Local increases in woody plant cover have been documented in many arid and semi-arid ecosystems d... more Local increases in woody plant cover have been documented in many arid and semi-arid ecosystems during the past century, and the apparent shift from herbaceous to woody plant dominance has often been linked to overgrazing of cattle, fire suppression, and climate change. However, there have been very few long-term, large-scale analyses of woody plant cover change in arid and semi-arid ecosystems, owing to the difficulty of analyzing aerial photograph and satellite data in these spatially complex regions. We report on a study to assess regional changes in woody plant cover and biomass in a 500 km2 North Texas rangeland from 1937 to 1999 using a combination of historical aerial photography and Landsat 7 ETM+ satellite data. Subsequent change analyses indicated a 26 percent increase in woody plant cover for the entire region from 1937 to 1999. There were strong topo-edaphic and land management controls over the woody cover changes, with unmanaged riparian zones undergoing woody cover increases of 30 percent, while highly managed upland clay loam soils had increases of 13 percent. Poorly developed shallow clay soils with little management intervention had woody plant cover increases of 23 percent over the 63-year period. Aboveground carbon stocks increased from 252 to 309 Tg C, which represented a 23 percent increase across the 500 km2 and an average increase of 810-1000 g m-2 in 63 years. Other important effects include increased soil carbon and nitrogen stocks and subsequent increases in emissions of nitric oxide (NO) from soils in areas that have undergone woody plant encroachment.
An experiment was conducted during 1995-1997 at Truscott Brine Lake, Truscott, TX to determine wh... more An experiment was conducted during 1995-1997 at Truscott Brine Lake, Truscott, TX to determine whether any of 38 salt tolerant plant species could survive salt conditions at Truscott Brine Lake and tolerate the summer heat and winter cold of northwest Texas. Little is known regarding survival and growth of species native to salt marsh conditions of southern United States coastal regions with respect to climate which is more xeric and having a wider range of temperature extremes than coastal areas. The objective of this study was to examine the effect of pond salinity and shoreline position (proximity to pond edge) on survival of salt tolerant species and to determine which species may have potential to survive in this environment. Species selection was based on salt tolerance in Gulf and Atlantic coastal areas and in arid western U.S. Species were planted in and around 1-acre circular research ponds, 3 filled with high saline water (termed 'salt water') and 3 filled with less saline water (termed 'fresh water'). Species were planted in spoke arrangements beginning near the center of each pond and radiating outward. Each pond contained a spoke for each species, spokes were spaced uniformly, and species were assigned randomly to spokes within a pond. Three slope positions were designated on each pond spoke to correspond to 3 levels of plant submergence. These levels were position 1: never submerged; position 2: periodically submerged; and position 3: continuously submerged. Water was pumped periodically into each pond to maintain predetermined water levels. Species survival was affected by species genetics, slope position, pond x slope, species x slope, and pond x species x slope interaction. Species survival decreased as the level of submergence increased. In the summer of 1995 there was little effect of slope on species survival, but by the fall of 1997 slope positions 1, 2, and 3 had only 48%, 29%, and 9% survival, respectively. Survival rate decreased by 20% with each slope increment. Based on percentage survival sand cordgrass, saltmeadow cordgrass selection 9067788, and common reed are viable species for use on all slopes in fresh water. Saltmeadow cordgrass var. 'Sharp', 'selection 9067788', and 'Flageo' along with common reed and sand cordgrass are viable species in salt water.
A positive relationship was found between moisture stress in honey mesquite, Prosopis glandulosa ... more A positive relationship was found between moisture stress in honey mesquite, Prosopis glandulosa T9rr., and degree of infestation by -the twig girdler, Oncideres cingulata texana Horn, in north central Texas. Individual mesquite trees were subjected to several soil moisture treatments and levels of intraspecific competition to produce differential moisture stress among trees. Predawn leaf petiole water potential was measured in late August 1989, just prior to the adult, twig girdling stage, which occurred in September. Observations of girdling damage in November revealed that trees which had greater water stress (i.e., lower predawn leaf water potential) had a greater number of girdled twigs per unit canopy volume.
Honey mesquite (Prosopis glandulosa Torr.} canopy responses to fire were measured following 20 si... more Honey mesquite (Prosopis glandulosa Torr.} canopy responses to fire were measured following 20 single winter fires conducted in north Texas. Weather conditions during the fires, understory herbaceous fine fuel (fine fuel) amount and moisture content, fire temperature at 0 cm, 10-30 cm and 1-3 m above ground, and canopy responses were compared. Ten fires occurred on a site where fine fuel was a mixture of cool and warm season grasses (mixed site). The other 10 fires occurred on a site dominated by warm season grasses (warm site). When both sites were included in regressions, peak fire temperature at all heights was positively related to fine fuel amount. Fine fuel amount, fine fuel moisture content, air temperature (AT) and relative humidity (RH) affected fire temperature duration in seconds over 100°C (FTD100) at 1-3 m height, but not at ground level. Mesquite percent above-ground mortality (topkill) increased with increasing fine fuel amount, decreasing fuel moisture content, increasing AT, and decreasing RH. Percent foliage remaining on non-topkilled (NTK) trees was inversely related to fine fuel amount and AT, and positively related to fine fuel moisture content. Effect of fire on mesquite topkill and foliage remaining of NTK trees was strongly affected by RH at the warm site (r2 = 0.92 and 0.82, respectively), but not at the mixed site. This difference was due to RH affecting fuel moisture content (and subsequently fire behavior) to a greater degree at the warm than at the mixed site, because of the lower green tissue content in warm site grasses at the time of burning. Under adequate fine fuel amounts to carry a fire, mesquite canopy responses to fire (i.e., topkill vs. partial canopy defoliation) were largely determined by AT and RH conditions during the fire. This has implications if the management goal is to preserve the mesquite overstory for a savanna result instead of topkilling all trees.
