Edwards Plateau Research Articles

Seasonal Variations in Modern Speleothem Calcite Growth in Central Texas, U.S.A.

Variations in growth rates of speleothem calcite have been hypothesized to reflect changes in a range of paleoenvironmental variables, including atmospheric temperature and precipitation, drip-water composition, and the rate of soil CO2 delivery to the subsurface. To test these hypotheses, we quantified growth rates of modern speleothem calcite on artificial substrates and monitored concurrent environmental conditions in three caves across the Edwards Plateau in central Texas. Within each of two caves, different drip sites exhibit similar annual cycles in calcite growth rates, even though there are large differences between the mean growth rates at the sites. The growth-rate cycles inversely correlate to seasonal changes in regional air temperature outside the caves, with near-zero growth rates during the warmest summer months, and peak growth rates in fall through spring. Drip sites from caves 130 km apart exhibit similar temporal patterns in calcite growth rate, indicating a controlling mechanism on at least this distance. The seasonal variations in calcite growth rate can be accounted for by a primary control by regional temperature effects on ventilation of cave-air CO2 concentrations and/or drip-water CO2 contents. In contrast, site-to-site differences in the magnitude of calcite growth rates within an individual cave appear to be controlled principally by differences in drip rate. A secondary control by drip rate on the growth rate temporal variations is suggested by interannual variations. No calcite growth was observed in the third cave, which has relatively high values of and small seasonal changes in cave-air CO2. These results indicate that growth-rate variations in ancient speleothems may serve as a paleoenvironmental proxy with seasonal resolution. By applying this approach of monitoring the modern system, speleothem growth rate and geochemical proxies for paleoenvironmental change may be evaluated and calibrated.

Phylogeographic Structure and Color Pattern Variation among Populations of Plethodon albagula on the Edwards Plateau of Central Texas

Texas populations of slimy salamanders are isolated from other members of the Plethodon glutinosus complex and are currently placed in the species P. albagula with populations from the Ozark Plateau and Ouachita Mountains of Missouri, Oklahoma, and Arkansas. We sequenced a portion of the mitochondrial protein-coding gene ND4 for 52 P. albagula from 25 localities in Texas and one locality in Missouri. Bayesian MCMC analysis identified five parapatric lineages of Plethodon in Texas. Several morphologically distinctive populations are phylogenetically embedded within the Texas radiation. Additional studies of the interactions among the lineages identified in this study will be necessary to resolve their taxonomic status.

Impacts of Juniper Vegetation and Karst Geology on Subsurface Flow Processes in the Edwards Plateau, Texas

Impacts of Ashe juniper (Juniperus ashei J. Buchholz) and karst geology on the regional water cycle in the Edwards plateau region of Texas are complex and not well understood. The objective of our study was to gain a comprehensive understanding of the subsurface flow processes occurring at a juniper woodland site on the Edwards Plateau near Honey Creek State Natural Area. A 2.3‐m‐deep, 7‐m‐long trench was excavated at the downslope end of a 7 by 14 m experimental plot, and time domain reflectometry (TDR) probes were installed at various locations within the trench face to measure volumetric water contents. A multi‐port telescopic boom type rainfall simulator was used to provide artificial rainfall on the plot. Six rainfall simulations and two dye‐tracer tests were conducted on the plot during a 7‐mo period. Subsurface flow was visually inspected at various locations on the trench face during artificial rainfall experiments and water content was monitored near slow and fast flow regions using TDR probes. The total volume of subsurface flow was also recorded after each rainfall simulation event. Results demonstrated that subsurface flow occurred in a bimodal manner, consisting of preferential/macropore flow around juniper root channels and planar fractures in the limestone, and pseudo‐matrix flow through the soil matrix (water flowing primarily through the intermediate layers and lenses of soil between the rock layers). Preferential/macropore flow at the trench face depended on imposed boundary conditions and was independent of antecedent moisture content in the soil matrix. Pseudo‐matrix flow response time decreased with high rainfall. During large rainfall events (>200 mm), water exchange was observed between the fractures and soil matrix. No apparent water exchange occurred between fractures and the soil matrix during small rainfall events. The dye studies indicated that fractures and juniper root channels are primary pathways for preferential/macropore flow occurring within the plot.

