2. As compared with published data, photosynthetic capacity of C. tenax is higher than that of other cyanobacterial and green-algal soil-crust species studied. At all temperatures and photon flux densities of ecological relevance, net photosynthesis (NP) shows a strong depression at high degrees of hydration; maximal apparent quantum-use efficiency of CO2 fixation is also reduced. Water requirements (moisture compensation point, WC for maximal NP) are higher than that of the green-algal lichens. Collema tenax exhibits extreme ‘sun plant’ features and is adapted to high thallus temperatures.

3. Erratic rain showers are the main source of moisture for soil crusts on the Colorado Plateau, quickly saturating the lichens with liquid water. High water-holding capacity of C. tenax ensures extended phases of favourable hydration at conditions of high light and temperature after the rain for substantial photosynthetic production. Under such conditions the cyanobacterial lichen appears superior over its green-algal competitors, which seem better adapted to habitats with high air humidity, dew or fog as prevailing source of moisture.

A wind-erosion model based on wind strength, atmospheric shear stress on the surface, and almospheric stability is being developed. This model will be constrained by remote sensing and ground-based observations and will then be linked with a regional climate model and interactive vegetation package to forecast how various climatic and land-use scenarios interact with critical wind speeds required to move surface materials. \Ve will attempt to answer the following questions: How does wind strength vary with natural climate cycles on decadal and cenhlry time scales? To what extent will winds become stronger or weaker under future climate scenarios? How have soil moisture and vegetation changes affected wind erosion in the past, and what can we expect in the future? As an example of possible future conditions, projections of doubled atmospheric CO₂ (above preindustrial levels) for the southwestern U.S. suggest a decrease in winter soil moisture. which may enhance wind erosion.

Many studies have addressed aspects of the relationship between crusts and vascular plants. These studies have been done at sites with different soil textures, physical and chemical crusting, soil surface roughness and stability, climate regimes, species composition of the biological crusts, and vascular plant species (each with different strategies for germination and survival). It is essential that all these differences be accounted for when generalizing about the relationship between soil crusts and vascular plants. In this chapter, we will discuss how crusts modify soils in ways that can affect vascular plants. We will then address the effects of biological soil crusts on seed dispersal and germination, and how crusts affect vascular plant survival and nutritional status in different climate regimes. Finally, we will discuss the overall effects of crusts on plant community structure.

Methods of assessing impacts of, and recovery from, disturbance have been highly variable in the past. Generally, measurements have been limited to visual estimates of crust cover. However, Belnap (1993) showed that visual assessment can accurately assess only moss and lichen cover, and cannot be used to measure recovery of cyanobacterial biomass, soil stability, and/or physiological functioning of crustal organisms. In addition, some studies have only considered total crust cover but have not delimited the relative cover of cyanobacteria, mosses, and lichens. The relationships between total crust cover and impacts of disturbance can be weak, as cyanobacterial cover generally increases, while moss and lichen cover decreases, after disturbance. This often makes total crust cover a poor measure of the dynamics of soilcrust recovery. Differentiating between crustal components is also important because alteration of species composition can heavily influence ecological functioning of the crusts (Eldridge 1998).

Comparing recovery rates from different studies can be problematic, as factors known to control recovery rates (such as site stability and precipitation following disturbance) are often not reported. More importantly, severity of disturbance is seldom quantified. Studies generally report disturbance levels as "light", "moderate", or "heavy" without any definition of these categories; thus, what is "moderate" in one study may be considered "heavy" in another. As studies cover a large range of climatic zones, soil types, levels of disturbance, and ways to calculate recovery, and because there has been no standard for measuring crust recovery, it is not surprising that recovery rates in the literature have ranged widely (2 years in cool deserts to over 3000 years in very dry deserts), and either show no pattern or often appear contradictory (Anderson et al. 1982; Callison et al. 1985; Jeffries and Klopatek 1987; Cole 1990; Belnap 1995,1996; Belnap and Warren 1998).

The trophic structure of soil food webs is very important in soil nutrient cycles. Soil primary producers are lichens, mosses, green algae, and cyanobacteria. These organisms, along with plant material, are both grazed directly and decomposed by soil biota. During decomposition, early-colonizing yeast and bacteria are grazed by nematodes and protozoans, while mites control nematode numbers. Later stages of decomposition are dominated by fungi, which are grazed by nematodes, collembola, and mites. Thus, trophic relationships among soil biota become major regulators of decomposition and mineralization in soils (Ingham et al. 1985).

