Edwards Plateau Of Central Texas Research Articles

Growth and longevity of two imperilled mussel species from the Edwards Plateau of Central Texas and its implications for freshwater mussel conservation and management

Abstract Life history information such as growth and longevity have been useful for understanding evolutionary relationships and predicting species responses to management and habitat alteration for aquatic species. For unionid mussels, which are globally imperilled, life history information remains unknown for a majority of unionid mussels and because of this has not been broadly used to guide mussel conservation efforts. To address this knowledge gap, growth and longevity were estimated for Cyclonaias petrina, Texas pimpleback, and Lampsilis bracteata, Texas fatmucket, using thin‐sectioning and validated using cross‐dating. Both species are proposed for listing under the US Endangered Species Act. Growth and longevity estimates differed between C. petrina (K = 0.065, 0.086, 0.101; L∞ = 55.03, 76.44, 94.43) and L. bracteata (K = 0.187, 0.208; L∞ = 61.40, 61.52), and growth for L. bracteata correlated to annual flow indices. Cross‐dating revealed high interseries correlations (R = 0.400–0.573), indicating estimates can be viewed with some certainty. Growth and longevity estimates indicate C. petrina is positioned near the K endpoint and L. bracteata near the r endpoint along the r/K life history continuum. This suggests C. petrina should be favoured in stable habitats where disturbance is minimal, whereas L. bracteata is expected to tolerate habitats with frequent and likely stochastic patterns of disturbance. Knowledge of growth and longevity along with life history position provides a qualitative basis to help scientists and practitioners better anticipate how species will respond to environmental change and management actions. Given the conservation status of C. petrina and L. bracteata the life history findings from this study are timely and should be useful in their conservation.

A Long Look Ahead: William L. Bray and Early Texas Conservationism

A Long Look Ahead: William L. Bray and Early Texas Conservationism Char Miller (bio) Click for larger view View full resolution Ecologist William L. Bray (1865–1953) was an expert on the vegetation of western Texas and one of the state’s first conservationists; he taught at the University of Texas and Syracuse University. Smithsonian Institution Archives, Accession 90-105, Science Service Records, Image No. SIA2008-0060. William L. Bray seemed called to the Edwards Plateau, the dominant landform in Central Texas. Not in the religious sense, exactly. He was botanist, not a minister. But the vast, uplifted, and semi-arid terrain was the scene of some of his most important work, research he approached with special fervor. The wellspring of his fervency was at once spiritual and scientific. Bray noted this link in an address at the May 1925 dedication of the University of Texas’s new Biological Laboratories Building: “It is unthinkable that a universe so vast, so wonderful, so beautiful, so orderly, and that a creature with power to discern and appreciate all this, should be matters of chance or without motive.” However centering Bray’s Christian faith was, its focus was “turned to the business of right living and acting in this present,” a social gospel that led Bray to pursue the “betterment of human environment and of social conditions.”1 Science thus played a formative role in shaping human affairs. For Bray, this meant that botany had utility, that it should be of use to society, an argument he had announced in his dissertation. Assessing “plant life in Texas as the product of its environment,” he wrote approvingly, “lies in the growing tendency to rely upon such study to furnish a rational basis for the exploitation of plant life in agriculture, horticulture, forestry, [End Page 251] agrostology, and various other economic fields.”2 Bray made the same case twenty-seven years later at the grand opening of the university’s new laboratory. Botanists, he argued, were “concerned in what way the plant world can be induced to yield toward material human welfare.”3 That was why he was deeply interested in the “practical benefits to the people of the State growing out of the service of botanical science,” a concept the title of his address highlighted: “Botany and the State.”4 Bray’s utilitarian perspective was consistent with the defining values of his academic discipline, its professional ethos and social advocacy. Established in 1894, the Botanical Society of America promoted botanical research in American universities and colleges that would be linked to the demands of industrial capitalism, affirmed the priority of academic expertise over amateurism, and publicly supported the creation of forest reserves as part of its utilitarian credo.5 Bray embodied these professionalizing commitments during his decade-long career at the University of Texas, from 1897 to 1907. While there, he mounted a series of robust botanical investigations and rigorous ecological analyses of two regions in the state led him to develop a sophisticated set of policy recommendations for their careful, scientific management. In particular, he made a strong case for the immediate application of conservation measures to control natural-resource exploitation on the Edwards Plateau of Central Texas and in the Piney Woods of East Texas to insure these regions’ ecological stability and economic productivity across time. Bray, in short, emerged as the leading academic advocate for the implementation of a state or national regulatory presence on the ground. He had no doubt such oversight of unregulated capitalism was necessary, even if little directly came of his convictions. Bray’s zeal was manifest in his man-on-a-mission approach to life. Born [End Page 252] in Burnside, Illinois, in September 1865, the ninth of thirteen children born to William and Martha Bray, he was educated in local public schools. After high school, Bray earned a teaching degree at Missouri’s Kirkland Normal School (now Truman State University) and spent the next set of years as a teacher and administrator in Iowa and Missouri. Showing his inclination toward the social gospel, he took a twelve-month leave to direct the YMCA in Fresno, California. During his free time, the...