Woody vegetation has encroached into areas once dominated by herbaceous land cover in arid and se... more Woody vegetation has encroached into areas once dominated by herbaceous land cover in arid and semiarid regions of the southwestern United States and around the world, resulting in documented changes to the biophysical and biogeochemical structure of these ecosystems during the past century. In North Texas rangelands, encroaching mesquite (Prosopis glandulosa var. glandulosa), a known nitrogen (N)-fixing species, has caused changes in aboveground biomass, which, in turn, have influenced carbon (C) and N storage in surface soils. However, the effect on N oxide (nitric-NO and nitrous-N 2 O oxide) emissions from the soils was unknown. We examined biotic (vegetation type and soil organic and inorganic N dynamics) and abiotic (soil moisture, temperature, and soil texture) controls over soil NO and N 2 O emissions across a gradient of aboveground Prosopis biomass growing on two soil types. Soil N oxide fluxes were dominated by NO emissions produced during nitrification. Aboveground biomass was the best spatial predictor of NO emissions, with values increasing 20-fold (0.04-0.78 mg NO-N·m Ϫ2 ·d Ϫ1 ) across a 70-fold biomass gradient (5-350 g/m 2 ). Emissions also covaried with soil pH and clay content. Microsite position, under or between the mesquite canopies, did not influence NO emission rates. NO fluxes were four times higher from clay loam than from shallow clay soils; however, soil N properties (total organic N and extractable inorganic N) and cycling rates (mineralization and nitrification) did not differ significantly across the sites. Temporally, NO emissions and nitrification potential were positively correlated with temperature, with precipitation events elevating NO emissions fourfold over a 24-h period and producing small amounts of N 2 O. We conclude that mesquite encroachment in these grasslands increases NO emissions in a spatially explicit manner influenced by the aboveground biomass and soil type, which is then temporally mediated primarily by temperature and secondarily by precipitation.
This study examined the potential of animals as vectors for mesquite seedling establishment via f... more This study examined the potential of animals as vectors for mesquite seedling establishment via fecal deposition. Cattle, deer, and coyote fecal material containing emerging mesquite was located in May 1994 on a clay loam site in north Texas and assigned to two grass competition treatments (clipped to 3-cm height, or undipped) and three moisture treatments (rainfall, rainfall + four 1.25-cm irrigations, or rainfall + one 10-cm irrigation). Except for a 2-week period in August 1993 when feces were deposited, cattle grazing was excluded from the site during the study and for several years prior to study initiation. Seedling emergence and survival were quantified throughout 1994, a dry year, and 1995, which had precipitation well above normal. By spring 1995, survival of 1994 seedlings (all treatments pooled) was 16.6, 9.3, and 1.3% at deer, cattle, and coyote fecal sites, respectively. Survival of 1994 seedlings was not affected by moisture or clipping treatments. New cohorts of emerging seedlings occurred from the same fecal sites during 1995. High rainfall during summer 1995 increased percent survival of 1995 seedlings when compared to survival of 1994 seedlings, even though grass growth was much greater in 1995 than in 1994. A strong trend emerged in which survival of 1995 seedlings appeared to be greater when surrounding perennial grasses were clipped (37.0% in clipped plots vs. 9.4% in undipped plots), but this was not statistically significant (P ^ 0.14). By spring 1996, average number of seedlings emerged per site (1994 and 1995 cohorts combined) was 7.8, 5.2, and 4.5; number of established seedlings per site was 0.8, 0.6, and 0.3; and percent survival was 12.1, 13.5, and 6.5% at cattle, deer, and coyote sites, respectively. Mesquite seedlings were capable of establishing from feces of all three animal vectors within dense, ungrazed stands of perennial grass.
Interest in the use of summer fires to restore southern Great Plains prairie ecosystems infested ... more Interest in the use of summer fires to restore southern Great Plains prairie ecosystems infested with woody plants and cactus is increasing, but information regarding effects on target and nontarget plant species is needed before this practice can be recommended. We review effects of summer and winter fires on mortality and growth of honey mesquite (Prosopis glandulosa), prickly pear (Opuntia phaeacantha), and grasses common to the southern mixed prairie, including C 4 midgrass sideoats grama (Bouteloua curtipendula), C 4 shortgrass buffalograss (Buchloe dactyloides), and C 3 midgrass Texas wintergrass (Nassella leucotricha). Summer fires were more effective than winter fires at top-killing mature mesquite, but plant mortality was Ͻ4% in all fire treatments. Prickly pear plant mortality was much greater following summer than winter fires. Sideoats grama, buffalograss, and Texas wintergrass were all tolerant of summer fire, although post-fire recovery rate was slower in sideoats grama than in the other species. Clipping once each spring reduced sideoats grama and Texas wintergrass standing crop in all treatments when measured 1 y after clipping, but effects of clipping ϩ summer fire were not negatively additive. The only time fire negatively affected Texas wintergrass was when spring clipping followed winter fire. In contrast, buffalograss standing crop was not reduced by clipping or clipping ϩ summer fire at 1 y after treatment. Results suggest that summer fire will not eradicate mesquite and that frequent fires either in summer or winter are necessary to maintain suppression. Grasses studied thus far have demonstrated a tolerance to fire in either season.
