Edith B. Allen

Ecological Effects of Nitrogen Deposition in the Western United States

Abstract In the western United States vast acreages of land are exposed to low levels of atmospheric nitrogen (N) deposition, with interspersed hotspots of elevated N deposition downwind of large, expanding metropolitan centers or large agricultural operations. Biological response studies in western North America demonstrate that some aquatic and terrestrial plant and microbial communities are significantly altered by N deposition. Greater plant productivity is counterbalanced by biotic community changes and deleterious effects on sensitive organisms (lichens and phytoplankton) that respond to low inputs of N (3 to 8 kilograms N per hectare per year). Streamwater nitrate concentrations are elevated in high-elevation catchments in Colorado and are unusually high in southern California and in some chaparral catchments in the southwestern Sierra Nevada. Chronic N deposition in the West is implicated in increased fire frequency in some areas and habitat alteration for threatened species. Between hotspots, N deposition is too low to cause noticeable effects or has not been studied.

SHIFTS IN ARBUSCULAR MYCORRHIZAL COMMUNITIES ALONG AN ANTHROPOGENIC NITROGEN DEPOSITION GRADIENT

We evaluated arbuscular mycorrhizal (AM) species diversity and abundance in nine locations along an anthropogenic nitrogen deposition gradient in coastal sage scrub (CSS) vegetation in southern California. The primary pollutants were nitrogen oxides derived from vehicular emissions. Extractable soil N on the gradient ranged from 5 to 87 μg/g during the summer months. For comparative purposes, we also assessed AM communities in nitrogen-fertilized (60 kg N·ha−1·yr−1) and unfertilized plots. Nitrogen enrichment induced a shift in AM community composition. In particular, an increasing input of nitrogen was associated with the displacement of the larger-spored species of Scutellospora and Gigaspora (due to a failure to sporulate) with a concomitant proliferation of small-spored Glomus species (e.g., Glomus aggregatum, Glomus leptotichum). A subsequent reduction in species richness and diversity (as measured by Shannon–Wiener index) accompanied eutrophication. Nitrogen enrichment also significantly reduced spo...

NITROGEN ENRICHMENT ALTERS MYCORRHIZAL ALLOCATION AT FIVE MESIC TO SEMIARID GRASSLANDS

Arbuscular mycorrhizal (AM) fungi are integral components of grasslands because most plants are associated with interconnected networks of AM hyphae. Mycorrhizae generally facilitate plant uptake of nutrients from the soil. However, mycorrhizal associations are known to vary in their mutualistic function, and there is currently no metric that links AM functioning with fungal colonization of roots. Mycorrhizal structures differ in their physiological and ecological functioning, so changes in AM allocation to intraradical (inside roots) and extraradical (in soil) structures may signal shifts in mycorrhizal function. We hypothesize that the functional equilibrium model applies to AM fungi and that fertilization should reduce allocation to arbuscules, coils, and extraradical hyphae, the fungal structures that are directly involved in nutrient acquisition and transfer to plants. This study compared AM responses to experimental N enrichment at five grasslands distributed across North America. Samples were collected from replicated N-enriched (and some P-enriched) and control plots throughout the growing season for three years. Intraradical AM structures were measured in over 1400 root samples, extraradical hyphal density was measured in over 590 soil samples, and spore biovolume was analyzed in over 400 soil samples. There were significant site × N interactions for spore biovolume, extraradical hyphae, intraradical hyphae, and vesicles. Nitrogen enrichment strongly decreased AM structures at Cedar Creek, the site with the lowest soil N:P, and it increased AM structures at Konza Prairie, the site with the highest soil N:P. As predicted by the functional equilibrium model, in soils with sufficient P, relative allocation to arbuscules, coils, and extraradical hyphae was generally reduced by N enrichment. Allocation to spores and hyphae was most sensitive to fertilization. At the mesic sites, this response was associated with a shift in the relative abundance of Gigasporaceae within AM fungal communities. This study demonstrates that N enrichment impacts mycorrhizal allocation across a wide range of grassland ecosystems. Such changes are important because they suggest an alteration in mycorrhizal functioning that, in turn, may impact plant community composition and ecosystem function.