Prescribed fires cause minimal damage to a threatened cactus (Tobusch fishhook cactus, Sclerocactus brevihamatus ssp. tobuschii, Texas, USA)

The effects of fire on cacti depend on individual plant characteristics, fire intensity, and local conditions. Tobusch fishhook cactus (Sclerocactus brevihamatus ssp. tobuschii) is a small (<~7 cm diameter), globose cactus endemic to the Edwards Plateau of central Texas. This species grows in open sites with shallow, rocky soils, surrounded by oak-juniper woodlands and shrublands. Fire is thought to maintain the open character of the sites, but few studies have examined the effects of fire on this species. We investigated the effects of two prescribed fires on Tobusch fishhook cacti (n > 600) in long-term monitoring plots. The fires killed only one cactus and visibly damaged <28% of cacti, none of which died within the first two growing seasons. Visibly damaged cacti grew less than undamaged cacti in the first and/or second growing seasons, but fruit set was not reduced. Fire damage was best predicted by cactus diameter (>1 cm), cactus height (>1 cm), and proximity to bunchgrasses (<3 cm). Prescribed fire can be used to maintain Tobusch fishhook cactus habitat but should avoid flowering/fruiting seasons (~January through May) and years with above-average rainfall that create high fuel loads.

Native tree and shrub canopy facilitates oak seedling regeneration in semiarid woodland

AbstractOaks (Quercus spp.) are currently declining worldwide due to a multitude of threats. Woodland management is often heavily focused on thinning and burning nontarget species to reduce competition and promote oak dominance in the overstory. These techniques have typically been developed in temperate regions, such as eastern U.S. forests, but whether they are the most effective strategy for promoting oak regeneration in semiarid woodlands has not been sufficiently examined. We conducted our study on the eastern Edwards Plateau in central Texas, where several oak species are codominant with Ashe juniper (Juniperus ashei) over karst limestone terrane. These juniper‐oak woodlands provide habitat for many endemic species and play an essential role in the maintenance of key aquifers. A history of canopy clearing and a severe drought in 2011 generated patches of living and dead juniper‐oak canopy interspersed with canopy gaps in our study area. In November 2013, we planted 200 shin oak (Quercus sinuata var. breviloba) acorns in each of three habitat treatments, replicated six times: (1) live canopy, (2) dead canopy, and (3) open invasive grassland. We monitored emergence and survival each year, harvesting five seedlings from each replicate in October 2017 to assess shoot height, rooting depth, biomass, ectomycorrhizal colonization, and foliar nutrients. Canopy trees, living or dead, significantly enhanced seedling emergence and survival. Survival was positively associated with increasing Ashe juniper and oak basal area, shrub cover, and soil organic matter, and negatively associated with increasing canopy gap size (complete mortality in gaps &gt;220 m2). Seedling biomass increased significantly in dead and open treatments along with foliar nutrients N, P, and S (dead treatments) or S and Fe (open treatments), whereas ectomycorrhizal colonization and foliar nutrients Ca, Mg, and Mn increased under living canopy. Our findings suggest that oak regeneration in these juniper‐oak woodlands closely resembles that of Mediterranean regions, where canopy facilitates seedling survival. Both living and dead trees and shrubs enhanced oak regeneration, with seedling survival depending on proximity to larger trees, living roots, shrubs, and juniper‐oak canopy. Planting acorns under canopy is an inexpensive, sustainable, and effective restoration practice in drought‐prone ecosystems.