Gardner saltbush [Atriplex gurdneri (Moq.) D. Dietr.] seeds collected from the Red Desert Basin o... more Gardner saltbush [Atriplex gurdneri (Moq.) D. Dietr.] seeds collected from the Red Desert Basin of Wyoming were subjected to pretreatments of scarification (Se), washing (W), and stratification (St) to alleviate dormancy. Laboratory germination was evaluated. Subsequently, seedling vigor was observed by determining field emergence of similarly pretreated seeds spring planted at 1 irrigated and 2 dryland sites in Wyoming. Effects of l-cm and 3-cm planting depths on emergence were also evaluated. Seed was pretreated, then dehydrated with minimal impact on seed germination. Field emergence was much less than laboratory germination for all treatments at all sites, indicating that establishment for this species is related to poor seedling vigor as much as to seed dormancy. Moreover, when compared to untreated controls, relative responses to seed pretreatments often differed between laboratory and field trials. In the laboratory SC = W = St provided the greatest germination, whereas the best seed pretreatment for field establishment was SC + St. Washing had little effect on enhancing field emergence and appeared to inhibit effects of St in scarified seed. The most effective planting depth varied with climatic/edaphic severity of the site.
ABSTRACT Fire-resistant woody species have invaded many grassland and savanna ecosystems worldwid... more ABSTRACT Fire-resistant woody species have invaded many grassland and savanna ecosystems worldwide. Adults of many of these species are fire resistant because they resprout following fire; however, seedlings may be vulnerable to fire. Here we quantify effects of seasonal timing of fire and fireline intensity on seedling mortality response to fire in honey mesquite (Prosopis glandulosa Torr.), a native woody legume that has increased in abundance in southern Great Plains, USA grasslands. Scarified mesquite seeds were planted in plots with either mid-grass or tall-grass fuels in each of 2 years. Plots were burned in winter when seedlings were 10 or 22 months old or late summer when they were 17 months old. In the mid-grass fuel type, seedling mortality was greater (P ≤ 0.05) in summer-burned, 17-month-old seedlings (85%) than in winter-burned, 10-month-old seedlings (35%), suggesting summer fire may be important in limiting seedling establishment under lower-grass, fine-fuel conditions. The tall-grass fuel type yielded similar mortality responses to summer (89% and 79%) and winter (77% and 66%) fires in Cohorts 1 and 2, respectively (differences not significant at P ≤ 0.05). The study found a positive relationship between fireline intensity and seedling mortality, especially when only winter fires were included in the regression (r2 = 0.96). Summer fire may be most useful in lower-grass fuel amounts and may accomplish the same level of seedling mortality without the need for lengthy grazing deferral to accumulate sufficient grass fine fuel for a successful winter fire, as seedling mortality from winter fire depends on fireline intensity.
Pregermination treatments applied to seed of Gardner saltbush, [ALriplexgar&cri(Moq.) D. Dietr.] ... more Pregermination treatments applied to seed of Gardner saltbush, [ALriplexgar&cri(Moq.) D. Dietr.] were evahtated for alleviation of seed dormancy. Treatments selected simulated conditions the seed might be exposed to in its natural environment, including dry afterripening, scarification, leaching, and cold stratification. Germination response to individual treatments was equal to or higher than nontreated seed. Germination percentage of afterripened seed was increased from 17% for nontreated filled seed to an average of 86% of filled seed by the combined treatments of scarification, leaching, and 4-week stratification. This combination of treatments allowed optimum germination. Response to treatments provided evidence as to the type of dormancy in Gardner saltbush seeds. The levels of germination response to specific treatments appears to be an adaptation to ensure a temporal dispersal of dormancy release and seedling emergence. Pregermination treatments used in this study were relatively easy to apply to the seeds and stimulated germination without potentially damaging the embryo. Thus, they would be useful in revegetation by direct seeding efforts.
ABSTRACT Mesquite and similar woody plant species occupy over 28 million hectares in the southwes... more ABSTRACT Mesquite and similar woody plant species occupy over 28 million hectares in the southwestern USA and often negatively affect the rural economic sector by reducing livestock forage production, increasing bare ground, and interfering with livestock handling. Current control methods that temporarily suppress the growth of these woody plants include herbicide sprays, mechanical treatments, and prescribed fire. One alternative to current control practices is to utilize mesquite biomass for electricity generation. This study examined the economic feasibility of mesquite biomass as a potential bioenergy feedstock for electricity production using a Faustmann-type model with five mesquite biomass accumulation scenarios, which take into account variations in rates of standing biomass accumulation and tree density reestablishment after harvest with the aboveground-only (AG) harvest option or the whole-plant (WP) harvest option. The estimated regrowth functions were then incorporated into a modified version of the Faustmann model to determine the optimal harvesting length of mesquite that maximizes the net present value of a power plant using mesquite biomass. Results indicated that mesquite biomass has economic potential to be a source for electricity generation. A long-term bioenergy harvest system with regrowth following AG harvest was much more cost-effective than reestablishment from germination of new seedlings after WP harvest, even though harvest cost is higher in the AG than that in the WP harvest option. In addition, the utilization of mesquite for bioenergy would enhance other more traditional income streams from these lands including livestock grazing and wildlife hunting leases (with patterned harvesting of mesquite) and would reduce herbicide or mechanical costs that ordinarily are necessary for woody plant control. Compared to conventional coal, a lower economic return was found when the mesquite biomass was used for electricity production, but a possible carbon emission tax would make mesquite biomass competitive with conventional coal.
