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Populations of Acer grandidentatum Nutt. (Bigtooth maple, Aceraceae = Sapindaceae) in central Texas are mostly found in isolated, deep, relatively remote, limestone canyons. Acer grandidentatum is found with a few other mostly deciduous species. Recruitment of juveniles has been reported to be lacking. One population of A. grandidentatum juveniles was found in a limestone canyon in a State Natural Area in Central Texas. Fifty juveniles were located. Wire enclosures were placed around half of the seedlings with half left in the open. In an adjacent canyon, 50 juvenile seedlings were planted in a similar habitat with adult A. grandidentatum trees nearby. Half were in enclosures and half in the open. Plant survival was followed for four growing seasons until November 2019. At the end of that time when survivals were compared between plants in enclosures and those in the open in both canyon communities, there was a significant difference in survival in both communities (χ2, P Odocoleus virginianus).

Quercus gambelii (Gambel’s oak) communities are found in the mountains of the western United States from Wyoming, Colorado, and Utah south into northern Mexico. Leaf gas exchange rates were measured for potential successional species in Q. gambelii communities. Daily average light level below the canopy was 125 μmol/m2/sec. Light response curves indicated that Pinus ponderosa and Q. gambelii had high maximum photosynthetic rates (14.13 and 11.21 μmol/m2/sec) and were sun species. Abies concolor (white fir) is a shade species with the lowest photosynthetic rate (3.71 μmol/m2/sec). At low light levels few differences in photosynthetic rates were found between the species. Pinus ponderosa and Q. gambelii maximum photosynthetic rates were reduced 71% - 73% in shade and the shade species maximum photosynthetic rates were reduced by 50% - 57%. Comparing annual gas exchange rates for all species showed that A. concolor had higher gas exchange rates and could replace Q. gambelii. Growth in height of Q. gambelii was a second order quadratic function reaching a plateau of about ten meters between 80 and 95 years. Growth estimates of height of A. concolor in canopy shade were exponential, which would allow seedlings to reach the Q. gambelii canopy in approximately 35 years. Abies concolor wood specific gravity is 56% lower than Q. gambelii, which means more carbon is put into growth in height to reach the canopy at low light levels and low photosynthetic rates. The additional shading it causes would further reduce Q. gambelii photosynthesis rates and prevent self-replacement in these Q. gambelii communities, leading to an A. concolor dominated community.

Bigtooth maple (Acer grandidentatum Aceraceae) is found in scattered relict populations across the southern Intermountain West of North America. The quadrat procedure was used to examine the woody plant population in a mesic site (canyon bottom near stream), a thirty-year-old deer exclosure (slightly upslope), and a xeric site (more upslope with shallower soil) at Lost Maples State Natural Area in Central Texas. All woody plants were identified, categorized as trees, saplings, or seedlings, and counted. The circumference of trees was measured. Bigtooth maple had the highest density of all seedling species, with its highest density at the mesic site and lowest in the exclosure. Both sapling and tree densities were highest in the exclosure. The mesic site did not contain bigtooth maple trees. Bigtooth maple total basal area was higher in the exclosure than at the xeric site, which had larger but fewer trees. Additionally, fifteen first-year seedlings were planted inside the exclosure and at the xe...

Coreopsis tinctoria (coreopsis, calliopsis, plains coreopsis, or golden wave) is an annual herbaceous Asteraceae with a broad geographic distribution mostly in the central and western United States. It co-occurs with Bouteloua curtipendula (sideoats grama) or other native C4 grasses. When grown together, C. tinctoria response variables decreased significantly in the presence of B. curtipendula aboveground, belowground, and total dry mass. The response variables included mean plant height, number of flower buds per plant, flowers per plant, as well as aboveground, belowground, and total dry mass. The presence of B. curtipendula belowground dry mass caused the greatest suppression of C. tinctoria belowground dry mass. When B. curtipendula tops were clipped to reduce grass aboveground dry mass (simulated herbivory), the percent survival of C. tinctoria plants increased from one percent in the no-clipping treatment to 18% in the neighbor removal treatment (100% clipped). Coreopsis tinctoria does not ...

Arbutus xalapensis Kunth (Texas madrone, Ericaceae) is found in western and central Texas, but its ecological characteristics are little known. Mature A. xalapensis plants were found in the Albert and Bessie Kronkosky State Natural Area in Kendall and Bandera counties, central Texas (29.740278 N, –98.838333 W). Habitats studied included Acer grandidentatum Nutt. (bigtooth maple) canyon bottoms, Juniperus ashei J.Buchholz/Quercus L. sp. (Ashe juniper/mixed oak) hillsides, and J. ashei uplands. Population characteristics were compared to habitat and surface geology. A total of 98 A. xalapensis plants were found in the communities studied. Differences were found for all parameters except height. Upland A. xalapensis trees had the lowest mean (±SD) density (2 ± 3 plants/ha), highest basal area (210 ± 190 cm2/plant), community elevation (573 ± 14 m) and height (4.09 ± 1.70 m/plant). Hillside communities had the highest density (61 ± 38 plants/ha), but lower basal area (110 ± 170 cm2/plant). The canyon bottom plant basal area was 80 ± 100 cm2/plant and density was 10 ± 5 plants/ha. The canyon bottoms had the deepest soil (27.2 ± 20.4 cm) and lowest elevation (510 ± 16 m). When geological substrates were examined, 128 additional plants were found. The largest plants were on the Fort Terrett member of the Edwards limestone (hard limestone, 230 ± 220 cm2/plant), but density was the lowest (2 ± 3 plants/ha). Whereas greater density was on the Upper Glen Rose limestone (softer limestone, 16 ± 7 plants/ha), where plants were smaller (110 ± 150 cm2/plant). Community type and geological substrate seem critical for growth of this species.