Growth and Habitat Use of Guadalupe Bass in the South Llano River, Texas

Abstract Predicting how stream fishes may respond to habitat restoration efforts is difficult, in part because of an incomplete understanding of how basic biological parameters such as growth and ontogenetic habitat shifts interact with flow regime and riverscape ecology. We assessed age-specific Guadalupe Bass Micropterus treculii habitat associations at three different spatial scales in the South Llano River, a spring-fed stream on the Edwards Plateau of central Texas, and the influence of habitat and flow regime on growth. We classified substrates using a low-cost side-scan sonar system. We used scale microstructure to determine age and to back-calculate size at age. Over 65% of captured Guadalupe Bass were age 2 or age 3, but individuals ranged from 0 to 7 y of age. Habitat associations overlapped considerably among age classes 1–3+, but age-0 Guadalupe Bass tended to associate with greater proportions of pool and run mesohabitats with submerged aquatic vegetation. Although habitat metrics across multiple scales did not have a large effect on growth, river discharge was negatively correlated with growth rates. Understanding age-specific Guadalupe Bass habitat associations at multiple scales will increase the effectiveness of restoration efforts directed at the species by assisting in determining appropriate ecological requirements of each life-history stage and spatial scales for conservation actions.

A comprehensive approach uncovers hidden diversity in freshwater mussels (Bivalvia: Unionidae) with the description of a novel species.

Major geological processes have shaped biogeographical patterns of riverine biota. The Edwards Plateau of central Texas, USA, exhibits unique aquatic communities and endemism, including several species of freshwater mussels. Lampsilis bracteata (Gould, 1855) is endemic to the Edwards Plateau region; however, its phylogenetic relationship with other species in the Gulf coastal rivers and Mississippi River basin is unknown. We evaluated phylogenetic relationships, shell morphologies and soft anatomy characters of L.bracteata and a closely related congener, Lampsilis hydiana (Lea, 1838) throughout their ranges. Our results showed the presence of an undescribed species: Lampsilis bergmanni sp.n. Lampsilis bracteata and L.bergmanni sp.n. share similar shell morphologies and soft anatomy characters; however, they are genetically distinct. Geological processes, such as faulting and sea-level changes during the Miocene to Pliocene, are likely to have facilitated diversification of Lampsilis species, resulting in isolation of L.bracteata on the Edwards Plateau and diversification between L.bergmanni sp.n. and L.hydiana. We conclude that L.bracteata range is restricted to the Colorado River basin, whereas L.bergmanni sp.n. occurs only in upstream reaches of the Guadalupe River basin. Conservation actions are warranted for both species due to their restricted distributions and potential anthropogenic threats.

The Early Life History of the Guadalupe Bass: Lessons for Culturing a Threatened Species

