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About 10,500 years ago at the beginning of the Holocene, the first humans in the north of Mexico found themselves in the middle of an aridification process that culminated about 4000 years ago in the modern Chihuahuan Desert, which is the largest desert in North America and the second most diverse on Earth with about 3400 plant species, including cacti that reach their greatest diversity in this region. Nearly 25% of the species are endemic, most notably in the Cuatro Cienegas Basin with 86 plant taxa. The climate is hot-dry with summer rain. Most of the region has calcareous soils, although there are important outcrops of gypsum in patchy arrangements throughout the region. These environmental pressures have generated a variety of adaptive strategies among the organisms evolving here, resulting in great species’ richness. Creosote bush (Larrea tridentata) dominates in the driest sites, frequently accompanied by lechuguilla (Agave lechuguilla) and tarbush (Flourensia cernua); in the bajadas (lowlands) we find the less drought-tolerant plants such as yuccas (Yucca spp.) and sotol (Dasylirion wheeleri). Grasslands integrate grass and shrub mosaics, with species such as bush muhly (Muhlenbergia porteri), bluegrass (Bouteloua gracilis), and purple three-awn (Aristida purpurea). In gypsum outcrops a diverse flora with several endemics is found. The vegetation is highly variable, responding to water availability, physical–chemical and biological dynamics of soils and fires, among other factors. Currently, there are serious pressures modifying the natural dynamics of the desert, such as the indiscriminate extraction of water, agricultural and livestock practices, water and soil contamination, and invasive species that threaten this unique place.

Graminivory by kangaroo rats (Dipodomys spp.) was investigated as a potential mechanism for the keystone role of these rodents in the dynamics of desert grasslands. Experiments confirmed that Ord's kangaroo rats (Dipodomys ordii) cut and consumed a large proportion of the tillers of three Chihuahuan Desert tussock-forming grass species. Field observations indicated that the characteristically cut grass tillers were absent from all-rodent and medium-sized kangaroo rat exclosures, but were frequent in large-sized kangaroo rat and rabbit exclosures, indicating that the medium-sized kangaroo rats (D. ordii, D. merriami) were responsible for grass cutting. Tiller waste as a percentage of peak standing crop ranged from 7% in grassland habitats to 0.7% in Flourensia cernua shrubland. Of the 13 species of perennial, tussock-forming grasses measured, only one, Muhlenbergia porteri, had no tillers cut by kangaroo rats. This study demonstrates that the keystone role of kangaroo rats in Chihuahuan Desert grassland ecosystems is probably the result of their graminivory.

The success of invasive aridland plants may depend on their utilization of precipitation not fully exploited by native species, which could lead to seasonally altered ecosystem carbon and water fluxes. We measured volumetric soil water across 25-cm profiles (θ25cm) and springtime whole-plant water- and carbon-fluxes of the exotic Lehmann lovegrass (Eragrostis lehmanniana) and a native bunchgrass, bush muhly (Muhlenbergia porteri), following typical (55 mm in 2009) and El Nino-enhanced accumulations (154 mm in 2010) in a SE Arizona savanna. Across both years, θ25cm was higher under lovegrass plots, with similar evapotranspiration (ET) between lovegrass and bush muhly plots. However, in 2010 transpiration (T) was higher in bush muhly than lovegrass, implying higher soil evaporation in lovegrass plots maintained similar ET. Net ecosystem carbon dioxide exchange (NEE) was similar between lovegrass and bush muhly plots in 2009, but was more negative in bush muhly plots following El Nino, indicating greater CO2 assimilation. Ecosystem respiration (R eco) and gross ecosystem photosynthesis (GEP) were similar between lovegrass and bush muhly plots in 2009, but were higher in bush muhly plots in 2010. As a result, lovegrass plots reduced ecosystem water-use efficiency (WUEe = NEE/ET), while bush muhly WUEe remained constant between 2009 and 2010. Concurrent whole-plant WUE (WUEp = GEP/T) did not change in lovegrass plots, but increased in bush muhly plots between these years. We concluded that cool-season precipitation use is not a component of Lehmann lovegrass invasive success, but that the change in ET partitioning and attendant shifts in cool-season WUEe may increase interannual variation in ecosystem water- and carbon-exchange dynamics in the water-limited systems it dominates.