The presence of grassland biomes and species cannot be predicted by examining bottom up causes such as precipitation and temperature. Top down causes including herbivory and fire seem to be major controlling aspects with other factors secondary. We examined soil depth and competitive ability of two North American C4 grasses in a greenhouse experiment. Changes in dry mass were determined and competitive intensity was calculated for both species. Species were grown separately or together in pots 30, 90, or 180 cm deep. When grown in monoculture, Schizachyrium scoparium total and belowground dry mass increased from the 30 to 90 cm depth, with no further significant increase from 90 to 180 cm. Aboveground dry mass did not increase significantly with depth. Total dry mass of Buchloe dactyloides increased significantly with depth when grown in monoculture. Aboveground dry mass increased from 30 to 90 cm depth but not from 90 to 180 cm. Belowground dry mass of B. dactyloides did not increase significantly with depth. In 180 cm pots, 53% of S. scoparium root dry mass was in the top 30 cm; 74% of B. dactyloides root dry mass was in the top 30 cm. Roots of B. dactyloides were not found deeper than 90 cm. Aboveground dry mass of S. scoparium was not different in mixture or monoculture at any depth. Buchloe dactyloides aboveground dry mass in mixture was significantly lower than monoculture at the 30 cm depth, but not at 90 or 180 cm. The greatest competitive intensity was in the shallow soil pots. Soil depth could partially explain mosaics found in C4 grasslands where both species were found together with S. scoparium on deeper soils and B. dactyloides on shallower soils.

Populus tremuloides (quaking aspen) is found from the Pacific to the Atlantic Ocean in the northern United States and Canada, and at higher elevations in the western United States and northern Mexico. While P. tremuloides can reproduce sexually or asexually, it is primarily a clonal species in the intermountain west, reproducing vegetatively via root sprouts, yielding genetically identical stems or ramets. In west Texas, isolated, outlier stands occur in the Guadalupe, Davis, and Chisos Mountains at an elevation of approximately 2300 m. This study utilized seven microsatellites or simple sequence repeats (SSRs) to examine leaf samples from 10 widely separated stems in 10 isolated P. tremuloides individual stands within the Davis Mountains to determine the level of clonal and genetic diversity. We then examined differentiation among stands. Each stem sampled within a stand was genetically identical to all stems examined in that stand or was part of a clone. There were eight genetically identical clones from these ten stands, with three stands being genetically identical or part of the same clone. Many of the genotypes shared several of the same alleles and the remaining alleles were only a few base pairs apart. Some of these alleles have been previously identified in other western North American P. tremuloides stands. Microsatellites identified several triploid patterns consistent with possible aneuploidy, which is concurrent with previous studies.

Streptanthus bracteatus A. Gray, bracted twistflower (Brassicaceae), is a rare, endemic of the Edwards Plateau ecoregion of central Texas. We examined effects of light levels on the growth of S. bracteatus in a field experiment and also the combined effects of a canopy (+/-) and exclosure (+/-, herbivory) in a 2 × 2 factorial field experiment. Plants in the light experiment in high light had significantly greater growth compared to plants in the low light treatments. For most variables, plants in the factorial field experiment were negatively affected by herbivory (likely white-tailed deer, Odocoileus virginianus) and the presence of woodland canopy with significant interactions between herbivory (+/exclosure) and canopy treatments (+/-). Survival was 100% in exclosures and 58% when unprotected. Plants protected from herbivory and in the open had the greatest basal diameter, height, aboveground, belowground, and total dry mass, while those not protected from herbivory had much lower responses. Pl...

We examined two co-occurring species of Malvaceae in the savanna of central Texas to determine their photosynthetic response to varying levels of light. Abutilon theophrasti had a mean (±1 SD) density of 4 ± 4 plants/m2 in the open-grassland phase of the savanna, and a density of 1 ± 2 plants/m2 under canopy of woody mottes. Malvaviscus arboreus var. drummondii was not in the open-grassland phase and had a density of 3 ± 6 plants/m2 under canopy of woody mottes. Mean midday levels of light in the open and canopy were significantly different at 2,004 versus 192 µmol/m2/s, respectively. Maximum photosynthetic rate of A. theophrasti (34.6 ± 3.6 µM CO2/m2/s) occurred at a photosynthetic-flux density of 2,000 ± 0.0 µM /m2/s and was significantly greater than the maximum photosynthetic rate of M. arboreus var. drummondii (14.8 ± 2.2 µM CO2/m2/s), which occurred at a photosynthetic-flux density of 1,350 ± 173.0 µM /m2/s. Light saturation, light-compensation point, dark respiration rates, stomatal conduc...

Helianthus paradoxus Heiser (Asteraceae, puzzle sunflower), is a federally threatened hybrid species found in salt marshes of west Texas and New Mexico. Helianthus annuus L. (Asteraceae, common sunflower) is one of the parent species and is found throughout North America, but it is not present in the inland salt marshes where H. paradoxus is found. Helianthus paradoxus has previously been described as a halophyte, but its tolerance to Na2SO4, one of the major salts found in its habitat, has not been investigated. However, salinity has been identified as a major abiotic factor influencing the limited distribution of H. paradoxus populations. In this greenhouse study, the effects of elevated concentrations of Na2SO4 and NaCl, at equal ionic strengths (0.00, 0.09, 0.17, 0.34, and 0.51), on the survival and dry mass of both H. paradoxus and H. annuus were examined. In the three-way factorial experiment, the effects on dry mass observed were dependent on the species, the type of salt and the ionic str...

Coreopsis tinctoria (Asteraceae) is a widely-distributed North American prairie flowering C3 annual. We hypothesized priority effects of planting C. tinctoria before a C4 grass would decrease interspecific competition. Coreopsis tinctoria was grown in south-central Texas (29°35′N, 98°37′W) with Bouteloua curtipendula (Poaceae), a widespread C4 prairie grass. The two species were planted in a de Wit replacement series with plant ratios of 12:0, 10:2, 8:4, 6:6, 4:8, 2:10, and 0:12 (C. tinctoriaa: B. curtipendula). There were five planting offsets with C. tinctoria planted 60 d before B. curtipendula (–60), 30 d before (–30), at the same time (0), 30 d after (30), and 60 d after (60). There were significant species by offset, species by frequency, and offset by frequency interactions. Coreopsis tinctoria produced the greatest total dry mass per plant in the 60 d before offset. Coreopsis tinctoria was competitively favored when planted 60 d before B. curtipendula and B. curtipendula was favored when planted 30 and 60 d before C. tinctoria. Results suggest a synergistic benefit from both spatial gaps and priority effects.