AbstractGuadalupe Bass Micropterus treculii fingerlings are produced at the A. E. Wood State Fish Hatchery in San Marcos, Texas, and stocked into streams draining the Edwards Plateau in central Texas as part of a conservation program to restore threatened populations and combat hybridization with Smallmouth Bass M. dolomieu. Despite an intensive conservation effort, much of the early life history (including morphological development and first feeding characteristics) required for the intensive production of Guadalupe Bass is unreported in the mainstream literature. A better understanding of this critical period may improve culture practices, fingerling production, and management efforts. We documented spawning and early developmental characteristics and milestones over a temperature range of 20–24°C and the prey composition of Guadalupe Bass in a hatchery setting. Eggs and fry were photographed under a microscope and observed in culture tanks and ponds from initial stocking to fingerling harvest. A dominant‐male, polygynous spawning structure was confirmed. Egg incubation (28–29 growing degree‐days), larval development, yolk depletion, swim‐up, and life stage transition proceeded faster at higher temperature. Eggs (mean diameter, 2.1 mm), fry (mean hatchling TL, 4.7 mm), and yolk reserves (mean hatchling yolk volume, 2.02 μL) were larger than reported for Largemouth Bass M. salmoides. Normally developed fry commenced exogenous feeding on a variety of zooplankton soon after swim‐up (120–144 h posthatch and 6.95–8.05 mm TL at 23.4°C), while yolk was still present, and consumed progressively larger aquatic invertebrates over time. Swim‐up fry with a range in gape width of 0.75–0.89 mm readily consumed brine shrimp Artemia spp. nauplii as first feed. Guadalupe Bass appear to be a relatively pliable species, and current culture practices are generally sound. The observations presented can be used to refine culture and management practices.

High-resolution inversion of OMI formaldehyde columns to quantify isoprene emission on ecosystem-relevant scales: application to the southeast US

Abstract. Isoprene emissions from vegetation have a large effect on atmospheric chemistry and air quality. “Bottom-up” isoprene emission inventories used in atmospheric models are based on limited vegetation information and uncertain land cover data, leading to potentially large errors. Satellite observations of atmospheric formaldehyde (HCHO), a high-yield isoprene oxidation product, provide “top-down” information to evaluate isoprene emission inventories through inverse analyses. Past inverse analyses have however been hampered by uncertainty in the HCHO satellite data, uncertainty in the time- and NOx-dependent yield of HCHO from isoprene oxidation, and coarse resolution of the atmospheric models used for the inversion. Here we demonstrate the ability to use HCHO satellite data from OMI in a high-resolution inversion to constrain isoprene emissions on ecosystem-relevant scales. The inversion uses the adjoint of the GEOS-Chem chemical transport model at 0.25∘ × 0.3125∘ horizontal resolution to interpret observations over the southeast US in August–September 2013. It takes advantage of concurrent NASA SEAC4RS aircraft observations of isoprene and its oxidation products including HCHO to validate the OMI HCHO data over the region, test the GEOS-Chem isoprene oxidation mechanism and NOx environment, and independently evaluate the inversion. This evaluation shows in particular that local model errors in NOx concentrations propagate to biases in inferring isoprene emissions from HCHO data. It is thus essential to correct model NOx biases, which was done here using SEAC4RS observations but can be done more generally using satellite NO2 data concurrently with HCHO. We find in our inversion that isoprene emissions from the widely used MEGAN v2.1 inventory are biased high over the southeast US by 40 % on average, although the broad-scale distributions are correct including maximum emissions in Arkansas/Louisiana and high base emission factors in the oak-covered Ozarks of southeast Missouri. A particularly large discrepancy is in the Edwards Plateau of central Texas where MEGAN v2.1 is too high by a factor of 3, possibly reflecting errors in land cover. The lower isoprene emissions inferred from our inversion, when implemented into GEOS-Chem, decrease surface ozone over the southeast US by 1–3 ppb and decrease the isoprene contribution to organic aerosol from 40 to 20 %.

