The Vegetation and Flora of Edwards Air Force Base, Western Mojave Desert, California

Edwards Air Force Base extends over 121,000 ha in the Antelope Valley of the western Mojave Desert, with much of the area part of a closed endorheic basin that held the Pleistocene Lake Thompson. Notable topographic features are Rogers, Rosamond and Buckhorn dry lakes, while rounded domes and scattered hills are present to the north and east. Elevation gradients are limited, ranging from a low of 690 m to 1044 m near the eastern margin. Diverse communities of saltbush scrub dominate the lower plains, while creosote bush scrub and Joshua tree woodlands are present away from the old lake basin. In many ways EAFB is a biogeographic crossroads with the broader central Mojave Desert to the east, Owens Valley and Great Basin to the north, and cismontane Central Valley and foothills to the west. The flora as currently known contains 403 vascular plant taxa, with 324 (80%) of these native. The eight largest families comprise more than 68% of the flora, led by the Asteraceae with 84 taxa (72 native). Annual plants make up over two-thirds of the total flora

Carbon isotope ratios of Atacama Desert plants reflect hyperaridity of region in northern Chile

Journal ArticleLeaf carbon isotope ratios were measured on plants from the coastal portions of the Atacama Desert at Pan de Azucar and Paposo, Chile. Most species possessed C3 photosynthesis, although there were several CAM species, indications of some facultative CAM species, and only one C4 species. The carbon isotope ratios of the C3 plants are unusually high, even for species from arid ecosystems. These observations indicated that C3 species of the Atacama Desert were characterized by very low intercellular C0 2 concentration that averaged between 159 and 190 ppm, depending on the severity of the drought

Plant Phenology and Water Relations in a Saline Pan-Dune Mosaic in the Western Mojave Desert

An extensive mosaic of small clay pans and low stable dunes exists within Edwards Air Force Base in the western Mojave Desert of California. This pan-dune ecosystem, positioned between the large Rosamond and Rogers dry lakes on the old Pleistocene lakebed of Lake Thompson, provides an opportunity to study the seasonal patterns in growth and flowering phenology and water relations in a saltbush scrub community dominated by a mix of C3 and C4 species. C3 shrubs initiated vegetative growth in February, with current shoot and leaf production largely ceasing in most species by mid-April. The deeply rooted Tetradymia stenolepis and Ericameria nauseosa continued vegetative growth through May. C4 species including Atriplex and the grass Distichilis spicata typically began vegetative growth in mid- to late March, 2–6 weeks later than the first group and continued this growth in most cases through June and July. Reproductive growth commonly did not begin until June and continued into July and/or August. Shrub species showed a general pattern of slowly declining water potentials (increased stress) through the late spring and summer months, reflecting more limited soil water availability. Smaller changes in predawn water potential over this period were seen in the deeply rooted species. Soil moisture availability in the widespread Atriplex confertifolia showed a pattern of variation between pan, dune and open flat microhabtats

Ecophysiological Observations on Lane Mountain Milkvetch, Astragalus Jaegerianus (Fabaceae), a Proposed Endangered Species of the Mojave Desert

Astragalus jaegerianus, the Lane Mountain milkvetch, a perennial herbaceous legume, is a rare and very narrow endemic of the central Mojave Desert in San Bernardino County, California, and currently proposed to be listed as an endangered species. This herb grows in the protection of low shrubs. Anatomical observations revealed that leaflets are amphistomatic and have isolateral mesophyll, typical of full-sun desert leaves, and the green stem is an important photosynthetic organ, having abundant stomata and a cylinder of cortical chlore nchyma. Ecophysiological studies showed that this species requires high PFD (1400-1500 μmol m-2s-1) to achieve maximum photosynthetic rates and, therefore, is probably not utilizing the nurse shrub for shade but, instead, as a trellis for upper shoots to intercept high PFD. A study of δ15N indicated that this legume is a nitrogen fixer, with higher tissue nitrogen content than other associated species (3.1% versus 1.8%). The herb-shrub association is speculated to be mutualistic, in that the shrub may benefit from higher soil nitrogen when it grows with a nitrogen-fixing legume, and A. jaegerianus likely uses the nurse shrub for protection from herbivores