Honey mesquite (Prosopis glandulosa Torr.) which grows on grasslands and rangelands in southweste... more Honey mesquite (Prosopis glandulosa Torr.) which grows on grasslands and rangelands in southwestern USA may have potential as a bioenergy feedstock because of existing standing biomass and regrowth potential. However, regrowth mesquite physiognomy is highly different from undisturbed mesquite physiognomy and little is known regarding growth rates and structural biomass allocation in regrowth mesquite. We compared canopy architecture, aboveground biomass and relative allocation of biomass components in regrowth (RG) trees of different known ages with undisturbed (UD) trees of similar canopy height to each RG age class. RG trees in most age classes (2-12 years old) had greater canopy area, leaf area, basal stem number, twig (o0.5 cm diameter) mass and small stem (0.5-3 cm diameter) mass than UD trees of the same height. Large stem (43 cm diameter) mass was similar between RG and UD trees in all height classes. Ages of UD trees were determined after harvest and further comparisons were made between age, canopy structure and biomass in RG and UD trees. Relationships between age and total mass, age and height, and age and canopy area indicated a faster growth rate in RG than in UD trees. Large stem mass as a percentage of total tree mass accumulated more rapidly with age in RG than UD trees. Leaf area index and leaf : twig mass ratio were maintained near 1 in all RG and UD trees. Regrowth potential may be one of the most important features of mesquite in consideration as a bioenergy feedstock.
ABSTRACT Remote detection of non-native invasive plant species using geospatial imagery may signi... more ABSTRACT Remote detection of non-native invasive plant species using geospatial imagery may significantly improve monitoring, planning and management practices by eliminating shortfalls, such as observer bias and accessibility involved in ground-based surveys. The use of remote sensing for accurate mapping invasion extent and pattern offers several advantages, including repeatability, large area coverage, complete instead of sub-sampled assessments and greater cost-effectiveness over ground-based methods. It is critical for locating, early mapping and controlling small infestations before they reach economically prohibitive or ecologically significant levels over larger land areas. This study was designed to explore the ability of hyperspectral imagery for mapping infestation of musk thistle (Carduus nutans) on a native grassland during the preflowering stage in mid-April and during the peak flowering stage in mid-June using the support vector machine classifier and to assess and compare the resulting mapping accuracy for these two distinctive phenological stages. Accuracy assessment revealed that the overall accuracies were 79% and 91% for the classified images at preflowering and peak flowering stages, respectively. These results indicate that repeated detection of the infestation extent, as well as infestation severity or intensity, of this noxious weed in a spatial and temporal context is possible using hyperspectral remote sensing imagery.
Light to moderate populations (50-200 per m2) of black grass bugs in a seeded monoculture of cres... more Light to moderate populations (50-200 per m2) of black grass bugs in a seeded monoculture of crested wheatgrass appeared to reduce plant vigor. Vigor of ungrazed plants was compared to plants grazed only by black grass bugs and plants grazed by black grass bugs and cattle. Generally, vigor decreased as levels of grazing increased. Leaf length, seedhead height, root crown nonstructural carbohydrates, and plant color correlated positively with vigor while functions of axillary tillering such as basal area and seedhead density correlated negatively with vigor. Cattle grazing was not in excess of intensities recognized as proper for the area.
Mesquite (Prosopis glandulosa) and redberry juniper (Juniperus pinchotii) are two woody species w... more Mesquite (Prosopis glandulosa) and redberry juniper (Juniperus pinchotii) are two woody species which dominate uncultivated lands in the south central Great Plains, USA (Texas, Oklahoma) and may have potential for bioenergy utilization. In this study, these two biomasses were gasified in an adiabatic, updraft fixed-bed gasifier using air as medium. The products include low heating value gas and liquid tar yield. The effects of fuel moisture content and the equivalence ratio (ER) on liquid yields including heavy tar and light tar and higher heating value (HHV) of the producer gas were investigated. It was found that updraft gasifiers produce a large amount of tar, in a range of 100 g/Nm 3 of gas produced. As the equivalence ratio (ER) increased (i.e. more rich char conditions), peak temperature (T peak ) within the bed decreased due to the lesser partial oxidization. The liquid yield showed peak value with change in moisture content between 6 and 24%. Heavy tar is a very high quality fuel with a higher heating value of over 29 MJ/kg on a dry, ash-free (DAF) basis which is a slightly higher than that of ethanol.
Assessment of herbaceous standing crop in heterogeneous range plant communities requires large nu... more Assessment of herbaceous standing crop in heterogeneous range plant communities requires large numbers of samples to account for inherent variability. The dry-weight-rank method (DWR) was developed to eliminate the need for clipping and sorting of herbage to determine relative proportions on a dry weight basis. The technique was assessed for applicability and accuracy in the mixed prairie of the Texas Rolling Plains. Much of the herbage within the communities investigated occurred in monospecific patches that resulted in only 15% of quadrats having 3 species ranked for which DWR was designed. Non-harvest methods of determining grass proportion by species were compared to harvested proportions in mesquite (Prosopis glandulosa Torr.) and redberry juniper (Juniperus pinchotii Sudw.) communities. Estimation methods evaluated were 1) harvest by species, 2) weight estimation by species, 3) DWR with quadrat weighting, 4) unweighted estimated proportion by species, and 5) unweighted DWR.