Early Miocene continental-scale sediment supply to the Gulf of Mexico Basin based on detrital zircon analysis

The early Miocene was a period of major continental margin progradation in the Gulf of Mexico Basin that accompanied prominent tectonic and climatic changes in North America. However, sediment pathways from continental upland sources to deep basinal sinks remain poorly constrained. This study presents 2192 new detrital zircon U-Pb analyses from 19 Lower Miocene samples spanning the entire northern Gulf of Mexico margin to elucidate early Miocene sediment provenance and paleodrainage systems. The U-Pb age patterns indicate that the Great Plains, southern Rocky Mountains, and mid-Cenozoic volcanic field were the major source terranes for the western-central Gulf of Mexico coast, whereas the Appalachian foreland basin and Appalachian Mountains mainly contributed sediment to the eastern Gulf of Mexico coast. Local source terranes included the Llano uplift and Edwards Plateau in central Texas and the Ouachita Mountains and foreland basin in Oklahoma and Arkansas. A comparison to previous detrital zircon studies around the Gulf of Mexico indicates that sediment recycling was important during the early Miocene. Sediment associated with major paleorivers, including the Rio Bravo, Rio Grande, Houston-Brazos, Red, Mississippi, Tombigbee, and Apalachicola Rivers, can be differentiated using the detrital zircon U-Pb analyses. These data help to better define the early Miocene source-to-sink system in the northern Gulf of Mexico, by relating the basin fill to hinterland tectonic and geological evolution. In comparison to the Paleocene−Eocene Wilcox drainage system, the early Miocene drainage system of the northern Gulf of Mexico was smaller and received less input from western Mexico arc terranes and Archean basement in Wyoming. This drainage area reduction, related to regional thermal uplift and Basin and Range−Rio Grande rifting, likely explains the reduced sediment volume of the Lower Miocene strata in the Gulf of Mexico relative to the Paleocene−Eocene Wilcox Group.

Phylogeographic inference using Bayesian model comparison across a fragmented chorus frog species complex.

Fragmented species complexes provide an interesting system for investigating biogeographic history and the present distribution of genetic variation. Recent advances in sequencing technology and statistical phylogeography enable the collection and rigorous analysis of large multilocus data sets, but designing studies that produce meaningful phylogeographic inferences remains challenging. We implemented a Bayesian model comparison approach to investigate previous biogeographic hypotheses while simultaneously inferring the presence of genetic structure in a chorus frog species complex. The Illinois chorus frog (Pseudacris illinoensis), originally described as a subspecies of the broadly distributed Strecker's chorus frog (Pseudacris streckeri), occurs in small, disjunct regions associated with scarce sand prairie habitats that have been impacted by human development. We used high-throughput sequencing to develop and collect a multitiered genetic data set comprised of three different marker types (23 anonymous nuclear sequence loci, four mitochondrial genes and 14 microsatellite loci) designed to address questions across different evolutionary timescales. Phylogenetic analyses uncovered a deep divergence between populations in the Edwards Plateau of central Texas and all other P.streckeri/P. illinoensis populations, but suggest the disjunct distribution of P.illinoensis occurred more recently. Our best-supported migration model is consistent with the hypothesis that central Texas represented a refugium from which populations expanded via multiple routes. This model also indicates that disjunct northern and southern regions of P.illinoensis should be considered genetically distinct management units. Our study provides an evolutionary context for future studies and conservation efforts in P.illinoensis and demonstrates the utility of model-based approaches for phylogeographic inference.

Evaluating habitat associations of a fish assemblage at multiple spatial scales in a minimally disturbed stream using low‐cost remote sensing

Abstract Habitat heterogeneity at multiple scales is a major factor affecting fish assemblage structure. However, assessments that examine these relationships at multiple scales concurrently are lacking. The lack of assessments at these scales is a critical gap in understanding as conservation and restoration efforts typically work at these levels. A combination of low‐cost side‐scan sonar surveys, aerial imagery using an unmanned aerial vehicle, and fish collections were used to evaluate the relationship between physicochemical and landscape variables at various spatial scales (e.g. micro‐mesohabitat, mesohabitat, channel unit, stream reach) and stream–fish assemblage structure and habitat associations in the South Llano River, a spring‐fed second‐order stream on the Edwards Plateau in central Texas during 2012–2013. Low‐cost side‐scan sonar surveys have not typically been used to generate data for riverscape assessments of assemblage structure, thus the secondary objective was to assess the efficacy of this approach. The finest spatial scale (micro‐mesohabitat) and the intermediate scale (channel unit) had the greatest explanatory power for variation in fish assemblage structure. Many of the fish endemic to the Edwards Plateau showed similar associations with physicochemical and landscape variables suggesting that conservation and restoration actions targeting a single endemic species may provide benefits to a large proportion of the endemic species in this system. Low‐cost side‐scan sonar proved to be a cost‐effective means of acquiring information on the habitat availability of the entire river length and allowed the assessment of how a full suite of riverscape‐level variables influenced local fish assemblage structure. Copyright © 2015 John Wiley &amp; Sons, Ltd.