Community Structure and Demography in a Saline Pan-Dune Mosaic in the Western Mojave Desert

A unique saline ecological system formed by an extensive mosaic of small clay pans and low stable dunes exists within Edwards Air Force Base in the western Mojave Desert of California. This ecosystem lies between the large Rosamond and Rogers dry lakes on the old Pleistocene lakebed of Lake Thompson. Plant communities on the low and relatively stable dunes were broadly classed as saltbush scrub, with a total canopy cover of 30–36%. Atriplex confertifolia was the typical dominant, with Ericameria nauseosa as an important associate. Taller dunes of younger age and less saline soils had similar plant cover, but a distinct plant community with Atriplex canescens and Krascheninnikovia lanata as dominants and Yucca brevifolia as a common associate. Flat areas adjacent to the dunes were dominated by a virtual monoculture of Atriplex confertifolia. Aboveground shrub and bunchgrass biomass including dead material varied from 503 to 1204 kg ha−1, low in comparison to similar plant communities in the Great Basin. The absence of small saplings and seedlings of many shrub species suggests successful establishment is highly episodic. Seedlings were abundant only in Isocoma acradenia

Impact of Ground Water Depletion on the Mesquite Community at Edwards Air Force Base, Western Mojave Desert, California

Edwards Air Force Base (EAFB) provides a habitat refugium for mesquite woodlands in the western Mojave Desert of the Antelope Valley. Although many mesquite communities in the arid southwest are considered invasive as they reduce the extent of grazing lands, the community at EAFB is composed primarily of large, widely spaced trees that provide food and shelter for local wildlife species and recreational opportunities for base personnel. Unfortunately, the range of these mesquite trees appears to be contracting as mature and old individuals dominate the community. Although anecdotal evidence suggests that the fall in the local groundwater table is responsible for the decline of the mesquite community at EAFB, no research has been carried out to confirm this. Our results corroborate the hypothesis that the groundwater table at EAFB has declined in the second half of the 20th century and that the area of the mesquite community is diminishing. Although the mesquite community expanded vigorously from 1956 to 1968, it contracted considerably from 1984 to 2000. The evidence for this observed decline is reinforced by the results of the age-class analysis as the community in 2003 is made up largely of mature, old, dying and dead trees. Few saplings (older than 2 years) and no new seedlings (1–2 years) are present in the study sites, suggesting that the mesquite community may not be able to replenish itself

Rarity in Astragalus: a California Perspective

Astragalus (Fabaceae), the largest genus of plants in the world with an estimated 3270 species, is known for large numbers of rare endemic species. An inventory of patterns of climatic, topographic, and edaphic diversity of Astragalus taxa in California (98 native species and 144 named taxa) provides a means to understand the occurrence of rarity in relation to climatic equitability and regional species richness of congeneric taxa. Most taxa in the genus have relatively small ranges of distribution, with 50% restricted geographically to a single Jepson Bioregion. The California Native Plant Society lists 51 Astragalus taxa (35% of the native Astragalus taxa) as rare, threatened, or endangered (RTE). Climate characteristics of geographic regions such as rainfall and temperature extremes show no obvious relationship to species richness or the proportion of listed taxa. Species richness is highest in the arid Great Basin (35 species and 53 taxa) combining both its components, followed by 29 species and 39 taxa in the Sierra Nevada East region that includes the White and Inyo Mountains. The Mojave Desert is also high in diversity with 32 species and 39 taxa, but in contrast the Sonoran Desert region is low with only 12 species and 14 taxa. Despite ranking highest in the number of Astragalus taxa present, the Great Basin regions are low in their proportion of RTE taxa (17%) compared to the South Coast Region (39.5%) and Mojave Desert (32%). Strong edaphic specialization is associated with the majority but not all RTE taxa. While no single ecophysiological adaptation can explain this pattern, it is significant that Astragalus taxa have the potential ability to develop symbiotic nitrogen fixation, and this trait is key to success in soils not conducive to growth of many potential competitors. Land use changes, alien grass invasion and grazing, among other threats, are increasing fragmentation of habitats for many rare taxa with consequent impacts on gene flow. The continued survival of rare and locally endemic taxa will require improved knowledge of their individual demographic traits and long-term population dynamics