Local increases in woody plant cover have been documented in many arid and semi-arid ecosystems d... more Local increases in woody plant cover have been documented in many arid and semi-arid ecosystems during the past century, and the apparent shift from herbaceous to woody plant dominance has often been linked to overgrazing of cattle, fire suppression, and climate change. However, there have been very few long-term, large-scale analyses of woody plant cover change in arid and semi-arid ecosystems, owing to the difficulty of analyzing aerial photograph and satellite data in these spatially complex regions. We report on a study to assess regional changes in woody plant cover and biomass in a 500 km2 North Texas rangeland from 1937 to 1999 using a combination of historical aerial photography and Landsat 7 ETM+ satellite data. Subsequent change analyses indicated a 26 percent increase in woody plant cover for the entire region from 1937 to 1999. There were strong topo-edaphic and land management controls over the woody cover changes, with unmanaged riparian zones undergoing woody cover increases of 30 percent, while highly managed upland clay loam soils had increases of 13 percent. Poorly developed shallow clay soils with little management intervention had woody plant cover increases of 23 percent over the 63-year period. Aboveground carbon stocks increased from 252 to 309 Tg C, which represented a 23 percent increase across the 500 km2 and an average increase of 810-1000 g m-2 in 63 years. Other important effects include increased soil carbon and nitrogen stocks and subsequent increases in emissions of nitric oxide (NO) from soils in areas that have undergone woody plant encroachment.
An experiment was conducted during 1995-1997 at Truscott Brine Lake, Truscott, TX to determine wh... more An experiment was conducted during 1995-1997 at Truscott Brine Lake, Truscott, TX to determine whether any of 38 salt tolerant plant species could survive salt conditions at Truscott Brine Lake and tolerate the summer heat and winter cold of northwest Texas. Little is known regarding survival and growth of species native to salt marsh conditions of southern United States coastal regions with respect to climate which is more xeric and having a wider range of temperature extremes than coastal areas. The objective of this study was to examine the effect of pond salinity and shoreline position (proximity to pond edge) on survival of salt tolerant species and to determine which species may have potential to survive in this environment. Species selection was based on salt tolerance in Gulf and Atlantic coastal areas and in arid western U.S. Species were planted in and around 1-acre circular research ponds, 3 filled with high saline water (termed 'salt water') and 3 filled with less saline water (termed 'fresh water'). Species were planted in spoke arrangements beginning near the center of each pond and radiating outward. Each pond contained a spoke for each species, spokes were spaced uniformly, and species were assigned randomly to spokes within a pond. Three slope positions were designated on each pond spoke to correspond to 3 levels of plant submergence. These levels were position 1: never submerged; position 2: periodically submerged; and position 3: continuously submerged. Water was pumped periodically into each pond to maintain predetermined water levels. Species survival was affected by species genetics, slope position, pond x slope, species x slope, and pond x species x slope interaction. Species survival decreased as the level of submergence increased. In the summer of 1995 there was little effect of slope on species survival, but by the fall of 1997 slope positions 1, 2, and 3 had only 48%, 29%, and 9% survival, respectively. Survival rate decreased by 20% with each slope increment. Based on percentage survival sand cordgrass, saltmeadow cordgrass selection 9067788, and common reed are viable species for use on all slopes in fresh water. Saltmeadow cordgrass var. 'Sharp', 'selection 9067788', and 'Flageo' along with common reed and sand cordgrass are viable species in salt water.
A positive relationship was found between moisture stress in honey mesquite, Prosopis glandulosa ... more A positive relationship was found between moisture stress in honey mesquite, Prosopis glandulosa T9rr., and degree of infestation by -the twig girdler, Oncideres cingulata texana Horn, in north central Texas. Individual mesquite trees were subjected to several soil moisture treatments and levels of intraspecific competition to produce differential moisture stress among trees. Predawn leaf petiole water potential was measured in late August 1989, just prior to the adult, twig girdling stage, which occurred in September. Observations of girdling damage in November revealed that trees which had greater water stress (i.e., lower predawn leaf water potential) had a greater number of girdled twigs per unit canopy volume.
Honey mesquite (Prosopis glandulosa Torr.} canopy responses to fire were measured following 20 si... more Honey mesquite (Prosopis glandulosa Torr.} canopy responses to fire were measured following 20 single winter fires conducted in north Texas. Weather conditions during the fires, understory herbaceous fine fuel (fine fuel) amount and moisture content, fire temperature at 0 cm, 10-30 cm and 1-3 m above ground, and canopy responses were compared. Ten fires occurred on a site where fine fuel was a mixture of cool and warm season grasses (mixed site). The other 10 fires occurred on a site dominated by warm season grasses (warm site). When both sites were included in regressions, peak fire temperature at all heights was positively related to fine fuel amount. Fine fuel amount, fine fuel moisture content, air temperature (AT) and relative humidity (RH) affected fire temperature duration in seconds over 100°C (FTD100) at 1-3 m height, but not at ground level. Mesquite percent above-ground mortality (topkill) increased with increasing fine fuel amount, decreasing fuel moisture content, increasing AT, and decreasing RH. Percent foliage remaining on non-topkilled (NTK) trees was inversely related to fine fuel amount and AT, and positively related to fine fuel moisture content. Effect of fire on mesquite topkill and foliage remaining of NTK trees was strongly affected by RH at the warm site (r2 = 0.92 and 0.82, respectively), but not at the mixed site. This difference was due to RH affecting fuel moisture content (and subsequently fire behavior) to a greater degree at the warm than at the mixed site, because of the lower green tissue content in warm site grasses at the time of burning. Under adequate fine fuel amounts to carry a fire, mesquite canopy responses to fire (i.e., topkill vs. partial canopy defoliation) were largely determined by AT and RH conditions during the fire. This has implications if the management goal is to preserve the mesquite overstory for a savanna result instead of topkilling all trees.