The use of δ13C values of leporid teeth as indicators of past vegetation

δ13C records from vertebrate teeth offer an opportunity to gain insight into changes in past vegetation. Increasingly, teeth from small mammals are used for such purposes, but because their teeth grow rapidly, seasonal changes in vegetation are potentially a large source of variability in their carbon isotope composition, complicating interpretations of isotope data. To investigate the influence of seasonality on the stable isotope composition of fossil teeth, we constructed a Monte-Carlo-based model to simulate the effects of changes in the seasonal pattern of diet in leporid lagomorphs (rabbits and hares) on the distribution of δ13C values in random populations of leporid teeth from the Edwards Plateau in central Texas. Changes in mean-state, seasonal vegetation range, and relative season length manifest themselves in predictable ways through changes in the median, standard deviation, and skewness of simulated tooth δ13C populations, provided sufficient numbers of teeth are analyzed. This Monte Carlo model was applied to the interpretation of a 20,000year record of leporid tooth δ13C values from Hall's Cave on the Edwards Plateau in central Texas. Populations of teeth from different climate intervals (e.g., the late Glacial, Younger Dryas, and the Holocene) display changes in tooth δ13C distributions that may reflect changes in seasonal vegetation. The inferred changes in seasonality of vegetation based upon our modeling are consistent with hypothesized climatic changes. These results indicate that small mammal teeth can potentially provide unique insights into climate and vegetation on seasonal and longer timescales that complement other approaches, but should be interpreted with a careful consideration of local conditions, taxon ecology and physiology, and the dominant timescales of isotope variability in a rigorous statistical framework.

Response of pocket gophers (Geomys) to late Quaternary environmental change on the Edwards Plateau of central Texas

Fossils of pocket gophers (Geomys) recovered from Hall's Cave, Texas, comprise a large sample of specimens spanning the late Pleistocene through Holocene. Statistically adequate sample sizes, a well-constrained chronology and independent palaeoenvironmental data allowed for the testing of both taxonomic change and predictions of ecophenotypic change in the dentary of Geomys. A time-transgressive reduction of diastema length (DL) in Geomys was predicted based on known, regional palaeoenvironmental changes and known ecophenotypic response in the closely related genus, Thomomys. Statistical evaluation suggests that although DL in Geomys is correlated to tooth row length (TRL), changes in DL may reflect an independent ecophenotypic response. Testing was hampered by the inadequacy of TRL as a diagnostic feature of most Geomys from Texas. A statistical test using coefficient of variation revealed the presence of multiple species of Geomys in some stratigraphic levels. Reduced local richness of Geomys to a single species coincides with the loss of other burrowing taxa in the vicinity of Hall's Cave. Based on the results of an analysis of variance (ANOVA) test, no significant changes in DL occurred in populations of Geomys from Hall's Cave.