Savannas are open communities with a woodland and grassland phase. Some species are restricted to one phase or the other with some in both. The central Texas Edwards Plateau is mainly a savanna community with an east to west rainfall gradient and a tree density gradient with both factors being reduced in the west. We examined gas exchange rates of sun and shade leaves of three perennial species of Asteraceae, Simsia calva (bush sunflower), Wedelia texana (hairy wedelia,) and Brickellia cylindracea (brickell-bush). Maximum photosynthetic rates (Amax), light saturation points (Lsat), ambient light levels, leaf mass per area, leaf conductance, and transpiration rates (E) for sun leaves were significantly greater than shade leaves for all three species. Mean Amax rates were 32.8, 23.2 and 21.2 µmolCO2/m2/s for sun leaves of S. calva, W. texana and B. cylindracea respectively. For shade leaves, rates were 21.3, 15.2 and 12.9 µmolCO2/m2/s respectively for the same species. Mean Rd (dark respiration) ra...

A greenhouse experiment was conducted to evaluate the competitive abilities of Schizachyrium scoparium (little bluestem) and Buchloe dactyloides (buffalo grass) in low and high nutrient soil. These species have overlapping ranges and co-occur in semi-arid North American grasslands. Both species showed increases of approximately 2 g plant −1 in aboveground dry mass when soil nutrients were added. Aboveground dry mass of S. scoparium was slightly higher in mixture than in monoculture in low nutrient soil; however, in high nutrient soil, aboveground dry mass in mixture was significantly less than in monoculture. Buchloe dactyloides aboveground dry mass was the same in mixture and monoculture in low nutrient soil; in high nutrient soil, aboveground dry mass was significantly higher in mixture than monoculture. The competitive abilities of these species in shallow soils are dependent upon soil resource levels. At low soil resources, competitive intensity is not great and the two species appear to be equal competitors. At high soil resources, competitive intensity is greater and B. dactyloides reduced the growth of S. scoparium in mixtures. Differences in competitive abilities suggest that during the establishment phase, B. dactyloides would dominate in shallow-high nutrient soils and that coexistence would occur in low nutrient soils.

Previous work with Quercus gambelii (Gambel's oak), a widespread oak species from the mountains of the western United States and northern Mexico, showed high juvenile density (< 3 cm in DBH [diameter at breast height] or less than 150 cm in height), but few or no small (3–5 cm in DBH) tree sized plants, suggesting a recruitment bottleneck. We postulate that there could be several factors preventing the recruitment of juveniles into the adult population, including reduced light levels, herbivory, competition for soil resources, or a combination of these factors. In this study, we evaluated the response of leaves of Q. gambelii to various light levels. Surface light levels and leaf gas exchange rates were measured for sun (open) and shade (understory) Q. gambelii seedlings in Q. gambelii communities in the Lincoln National Forest, New Mexico, USA. Mean daily photosynthetic flux densities (PFD) in the understory during the monsoon period (wet season) were 40 ± 46 µmol m−2 sec−1, or 6% of the open co...

Juniperus communities are found on over 50 × 106 ha in arid and semiarid habitats in southwestern North America. The drought tolerant sedge Carex planostachys occurs below the canopy in some of these communities. Cover and biomass of C. planostachys are high below the canopy and low in associated gaps. The purposes of this study were to investigate the temporal and spatial physiologic response of C. planostachys to abiotic changes, and determine it's light response characteristics from four contiguous microsites. Net photosynthesis was highest in spring when temperature was cooler and soil water higher, but low carbon uptake continued during summer drought. In addition, C. planostachys demonstrates a capacity to recover from extreme drought, despite water potential measured below −9.0 MPa. Based on physiological light response curves and gas-exchange measurements, C. planostachys appears tolerant of shaded and full sun habitats. Light levels below the canopy were reduced compared to the gaps, but light saturation of C. planostachys did not change and net CO2 uptake was only reduced slightly. Carbon uptake was coupled to light levels and not soil moisture. Observed differences in physiological attributes and variation in C. planostachys cover and biomass correspond to the presence or absence of the canopy. Low light compensation points, coupled with reduced respiratory demand, maximize photosynthetic gain in low light microsites. C. planostachys appears to acclimate across a range of light regimes, suggesting photosynthetic plasticity, allowing growth and survival in diverse light microhabitats. C. planostachys, tolerant of drought, appears anisohydric and demonstrates a capacity to acclimate to sun and shaded habitats, which could allow it to occur across a wider range of arid areas.

Populus tremuloides is one of the most widespread woody species in North America, occurring across Canada, the northern United States, and at higher elevations in the southwestern United States and parts of Mexico. In Texas, it is found only at the highest elevations in a few disjunct populations in the Guadalupe, Davis, and Chisos Mountains. We examined the structure of six isolated P. tremuloides stands in the Davis Mountains. Stand age ranged from 25 to 65 years. The density of tree-sized P. tremuloides was 429 to 15,520 stems/ha, average basal area was 8.2 to 305.9 cm2/plant, and total basal area 5.1 to 22.9 m2/ha. As stand age increased, density of P. tremuloides decreased and average basal area and total basal area increased. The highest density of seedlings and saplings was in the younger stands. Seventeen woody species were found, with four to 10 woody species in any one stand. While these stands were dominated by P. tremuloides, two stands had Quercus gambelii relative basal area of 34 a...