Comparison of Two Techniques for Assessing Invertebrate Availability for Wild Turkeys in Texas

Although the importance of invertebrates to young galliforms has been demonstrated by numerous researchers, few comparisons of techniques designed to assess invertebrate availability for these species have been published. Our objective was to compare suction sampling and sweep-netting as methods for evaluating invertebrates available for Rio Grande wild turkey (Meleagris gallopavo intermedia) poults on the Edwards Plateau of Texas, USA. We collected invertebrates via sweep-netting (n = 102) and suction sampling (n = 100) during summers 2002 and 2003 at brood locations. Suction sampling collected the same orders of invertebrates as sweep-netting, but invertebrate dry mass and frequency of occurrence were lower for nearly every order compared to sweep-netting. Suction sampling provided no additional information to that obtained by sweep-netting in the broken grasslands where our study was conducted. For this reason, we believe the additional cost and time required to implement this technique was not warranted for our study areas.

Springs on rangelands: runoff dynamics and influence of woody plant cover

AbstractTo date, little effort has been devoted to understanding the nature of streamflow from dryland catchments where springs are found, and little is known about how changes in vegetation may alter that streamflow. But where they do occur in drylands, springs are important—ecologically as well as hydrologically. Areas that naturally support spring flow, such as those having an underlying karst geology, hold the most promise for increasing streamflow through control of woody plants. For rangelands, this possibility is a particularly appealing one. However, few studies have documented that such a method is effective (except in Mediterranean climates). For this reason, we studied runoff generation for a site on the Edwards Plateau of Texas: a first‐order (19‐ha) rangeland catchment supplied with intermittent streamflow from a spring at its base. Using hydrometric as well as isotopic characterization, we evaluated the extent to which runoff changed following removal of most of the woody plant cover (predominantly Ashe juniper). After monitoring streamflow for 4 years (two before plant removal and two following removal), we found that (1) streamflow increased following removal of juniper, by about 46 mm annually, except during the summer; (2) for the 4 years of observation, runoff made up about 22% of the water budget, with baseflow from the spring accounting for about half the total flow; and (3) hydrograph separation analysis conducted for two events following juniper removal indicated that pre‐event water made up 25% and 50% of the runoff (these numbers represent lower bounds for the percentage of water derived from the spring during storm events). These findings are important, not only because they add to our understanding of runoff generation from spring‐fed catchments, but also in particular because they demonstrate that where springs are present, decreasing woody plant cover may augment runoff. Copyright © 2006 John Wiley & Sons, Ltd.

Immigration Crisis in Texas: The Impact of the Exotic Axis Deer on the Texas Hill Country

T_ ~5exas is facing a serious immigration crisis. I don't mean the immigration of people, as is currently in the news, but the importation and proliferation of nonnative deer species, specifically the axis deer (Axis axis). We are the best of a bad example. These are the words of Wayne Haley, Superintendent of the South Llano River State Park. His words not only speak for the condition of the park, but serve as a reminder of the condition most of the area is in. The park is overpopulated with white-tailed deer and exotics, and struggles to provide enough forage to feed them. Additionally the Texas Hill Country is currently in the midst of a heavy drought, which compounds the problem. The South Llano River State Park is located in Kimble County just outside the town of Junction in the heart of the Texas Hill Country. The Hill Country, or Edwards Plateau region, contains approximately 24 million acres and is located in the southwestern part of the state. Junction, located at the confluence of the North and South Llano rivers, has an elevation of 1,750 feet, and an average annual rainfall of only 22 inches per year. Located approximately 150 miles west of Austin, Junction is accessed by US Highway 83, Interstate 10, and US Highway 377. Historically an agricultural and ranching area, the Hill Country is being transformed into a popular place to visit for recreation, and to retire. The hunting of white-tailed deer, Rio Grande turkey, white wing and mourning dove, and more recently the exotic imports, brings a significant increase in the number of visitors, and has become the major source of income in the local economy. River activities such as fishing, canoeing, tubing, and kayaking bolster the increasingly tourist-oriented atmosphere. Because of the gaining popularity of the area, property values have risen over the past decade, and the use of the |+ , . r = ~ >t x la n d is steadily shifting from agx ? _ -~ riculture to recreation. With this change to increased recreational land use, ranches are I being subdivided into smaller * tracts, and many landowners look to hunting as a source of income. EIDWARDS ~.A number of these landowners PLATEAU have also added the hunting of exotics in order to boost that in-

Wide Ecological Amplitude of a Diversity-Reducing Invasive Grass

The goals of this study were to identify habitats occupied by the Eurasian grass Bothriochloa ischaemum (King Ranch bluestem) on the eastern Edwards Plateau of central Texas, USA, and to measure the effects of this invasive species on plant diversity. A set of descriptive field studies were conducted to determine the relationships between habitat characteristics (woody cover, slope, presence of roads and trails, history of prescribed burning, and presence of grazing) and the presence and abundance of this species. Species richness and diversity of perennial herbaceous species were compared between plots in which B. ischaemum was absent and plots that it dominated.