Patterns and regulation of mycorrhizal plant and fungal diversity

The diversity of mycorrhizal fungi does not follow patterns of plant diversity, and the type of mycorrhiza may regulate plant species diversity. For instance, coniferous forests of northern latitudes may have more than 1000 species of ectomycorrhizal (EM) fungi where only a few ectomycorrhizal plant species dominate, but there are fewer than 25 species of arbuscular mycorrhizal (AM) fungi in tropical deciduous forest in Mexico with 1000 plant species. AM and EM fungi are distributed according to biome, with AM fungi predominant in arid and semiarid biomes, and EM fungi predominant in mesic biomes. In addition, AM fungi tend to be more abundant in soils of low organic matter, perhaps explaining their predominance in moist tropical forest, and EM fungi generally occur in soils with higher surface organic matter.EM fungi are relatively selective of host plant species, while AM tend to be generalists. Similar morphotypes of AM fungi collected from different sites confer different physiological benefits to the same plant species. While the EM fungi have taxonomic diversity, the AM fungi must have physiological diversity for individual species to be so widespread, as supported by existing studies. The environmental adaptations of mycorrhizal fungi are often thought to be determined by their host plant, but we suggest that the physiology and genetics of the fungi themselves, along with their responses to the plant and the environment, regulates their diversity. We observed that one AM plant species,Artemisia tridentata, was associated with different fungal species across its range, indicating that the fungi can respond to the environment directly and must not do so indirectly via the host. Different species of fungi were also active during different times of the growing season on the same host, again suggesting a direct response to the environment.These patterns suggest that even within a single “functional group” of microorganisms, mycorrhizal fungi, considerable diversity exists. A number of researchers have expressed the concept of functional redundancy within functional groups of microorganisms, implying that the loss of a few species would not be detectable in ecosystem functioning. However, there may be high functional diversity of AM fungi within and across habitats, and high species diversity as well for EM fungi. If one species of mycorrhizal fungus becomes extinct in a habitat, field experimental data on AM fungi suggest there may be significant shifts in how plants acquire resources and grown in that habitat.

Effects of nitrogen deposition and empirical nitrogen critical loads for ecoregions of the United States

Human activity in the last century has led to a significant increase in nitrogen (N) emissions and atmospheric deposition. This N deposition has reached a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. One approach for quantifying the deposition of pollution that would be harmful to ecosystems is the determination of critical loads. A critical load is defined as the input of a pollutant below which no detrimental ecological effects occur over the long-term according to present knowledge. The objectives of this project were to synthesize current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and to estimate associated empirical N critical loads. The receptors considered included freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Ecosystem impacts included: (1) biogeochemical responses and (2) individual species, population, and community responses. Biogeochemical responses included increased N mineralization and nitrification (and N availability for plant and microbial uptake), increased gaseous N losses (ammonia volatilization, nitric and nitrous oxide from nitrification and denitrification), and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root : shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species.

MYCORRHIZAL COMMUNITY DYNAMICS FOLLOWING NITROGEN FERTILIZATION: A CROSS‐SITE TEST IN FIVE GRASSLANDS

Arbuscular mycorrhizal fungi (AMF) are considered both ecologically and physiologically important to many plant communities. As a result, any alteration in AMF community structure following soil nitrogen (N) enrichment may impact plant community function and contribute to widespread changes in grassland productivity. We evaluated the responses of AMF communities to N fertilization (≥100 kg N·ha−1·yr−1) in five perennial grasslands within the Long-Term Ecological Research network to generate a broader understanding of the drivers contributing to AMF species richness and diversity with increasing soil N fertility, and subsequent effects to host-plant communities. AMF spore and hyphal community data at three mesic sites (Cedar Creek, Kellogg Biological Station, Konza Prairie) and two semiarid sites (Sevilleta, Shortgrass Steppe) were collected over two consecutive years and used to test four hypotheses about AMF responses to N fertilization. Under ambient soil N, plant annual net primary productivity and soi...

Nitrogen critical loads and management alternatives for N-impacted ecosystems in California

Empirical critical loads for N deposition effects and maps showing areas projected to be in exceedance of the critical load (CL) are given for seven major vegetation types in California. Thirty-five percent of the land area for these vegetation types (99,639 km(2)) is estimated to be in excess of the N CL. Low CL values (3-8 kg N ha(-1) yr(-1)) were determined for mixed conifer forests, chaparral and oak woodlands due to highly N-sensitive biota (lichens) and N-poor or low biomass vegetation in the case of coastal sage scrub (CSS), annual grassland, and desert scrub vegetation. At these N deposition critical loads the latter three ecosystem types are at risk of major vegetation type change because N enrichment favors invasion by exotic annual grasses. Fifty-four and forty-four percent of the area for CSS and grasslands are in exceedance of the CL for invasive grasses, while 53 and 41% of the chaparral and oak woodland areas are in exceedance of the CL for impacts on epiphytic lichen communities. Approximately 30% of the desert (based on invasive grasses and increased fire risk) and mixed conifer forest (based on lichen community changes) areas are in exceedance of the CL. These ecosystems are generally located further from emissions sources than many grasslands or CSS areas. By comparison, only 3-15% of the forested and chaparral land areas are estimated to be in exceedance of the NO(3)(-) leaching CL. The CL for incipient N saturation in mixed conifer forest catchments was 17 kg N ha(-1) yr(-1). In 10% of the CL exceedance areas for all seven vegetation types combined, the CL is exceeded by at least 10 kg N ha(-1) yr(-1), and in 27% of the exceedance areas the CL is exceeded by at least 5 kg N ha(-1) yr(-1). Management strategies for mitigating the effects of excess N are based on reducing N emissions and reducing site N capital through approaches such as biomass removal and prescribed fire or control of invasive grasses by mowing, selective herbicides, weeding or domestic animal grazing. Ultimately, decreases in N deposition are needed for long-term ecosystem protection and sustainability, and this is the only strategy that will protect epiphytic lichen communities.