Woody vegetation has encroached into areas once dominated by herbaceous land cover in arid and se... more Woody vegetation has encroached into areas once dominated by herbaceous land cover in arid and semiarid regions of the southwestern United States and around the world, resulting in documented changes to the biophysical and biogeochemical structure of these ecosystems during the past century. In North Texas rangelands, encroaching mesquite (Prosopis glandulosa var. glandulosa), a known nitrogen (N)-fixing species, has caused changes in aboveground biomass, which, in turn, have influenced carbon (C) and N storage in surface soils. However, the effect on N oxide (nitric-NO and nitrous-N 2 O oxide) emissions from the soils was unknown. We examined biotic (vegetation type and soil organic and inorganic N dynamics) and abiotic (soil moisture, temperature, and soil texture) controls over soil NO and N 2 O emissions across a gradient of aboveground Prosopis biomass growing on two soil types. Soil N oxide fluxes were dominated by NO emissions produced during nitrification. Aboveground biomass was the best spatial predictor of NO emissions, with values increasing 20-fold (0.04-0.78 mg NO-N·m Ϫ2 ·d Ϫ1 ) across a 70-fold biomass gradient (5-350 g/m 2 ). Emissions also covaried with soil pH and clay content. Microsite position, under or between the mesquite canopies, did not influence NO emission rates. NO fluxes were four times higher from clay loam than from shallow clay soils; however, soil N properties (total organic N and extractable inorganic N) and cycling rates (mineralization and nitrification) did not differ significantly across the sites. Temporally, NO emissions and nitrification potential were positively correlated with temperature, with precipitation events elevating NO emissions fourfold over a 24-h period and producing small amounts of N 2 O. We conclude that mesquite encroachment in these grasslands increases NO emissions in a spatially explicit manner influenced by the aboveground biomass and soil type, which is then temporally mediated primarily by temperature and secondarily by precipitation.
This study examined the potential of animals as vectors for mesquite seedling establishment via f... more This study examined the potential of animals as vectors for mesquite seedling establishment via fecal deposition. Cattle, deer, and coyote fecal material containing emerging mesquite was located in May 1994 on a clay loam site in north Texas and assigned to two grass competition treatments (clipped to 3-cm height, or undipped) and three moisture treatments (rainfall, rainfall + four 1.25-cm irrigations, or rainfall + one 10-cm irrigation). Except for a 2-week period in August 1993 when feces were deposited, cattle grazing was excluded from the site during the study and for several years prior to study initiation. Seedling emergence and survival were quantified throughout 1994, a dry year, and 1995, which had precipitation well above normal. By spring 1995, survival of 1994 seedlings (all treatments pooled) was 16.6, 9.3, and 1.3% at deer, cattle, and coyote fecal sites, respectively. Survival of 1994 seedlings was not affected by moisture or clipping treatments. New cohorts of emerging seedlings occurred from the same fecal sites during 1995. High rainfall during summer 1995 increased percent survival of 1995 seedlings when compared to survival of 1994 seedlings, even though grass growth was much greater in 1995 than in 1994. A strong trend emerged in which survival of 1995 seedlings appeared to be greater when surrounding perennial grasses were clipped (37.0% in clipped plots vs. 9.4% in undipped plots), but this was not statistically significant (P ^ 0.14). By spring 1996, average number of seedlings emerged per site (1994 and 1995 cohorts combined) was 7.8, 5.2, and 4.5; number of established seedlings per site was 0.8, 0.6, and 0.3; and percent survival was 12.1, 13.5, and 6.5% at cattle, deer, and coyote sites, respectively. Mesquite seedlings were capable of establishing from feces of all three animal vectors within dense, ungrazed stands of perennial grass.
Interest in the use of summer fires to restore southern Great Plains prairie ecosystems infested ... more Interest in the use of summer fires to restore southern Great Plains prairie ecosystems infested with woody plants and cactus is increasing, but information regarding effects on target and nontarget plant species is needed before this practice can be recommended. We review effects of summer and winter fires on mortality and growth of honey mesquite (Prosopis glandulosa), prickly pear (Opuntia phaeacantha), and grasses common to the southern mixed prairie, including C 4 midgrass sideoats grama (Bouteloua curtipendula), C 4 shortgrass buffalograss (Buchloe dactyloides), and C 3 midgrass Texas wintergrass (Nassella leucotricha). Summer fires were more effective than winter fires at top-killing mature mesquite, but plant mortality was Ͻ4% in all fire treatments. Prickly pear plant mortality was much greater following summer than winter fires. Sideoats grama, buffalograss, and Texas wintergrass were all tolerant of summer fire, although post-fire recovery rate was slower in sideoats grama than in the other species. Clipping once each spring reduced sideoats grama and Texas wintergrass standing crop in all treatments when measured 1 y after clipping, but effects of clipping ϩ summer fire were not negatively additive. The only time fire negatively affected Texas wintergrass was when spring clipping followed winter fire. In contrast, buffalograss standing crop was not reduced by clipping or clipping ϩ summer fire at 1 y after treatment. Results suggest that summer fire will not eradicate mesquite and that frequent fires either in summer or winter are necessary to maintain suppression. Grasses studied thus far have demonstrated a tolerance to fire in either season.