The South African grass, Lehmann lovegrass (Eragrostis lehmanniana), may alter ecosystem processes across extensive semiarid grasslands and savannahs of western North America. We compared volumetric soil moisture (theta), total and green tissue leaf area index (LAI), ecosystem (i.e. whole-plant and soil), and leaf-level gas exchange of Lehmann lovegrass and the native bush muhly (Muhlenbergia porteri) over the 2008 monsoon season in a semiarid savanna in southern Arizona, USA, to see if these were consistent with high productivity associated with lovegrass invasive success. theta across 0-5 and 0-25 cm was higher while evapotranspiration (ET) was similar between lovegrass and bush muhly plots, except shortly after rainfall, when ET was 32-81% higher in lovegrass plots. Lehmann lovegrass had lower, quickly developing LAI with greater leaf proportions than bush muhly. When early season theta was high, net ecosystem CO(2) exchange (NEE) was similar, but as storm frequency and theta declined, NEE was more negative in lovegrass (-0.69 to -3.00 micromol m(-2) s(-1)) than bush muhly (+1.75 to -1.55 micromol m(-2) s(-1)). Ecosystem respiration (R (eco)) responded quickly to monsoon onset and late-season rains, and was lower in lovegrass (2.44-3.74 micromol m(-2) s(-1)) than bush muhly (3.60-5.3 micromol m(-2) s(-1)) across the season. Gross ecosystem photosynthesis (GEP) was greater in Lehmann lovegrass, concurrent with higher leaf-level photosynthesis and stomatal conductance. We conclude that canopy structure facilitates higher theta under Lehmann lovegrass, reducing phenological constraints and stomatal limitations to whole-plant carbon uptake through the short summer monsoon growing season.

Relationships between woody plants and grass are typically described at either the stand (≥100 m2) or plant (≤10 m2) spatial resolutions. Descriptions of the Prosopis velutina (mesquite)–grass relationships in the North American Desert Grassland differ between spatial scales: P. velutina often inhibits grass at the stand scale compared to facilitation or neutral relations at the plant scale. We use simultaneous measures of grass density at both spatial resolutions (made at 3 y intervals from 1997 to 2003) to evaluate the influence of the spatial resolution on detecting P. velutina–grass relationships. We confine our observations to the parsimonious conditions of (1) two distinct P. velutina age classes that are exclusive occupants of study sites and (2) total cover of P. velutina was not different between study sites in 2000 and 2003. Thus, total P. velutina cover is constant in those 2 years but P. velutina ages contributing to that cover differed between the study sites. We confirmed the importance of spatial scale because no grass species was related to P. velutina age at the stand scale, but 2 of 5 species were significantly related at the plant scale. Aristida spp. was greater under young than old P. velutina, and Muhlenbergia porteri was greater under old P. velutina. These results suggest that the influence of old P. velutina on grass abundance is restricted to the area under the canopy and does not extend into inter-canopy areas, and therefore the observations at the stand scale may not detect the P. velutina–grass relationships at the plant scale.

Density of 23 perennial grass species was measured in 25 permanent plots nine times between 1972–2000. Grass density was not related to the intensity of livestock grazing. Only one species expressed a difference between the summer rest and no summer rest with heavier stocking grazing treatments: bush muhly (Muhlenbergia porteri Scribn. ex Beal) density was less under the no summer rest with heavier stocking treatment. Beginning in 1975, the non-native Lehmann lovegrass (Eragrostis lehmanniana Nees) spread from distant seedings to one plot, and by 1991, it was the dominant species on most plots. The density of native species was not related to the length of time that the non-native lovegrass was present on a plot. In general, native species declined prior to the arrival and increase of the non-native lovegrass.

In arid regions, overstory plants may influence establishment and persistence of understory species. Identifying relationships between overstory and understory plants may increase success of revegetation efforts. This research considered whether three desirable grasses native to the Sonoran Desert, Arizona cottontop (Digitaria californica), plains lovegrass (Eragrostis intermedia) and bush muhly (Muhlenbergia porteri), differ in density or cover under overstory species compared to open areas. Selection of sites in southern Arizona (two or three per species) was based on abundance of study species and elevation. Density and cover of Arizona cottontop and plains lovegrass was highly variable between canopied and open areas among sites. Bush muhly had greater density and cover under canopies at all sites. No association was noted between precipitation or elevation and density or cover ratios. Identifying understory species well-adapted to shaded sites may aid in determining the best use of existing vegetation for revegetation in this region.

