconstructions produce models that have rapidly declining validation statistics in recent decades. Split-sample calibration-verification that uses the first and second halves of the record can be problematic, as those two samples may not represent a sufficiently wide range of soil moisture conditions. To investigate this problem, we develop a randomized validation procedure that generates an empirical distribution of calibration and validation statistics. We place validation statistics derived from traditional methods in the generated distribution and compare them to a stratified approach that ensures each calibration model is composed of a sample that includes both dry and wet years. We find that the deteriorating relationship between tree growth and soil moisture is an artifact of the absence of drought over an extended period of time. A model that forces each calibration period to contain extreme drought years is statistically validated.. Nonetheless, if the current pluvial continues in the Midwest US, the linear relationship between tree rings and soil moisture will likely continue to deteriorate to the point where tree rings in the region will have a reduced ability to estimate past drought conditions.

Drought and Other Driving Forces behind Population Change in Six Rural Counties in the United States

The possible impacts of climatic change on postindustrial population patterns in the United States have been largely ignored. In this study, we examined population change in six rural counties of three different regions in the United States. In addition to determining the role of traditional variables known to influence population change, we also examined the potential influence of drought as measured by the study. We used correlation and regression analysis to determine the driving forces behind population change in each case study county. The traditional variables accounted for the majority of population change. While drought explained a small percentage of the variance in population change, it was significant in three out of the six counties and in each region we examined. Spatially, with the exception of the climatic variables, counties within the same region tended to have similar driving forces for population change.

Increased tree-ring network density reveals more precise estimations of sub-regional hydroclimate variability and climate dynamics in the Midwest, USA

Understanding the historic variability in the hydroclimate provides important information on possible extreme dry or wet periods that in turn inform water management plans. Tree rings have long provided historical context of hydroclimate variability of the U.S. However, the tree-ring network used to create these countrywide gridded reconstructions is sparse in certain locations, such as the Midwest. Here, we increase (nxa0=xa020) the spatial resolution of the tree-ring network in southern Indiana and compare a summer (June–August) Palmer Drought Severity Index (PDSI) reconstruction to existing gridded reconstructions of PDSI for this region. We find both droughts and pluvials that were previously unknown that rival the most intense PDSI values during the instrumental period. Additionally, historical drought occurred in Indiana that eclipsed instrumental conditions with regard to severity and duration. During the period 1962–2004 CE, we find that teleconnections of drought conditions through the Atlantic Meridional Overturning Circulation have a strong influence (rxa0=xa0−0.60, pxa0<xa00.01) on secondary tree growth in this region for the late spring-early summer season. These findings highlight the importance of continuing to increase the spatial resolution of the tree-ring network used to infer past climate dynamics to capture the sub-regional spatial variability. Increasing the spatial resolution of the tree-ring network for a given region can better identify sub-regional variability, improve the accuracy of regional tree-ring PDSI reconstructions, and provide better information for climatic teleconnections.

Placing modern droughts in historical context in the Ohio Valley using tree-rings

Abstract The temporal variability and severity of pre-instrument record summer droughts in the Ohio River Valley (Illinois, Indiana, and Ohio, USA) are not well understood. This study attempts to help fill this gap in Ohio Valley drought knowledge by using tree-ring chronologies from Illinois, Indiana, and Ohio to reconstruct summer (June through August) PDSI. We found that recent meteorological droughts of 1988 and 2012 are not unusual in the context of those reconstructed for the interval of 1680–2012. Droughts prior to 1895 (when the instrument-based record began) were more severe and lasted, on average, 1.5 times longer than those after 1895. The North American Drought Atlas represents droughts well for this region, but we found that drought severity was not homogeneous across the three sites. This indicates drought in the Ohio River Valley should be examined at a sub-regional level and suggests a need for a finer spatial representation of tree-ring chronologies in the Ohio Valley. Given the context of historical drought variability, the reconstructions suggest this region should be prepared for droughts that may be more severe and longer lasting than those recently observed.

Linking variation in intrinsic water‐use efficiency to isohydricity: a comparison at multiple spatiotemporal scales

Species-specific responses of plant intrinsic water-use efficiency (iWUE) to multiple environmental drivers associated with climate change, including soil moisture (θ), vapor pressure deficit (D), and atmospheric CO2 concentration (ca ), are poorly understood. We assessed how the iWUE and growth of several species of deciduous trees that span a gradient of isohydric to anisohydric water-use strategies respond to key environmental drivers (θ, D and ca ). iWUE was calculated for individual tree species using leaf-level gas exchange and tree-ring δ13 C in wood measurements, and for the whole forest using the eddy covariance method. The iWUE of the isohydric species was generally more sensitive to environmental change than the anisohydric species was, and increased significantly with rising D during the periods of water stress. At longer timescales, the influence of ca was pronounced for isohydric tulip poplar but not for others. Trees physiological responses to changing environmental drivers can be interpreted differently depending on the observational scale. Care should be also taken in interpreting observed or modeled trends in iWUE that do not explicitly account for the influence of D.

Drought timing and local climate determine the sensitivity of eastern temperate forests to drought

Projected changes in temperature and drought regime are likely to reduce carbon (C) storage in forests, thereby amplifying rates of climate change. While such reductions are often presumed to be greatest in semi-arid forests that experience widespread tree mortality, the consequences of drought may also be important in temperate mesic forests of Eastern North America (ENA) if tree growth is significantly curtailed by drought. Investigations of the environmental conditions that determine drought sensitivity are critically needed to accurately predict ecosystem feedbacks to climate change. We matched site factors with the growth responses to drought of 10,753 trees across mesic forests of ENA, representing 24 species and 346 stands, to determine the broad-scale drivers of drought sensitivity for the dominant trees in ENA. Here we show that two factors-the timing of drought, and the atmospheric demand for water (i.e., local potential evapotranspiration; PET)-are stronger drivers of drought sensitivity than soil and stand characteristics. Drought-induced reductions in tree growth were greatest when the droughts occurred during early-season peaks in radial growth, especially for trees growing in the warmest, driest regions (i.e., highest PET). Further, mean species trait values (rooting depth and ψ50 ) were poor predictors of drought sensitivity, as intraspecific variation in sensitivity was equal to or greater than interspecific variation in 17 of 24 species. From a general circulation model ensemble, we find that future increases in early-season PET may exacerbate these effects, and potentially offset gains in C uptake and storage in ENA owing to other global change factors.

Changes in the Mechanisms Causing Rapid Drought Cessation in the Southeastern United States

The synoptic processes that end droughts are poorly understood, yet have significant climatological implications. Here we examined the spatiotemporal patterns of rapid drought cessation (RDC) in the southeastern United States during the1979–2013 warm season (April–November) for three storm types: Frontal, Tropical, and Air mass. We defined RDC as a 1xa0month shift in soil moisture sufficient to alleviate an existing drought. We found that 73% of all warm-season droughts were ended by RDC events and the three storm-type groups ended droughts over similar spatial areas. Frontal storms were the most frequent mechanism for RDC events, yet their occurrences significantly decreased and were negatively related to increases in Northern Hemisphere air temperatures. Projected future warming in the Northern Hemisphere suggests a continued decline in the frequency and relative contribution of Frontal storms as RDC events, potentially influencing the timing and spatial scale of drought cessation in the southeastern U.S.