Gardner saltbush [Atriplex gurdneri (Moq.) D. Dietr.] seeds collected from the Red Desert Basin o... more Gardner saltbush [Atriplex gurdneri (Moq.) D. Dietr.] seeds collected from the Red Desert Basin of Wyoming were subjected to pretreatments of scarification (Se), washing (W), and stratification (St) to alleviate dormancy. Laboratory germination was evaluated. Subsequently, seedling vigor was observed by determining field emergence of similarly pretreated seeds spring planted at 1 irrigated and 2 dryland sites in Wyoming. Effects of l-cm and 3-cm planting depths on emergence were also evaluated. Seed was pretreated, then dehydrated with minimal impact on seed germination. Field emergence was much less than laboratory germination for all treatments at all sites, indicating that establishment for this species is related to poor seedling vigor as much as to seed dormancy. Moreover, when compared to untreated controls, relative responses to seed pretreatments often differed between laboratory and field trials. In the laboratory SC = W = St provided the greatest germination, whereas the best seed pretreatment for field establishment was SC + St. Washing had little effect on enhancing field emergence and appeared to inhibit effects of St in scarified seed. The most effective planting depth varied with climatic/edaphic severity of the site.
ABSTRACT Fire-resistant woody species have invaded many grassland and savanna ecosystems worldwid... more ABSTRACT Fire-resistant woody species have invaded many grassland and savanna ecosystems worldwide. Adults of many of these species are fire resistant because they resprout following fire; however, seedlings may be vulnerable to fire. Here we quantify effects of seasonal timing of fire and fireline intensity on seedling mortality response to fire in honey mesquite (Prosopis glandulosa Torr.), a native woody legume that has increased in abundance in southern Great Plains, USA grasslands. Scarified mesquite seeds were planted in plots with either mid-grass or tall-grass fuels in each of 2 years. Plots were burned in winter when seedlings were 10 or 22 months old or late summer when they were 17 months old. In the mid-grass fuel type, seedling mortality was greater (P ≤ 0.05) in summer-burned, 17-month-old seedlings (85%) than in winter-burned, 10-month-old seedlings (35%), suggesting summer fire may be important in limiting seedling establishment under lower-grass, fine-fuel conditions. The tall-grass fuel type yielded similar mortality responses to summer (89% and 79%) and winter (77% and 66%) fires in Cohorts 1 and 2, respectively (differences not significant at P ≤ 0.05). The study found a positive relationship between fireline intensity and seedling mortality, especially when only winter fires were included in the regression (r2 = 0.96). Summer fire may be most useful in lower-grass fuel amounts and may accomplish the same level of seedling mortality without the need for lengthy grazing deferral to accumulate sufficient grass fine fuel for a successful winter fire, as seedling mortality from winter fire depends on fireline intensity.
Pregermination treatments applied to seed of Gardner saltbush, [ALriplexgar&cri(Moq.) D. Dietr.] ... more Pregermination treatments applied to seed of Gardner saltbush, [ALriplexgar&cri(Moq.) D. Dietr.] were evahtated for alleviation of seed dormancy. Treatments selected simulated conditions the seed might be exposed to in its natural environment, including dry afterripening, scarification, leaching, and cold stratification. Germination response to individual treatments was equal to or higher than nontreated seed. Germination percentage of afterripened seed was increased from 17% for nontreated filled seed to an average of 86% of filled seed by the combined treatments of scarification, leaching, and 4-week stratification. This combination of treatments allowed optimum germination. Response to treatments provided evidence as to the type of dormancy in Gardner saltbush seeds. The levels of germination response to specific treatments appears to be an adaptation to ensure a temporal dispersal of dormancy release and seedling emergence. Pregermination treatments used in this study were relatively easy to apply to the seeds and stimulated germination without potentially damaging the embryo. Thus, they would be useful in revegetation by direct seeding efforts.
ABSTRACT Mesquite and similar woody plant species occupy over 28 million hectares in the southwes... more ABSTRACT Mesquite and similar woody plant species occupy over 28 million hectares in the southwestern USA and often negatively affect the rural economic sector by reducing livestock forage production, increasing bare ground, and interfering with livestock handling. Current control methods that temporarily suppress the growth of these woody plants include herbicide sprays, mechanical treatments, and prescribed fire. One alternative to current control practices is to utilize mesquite biomass for electricity generation. This study examined the economic feasibility of mesquite biomass as a potential bioenergy feedstock for electricity production using a Faustmann-type model with five mesquite biomass accumulation scenarios, which take into account variations in rates of standing biomass accumulation and tree density reestablishment after harvest with the aboveground-only (AG) harvest option or the whole-plant (WP) harvest option. The estimated regrowth functions were then incorporated into a modified version of the Faustmann model to determine the optimal harvesting length of mesquite that maximizes the net present value of a power plant using mesquite biomass. Results indicated that mesquite biomass has economic potential to be a source for electricity generation. A long-term bioenergy harvest system with regrowth following AG harvest was much more cost-effective than reestablishment from germination of new seedlings after WP harvest, even though harvest cost is higher in the AG than that in the WP harvest option. In addition, the utilization of mesquite for bioenergy would enhance other more traditional income streams from these lands including livestock grazing and wildlife hunting leases (with patterned harvesting of mesquite) and would reduce herbicide or mechanical costs that ordinarily are necessary for woody plant control. Compared to conventional coal, a lower economic return was found when the mesquite biomass was used for electricity production, but a possible carbon emission tax would make mesquite biomass competitive with conventional coal.