This study describes the distribution of species in a 21.6 ha inland salt marsh, consisting of three plant communities. The marsh community is along Leon Creek in the Diamond Y Spring Preserve in Pecos County near Fort Stockton, in western Texas (31°00.54′N, 102°55.49′W). From driest (highest elevation) to wettest (lowest elevation), the sequence is a Sporobolus airoides (alkali sacaton, Poaceae) grassland, a Distichlis spicata (saltgrass, Poaceae) grassland, and in the drainage a Schoenoplectus americanus (bulrush, Cyperaceae) marsh community. The communities were simple, with a total of 15 species found in the marsh. The mean plant cover in the salt marsh was 59%. The Sporobolus grassland covered 26% of the marsh and contained six species including S. airoides with 52 ± 36% cover and D. spicata with 13 ± 21% cover. The four other species were Helianthus paradoxus (Pecos or puzzle sunflower, Asteraceae), Suaeda calceoliformis (seepweed, Chenopodiaceae), Limonium limbatum (sea lavender, Plumbagin...

The structure of eight Quercus gambelii (Gambel's oak) communities in the Lincoln National Forest, New Mexico, USA were examined. Belt transects were used to estimate the density and basal area of the trees and the density of juvenile woody plants. In addition, diameter size-class distributions of Q. gambelii were examined to determine community development. The communities were estimated to be 109–137 years old and mid to late-successional. Total tree density was 3586–6480 plants/ha, with Q. gambelii having a relative density of 94–100%. Total basal area was 20.1–42.0 m2/ha, with Q. gambelii relative basal area 82–100%. The density and basal area of all other species present was low. Quercus gambelii juvenile density ranged from 1760 to 9160 plants/ha. Juveniles of all other species found were zero to 847 plants/ha. Based on Weibull analyses, all of the diameter size-class distributions of Q. gambelii were unimodal. There were few or no individuals in the smallest (1–5 cm) diameter size-classes, suggesting that recruitment of small Q. gambelii plants into the adult population may be below the replacement rate for these stands. Pooled size-class distributions for the other species were non-normal with most individuals in the smallest diameter size-classes. Low light levels below the canopy, a lack of canopy gaps, or browsing by Cervus elaphus (elk = red deer) may be the primary causes of poor recruitment because there were large numbers of Q. gambelii juveniles, but these individuals are not entering the small-tree size-class in any of the communities.

The influence of environmental factors on Juniperus seedling growth and mortality in Juniperus woodlands and their role in community replacement or succession is poorly understood. A full factorial field experiment was set up to investigate the influence of adult tree canopies, light levels and water and nutrient additions on Juniperus ashei seedling growth and mortality. Relative growth rates calculated from basal diameter, height and number of seedling branches were used as response variables in a four- way multivariate analysis of variance, using canopy, light levels, water and nutrient manipulations as factors. The pattern of seedling growth in the experiment was best explained with two significant three- way interactions between the canopy, light level and water treatments (F = 2.21, P = 0.04) and light level, water and nutrient treatments (F = 3.01, P = 0.03). Greatest seedling relative growth rates occurred with modified light levels associated with canopies removed, with supplemental water and nutrients. Mortality was increased by shading with supplemental water, added nutrients or a combination of supplemental water and nutrients. Conversely, mortality was not affected by canopy treatment, but it was reduced with high light levels, the additions of nutrients, water or combinations of nutrients and water. Evidence suggests that growth of J. ashei seedlings and eventual replacement of the adults is most strongly influenced by light levels, and at higher light levels, both water and nutrients interact to support continued growth.

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.

The effect of soil salinity on the growth and competition among Helianthus paradoxus, H. annuus, and H. petiolaris was examined in a greenhouse experiment to determine the role competition might have in leading to the narrow endemism of H. paradoxus. Helianthus paradoxus (the Pecos sunflower or puzzle sunflower) is a threatened annual species that is federally listed and only found in a few areas in west Texas and New Mexico. It is a diploid hybrid species that occurs in saline soils where its progenitors, H. annuus and H. petiolaris, are absent. The response of the target species to the competing species was usually dependent on soil salinity. Helianthus paradoxus was the better competitor in high‐saline soil and H. annuus the better competitor in low‐saline soil. Aggressivity values in low‐saline soil indicated the following competitive hierarchy: H. annuus > H. paradoxus > H. petiolaris. In the higher‐saline soils the competitive hierarchy was H. paradoxus > H. annuus > H. petiolaris. The ability of H....

▪ Abstract The composition and structure of the semiarid or desert grasslands of southwestern North America have changed over the past 150 y. Brushy or woody species in these communities have increased in density and cover. This increase in density of woody species is called brush encroachment because most of these species have been present in these communities at lower densities for thousands of years. The brushy or woody species were not introduced from other continents or from great distances. They are indigenous species that have increased in density or cover because of changes in local abiotic or biotic conditions. The brushy and woody plants are not the cause of these changes, but their increase is the result of other factors. The causes of changes that have led to the present woody-brushy composition of these semiarid grasslands has been difficult to determine. Warming of the climate seems to be a background condition, but the driving force seems to be chronic, high levels of herbivory by domestic animals. This herbivory has reduced the aboveground grass biomass, leading to the reduction of fine fuel and a concomitant reduction or complete elimination of grassland fires. This combination of factors favors the encroachment, establishment, survival and growth of woody plants. Less competition from grasses, dispersal of seeds of woody plants by domestic animals, and changes in rodent, lagomorph, and insect populations seem to modify the rate of change. Elevated levels of atmospheric CO2are not necessary to explain shrub encroachment in these semiarid grasslands. The direction of future change is difficult to predict. The density of brushy and woody plants will probably increase as will the stature and number of species. However, if soil nutrients increase, woody legumes may be replaced by other brushy or woody species. Reversing the changes that have been going on for 150 y will be a difficult, long-term, and perhaps impossible, task.

