The Ecology and Evolution of Rare, Soil Specialist Astragalus Plants in the Arid Western U.S.

Organisms that specialize in uncommon habitats are, by their very nature, inherently uncommon. Specialization has its advantages, namely reduced competition and predation, but it also incurs costs. Specialists often have small population sizes, narrow ranges, and fragmented habitat, all of which engender negative consequences on an evolutionary timescale. Herein, I examine benefits and costs of specialization in selenium-hyperaccumulating plants in the genus Astragalus (Fabaceae). These plants are disproportionately likely to be rare and of conservation concern. Thus, I optimized germination pretreatments for Astragalus species such that seed loss can be minimized during ex situ cultivation, and found that physical scarification is most effective in breaking hard-seed dormancy. Through analysis of soil in seleniferous habitats, I found that soil hydrology can rapidly deplete bioavailable selenium, potentially further reducing the habitat available for accumulators. To better understand the relationship between soil bioavailable selenium and plant performance, I subjected Astragalus species to a gradient of selenium concentrations in the greenhouse. Both non-accumulators and hyperaccumulators had less herbivory with increasing selenium concentrations, and also grew larger, despite the energetic cost of selenium uptake. One potential explanation for their larger growth is that selenium reduced inadvertent drought stress during the experiment, so I tested that hypothesis using a full factorial experiment of drought stress and selenium dosage. Although drought stress reduced lifespan and selenium extended it, there was no evidence that selenium ameliorated drought stress. As a case study of the potential population genetic consequences of specialization, I examined the genetic structure and diversity of two allopatric cryptic sister species of Astragalus. Despite known low pollen and seed dispersal and strong genetic isolation by distance, populations were relatively diverse and not substantially inbred. Additionally, the genetic data did not support a two-species arrangement, so I recommend the species be consolidated, although several populations are somewhat isolated and merit special conservation attention. In summary, hyperaccumulators derive ecological benefits from their specialization that outweigh its metabolic cost, but may suffer low connectivity between populations, if not necessarily inbreeding depression. Conservation efforts should thus focus primarily on minimizing threats to and preserving connectivity of specialist habitats

Germination pretreatments to break hard-seed dormancy in Astragalus cicer L. (Fabaceae)

Conservationists often propagate rare species to improve their long-term population viability. However, seed dormancy can make propagation efforts challenging by substantially lowering seed germination. Here I statistically compare several pretreatment options for seeds of Astragalus cicer L.: unscarified controls and scarification via physical damage, hot water, acid, and hydrogen peroxide. Although only 30% of unscarified seeds germinated, just physical scarification significantly improved germination, whereas one treatment, hot water, resulted in no germination at all. I recommend that rare species of Astragalus, as well as other hard-seeded legumes, be pretreated using physical scarification. Other methods may require considerable optimization, wasting precious time and seeds

Selenium in Soils of Western Colorado

Seleniferous soils are host to a diverse and unique community of plants, animals, and microorganisms. Often, studies of these organisms, if they report selenium at all, only report the total selenium content of the soil. We conducted a field survey of soils to determine a) whether total selenium is a reliable proxy for bioavailable selenium, and b) the general characteristics of typical seleniferous soils. We analyzed soils from 32 seleniferous and nearby non-seleniferous habitats across western Colorado. In normal, low-selenium soils, the relationship between total and bioavailable selenium is roughly linear. In seleniferous soils however (total Se \u3e 2 mg/kg), there is no relationship between total and bioavailable selenium. Also, these soils can be broadly characterized by two principal axes: a metals-rich axis likely explained by the mineralogy and depositional environment of the parent rock, and a soluble, salt-rich axis likely explained by soil weathering and hydrology. There is considerably more variation along the former axis, which also appears to predict primary productivity, but selenium content, particularly bioavailable selenium, is influenced by the latter. Researchers in seleniferous environments must recognize that seleniferous soils are heterogeneous, and may be shaped by current environmental factors as much as by the geological past

A review of water and greenhouse gas impacts of unconventional natural gas development in the United States

This paper reviews recent developments in the production and use of unconventional natural gas in the United States with a focus on water and greenhouse gas emission implications. If unconventional natural gas in the U.S. is produced responsibly, transported and distributed with little leakage, and incorporated into integrated energy systems that are designed for future resiliency, it could play a significant role in realizing a more sustainable energy future; however, the increased use of natural gas as a substitute for more carbon intensive fuels will alone not substantially alter world carbon dioxide concentration projections. This paper reviews recent developments in the production and use of unconventional natural gas in the United States with a focus on environmental impacts. Specifically, we focus on water management and greenhouse gas emission implications. If unconventional natural gas in the United States is produced responsibly, transported and distributed with little leakage, and incorporated into integrated energy systems that are designed for future resiliency, it could play a significant role in realizing a more sustainable energy future. The cutting-edge of industry water management practices gives a picture of how this transition is unfolding, although much opportunity remains to minimize water use and related environmental impacts. The role of natural gas to mitigate climate forcing is less clear. While natural gas has low CO2 emissions upon direct use, methane leakage and long term climate effects lead to the conclusion that increased use of natural gas as a substitute for more carbon intensive fuels will not substantially alter world carbon dioxide concentration projections, and that other zero or low carbon energy sources will be needed to limit GHG concentrations. We conclude with some possible avenues for further work