There is also a synergy between bacteria, fungi, protozoa, and cyanobacteria. Grazing by protozoa stimulate cyanobacterial nitrogen fixation (Ghabbour et al. 1980). Addition of heterotrophic bacteria and fungi to soils significantly increases cyanobacterial biomass (Schiefer and Caldwell 1982 ). Bacteria and fungi release nutrients and scavenge cyanobacterial "wastes", including 02' enhancing cyanobacterial N fixation (see Chap. 19). Nitrogen provided by the cyanobacteria, in turn, increases microbial decomposition activity (Lynch and Harper 1983). The objective of this chapter is to discuss the relationship between biological soil crusts and other heterotrophic components of the soil food web, such as bacteria, fungi, protista, nematodes, and soil invertebrates.

N fixation activity is generally measured as ¹⁵N₂ incorporation or by the acetylene (C₂H₂) reduction assay (ARA). As ¹⁵N₂ uptake is a direct measure of N fixation, it is the most reliable method. However, it is costly and requires highly controlled conditions; thus, most studies use ARA. In the presence of C2H2, the nitrogenase enzyme produces C₂H₄ (ethylene); thus, C₂H₄ production is a measure of nitrogenase enzyme activity (NA). Because NA is so widely used as a surrogate for nitrogen fixation, these terms will be used synonymously in this chapter. There are some problems associated with ARA. C₂H₂ can affect physiological functioning of the organism, especially when exposure lasts longer than 6h (David and Fay 1977). The biggest issue is that conversion of ARA data to absolute amounts of N2 fixed requires calibration by lSN2, which is seldom done. This is essential, as reported conversion rates in the literature for the C₂H₄:N₂ ratio vary from 2 to 56, while conversion ratios for naturally occurring soil cyanobacteria generally range from 1.9 to 6.1 (Potts 1984). However, two recent studies indicate that conversion ratios can be much lower and also can be seasonally variable. S. Phillips and J. Belnap (unpubl. data) found that desert Nostoc commune sheets had an average conversion ratio of 0.31. Liengen (1999) reported conversion ratios varied with environmental conditions for both Nostoc sheets (0.11-0.4) and free-living soil cyanobacteria (0.02-0.07) in the Arctic. He also analyzed Anabaena in culture and obtained a ratio close to 4. These studies may explain why reported rates are so variable, as many studies have used cultured specimens or aquatic species, while actual rates in soils may be much lower. In addition, environmental conditions may lead to large variations in conversion rates. If these more recent results apply to other ecosystems or soil cyanobacterial species, N fixation rates may be currently underestimated by an order of magnitude. This matter clearly needs additional investigation.

Most of the soil photosynthetic productivity and nitrogen (N) fixation in desert soils is concentrated within 3 mm of the surface (Garcia-Pichel and Belnap 1996; see Chap. 16). Thus, only a little soil loss can significantly reduce C and N inputs from these organisms (see Chaps. 18, 19). In addition, the top few mm of soil contain a much higher percentage of soil fine particles than underlying soils (Danin and Ganor 1991; Verrecchia et al. 1995). Loss of soil fines can reduce site productivity, as plant -essential nutrients are often bound to these particles. Burial of nearby biological soil crusts from windblown sediments generally means death for the photosynthetic components of the soil crusts, further reducing fertility. Reduced fertility of systems is one of the most definitive, and problematic, aspects of desertification. Worldwide increases in windborne sediments are amply documented (Goudie 1978; Kovda 1980; Tsoar and Pye 1987).

Both plant and soil characteristics influence wind erosion. In deserts, vascular plants are generally short with sparsely vegetated stems, and very large spaces occur between individuals. Plant litter cover is also very low. Thus, plant materials in semiarid and arid regions offer limited protection to soils from wind erosion. In such environments, biological soil crusts can playa critical role in soil stabilization.