Achenes of Aster ericoides, Baccharis neglecta, Brickellia dentata, Eupatorium havavense, Gymnosperma glutinosum, Helianthus maximilianii, Verbesina virginica and Viguiera dentata were collected at maturity in autumn in southcentral Texas and tested for germination over a range of thermoperiods (15/6, 20/10, 25/15, 30/15 and 35/20°C)simulating those in the habitat from September to May. Achenes of Aster, Baccharis, Brickellia, Eupatorium and Gymnosperma were nondormant, thus cold stratification did not change the range of thermoperiods over which they germinated. However, cold stratification increased the percentage of Brickellia achenes germinating at 15/6 and 20/10°C. Achenes of Helianthus, Verbesina and Viguiera were conditionally dormant at maturity, and cold stratification decreased the minimum thermoperiod for germination of Helianthus, Verbesina and Viguiera achenes to 15/6°C and increased the maximum thermoperiod for germination of Verbesina achenes to 35/20°C.Thus, achenes of Helianthus and Viguiera exhibited a type 2 germination response pattern during dormancy break and those of Verbesina a type 3. Germination percentages of Helianthus, Verbesina and Viguiera achenes increased during 6–8 wk of incubation at 20/10 and 15/60°C, which simulate habitat temperatures from November to February. Therefore, winter field temperatures are favorable for dormancy loss as well as for germination. If the soil is too dry for germination of conditionally-dormant achenes in autumn, a reduction of the minimum temperature for germination during winter would allow them to germinate in midto late winter, several months before the onset of the high temperature and drought conditions of early summer.

Recent increases in woody plant density have occurred in grasslands worldwide, but both the cause and mechanisms involved in these changes have been elusive. Changes in grass biomass mediated by high levels of constant herbivory seem to be the pivotal reason. In this field study in central Texas, USA, effects of aboveground and belowground interference on the growth of seedlings of a deep-rooted, woody heliophyte, Prosopis glandulosa, were tested. The effects of two positions (gap or Bouteloua curtipendula grassland), two levels of aboveground interference (high or low light), and three levels of grass root interference (2, 20, and 40 cm deep root excluders) on P. glandulosa aboveground, belowground, and total dry mass were measured. The exclusion of belowground interference significantly increased aboveground, belowground, and total P. glandulosa dry mass, with the reduction of belowground interference to a depth of 20 cm maximizing P. glandulosa dry mass. Aboveground, belowground, and total dry mass of P. glandulosa were not significantly different when grown in gaps compared to grasslands, nor were there any significant differences when aboveground interference (shade) was imposed. However, the trend was for greater dry mass in gaps and high light. Data indicate that belowground interference from grass roots significantly reduces the dry mass of P. glandulosa, while aboveground interference has a lesser effect.

In the coastal prairies of Texas, C4 grasses dominate. However, reasons for spatial differences in composition of these prairies are uncertain, and the reasons for codominance of Schizachyrium scoparium and Paspalum plicatulum are unexplained. We completed a greenhouse experiment that examined the effects of neighbors, soil pH, added P, and N on the growth of P. plicatulum and S. scoparium, two C4 grasses. Intraspecific competition was essentially equal to interspecific competition, and soil factors seem to control dry mass of these two species. For P. plicatulum, addition of P to native soil increased above- and belowground dry mass 1.8-2.5 times, while P and N together increased dry mass 2.9-6.5 times. The response was pH dependent, with the greatest dry mass at low pH. Root: shoot ratios changed from 0.3 to ca. 1.0 as the pH decreased in native soil regardless of the P level. When N was added, ratios were ca. 0.3 across all pH and P treatments. For S. scoparium in native soil, there was little change i...

The chronological appearance and function of the woody legumes in many of the C4 grassland communities is still not completely understood or accepted by all ecologists or rangeland scientists. Vachellia, particularly V. farnesiana and V. rigidula; S. berlandieri; and Prosopis glandulosa appear to be early successional species that establishes on low nutrient or low nitrogen soils (Van Auken et al. 1985) or in grassland communities that have had high levels of disturbances via extensive continuous domestic grazing and no or few fires or probably both (Archer et al. 1988; Van Auken 2009). Two factors or conditions that may be limiting and that are very important for the establishment and growth of these species or community are surface light levels and soil nitrogen levels. These are factors suggested by Tilman as potentially responsible for the sequence of species in the resource ratio theory of community development in succession (Tilman 1985).

Apparently many former grasslands and savannas have changed because of anthropogenic disturbances caused by the introduction of numerous domestic grazing animals (cattle and other species), the reduction of light, fluffy fuel (grasses by herbivory), and a concomitant reduction of fire frequency (Van Auken 2000, 2009). In southwestern North America, these changes have led to encroachment of various woody species including species of Prosopis, Senegalia, and Vachellia (the latter two previously acacias). The conditions required for the establishment (encroachment), growth, and dominance of the woody plants in many of these former grasslands have been difficult to understand. Global change phenomena including increased levels of atmospheric CO2 and concomitant elevated temperature do not seem to be the main cause of the encroachment, which appears to be a management problem or phenomena (Van Auken 2000, 2009).

There are estimations suggesting that approximately 60 million ha of grasslands in the American southwest have been encroached by various woody species and converted to savannas, shrublands, or woodlands (Humphrey 1958; Grover and Musick 1990). Interestingly, this is more than the total estimated area of these grasslands (Laurenroth 1979). Thus, the composition, cover, density, productivity, and structure of most if not all of these grasslands have changed. In addition, it has been suggested that 220–330 million ha of all North American grasslands have been encroached by woody species (Knapp et al. 2008). Not all of this woody plant encroachment is by woody legumes, but a considerable amount is (Van Auken 2009). Prosopis, Senegalia, and Vachellia are genera with major encroaching species in southwestern North American grasslands, but they are not the only species.