Using hydrogeochemical and ecohydrologic responses to understand epikarst process in semi-arid systems, Edwards plateau, Texas, USA

The epikarst is a permeable boundary between surface and subsurface environments and can be conceptualized as the vadose critical zone of epigenic karst systems which have not developed under insoluble cover. From a hydrologic perspective, this boundary is often thought of as being permeable in one direction only (down), but connectivity between the flow paths of water through the epikarst and the root systems of woody plants means that water moves both up and down across the epikarst. However, the dynamics of these flows are complex and highly dependent on variability in the spatial structure of the epikarst, vegetation characteristics, as well as temporal variability in precipitation and evaporative demand. Here we summarize insights gained from working at several sites on the Edwards Plateau of Central Texas, combining isotopic, hydrogeochemical, and ecophysiological methodologies. 1) Dense woodland vegetation at sites with thin to absent soils (0-30 cm) is in part supported by water uptake from the epikarst. 2) However, tree transpiration typically becomes water-limited in dry summers, suggesting that the plant-available fraction of stored water in the epikarst depletes quickly, even when sustained cave drip rates indicate that water is still present in the epikarst. 3) Flow paths for water that feeds cave drips become rapidly disconnected from the evaporation zone of the epikarst and out of reach for plant roots. 4) Deep infiltration and recharge does not occur in these systems without heavy or continuous precipitation that exceeds some threshold value. Thresholds are strongly correlated with antecedent potential evapotranspiration and rainfall, suggesting control by the moisture status of the epikarst evapotranspiration zone. The epikarst and unsaturated zone in this region can be conceptualized as a variably saturated system with storage in fractures, matrix porosity, and in shallow perched aquifers, most of which is inaccessible to the root systems of trees, although woody vegetation may control recharge thresholds. Keywords: hydraulic disconnection, precipitation thresholds, root zone, plant water use, recharge, epikarst storage, barometric pressure. DOI: 10.3986/ac.v42i2-3.670

The Siren Site and the Long Transition from Archaic to Late Prehistoric Lifeways on the Eastern Edwards Plateau of Central Texas

The Siren Site and the Long Transition from Archaic to Late Prehistoric Lifeways on the Eastern Edwards Plateau of Central Texas

Hydraulic responses to extreme drought conditions in three co-dominant tree species in shallow soil over bedrock

An important component of the hydrological niche involves the partitioning of water sources, but in landscapes characterized by shallow soils over fractured bedrock, root growth is highly constrained. We conducted a study to determine how physical constraints in the root zone affected the water use of three tree species that commonly coexist on the Edwards Plateau of central Texas; cedar elm (Ulmus crassifolia), live oak (Quercus fusiformis), and Ashe juniper (Juniperus ashei). The year of the study was unusually dry; minimum predawn water potentials measured in August were -8 MPa in juniper, less than -8 MPa in elm, and -5 MPa in oak. All year long, species used nearly identical water sources, based on stable isotope analysis of stem water. Sap flow velocities began to decline simultaneously in May, but the rate of decline was fastest for oak and slowest for juniper. Thus, species partitioned water by time when they could not partition water by source. Juniper lost 15-30 % of its stem hydraulic conductivity, while percent loss for oak was 70-75 %, and 90 % for elm. There was no tree mortality in the year of the study, but 2 years later, after an even more severe drought in 2011, we recorded 34, 14, 6, and 1 % mortality among oak, elm, juniper, and Texas persimmon (Diospyros texana), respectively. Among the study species, mortality rates ranked in the same order as the rate of sap flow decline in 2009. Among the angiosperms, mortality rates correlated with wood density, lending further support to the hypothesis that species with more cavitation-resistant xylem are more susceptible to catastrophic hydraulic failure under acute drought.

Morphological Variation among Populations of the Western Slimy Salamander on the Edwards Plateau of Central Texas

We conducted a morphometric analysis on Western Slimy Salamanders, Plethodon albagula, from each of the five mitochondrially defined groups that occur on the Edwards Plateau of central Texas. Although several groups are similar in body size and/or shape, multivariate analyses do find significant differences among groups, and several groups have clear differences for one or several characters. Thus, for several between-group comparisons, the morphological data are consistent with the mitochondrial data in arguing for cryptic lineages of slimy salamanders on the Edwards Plateau. These results demonstrate that despite the common interpretation of morphological conservatism in plethodontids, detailed morphometric studies can be used in taxonomic and evolutionary investigations of these salamanders. Lastly, male central Texas P. albagula are found to have a larger mean body size than females; this pattern of sexual size dimorphism (SSD) is reverse from that observed in nearly all other plethodontids with SSD.