The semidesert grassland in southern Arizona has changed from a native grassland to a scattered Prosopis juliflora var. velutina (mesquite) woodland with an understory of African Eragrostis lehmanniana (Lehmann lovegrass) on many sites. To determine native grass restoration potential, seven species were direct seeded into E. lehmanniana stands that were left alive, burned, sprayed with an herbicide and then either left standing, or mowed. Initial native grass establishment was limited in the live standing treatment but was successful for all other treatments when either June or August sowing was followed by consistent summer precipitation and soil water availability. Four species, Bothriochloa barbinodis (cane beardgrass), Bouteloua curtipendula (sideoats grama), Digitaria californica (Arizona cottontop), and Leptochloa dubia (green spangletop) initially established most successfully, while only Muhlenbergia porteri (bush muhly) had consistently limited or no establishment. E. lehmanniana establishment from the seed bank was increased by canopy removal associated with burning. Densities of native grasses one year after successful initial establishment were much lower than that of E. lehmanniana. A possible revegetation strategy would be to spray emergent E. lehmanniana seedlings and surviving plants with an herbicide during the summer rainy season after spring burning. Native grasses could then be established by sowing in early August of that year or June and August of subsequent years until consistent precipitation produces a native grass stand.

Second through fifth instars and adult male and female genitalia of Rhabdocephala brunnea Van Duzee are described and illustrated. In Arizona, nymphs and adults were collected from a perennial bunchgrass, bush muhly, Muhlenbergia porteri Scribn. ex Beal, which is the first documented host for this fulgorid. Grass-feeding habits have not been reported previously in the Fulgoridae, host plant records of whose members are reviewed.

The purpose of this study was to determine the long-term response of understory vegetation in the desert grassland of southeastern Arizona, USA, to removal and regrowth of mesquite Prosopis juliflora (Swartz) DC. var. velutina (Wooton) Sarg. trees. The study involved 3 treatments applied to mesquite in 1966; mesquite left intact (MI), mesquite removed (MR), and mesquite removed, sprouted, and regrown (MRS). Vegetation responses to 2 litter treatments, litter intact (LI) and litter removed (LR), also were examined. Cover of understory vegetation and juvenile mesquite (< 1.5 m height) were measured in canopy and open (intercanopy) locations. Major changes between 1967 and 1991 were increased cover of juvenile mesquite, shrubs, halfshrubs, bush muhly Muhlenbergia porteri Scribn., and Lehmann lovegrass Eragrostis lehmanniana Nees., and a decline in cover of Arizona cottontop Digitaria californica (Benth.) Chase, and plains bristlegrass Setaria macrostachya H.B.K. Arizona cottontop, plains bristlegras...

Studies of vegetation patterns on small desert mountains have rarely been conduct- ed. The purpose of this study was to evaluate the influence of aspect and elevation on vegetation of Mt. Summerford, a small desert mountain characteristic of the Basin and Range Province of southwestern United States and northern Mexico. Vegetation was randomly sampled on 4 aspects and 3 elevations (0-80 m, 80-160 m, and 160-240 m) of the mountain. South (S) and west (W) aspects had lower grass cover and higher shrub density than east (E) and north (N) aspects. Grass cover decreased as elevation increased on E and N exposures. In contrast, grass cover increased as elevation increased on S and W exposures. A high density of succulent species was found on lower elevations of the S and W aspects. The most frequent species on the mountain were Bouteloua curtipendula, Bouteloua eriopoda, Muhlenbergia porteri, Aristida ternipes, Aloysia wrightii, Ericameria lari- cifolia, Gutierrezia sarothrae, Fouquieria splendens, Dasylirion wheeleri, and Opuntia. Composition of these species, as a mosaic unit, was significantly different among the aspect-elevation units. Species richness was higher in S and W than in E and N aspects. Differences in vegetation patterns were attributed to differential water and nutrient availability on aspects and elevations. This study sug- gests that topographical change is responsible for vegetation patterns in mountains less than 500 m high.

Knowledge of comparative diet selection by cattle and black-tailed jackrabbits (Lepus californicus) would permit better estimation of grazing capacity on Chihuahuan desert ranges. Cattle and black-tailed jackrabbit diets were evaluated seasonally on good and fair condition ranges over a 2-year period. Fecal samples analyzed by the microhistological technique were used to determine diets of both animals. Key forage species in cattle diets were dropseeds (Sporobolus sp.), black grama (Bouteloua eriopoda Torr.), leatherweed croton (Croton pottsii Lam.), and bush muhly (Muhlenbergia porteri Scribn.). Key forage species in jackrabbit diets were honey mesquite (Prosopis glandulosa Torr.), cactus (Opuntia sp.), dropseed, broom snakeweed (Gutierrezia sarothrae Pursh.), and black grama. Overall diet botanical composition data showed cattle consumed 58% grass compared to 22% for jackrabbits (P < 0.05). Forb consumption was similar between the 2 animals and averaged about 31%. Shrub consumption averaged 47% and 12% for jackrabbits and cattle, respectively (P < 0.05). Range condition did not influence total grass consumption by either animal. Both animals, however, had lower forb and higher shrub consumption on fair compared to good condition range. Overall dietary overlaps between jackrabbits and cattle were 40 and 42% on good and fair condition ranges, respectively. Poisonous plants contributed up to 14 and 36% of cattle and jackrabbit diets, respectively. Data from this study show little forage competition occurs between cattle and jackrabbits when stocking rates and jackrabbit numbers are moderate. Several plants poisonous and unpalatable to cattle were important jackrabbit foods. These plants were more prevalent on the fair compared to the good condition range.