Honey mesquite (Prosopis glandulosa Torr.) which grows on grasslands and rangelands in southweste... more Honey mesquite (Prosopis glandulosa Torr.) which grows on grasslands and rangelands in southwestern USA may have potential as a bioenergy feedstock because of existing standing biomass and regrowth potential. However, regrowth mesquite physiognomy is highly different from undisturbed mesquite physiognomy and little is known regarding growth rates and structural biomass allocation in regrowth mesquite. We compared canopy architecture, aboveground biomass and relative allocation of biomass components in regrowth (RG) trees of different known ages with undisturbed (UD) trees of similar canopy height to each RG age class. RG trees in most age classes (2-12 years old) had greater canopy area, leaf area, basal stem number, twig (o0.5 cm diameter) mass and small stem (0.5-3 cm diameter) mass than UD trees of the same height. Large stem (43 cm diameter) mass was similar between RG and UD trees in all height classes. Ages of UD trees were determined after harvest and further comparisons were made between age, canopy structure and biomass in RG and UD trees. Relationships between age and total mass, age and height, and age and canopy area indicated a faster growth rate in RG than in UD trees. Large stem mass as a percentage of total tree mass accumulated more rapidly with age in RG than UD trees. Leaf area index and leaf : twig mass ratio were maintained near 1 in all RG and UD trees. Regrowth potential may be one of the most important features of mesquite in consideration as a bioenergy feedstock.
ABSTRACT Remote detection of non-native invasive plant species using geospatial imagery may signi... more ABSTRACT Remote detection of non-native invasive plant species using geospatial imagery may significantly improve monitoring, planning and management practices by eliminating shortfalls, such as observer bias and accessibility involved in ground-based surveys. The use of remote sensing for accurate mapping invasion extent and pattern offers several advantages, including repeatability, large area coverage, complete instead of sub-sampled assessments and greater cost-effectiveness over ground-based methods. It is critical for locating, early mapping and controlling small infestations before they reach economically prohibitive or ecologically significant levels over larger land areas. This study was designed to explore the ability of hyperspectral imagery for mapping infestation of musk thistle (Carduus nutans) on a native grassland during the preflowering stage in mid-April and during the peak flowering stage in mid-June using the support vector machine classifier and to assess and compare the resulting mapping accuracy for these two distinctive phenological stages. Accuracy assessment revealed that the overall accuracies were 79% and 91% for the classified images at preflowering and peak flowering stages, respectively. These results indicate that repeated detection of the infestation extent, as well as infestation severity or intensity, of this noxious weed in a spatial and temporal context is possible using hyperspectral remote sensing imagery.
Light to moderate populations (50-200 per m2) of black grass bugs in a seeded monoculture of cres... more Light to moderate populations (50-200 per m2) of black grass bugs in a seeded monoculture of crested wheatgrass appeared to reduce plant vigor. Vigor of ungrazed plants was compared to plants grazed only by black grass bugs and plants grazed by black grass bugs and cattle. Generally, vigor decreased as levels of grazing increased. Leaf length, seedhead height, root crown nonstructural carbohydrates, and plant color correlated positively with vigor while functions of axillary tillering such as basal area and seedhead density correlated negatively with vigor. Cattle grazing was not in excess of intensities recognized as proper for the area.
Mesquite (Prosopis glandulosa) and redberry juniper (Juniperus pinchotii) are two woody species w... more Mesquite (Prosopis glandulosa) and redberry juniper (Juniperus pinchotii) are two woody species which dominate uncultivated lands in the south central Great Plains, USA (Texas, Oklahoma) and may have potential for bioenergy utilization. In this study, these two biomasses were gasified in an adiabatic, updraft fixed-bed gasifier using air as medium. The products include low heating value gas and liquid tar yield. The effects of fuel moisture content and the equivalence ratio (ER) on liquid yields including heavy tar and light tar and higher heating value (HHV) of the producer gas were investigated. It was found that updraft gasifiers produce a large amount of tar, in a range of 100 g/Nm 3 of gas produced. As the equivalence ratio (ER) increased (i.e. more rich char conditions), peak temperature (T peak ) within the bed decreased due to the lesser partial oxidization. The liquid yield showed peak value with change in moisture content between 6 and 24%. Heavy tar is a very high quality fuel with a higher heating value of over 29 MJ/kg on a dry, ash-free (DAF) basis which is a slightly higher than that of ethanol.
Assessment of herbaceous standing crop in heterogeneous range plant communities requires large nu... more Assessment of herbaceous standing crop in heterogeneous range plant communities requires large numbers of samples to account for inherent variability. The dry-weight-rank method (DWR) was developed to eliminate the need for clipping and sorting of herbage to determine relative proportions on a dry weight basis. The technique was assessed for applicability and accuracy in the mixed prairie of the Texas Rolling Plains. Much of the herbage within the communities investigated occurred in monospecific patches that resulted in only 15% of quadrats having 3 species ranked for which DWR was designed. Non-harvest methods of determining grass proportion by species were compared to harvested proportions in mesquite (Prosopis glandulosa Torr.) and redberry juniper (Juniperus pinchotii Sudw.) communities. Estimation methods evaluated were 1) harvest by species, 2) weight estimation by species, 3) DWR with quadrat weighting, 4) unweighted estimated proportion by species, and 5) unweighted DWR.
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