The density of woody plants has increased in grasslands throughout the world, but the cause has been elusive, although changes in herbivory seem central. In this study, the importance of relative time of planting and clipping of Bouteloua curtipendula, a C 4 grass, on Prosopis glandulosa, a woody seedling, was evaluated in a 2-yr field study in central Texas, USA. Secondarily, effects of planting date, presence of the woody seedling, and clipping of the grass on the growth of the grass were evaluated. Relative growth rates (RGRs) of Prosopis glandulosa (honey mesquite) seedlings grown alone were seasonal, generally high in spring and decreasing as summer progressed. When P. glandulosa was planted earlier than Bouteloua curtipendula (side-oats grama), the grass had little or no effect on the woody seedling RGRs. However, the dry mass of P. glandulosa increased 2.3 times when the grass was clipped but was reduced 54% when the grass was unclipped. When the two species were started at the same time and the grass was not clipped, P. glandulosa RGRs decreased and remained zero over the 2 yr, dry mass was reduced 99.8%, and mortality was 80%. If both species were started at the same time and the grass was clipped, 1st- and 2nd-yr growth of P. glandulosa was comparable to that of seedlings grown alone; however dry mass was reduced 79%. When planted later than the clipped grass, dry mass of P. glandulosa was reduced 98% compared to dry mass when planted alone, while in unclipped plots there was 100% mortality. When B. curtipendula was planted early in the growing season, clipping increased aboveground grass dry mass 1.63 times; there was no increase when the grass was planted late. Evidence suggests that woody seedling growth in undisturbed C 4 grasslands would be very limited, and establishment requires gap formation.

When subjected to simulated habitat temperatures, achenes of six perennial Asteraceae from southcentral Texas came out of dormancy during summer. In the early stages of dormancy loss, achenes of Erigeron modestus, Gaillardia suavis and Hymenoxys scaposa germinated (to ≥ 10%) in light at 12/12 hr daily thermoperiods of 15/6, 20/10 and 25/15°C and those of Pinaropappus roseus at 15/6 and 20/10°C. After additional dormancy loss, achenes of these four species also germinated at 30/15 and 35/20°C. Achenes of these four species had a Type 1 germination response pattern, which heretofore has not been reported in perennial Asteraceae. Achenes of Chaptalia nutans first germinated (to ≥ 10%) at 20/10 and 25/15°C and those of Hymenopappus scabiosaeus at 20/10°C, but with further loss of dormancy achenes of both species also germinated at 15/6, 30/15 and 35/20°C. Thus, achenes of these two species had a Type 3 pattern. This is the first report of perennials in any family with a Type 3 response pattern whose seeds come out of dormancy during summer. Presence of annual and perennial Asteraceae with Type 2 in temperate eastern North America and annual and perennial Asteraceae with Type 1 in southcentral Texas causes us to conclude that climate is more important than the type of life cycle in determining the type of germination response pattern.

The growth of Schizachyrium scoparium (Michx.) Nash (little bluestem) and Paspalum plicatulum Michx. (brownseed paspalum), two C4 grasses, was examined in monoculture and in mixed culture in three soil depths. Effects of soil depth and competition were both significant. Growth of both species increased as soil depth increased from 30 to 180 cm in pot experiments. However, P. plicatulum dry mass increased approximately four times more than S. scoparium as soil depth increased. Schizachyrium scoparium dry mass was always greater in monoculture while P. plicatulum dry mass was always greater in mixture. Significant interspecific competition was not measured in the shallow soil treatment (30 cm) suggesting equal competition. Significant interspecific competition was measured in the deeper soil treatments (90 and 180 cm), and P. plicatulum dry mass was clearly greater in mixtures in the two deeper soils. Paspalum plicatulum does not appear to prevent the growth of S. scoparium roots in the deeper soil, but seems able to exploit deeper soil resources by proliferating roots at depths where S. scoparium roots do not seem capable of growth.

Patterns of change in temperature requirements for germination of achenes of four Texas winter annual Asteraceae were investigated as they afterripened at simulated summer habitat temperatures. In addition, temperature requirements for after-ripening were determined. In Gaillardia pulchella, Krigia gracilis, and Pyrrhopappus multicaulis, the maximum temperature for germination increased during afterripening (type 1 response pattern). This is the first report of a type 1 pattern in the Asteraceae. In Hymenoxys linearifolia, the maximum and minimum temperatures for germination increased and decreased, respectively, increasing to two the number of winter annuals with a type 3 response pattern. As in winter annuals in other plant families, achenes of the four species required exposure to moderately high temperatures (25:15, 30:15 °C) to gain the ability to germinate to high percentages at autumn temperatures in autumn. Of the 32 species of Asteraceae whose afterripening pattern has been investigated, 3 have type 1, 22 have type 2, and 7 have type 3. Evidence suggests that types 1 and 3 and types 2 and 3 are more closely related physiologically than are types 1 and 2. Key words: winter annuals, Asteraceae, achenes, seed germination, afterripening.

A 2 × 2 factorial experiment using light and nutrients as factors was carried out over 2½ years in a remnant, native Coastal Prairie of Texas. Light level, nutrient level, and date of sampling all had significant influences on total biomass, but changes were subtle, suggesting additional limiting factors. More obvious were changes in biomass of individual species, especially the dominant C4 grass species. Schizachyrium scoparium (Michx.) Nash (little bluestem), Paspalum plicatulum Michx. (brownseed paspalum), and Sorghastrum nutans (L.) Nash (Indian grass), three C4 grasses, maintained their biomass at low nutrient levels throughout the study. Sorghastrum nutans responded to higher nutrient levels with increased biomass, whereas the biomass of S. scoparium and P. plicatulum decreased. Results suggest that remnant grassland communities dominated by P. plicatum and S. scoparium are relatively stable communities maintained by yearly mowing, which removes biomass and nutrients. With the accumulation of litter, surface light levels would decrease, soil nutrients would increase, and S. nutans biomass would increase at the expense of P. plicatulum and S. scoparium. Key words: C4 grasses, fertilization, grassland, productivity, shading.