Quantitative Relationships Among Golden-Cheeked Warbler Occurrence and Landscape Size, Composition, and Structure

Abstract The golden-cheeked warbler (Dendroica chrysoparia) is a federally listed endangered species with an extremely limited breeding range (Edwards Plateau, Tex., USA) and specific habitat requirements. Although localized habitat composition and structure correlate well with golden-cheeked warbler occurrence and nest location, the effects of landscape composition and structure are largely untested. To investigate landscape-scale influences on habitat occupancy, we surveyed 202 point locations for golden-cheeked warbler presence. We used logistic regression to compare the relative influence of 4 landscape-scale variables (largest patch index, percent of the landscape in woodland, mean-nearest neighbor, and edge density) measured at 4 spatial scales (3 ha, 12 ha, 50 ha, and 200 ha) in predicting occurrence. The amount of mature juniper–oak (Juniperus ashei and Quercus spp.) woodland in the landscape was the single most important variable across spatial scales and average values of woodland cover remained...

Poult Adoption and Nest Abandonment by a Female Rio Grande Wild Turkey in Texas

While evaluating reproductive parameters in Rio Grande Wild Turkeys (Meleagris gallopavo intermedia) in the Edwards Plateau region of Texas, we observed a case of poult adoption and abandonment of an active nest. In wild turkeys, adoption of poults has been described previously, but during our observation the hen also abandoned her nest at a late stage of incubation. Most research discussing adoption in gallinaceous birds has focused on brood abandonment after hatch. Although poult adoption in conjunction with nest abandonment is probably rare, our observations indicate that it can occur, at least in Rio Grande Wild Turkeys.

Stable Carbon Isotopes from Archaic Human Remains in the Chihuahuan Desert and Central Texas

The distribution of δ13C values from human remains recovered in the eastern Chihuahuan Desert and central Texas demonstrates the existence of at least two distinct coeval dietary populations during the Early and Late Archaic. We employed a novel statistical technique, norma probability plots, to identify distinct dietary populations. δ13C values from one population in the Lower Pecos and Trans-Pecos regions of Texas reflect the long-term use of Chihuahuan Desert C4 and CAM plants. A second group, consisting of all the central Texas individuals and some individuals from the Lower Pecos, depended heavily on C3 plants more common in the Edwards Plateau or in the oak-pine forests in Mexico. The wide range of stable carbon isotope ratios from the Lower Pecos region of the eastern Chihuahuan Desert forces a reconsideration of hunter-gatherer settlement and social organization models proposed for that region. This study highlights the need to conduct more research on hunter-gatherer prehistoric diets and social organization.

Heinrich H1 and 8200 yr B.P. climate events recorded in Hall's Cave, Texas

Research Article| September 01, 2006 Heinrich H1 and 8200 yr B.P. climate events recorded in Hall's Cave, Texas Brooks B. Ellwood; Brooks B. Ellwood 1Department of Geology and Geophysics, Louisiana State University, E235 Howe-Russell Geoscience Complex, Baton Rouge, Louisiana 70803, USA Search for other works by this author on: GSW Google Scholar Wulf A. Gose Wulf A. Gose 2Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar Geology (2006) 34 (9): 753–756. https://doi.org/10.1130/G22549.1 Article history received: 20 Dec 2005 rev-recd: 08 Apr 2006 accepted: 25 Apr 2006 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Brooks B. Ellwood, Wulf A. Gose; Heinrich H1 and 8200 yr B.P. climate events recorded in Hall's Cave, Texas. Geology 2006;; 34 (9): 753–756. doi: https://doi.org/10.1130/G22549.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Measurements of magnetic susceptibility (MS) have been performed on a continuous set of samples from a well-dated ∼3 m sequence of sediments collected from Hall's Cave, Edwards Plateau, central Texas. Three major climatic events are represented by distinctive MS increases in the sequence: (1) the Heinrich event H1 (from 17,500 to 17,000 yr B.P.); (2) the end of full glacial conditions on the Edwards Plateau at 14,200 yr B.P.; and (3) the 8200 yr B.P. climatic event (from 8250 to 8050 yr B.P.). In addition, a minor event at 4400 yr B.P. is also well defined here and in Canada, indicating that it is a broadly regional event. These data indicate that all four events represent a shift toward milder climatic conditions accompanied by increased rainfall at this location. The general climatic trends interpreted from the MS data are consistent with independent vertebrate assemblages, floral indicators, and isotopic data recovered from the cave. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Ecology of the Jollyville Plateau Salamander (Eurycea tonkawae: Plethodontidae) with an Assessment of the Potential Effects of Urbanization