Control of Invasive Weeds with Prescribed Burning

Prescribed burning has primarily been used as a tool for the control of invasive late-season annual broadleaf and grass species, particularly yellow starthistle, medusahead, barb goatgrass, and several bromes. However, timely burning of a few invasive biennial broadleaves (e.g., sweetclover and garlic mustard), perennial grasses (e.g., bluegrasses and smooth brome), and woody species (e.g., brooms and Chinese tallow tree) also has been successful. In many cases, the effectiveness of prescribed burning can be enhanced when incorporated into an integrated vegetation management program. Although there are some excellent examples of successful use of prescribed burning for the control of invasive species, a limited number of species have been evaluated. In addition, few studies have measured the impact of prescribed burning on the long-term changes in plant communities, impacts to endangered plant species, effects on wildlife and insect populations, and alterations in soil biology, including nutrition, mycorrhizae, and hydrology. In this review, we evaluate the current state of knowledge on prescribed burning as a tool for invasive weed management. Nomenclature: Barb goatgrass, Aegilops triuncialis L. #3 AEGTR; Canada bluegrass, Poa compressa L. # POACO; Chinese tallow tree, Sapium sebiferum (L.) Roxb. # SAQSE; downy brome, Bromus tectorum L. # BROTE; French broom, Genista monspessulana (L.) L. Johnson # TLNMO; garlic mustard, Alliaria petiolata Andrz. # ALAPE; Kentucky bluegrass, Poa pratensis L. # POAPR; medusahead, Taeniatherum caput-medusae (L.) Nevski; red brome, Bromus madritensis L. ssp. rubens (L.) Husnot # BRORU; ripgut brome, Bromus diandrus Roth # BRODI; Scotch broom, Cytisus scoparius (L.) Link # SAOSC; smooth brome, Bromus inermis Leysser # BROIN; sweetclover, Melilotus spp.; yellow starthistle, Centaurea solstitialis L. # CENSO. Additional index words: Fire, integrated vegetation management, rangelands, wildlands.

Ecological restoration for future sustainability in a changing environment

Abstract Since its emergence in the past decades, restoration ecology has demonstrated an astounding growth as a new discipline of applied science. At the same time, this young discipline has been criticized for its retrospective goals largely based on the past, its fragmented approach, and its idealistic goals, which do not relate to the real world context. Restoration with past-focused, idealistic, and/or ad hoc goals may not work in the future because an ecosystem that is restored for the past environment is not likely to be sustainable in the changing environment of the future, simple recomposition of isolated and fragmented naturalistic patches is not likely to restore ecosystem functions, and unrealistic goals and work plans are not likely to gain public support. We advocate directing the principles and practice of ecological restoration to the future. Future- aimed restoration should acknowledge the changing and unpredictable environment of the future, assume the dynamic nature of ecological communities with multiple trajectories, and connect landscape elements for improving ecosystem functions and structures. In this paper, we discuss the predictability of restoration trajectories under changing environmental conditions, the application of ecological theories to restoration practice, the importance of interdisciplinary approaches and human interventions in ecosystem recovery, and the social context of ecological restoration.

Nitrogen fertilization alters the functioning of arbuscular mycorrhizas at two semiarid grasslands

The effects of nitrogen (N) fertilization on arbuscular mycorrhizas were studied at two semiarid grasslands with different soil properties and N-enrichment history (Shortgrass Steppe in Colorado, and Sevilleta National Wildlife Refuge in New Mexico). These sites are part of the National Science Foundations Long-Term Ecological Research Network. The experimental plots at Shortgrass Steppe were fertilized with ammonium nitrate (NH4NO3) from 1971 to 1975, and have not received additional N since then. The experimental plots at Sevilleta were also fertilized with NH4NO3, but were established in 1995, 2 years before the soils were used for this study. Greenhouse experiments were conducted to compare the growth response of local grasses to arbuscular mycorrhizal (AM) fungi from fertilized (FERT) and unfertilized (UNFERT) field soils, at each site. Two species per site were chosen, Bouteloua gracilis and Elymus elymoides from Shortgrass Steppe, and B. gracilis and B. eriopoda from Sevilleta. Plants were grown for 3 months at HIGH N and LOW N levels, with FERT or UNFERT soil inoculum and in a non-mycorrhizal condition. Fertilization with N altered the functioning of AM fungi at both sites. Grasses inoculated with AM fungi from UNFERT soils had the most tillers, greatest biomass and highest relative growth rates. There were no significant differences in the growth response of plants inoculated with AM fungi from FERT soils and the non-mycorrhizal controls. These results were consistent across sites and species except for the plants grown at LOW N in Sevilleta soils. These plants were deficient in N and phosphorus (P) and did not show growth enhancement in response to AM inoculation with either FERT or UNFERT soils. Percent root length colonized by AM fungi was not directly related to plant performance. However, enrichment with N consistently decreased root colonization by AM fungi in the grasses grown in soils from Shortgrass Steppe with high P availability (18.4 mg kg−1), but not in the grasses grown in Sevilleta soils with low P availability (6.6 mg kg−1). Our study supports the hypotheses that (1) fertilization with N alters the balance between costs and benefits in mycorrhizal symbioses and (2) AM fungal communities from N fertilized soils are less beneficial mutualists than those from unfertilized soils.