In an investigation of the causes of the invasion of woody plants into grasslands, competition between seedlings of Prosopis glandulosa Torr. and Bouteloua curtipendula (Michx.) Torr. was examined. Introduction of P. glandulosa into a B. curtipendula neighborhood significantly reduced P. glandulosa dry mass when compared to P. glandulosa growth alone. The greater the time interval from P. glandulosa germination to addition of B. curtipendula, the less interference the grass had on woody plant growth. Reciprocally, the greater the time interval from B. curtipendula germination to addition of P. glandulosa, the more interference the grass had on woody plant growth. Prosopis glandulosa belowground dry mass was

Both shade and herbaceous competition reduced the germination and growth of Prosopis glandulosa Torr. (honey mesquite). Natural and artificial shade was used and herbaceous competition was manipulated by clipping and herbicide. Greatest growth of P. glandulosa seedlings was in full sunlight when there were no competitors. Plants in shade without competitors or in full sunlight with competitors had intermediate growth, while those in shade with competitors were the smallest. Natural shading by P. glandulosa trees caused similar trends, although significant differences were not always found. Prosopis glandulosa germination in the laboratory was 93 ± 5% versus 2%-19% in the field. No P. glandulosa seedlings survived after one growing season in two grassland plots with high biomass, compared with 8% in the herbicided plot. Lack of P. glandulosa seedling germination and growth under mature P. glandulosa trees appears to be the result of reduced light levels and herbaceous plant competition. These factors also ...

Juniperus ashei woodlands are found extensively throughout the Edwards Plateau of central Texas. Prior studies indicate that seed germination and seedling survival in J. ashei was greatest under the canopy whereas growth was greatest at the canopy edge. To investigate the cause of these spatial differences, measurements of percent soil moisture and plant predawn xylem water potential (ψ p ) for J. ashei seedlings were made along transects from under a woodland canopy into an intercanopy patch. A soil moisture gradient was detected, with soil moisture highest under the woodland canopy and decreasing into the intercanopy patch. Soil moisture was lowest during the summer drought along this gradient. Mean soil moisture was highest at 68.4 ′ 7.0% (′SE) under the Juniperus canopy in May 1997 and lowest at 12.5 ′ 1.9% in the intercanopy patch during July 1997. Seedling ψ p under the Juniperus canopy ranged from values of -0.51 ′ 0.02 MPa in May to -6.59 ′ 0.00 MPa in August Plant ψ p at the canopy edge was -0.72 ′ 0.03 MPa in May and -6.50 ′ 0.00 MPa in August. No seedlings were found in the intercanopy patch. Spatial and temporal differences in seedling ψ p between the Juniperus woodland and canopy edge did exist, but differences were small (

Intercanopy bare patches are small, relatively circular communities with an exposed central bedrock, shallow soil, low plant cover, and dominated by non-vascular plants, annual grass- es, and forbs. Although these patches are relatively common in the Edwards Plateau of central Texas, their characteristics have never been described. Twelve circular, intercanopy bare patches in the Hill Country State Natural Area in Bandera and Medina counties, were examined using the line-point procedure. Mean cover (?SD) of plant, litter, rock, and bare soil was 23 - 3%, 23 ? 5%, 44 ? 9%, and 10 ? 4%, respectively. Approximately half of the total plant relative cover consisted of forbs with 25% grasses and 25% non-vascular plants. Fifty-seven species of flowering plants from 30 families were identified, including 27 annuals and 30 perennials. The most com- mon families were the Poaceae (9 species), Asteraceae (5 species), Euphorbiaceae (5 species) and Labiatae (4 species). Flowering plant species encountered in 10 to 12 of the intercanopy bare patches sampled were Chaetopappa bellidifolia, Evax prolifera, Croton monanthogynus, Spermolepis inter- mis, Centaurium texense, Galium virgatum, Sida abutifolia, and Schizachyrium scoparium. Nostoc commune, a common soil blue-green algae, was found in every intercanopy bare patch. Non-vascular plant cover was highest near the edge of the central bedrock and decreased with distance. Total plant and litter cover increased with distance from the central bedrock. Total annual cover was five times higher than total perennial cover and both increased with distance from the central bedrock. Mean number of species found per intercanopy patch was 31 ? 4 with a mean Simpson's diversity of 9.2 ?- 2.8. Soil depth increased with distance from the central bedrock, whereas rock cover decreased. Total plant, annual and perennial cover, and litter cover were positively related to soil depth; rock cover was negatively related to soil depth. Nostoc commune relative cover decreased with distance, but Chaetopappa bellidifolia relative cover increased with distance from the patch center, and then decreased near the outer edge of the patch. Heliotropium tenellum and Bothrichloa ischaemum relative cover increased with distance from the center of the patch, with no indication of a decrease near the outer edge. Nostoc commune relative cover was inversely related to soil depth. Relative cover of C. bellidifolia, H. tenellum and B. ischaemum was positively related to soil depth. Almost all soil chemical characteristics were the same from near the central bedrock area to the outer edge of the patch. Soils were slightly basic, high in calcium, low in nitrogen and organic matter. Shallow soil depth and intermittent water availability are probably the main factors that determine composition, including low cover and the high proportion of annuals, in these com- munities.

VAN AUKEN, 0. W. AND J. K. BUSH (Division of Life Sciences, The University of Texas at San Antonio, San Antonio, TX 78285). Influence of light levels, soil nutrients, and competition on seedling growth of Baccharis neglecta (Asteraceae). Bull. Torrey Bot. Club 117: 438-444. 1990.-All growth parameters measured for Baccharis neglecta Britt. (Roosevelt weed) increased as light levels increased with no indication of light saturation. In competition studies with Cynodon dactylon (L.) Pers. (bermuda grass), B. neglecta biomass increased significantly with increased soil resources, but only in monoculture. In all nutrient treatments and proportions when C. dactylon was present, B. neglecta growth was significantly reduced. Cynodon dactylon growth in monoculture also increased significantly with increased soil resources, but its growth was greater when grown with B. neglecta in all nutrient treatments and proportions. In the field, B. neglecta growth was greatest in full sunlight without herbaceous plants and least in full sunlight with herbaceous plants. Results suggest that B. neglecta is a heliophyte or sun plant that would not become established and grow in grasslands except in vegetation gaps and would only occur in old fields after disturbances.