1. Desertification negatively impacts a large proportion of the global human population and > 30% of the terrestrial land surface. Better methods are needed to detect areas that are at risk of desertification and to ameliorate desertified areas. Biological soil crusts are an important soil lichen-moss-microbial community that can be used toward these goals, as (i) bioindicators of desertification damage and (ii) promoters of soil stability and fertility.
2. We identified environmental factors that correlate with soil crust occurrence on the landscape and might be manipulated to assist recovery of soil crusts in degraded areas. We conducted three studies on the Colorado Plateau, USA, to investigate the hypotheses that soil fertility [particularly phosphorus (P), manganese (Mn) and zinc (Zn)] and/or moisture limit soil crust lichens and mosses at four spatial scales.
3. In support of the soil fertility hypothesis, we found that lichen–moss crusts were positively correlated with several nutrients [Mn, Zn, potassium (K) and magnesium (Mg) were most consistent] at three of four spatial scales ranging from 3·5 cm2 in area to c. 800 km2. In contrast, P was negatively correlated with lichen–moss crusts at three scales.
4. Community composition varied with micro-aspect on ridges in the soil crust. Three micro-aspects [north-north-west (NNW), east-north-east (ENE) and TOP] supported greater lichen and moss cover than the warmer, windward and more xeric micro-aspects [west-south-west (WSW) and south-south-east (SSE)]. This pattern was poorly related to soil fertility; rather, it was consistent with the moisture limitation hypothesis.
5. Synthesis and application. Use of crusts as desertification bioindicators requires knowledge of a site's potential for crust cover in the absence of desertification. We present a multi-scale model of crust potential as a function of site properties. Future quantitative studies can use this model to guide sampling efforts. Also, our results suggest new directions in restoration research: enhancement of moisture residence time and fertilization with key nutrients (Mn, Zn, K and Mg). Re-establishment of soil crusts in desertified lands will help regain lost soil stability and fertility, and facilitate plant re-establishment.

Location:
Grand Staircase-Escalante National Monument, an arid region of the Colorado Plateau, USA.

Methods:
Biological soil crusts are highly functional soil surface communities of mosses, lichens and cyanobacteria that are vulnerable to soil surface disturbances such as grazing. We sampled BSC communities at 114 relatively undisturbed sites. We developed an eight-tier BSC habitat classification based upon soil properties including texture, carbonate and gypsum content, and presence of shrinking-swelling clays. We used simple structural equation models to determine how well this classification system predicted the evenness, richness, and community structure of BSC relative to elevation and annual precipitation.

Results:
We found that our habitat classification system explained at least 3.5 × more variance in BSC richness (R2= 0.57), evenness (R2= 0.59), and community structure (R2= 0.34) than annual precipitation and elevation combined. Gypsiferous soils, non-calcareous sandy soils, and limestone-derived soils were all very high in both species richness and evenness. Additionally, we found that gypsiferous soils were the most biologically unique group, harboring eight strong to excellent indicator species.

Conclusions:
Community properties of BSCs are overwhelmingly influenced by edaphic factors. These factors can be summarized efficiently by land managers and laypeople using a simple soil habitat classification, which will facilitate incorporation of BSCs into assessment and monitoring protocols and help prioritize conservation or restoration efforts.

The natural science issues facing today’s world are complex and cut across many scientific disciplines. The Earth is a system in which atmosphere, oceans, land, and life are all connected. Rocks and soils contain the answers to important questions about the origin of energy and mineral resources, the evolution of life, climate change, natural hazards, ecosystem structures and functions, and the movements of nutrients and toxicants. The science of geology has the power to help us understand the processes that link the physical and biological world so that we can model and forecast changes in the system.

Ensuring the success of this strategy will require integration of geological knowledge with the other natural sciences and extensive collaboration across USGS science centers and with partners in Federal, State, and local agencies, academia, industry, nongovernmental organizations and, most importantly, the American public. The first four goals of this report describe the scientific issues facing society in the next 10 years and the actions and products needed to respond to these issues. The final two goals focus on the expertise and infrastructure needed to ensure the long-term sustainability of the geological sciences in the USGS.

The ultimate goal of USGS science and of the strategy laid out in this document is to contribute to the development of a sustainable society that operates in harmony with the Earth systems that society depends upon. As we begin the second decade of the 21st century, our Nation faces growing challenges in resource availability, climate and environmental change, and natural hazards. Meeting these challenges will require strong collaboration across the natural and social sciences and extensive partnerships with both the public and private sectors. The six goals described in this document represent a mix of scientific focus areas and operational necessities that together provide a comprehensive roadmap for USGS geologic science to effectively contribute to the USGS mission, providing science for a changing world.