Treatment of a nine-hectare desert shrub community with an herbicide (dicamba) resulted in reduction of the live canopy of creosotebush, Larrea tridentata, and an increase in coverage of bush muhly, Muhlenbergia porteri. After treatment, Dipodomys merriami was replaced by Dipodomys ordii as the dominant species. On untreated control areas, D. merriami maintained dominance. These changes are interpreted to result from a release of growth inhibition of M. porteri by creosotebush and habitat structure selection in Dipodomys.

A pot-culture technique was used to calculate relative yields for N, P, K, and S as an index of availability of these nutrients in soil from under mesquite [Prosopis juliflora (Swartz.) DC] trees and from adjacent open areas on a desert grassland site. Arizona cottontop [Trichachne californica (Benth.) Chase], plains bristlegrass (Setaria macrostachya H.B.K.), and bush muhly (Muhlenbergia porteri Scribn.), all native perennial grasses which grow abundantly under mesquite, were used as test plants. Relative yields for N were up to 15 times higher in soil under mesquite trees than in nonmesquite soil, whereas the level of total N was only three times greater. The soil under mesquite trees apparently provides a more favorable environment for supply of N than nonmesquite soil. Relative yields for S in mesquite soil were three times greater than in nonmesquite soil and were proportional to the difference in total S between soils, indicating that S is supplied more slowly than N under mesquite trees. Phosphorus showed a slight, although significant, difference in availability between soils, but total P was the same in the two soils. A threefold higher level of organic matter in soil under mesquite probably contributes to increased availability of P. Differences in total potassium and its supply to test plants were not detected between the two soils.

The effects of aqueous extracts from creosotebush (Larrea tridentata) material on germination and initial growth of black grama (Boutelous eriopoda), bush muhly (Muhlenbergia porteri), and creosotebush were studied. There was no apparent difference among extracts obtained from the various creosotebush plant parts. An aqueous extract from leaves and twigs significantly reduced germination of black grama caryopses, but the germination of bush muhly caryopses and of creosotebush seeds (removed from the carpels) was not affected. Less concentrated extracts did not significantly reduce germination of test species. However, radicle and plumule growth of black grama and bush muhly were significantly reduced by all extracts. Apparently the relatively low osmotic concentrations or the moderate pH's were not responsible for these reductions in growth. Comparison among creosotebush seeds (removed from the carpel) watered with a carpel extract and a water—mannitol solution of the same osmotic concentration, and carpels treated with water indicated that there is a structural characteristic or a non—water—soluble chemical compound in the carpel which inhibits germination of creosotebush. Germination of creosotebush seeds treated with water—mannitol solutions of 1.50, 3.50, 5.00, and 7.00 atm osmotic concentration was significantly reduced as compared to those treated with low concentrations simulating field capacity. Nutrient solution recycled for 37 days around the roots of creosotebush plants growing in a gravel—sand substratum was not inhibitory to germination or initial growth. In a pot test, creosotebush extracts created a crust on the soil surface which reduced the infiltration rate of water. Some of the preceding factors may possibly contribute to the degeneration of grassland areas where creosotebush is present.

Removing 65% of the leaf area of bush muhly in three consecutive years during the growing season reduced plant vigor regardless if effective management is to be implemented. of season of clipping. Late or continuous season defoliation had The purpose of this study was to evaluate the effect of season of

Cover of plants was measured in creosote bush (Larrea tridentata) and tarbush (Flourensia cernua) vegetations within a newly established cattle exclosure in 1958, and again in 1976-1977, within the exclosure and in contiguous grazed areas. Protection from cattle grazing for 18-19 years did not alter the direction of vegetation change. Unpalatable species, particularly snakeweed (Gutierrezia sarothrae) and fluff grass (Tridens pulchellus), increased significantly within and without the exclosure from less than 1% cover in 1958 to as much as 20% in 1977. There was significant, but limited, increase of palatable perennial grasses within the exclosure, particularly in shallow washes. These grasses also increased in grazed creosote bush vegetation, particularly bush muhly (Muhlenbergia porteri), but decreased in grazed tarbush vegetation. There is evidence that there was less moisture stress in the shallow soil over caliche in the creosote bush vegetation than in the deep soil of tarbush. There was high mortality of fluff grass after a prolonged freeze in December 1978. Such infrequent events confound the interpretation of data on long-term vegetational changes.