The translocation, distribution, metabolism and environmental degradation of hexachlorophene were investigated in tomato plants (Lycopersicon esculentum Mill cv. beefsteak). All plants were grown under standardized conditions and treated with leaf-applied 14C-ring-labeled hexachlorophene (HCP). Treatment time ranged from 0 to 70 days. Autoradiographic analyses were performed on all plants. Selected plant tissues were extracted and chromatographed, using thin layer (TLC) and gas liquid chromatography (GLC). Hexachlorophene was not translocated from the plant leaves. No metabolites of hexachlorophene was found. A slight, but statistically nonsignificant, amount of HCP was lost from the leaves and the inert controls. At the end of the 70-day treatment, based on TLC and regression analysis of thin layer chromatographic plates, averages of 89.5% and 75.9% of the applied HPC remained unaltered on the treated plants and controls, respectively. This indicated that 10.5% and 24.1%, respectively, of the original HCP had been altered. Differences between the treatments and controls were statistically significant after 28 days of exposure. Further analyses of the above data, using gas chromatographic methods, showed that as many as 14 peaks were found in the treated samples and the controls, including the parent material. Ultraviolet photolysis seemed to be the mechanism responsible for alteration of the hexachlorophene. Three extracted chlorinated compounds have been identified by GC-mass spectral (MS) analysis including 2,2'-dihydroxy-3,5',6,6'-tetrachlorodiphenylmethane, 2,2'-dihydroxy-3,5,5',6,6'-pentachlorodiphenylmethane, and 2,2'-dihydroxy-3,3',5,5',6,6'-hexachlorodiphenylmethane (parent HCP). Eleven other electrophylic compounds have been found in various treated plant or control extracts. Further analyses will be necessary to verify the identification of the other degradation products.

forest in Amazonia, Venezuela. Acta Cientifica Venezolona 33:72-77. Uhl, C., K. Clark, H. Clark, and P. Murphy. 1981. Early plant succession after cutting and burning in the upper Rio Negro region of the Amazon basin. Journal of Ecology 69: 63 1-649. Webb, L. J., J. G. Tracey, and W. T. Williams. 1972. Regeneration and pattern in the subtropical rain forest. Journal of Ecology 60:675-695. Wilson, B. F. 1968. Red maple stump sprouts: development the first year. Harvard Forest Paper 18. Wyatt-Smith, J., and E. C. Foenander. 1962. Damage to regeneration as a result of logging. Malayan Forester 25: 40-44. Zahner, R., R. K. Myers, and C. J. Hutto. 1985. Crop tree quality in young piedmont oak stands of sprout origin. Southern Journal of Applied Forestry 9:15-20.

Translocation, distribution, metabolism and photolysis of hexachlorophene (HCP) were investigated in peanut plants (Arachis hypogea L., Spanish type) grown under standardized conditions and treated with l4C-ring-labeled HCP. Treatment time ranged from 0–114 days. Autoradiographic analyses were performed on all plants. Selected plant tissues were extracted and chromatographed, using both thin layer and gas liquid chromatography. No translocation of HCP was detected in the plant tissue. No HCP metabolites were found. Some HCP was lost from the leaves and inert controls at a specific rate per unit time. The rates were slightly different, being slower on the leaves than on the controls. At the end of the 114-day treatment and based on regression analysis of thin layer chromatographic plates, an average of 68% of the applied HCP remained unaltered on the treated plants and an average of 77% remained on the controls. This indicated that, respectively, 32 and 23% of the original HCP had been altered. This 9% difference was statistically significant. Upon further analyses of the above data, using gas chromatographic methods, as many as 14 peaks were found in the treated samples and the controls, including some parent material. Ultraviolet photolysis seems to be the mechanism responsible for alteration of the HCP on the treated plants and controls. Two photolysis products have been identified by gas liquid chromatography-mass spectral analysis. Twelve other electrophylic compounds have been found in various treated plant or control extracts. Further analyses will be necessary to verify the identification and quantification of the other degradation products.

The antimicrobial properties of trichlorofluoromethane, (Freon® 11), dichlorofluoromethane (Freon 21), and monochlorodifluoromethane (Freon 22) were tested at room temperature (22 °C) on various foodstuffs over a 12-week period. Changes in color, odor, and texture as well as precipitate formation, liquefaction, and pH were measured using samples of milk, eggs, fish, and shrimp. Dichlorofluoromethane exhibited the greatest microbial inhibition requiring 6.6 mg/ml to retard bacteria and fungal development in eggs and fish for 12 weeks, 13.1 mg/ml for shrimp, and 68 mg/ml to preserve milk. Trichlorofluoromethane was similar to dichlorofluoromethane, but monochlorodifluoromethane did not have any food preservation properties at the concentrations tested.Colony development of Streptococcus lactis was also investigated with the three previously mentioned fluorocarbons plus dichlorodifluoromethane (Freon 12). The decreasing order of inhibition of colony development was as follows: dichlorofluoromethane (Freon 21) > trichlorofluoromethane (Freon 11) = dichlorodifluoromethane (Freon 12) > monochlorodifluoromethane (Freon 22). The change in pH of S. lactis-seeded pasteurized milk was also inhibited by certain concentrations of dichlorofluoromethane.

Three fluorocarbons were tested to determine their effect on bacterial growth. Freon 11 and 21 in various concentrations had an inhibitory effect on selected test organisms, but Freon 22 had no effect. Both aerobic and anaerobic microorganisms, as well as gram-positive and gram-negative species, were included among the bacteria tested. Freon 11 and 21 caused a similar response with Freon 11 being more inhibitory to some species and Freon 21 more inhibitory to others. Inhibition was dependent on the concentration of the halocarbon and resulted in decreased respiration rates at all concentrations tested. Results reported here indicate that the action of the fluorocarbons tested is bactericidal rather than bacteriostatic. Serratia marcescens and Clostridium botulinum were the species most sensitive to the halocarbons tested.