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

Flowering Plants, the Santa Monica Mountains, Coastal and Chaparral Regions of Southern California by Nancy Dale

Volume: 33Start Page: 315End Page: 31

NEW SPECIES OF MENTZELIA (LOASACEAE) FROM GRAND COUNTY, UTAH

Volume: 46Start Page: 361End Page: 36

A NEW SPECIES OF MENTZELIA SECTION BARTONIA (LOASACEAE) FROM THE GREAT BASIN DESERT OF CALIFORNIA AND NEVADA

Volume: 51Start Page: 379End Page: 38

A new species of Mentzelia section Bartonia (Loasaceae) from western Nevada [M. candelariae]

Volume: 55Start Page: 281End Page: 28

A survey of the microbial community in the rhizosphere of two dominant shrubs of the Negev Desert highlands, Zygophyllum dumosum (Zygophyllaceae) and Atriplex halimus (Amaranthaceae), using cultivation-dependent and cultivation-independent methods.

UnlabelledPremise of the studyPlant roots comprise more than 50% of the plant's biomass. Part of that biomass includes the root microbiome, the assemblage of bacteria and fungi living in the 1-3 mm region adjacent to the external surface of the root, the rhizosphere. We hypothesized that the microorganisms living in the rhizosphere and in bulk soils of the harsh environment of the Negev Desert of Israel had potential for use as plant-growth-promoting bacteria (PGPB) to improve plant productivity in nutrient-poor, arid soils that are likely to become more common as the climate changes. •MethodsWe used cultivation-dependent methods including trap experiments with legumes to find nitrogen-fixing rhizobia, specialized culture media to determine iron chelation via siderophores and phosphate-solubilizing and cellulase activities; cultivation-independent methods, namely 16S rDNA cloning and sequencing; and also community-level physiological profiling to discover soil microbes associated with the Negev desert perennials Zygophyllum dumosum and Atriplex halimus during the years 2009-2010. •Key resultsWe identified a number of PGPB, both epiphytes and endophytes, which fix nitrogen, chelate iron, solubilize phosphate, and secrete cellulase, as well as many other bacteria and some fungi, thereby providing a profile of the microbiomes that support the growth of two desert perennials. •ConclusionWe generated a snapshot of the microbial communities in the Negev Desert, giving us an insight in its natural state. This desert, like many arid environments, is vulnerable to exploitation for other purposes, including solar energy production and dry land farming