The Jollyville Plateau salamander, Eurycea tonkawae Chippindale, Price, Wiens, and Hillis, inhabits springs and wet caves of the Jollyville segment of the Edwards Plateau, Texas. The known range of this species is limited to six stream drainages, and most known localities are at risk of impairment from urban development. Our purpose was to gather needed autecological information on E. tonkawae and evaluate factors that may affect the distribution and abundance of the species. We conducted visual salamander surveys at nine stream sites across the Jollyville Plateau between December 1996 and December 1998. The survey sites were classified as undeveloped or developed based on watershed impervious cover estimates. We characterized the habitat for each site, including substrate type, discharge, and water quality. Salamander counts varied seasonally, but generally were higher during spring and summer. Salamander densities across sites were positively correlated with rubble and cobble substrate density as preferred cover, and negatively correlated with the standard deviation of water temperature, as expected for a spring-adapted species. In addition, we found that mean salamander densities at sites occurring in undeveloped watersheds were significantly higher than at developed sites, where specific conductance of the water was higher. The results of this study suggest that while habitat and seasonal factors influence surface salamander densities, E. tonkawae populations may be most vulnerable to effects associated with urbanization.

To Burn or Not to Burn…That Is the Question

By Kelly Haile Editor's Note: This paper is the 5th Place winner of the High School Youth Forum contest at the Societyfor Range ManagementAnnual Meeting, February 2005, Fort Worth, Texas. In presettlement times in North America fire was as much a part of the landscape as rainfall, snow, grazing, and insects. Fire was a natural occurrence caused by lightning strikes. These naturally set fires were common in the summer when it was hot and dry. Under these conditions the fuel was easily ignited. Native Americans also deliberately set fires to reduce ground cover for the ease of traveling and hunting. This could be considered a prescribed burn. A modern definition of a prescribed burn is the application of fire to a vegetated site in a predetermined area under specific fuel load and weather conditions in accordance with a written prescription plan. The Native Americans did not have a written prescription but they did have a plan. Burning is one of the oldest rangeland management practices. It has been known for years that prescribed burns have many benefits. Following are 3 benefits: 1) burning helps control brush and invading grasses to improve rangeland condition and wildlife habitat, 2) burning helps prevent wildfires, and 3) burning helps improves seed germination and seedling establishment. In the Edwards Plateau where I live we have several invading brush species. We use prescribed burning to increase the production of native grasses and forbs while reducing the growth of unwanted brush species. For instance, Ashe juniper, also known as cedar, is one of the main invading brush species. It can take over the rangeland in a relatively short period of time. If left unchecked it can get so dense that the production of grasses and forbs are reduced to a minimum amount. Further study shows that a rain of less than 1 inch can be caught in its thick canopy, keeping the rain from reaching the soil underneath. However, Ashe juniper can be controlled by fire because it does not resprout after being burned for the right amount of time and at a high enough temperature. Although not all brush species are killed by fire like Ashe juniper, their growth is suppressed, which makes the rangeland more accessible for

Experimental Burned Rock Studies on the Edwards Plateau: A View from Camp Bullis, Texas

This article presents an experimental study concerning the use of thermally altered stone, commonly referred to as fire-cracked, or burned rock. A series of controlled cooking experiments were initiated as limestone cobbles were placed within a prepared earth-oven and re-heated; this type of earth-oven is similar to those found within prehistoric burned rock mounds on the Edwards Plateau of central Texas. Twelve experiments were conducted in this study, as four different sets of cobble assemblages were each re-heated three times, at six, twelve, twenty-four, and forty-eight hour intervals. These heated limestone cobbles were then analyzed for thermal alteration characteristics such as color change, cracking, and spalling, for comparative purposes. Quantitative analyses were then applied to the data in order to establish whether different patterns of thermal alteration exist dependent upon short-term and long-term re-uses.