Here, we review studies of the responses of population-level plant-plant, plant-herbivore, and predator-prey interactions to changes in water availability in dryland environments in order to develop new hypotheses and predictions to guide future research. To help explain patterns of interaction outcomes, we developed a conceptual model that views interaction outcomes as shifting between (1) competition and facilitation (plant-plant), (2) herbivory, neutralism, or mutualism (plant-herbivore), or (3) neutralism and predation (predator-prey), as water availability crosses physiological, behavioural, or population-density thresholds. We link our conceptual model to hypothetical scenarios of current and future water availability to make testable predictions about the influence of changes in water availability on species interactions. We also examine potential implications of our conceptual model for the relative importance of top-down effects and the linearity of patterns of change in trophic biomass with changes in water availability. Finally, we highlight key research needs and some possible broader impacts of our findings. Overall, we hope to stimulate and guide future research that links changes in water availability to patterns of species interactions and the dynamics of populations and communities in dryland ecosystems.

Methods
With a combination of experimental 2°C warming and altered summer precipitation frequency applied over 2 years, we measured soil nutrient cycling and the structure and function of crust communities.

Results
We saw no change in crust cover, composition, or other measures of crust function in response to 2°C warming and no effects on any measure of soil chemistry. In contrast, crust cover and function responded to increased frequency of summer precipitation, shifting from moss to cyanobacteria-dominated crusts; however, in the short timeframe we measured, there was no accompanying change in soil chemistry. Total bacterial and fungal biomass was also reduced in watered plots, while the activity of two enzymes increased, indicating a functional change in the microbial community.

Conclusions
Taken together, our results highlight the limited effects of warming alone on biological soil crust communities and soil chemistry, but demonstrate the substantially larger effects of altered summertime precipitation.

Calculation of far-traveled dust contents of the surficial deposits is based on measurements of the magnetic properties of rock, surficial deposits, and modern dust using a binary mixing model. At the undisturbed site, IRM-based calculations show a systematic down-slope increase in aeolian dust (ranging from 2% to 18% of the surface soil mass), similar to the down-slope increase in total fines (18–39% of surface soil mass). A combination of winnowing by wind during the past and down-slope movement of sediment likely accounts for the modern distribution of aeolian dust and associated nutrients. At the previously grazed site, dust also increases down slope (5–11%) in sediment with corresponding abundances of 13–25% fines. Estimates of the contributions of aeolian dust to the total soil nutrients range widely, depending on assumptions about grain-size partitioning of potential nutrients in weathered bedrock. Nevertheless, aeolian dust is important for this setting, contributing roughly 40–80% of the rock-derived nutrient stocks (P, K, Na, Mn, Zn, and Fe) in uppermost soil over most of the sampled slope at the undisturbed site, which shows no evidence of recent wind erosion.

Differences in colonization frequencies, abundances, and distributions of microorganisms comprising the crusts, as a function of mound aspect or exposure, suggest that these organisms are associated with particular aspects due to distinctive and favorable microhabitats on these exposures. Polysaccharide sheath material, deposited by cyanobacteria, and associated filaments occurred in greater quantities on ENE than WSW mound exposures, and cover by Collema spp. lichens exhibited the same pattern. Colonization of mounds by common lichen species occurred significantly more frequently on ENE than WSW mound aspects at 2 of 3 sites. In contrast, the 3 most common lichen species, aside from Collema spp., did not exhibit a tendency for greater cover on ENE than WSW mound aspects. Physiological differences between gelatinous cyanolichens and green-algal lichens may explain the different distributional patterns of Collema spp. and the 3 other lichens.

The oldest Tamarix established along several sampling reaches in 1938, and 1,50-2.25 m of alluvium has accreted above their germination surfaces. Nearly 90% of the Tamarix and Populus samples established during flood years that exceeded the 2.5-year recurrence interval. Recruitment was most common when large floods were followed by years with smaller peak flows. The majority of Tamarix establishment and Green River channel narrowing occurred long before riverregulation by Flaming Gorge Dam. Tamarix initially colonized bare instream sand deposits (e.g., islands and bars), and most channel and floodplain changes followed the establishment of Tamarix. Our results suggest that Tamarix recruitmentwas triggeredby large annual floods that were followed by years with lower peak flows, not by periods of low flow alone. Tamarix appears to have actively invaded floodplains, while Populus colonization has been limited. Thus, Tamarix invasion may have greatly influenced floodplain development and riparianvegetation composition along the lower Green River since the early 20th century.