Early Decomposition of Ashe Juniper (Juniperus ashei) Wood in Open and Shaded Habitat

Grasslands of the Edwards Plateau of central Texas have been extensively altered through woody species encroachment, particularly as a result of increasing abundance of the invasive native shrub, Ashe juniper (Juniperus ashei). Over the last several decades there has been widespread mechanical removal of the species. The wood is often left in place to decompose, either mulched or not. Where the wood is left to decompose might have some bearing on its rate of decomposition. This study was conducted to determine the rates of Ashe juniper wood decomposition as a function of open vs. shaded habitat and the potential effect of wood decomposition on nutrient inputs into this system. Wood decomposition in this arid ecosystem might be expected to occur more rapidly in shaded habitat where the moisture and temperature regimes would be more favorable for wooddecomposing fungi. On the other hand, during times of low rainfall we might expect wood to decompose more rapidly when exposed to high levels of ultraviolet radiation. In our experiment, we found no difference between open and shaded treatments. Wood biomass loss occurred rapidly over the first 3–4 mo of the study and slowed for the remaining 2 yr. Wood carbon (C) increased only slightly (7.3%), but nitrogen (N) increased significantly (176%). As a consequence of changes in wood nitrogen, C:N decreased through time. Results of this study suggest that the wood decomposition process in open and shaded habitats in this arid ecosystem during a time of low rainfall do not differ. Our findings also suggest that land managers aiming to establish native species following felling of Ashe juniper should do so in the first year when nutrient release from decomposing wood is the highest.

Geographic and genetic isolation in spring-associated Eurycea salamanders endemic to the Edwards Plateau region of Texas

Populations of neotenic, spring-associated salamanders of the genus Eurycea occupy discontinuous sites throughout the Edwards Plateau of central Texas and many warrant conservation attention. Here we used DNA sequence data from a nuclear (rag1) and a mitochondrial (ND4) gene to determine (1) the extent of genetic isolation among seven Edwards Plateau Eurycea populations and (2) the relationship between genetic distance and both geographic distance and hydrogeological features. Coalescent-based methods detected little gene flow among the sampled Eurycea populations, and we were unable to reject a model of complete isolation for any pair of populations. These findings were consistent with the relatively high genetic distances we detected among the sampled Eurycea populations (pairwise ϕST ranged from 0.249 to 0.924). We detected a positive correlation between genetic distance and geographic distance, which is consistent with a pattern of isolation by distance. However, while controlling for geographic distance, we did not detect a positive relationship between genetic distance and aquifer or river distance. Thus, we found no evidence that aquifers and/or rivers serve as dispersal corridors among isolated Eurycea populations. Based on these results, we have no evidence that re-colonization of spring sites by migrant salamanders following local extirpation would be likely. Our findings indicate that spring-associated Eurycea salamander populations occupying the Edwards Plateau region are genetically isolated, and that each of these populations should be considered a distinct management unit.

Subsurface stormflow is important in semiarid karst shrublands

In this paper we describe hillslope‐scale, rainfall‐simulation experiments on karst shrublands dominated by Ashe juniper. These simulations, designed to mimic flood‐producing rainfall events, were carried out at two sites separated by 206 km within the Edwards Plateau of Central Texas. Five hillslope plots were instrumented—two shrub‐covered (canopy) plots and three intercanopy plots measuring 12–14 m in length. We repeated the experiments on the canopy plots after removing the shrubs. For the canopy plots, both before and after shrub removal, 50% or more of the water applied exited the plots as subsurface stormflow and no overland flow occurred. For the intercanopy plots, subsurface stormflow amounted to less than 10% of the water applied and overland flow was between 10 and 50%. These experiments demonstrate the importance of subsurface stormflow in semiarid karst shrublands during flood events, and more generally highlight the fact that subsurface stormflow is important in some semiarid landscapes.