Tropical cyclones and the flood hydrology of Puerto Rico

Some of the largest unit discharge flood peaks in the stream gaging records of the U.S. Geological Survey (USGS) have occurred in Puerto Rico. Many of these flood peaks are associated with tropical cyclones. Hurricane Georges, which passed directly over the island on 21–22 September 1998, produced record flood peaks at numerous USGS stations in Puerto Rico. The hydrology and hydrometeorology of extreme flood response in Puerto Rico are examined through analyses of rainfall, based on Weather Surveillance Radar–1988 Doppler (WSR‐88D) radar reflectivity observations and USGS rain gage observations and discharge from USGS stream gaging stations. Peak rainfall accumulations of more than 700 mm occurred in the central mountain region of the island. The largest unit discharge flood peaks, however, were located in the eastern portion of the island in areas with smaller storm total rainfall accumulations but markedly larger rainfall rates at 5–60 min timescale. Orographic precipitation mechanisms played an important role in rainfall distribution over the island of Puerto Rico. Amplification of rainfall accumulations was associated with areas of upslope motion. Elevated low‐level cloud water content in regions of upslope motion played an important role in the maximum rainfall accumulations in the central mountain region of Puerto Rico. The largest unit discharge flood peaks, however, were produced by a decaying eye wall mesovortex, which resulted in a 30–45 min period of extreme rainfall rates over the eastern portion of the island. This storm element was responsible for the record flood peak of the Río Grande de Lóiza. The role of terrain in development and evolution of the eye wall mesovortex is unclear but is of fundamental importance for assessing extreme flood response from the storm. Hydrologic response is examined through analyses of rainfall and discharge from five pairs of drainage basins, extending from east to west over the island. These analyses point to the importance of short‐term rainfall rates for extreme flood response. The hydrologic response of Puerto Rico is compared with two other extreme flood environments, the central Appalachians and Edwards Plateau of Texas. These analyses suggest that the high rainfall environment of Puerto Rico is linked to the development of a hydraulically efficient drainage system.

DO WOODY PLANTS AFFECT STREAMFLOW ON SEMIARID KARST RANGELANDS?

There is considerable public and political pressure to reduce woody plant cover on rangelands as a means of increasing water yield, despite the lack of studies documenting that such a strategy is effective. In the Texas Hill Country, runoff from the Edwards Plateau recharges the highly productive and regionally vital Edwards Aquifer. The dominant woody plant on the Plateau is Ashe juniper (Juniperus ashei Buchholz). To understand how woody plant cover may affect the amount and timing of runoff in this region, we monitored streamflow from nine small (3- to 6-ha) watersheds over a 13-year period. After the first two years (initial observations), 100% of the shrub cover was removed from three of the watersheds and ∼70% from another three. Following these treatments we continued to monitor runoff for four years, suspended monitoring for four and a half years, and then resumed monitoring for an additional three years. Runoff from these nine first-order watersheds generally accounted for <5% of the total precip...

MODELING EFFECTS OF BRUSH MANAGEMENT ON THE RANGELAND WATER BUDGET: EDWARDS PLATEAU, TEXAS

ABSTRACT: The Soil and Water Assessment Tool (SWAT) was used to evaluate the influence of woody plants on water budgets of semi-arid rangelands in karst terrain. The model was configured for the hydrologic evaluation of the North Fork of the Upper Guadalupe River watershed and was calibrated and verified using measured flow data. Nash and Sutcliffe fit efficiencies for daily and monthly verification periods were 0.09 and 0.50, respectively. Streamflow, baseflow, and evapotranspiration (ET) rates were comparable to published field data. Simulated deep recharge was considerably higher than the published values for the Edwards Plateau. The model was then used to simulate conditions with existing brush cover and four different brush removal scenarios. Scenarios were created to represent existing brush and the removal of brush from only locations that were either covered by heavy brush, were on a moderate slope, or were in shallow soils. Resulting data was compared to previous studies of both field experiments and model simulations. Maximum brush removal resulted in a reduction in ET equal to 31.94 mm/yr depth over the entire basin, or 46.62 mm/yr depth over the treated area. Removal of heavy brush cover resulted in the greatest changes in evapotranspiration, surface runoff, base-flow, and deep recharge. Slope was found to have the greatest effect on lateral subsurface flow.