Bottom-up Effects of Substrate on Two Adjacent Shrub Communities and the Distribution of a Rare and Endangered Plant Species, Astragalus jaegerianus Munz.

Edaphic habitats are botanically interesting because of differences in vegetation with neighboring sites and because they tend to harbor rare species. In the central Mojave Desert, there are granite colluvial substrates where creosote bush, the dominant shrub in the area, is sparser and generally smaller than in the neighboring creosote bush communities. It is on these sites that the Lane Mountain milkvetch, a rare and federally endangered species, is restricted. The milkvetch is a nitrogen-fixer and grows under and within the canopy of host shrubs. Our previous studies have demonstrated that the milkvetch has no preference for species of host shrub, except Larrea tridentata, which appears to be an unsuitable host plant for the milkvetch. In this study, we surveyed three transects within milkvetch habitats and three transects in adjacent creosote bush habitats in the year 2000 and again in 2010, a period coincident with long-term drought conditions in the Mojave Desert. Our results show that adjacent milkvetch and creosote bush shrub communities differ significantly in shrub height, shrub volume, and shrub density in the year 2000: the shrubs in milkvetch communities were more numerous but smaller compared to adjacent creosote bush scrub. Species richness also differed between communities in the year 2000: milkvetch communities contained 19 different shrub species and creosote bush communities had only 9 species. Surveys in 2010 show that the drought had significant negative effects on both shrub communities. Total shrub mortality (166 shrubs) was high compared to shrub recruitment (16 shrubs), and the majority of mortality and recruitment occurred in milkvetch communities (131 deaths and 16 recruits). Shrub densities decreased significantly in milkvetch communities in 2010, but were still considerably higher than in creosote bush communities. These results suggest that the restricted distribution of the Lane Mountain milkvetch may be the result of higher shrub densities in milkvetch shrub communities; increased shrub densities increases the proximity of suitable host shrubs, which in turn increase the probability of successful seed dispersal and establishment

Dynamic Microclimate Boundaries across a Sharp Tropical Rainforest–Clearing Edge

As landscapes become increasingly fragmented, research into impacts from disturbance and how edges affect vegetation and community structure has become more important. Descriptive studies on how microclimate changes across sharp transition zones have long existed in the literature and recently more attention has been focused on understanding the dynamic patterns of microclimate associated with forest edges. Increasing concern about forest fragmentation has led to new technologies for modeling forest microclimates. However, forest boundaries pose important challenges to not only microclimate modeling but also sampling regimes in order to capture the diurnal and seasonal dynamic aspects of microclimate along forest edges. We measured microclimatic variables across a sharp boundary from a clearing into primary lowland tropical rainforest at La Selva Biological Station in Costa Rica. Dynamic changes in diurnal microclimate were measured along three replicated transects, approximately 30 m in length with data collected every 1 m continuously at 30 min intervals for 24 h with a mobile sensor platform supported by a cable infrastructure. We found that a first-order polynomial fit using piece-wise regression provided the most consistent estimation of the forest edge, relative to the visual edge, although we found no best sensing parameter as all measurements varied. Edge location estimates based on daytime net shortwave radiation had less difference from the visual edge than other shortwave measurements, but estimates made throughout the day with downward-facing or net infrared radiation sensors were more consistent and closer to the visual edge than any other measurement. This research contributes to the relatively small number of studies that have directly measured diurnal temporal and spatial patterns of microclimate variation across forest edges and demonstrates the use of a flexible mobile platform that enables repeated, high-resolution measurements of gradients of microclimate