Survival and growth of Juniperus seedlings in Juniperus woodlands

Juniperus woodlands are widely distributed in western North America. Few studies of seedling emergence, long-term survival, growth or mortality of the dominant Juniperus spp. in these woodlands have been carried out. Consequently, regeneration dynamics in these woodlands are poorly understood. Juniperus ashei is the dominant woody plant in the majority of woodland and savanna communities of the Edwards Plateau region in central Texas. We examined the emergence, mortality and growth of various spatial and temporal cohorts of J. ashei seedlings over an eight or nine-year period. Greatest emergence was found during the cool, mostly winter months and under the canopy of mature J. ashei trees. Emergence was significantly inversely related to temperature and significantly linearly related to rainfall, but only if the monthly rainfall and emergence were offset by one to four months. Greatest survival occurred below the J. ashei canopy, but greatest growth was at the canopy edge. Emerging seedlings were not from the current year’s seed crop, but from one or more previous year’s seed crops. Greatest mortality occurred mostly during the summer months and in the grassland habitat. There was a significant inverse logarithmic or exponential relationship between mean monthly temperature and mean monthly mortality. A large number of J. ashei seedlings or immature plants with reduced growth were found beneath the canopy of mature trees. These plants seem to serve as a seedling bank, providing the source of recruitment into the population should the overstory trees be removed. Survival of the two canopy cohorts with known emergence dates declined with time (negative exponential function) and was 1.0–3.4% after eight or nine years depending on the cohort. The pre-existing cohort seemed to have constant mortality (and presumably replacement), with about 8% of the population dying each year. Higher growth rates for seedlings were found at the edge of the established woodland canopy, which suggests that conditions in the edge habitat or possibly in canopy gaps are best for growth beyond the seedling stage.

Effects of white-tailed deer on the population dynamics of acorns, seedlings and small saplings of Quercus buckleyi

To measure the effects of white-tailed deer (Odocoileus virginianus) herbivory on seeds, seedlings, and young saplings of Quercus buckleyi on the eastern Edwards Plateau of central Texas, USA, experimental fenced deer exclosures were constructed. Acorns or small Q. buckleyi transplants were placed in each exclosure and in each unfenced control plot. Deer did not significantly affect acorn survival and germination, but did significantly reduce transplant survival and growth rates. The results support the hypothesis that deer are responsible for the failure of recruitment into adult size classes in Q. buckleyi populations in this region. Without adult recruitment of Q. buckleyi, the species composition and possibly even the physiognomy of woodlands on the eastern Edwards Plateau will change markedly. The results of this experiment also indicate that, although juniper (Juniperus ashei) and Q. buckleyi presumably compete for water, light, and nutrients, in the presence of deer junipers can have a positive effect on seedlings and saplings of Q. buckleyi, a case of facilitation. In the presence of deer the transplants increased in height significantly more slowly away from juniper saplings than they did beneath juniper saplings, probably due to the physical protection from browsing that junipers provided to the transplants.

Diploid and polyploid cytotype distribution inMelampodium cinereumandM. leucanthum(Asteraceae, Heliantheae)

Previous chromosomal studies within Melampodium (Asteraceae, Heliantheae) of Mexico and Central America have documented chromosome numbers n = 9, 10, 11, 12, 18, 20, 23, 25 ± 1, 27, 30, and 33. Some species also have been shown to exhibit infra- and interpopulational polyploidy. The presence of cytotype mixtures is especially pronounced in the white-rayed complex, which occurs in the southwestern United States and adjacent Mexico. This group includes M. cinereum (n = 10 and 20), M. leucanthum (n = 10 and 20), and M. argophyllum (n = 30). Cytotype distribution has been newly analyzed in 415 plants from 152 populations and added to existing data from 185 plants from 113 populations, yielding information from a total of 600 individuals from 265 populations. Within M. cinereum and M. leucanthum are parapatric distributions of cytotypes, with tetraploids centered in the eastern and diploids in the western portions of their ranges. Tetraploids are most likely of autopolyploid origin, forming recurrently, with adaptations that allow colonization and establishment in new ecological regions. Contact zones are relatively narrow and only two triploid individuals have been detected. The tetraploid cytotypes probably extended eastward into central and southern Texas to the natural barriers at the edge of the Edward's Plateau in M. leucanthum and the low sandy plains in M. cinereum. The hexaploid M. argophyllum is interpreted as a relict surviving in the low mountains of northern Mexico; it may be an allopolyploid of hybrid origin between ancestors of the evolutionary lines that eventually yielded M. cinereum and M. leucanthum.