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Critical loads of atmospheric deposition help decision-makers identify levels of air pollution harmful to ecosystem components. But when critical loads are exceeded, how can the accompanying ecological risk be quantified? We use a 90% quantile regression to model relationships between nitrogen and sulfur deposition and epiphytic macrolichens, focusing on responses of concern to managers of US forests: Species richness and abundance and diversity of functional groups with integral ecological roles. Analyses utilized national-scale lichen survey data, sensitivity ratings, and modeled deposition and climate data. We propose 20, 50, and 80% declines in these responses as cut-offs for low, moderate, and high ecological risk from deposition. Critical loads (low risk cut-off) for total species richness, sensitive species richness, forage lichen abundance and cyanolichen abundance, respectively, were 3.5, 3.1, 1.9, and 1.3 kg N and 6.0, 2.5, 2.6, and 2.3 kg S ha−1 yr−1. High environmental risk (80% decline), excluding total species richness, occurred at 14.8, 10.4, and 6.6 kg N and 14.1, 13, and 11 kg S ha−1 yr−1. These risks were further characterized in relation to geography, species of conservation concern, number of species affected, recovery timeframes, climate, and effects on interdependent biota, nutrient cycling, and ecosystem services.

Exotic invasive plants threaten ecosystem integrity, and their success depends on a combination of abiotic factors, disturbances, and interactions with existing communities. In dryland ecosystems, soil biocrusts (communities of lichens, bryophytes, and microorganisms) can limit favorable microsites needed for invasive species establishment, but the relative importance of biocrusts for landscape-scale invasion patterns remains poorly understood. We examine effects of livestock grazing in habitats at high risk for invasion to test the hypothesis that disturbance indirectly favors exotic annual grasses by reducing biocrust cover. We present some of the first evidence that biocrusts increase site resistance to invasion at a landscape scale and mediate the effects of disturbance. Biocrust species richness, which is reduced by livestock grazing, also appears to promote native perennial grasses. Short mosses, as a functional group, appear to be particularly valuable for preventing invasion by exotic annual grasses. Our study suggests that maintaining biocrust communities with high cover, species richness, and cover of short mosses can increase resistance to invasion. These results highlight the potential of soil surface communities to mediate invasion dynamics and suggest promising avenues for restoration in dryland ecosystems.

Because of their unique physiology, lichen communities are highly sensitive to climatic conditions, making them ideal bioindicators for climate change. Southeast and south-central Alaska host diverse and abundant lichen communities and are faced with a more rapidly changing climate than many more southerly latitudes. We develop sensitive lichen-based indicators for tracking the effects of climate change in south-central and southeast Alaska. Using 196 plots, we model community composition and 12 individual species abundances in relation to synthetic climate variables. Both types of lichen indicator are closely related to the climate variable describing a transition from warm, wet oceanic climates to cooler, drier suboceanic climates. Lichen communities and individual species exhibited thresholds associated with average December minimum temperatures between −10.2 and −7.8°C and annual precipitation between 106 and 172 cm, suggesting rapid turnover with relatively small changes within these ranges. These climate conditions occur close to the coast in northern portions of the region and further inland in southeast Alaska. Because lichen communities in the threshold region may be most sensitive to a changing future climate, they should be targeted for monitoring efforts.

We used data on epiphytic lichen communities in 1215, 0.4-ha plots in the Southwest U.S.A. collected by the Forest Inventory and Analysis (FIA) program to analyze relationships with climate. We sought the climate variables most strongly associated with differences in epiphytic macrolichen communities and described the nature of those relationships, including diversity, community composition, and patterns in individual species. Five lichen community groups were strongly related to temperature and elevation gradients, overall moisture, and summer rain. Lichen abundance was highest in the wettest groups and lowest in the hottest and driest groups. Warm summer monsoonal climates supported the greatest number of species across all plots and within plots. The monsoonal pattern did not occupy a discrete geographic area, but instead formed a gradient, strongest in the southern part of our study area, diminishing to the north and west. In contrast, hot summer monsoonal climates had much lower within-plot richness. Hot, dry climates had the most variation in species composition among plots, but the fewest species within each plot and across all plots. Lichen community gradients had nonlinear relationships with combinations of climate variables rather than strong linear relationships with any single variable, including those derivative climate variables meant to have direct biological relevance. Relationships between air quality and community gradients were weak, potentially overwhelmed by regional climatic variation and complex topographic gradients. Richness of particular functional groups was more strongly related to climate than was overall species richness; functional groups have their own climatic tolerances, owing to the physiological consequences of growth form and photobiont. Presumably species in different functional groups have experienced their own evolutionary tradeoffs, developing peak performance in different climates. On the other hand, overall richness was driven by an even more complex combination of performances relative to climate and was in some functional groups more strongly related to geographic coordinates than to climate variables. Because climatic variables are themselves geographically structured, stronger model fit for geographic coordinates than for climate implies some influence of large-scale historical factors (i.e., factors not clearly expressed in modern climates, such as past climates, vegetation structure, or disturbance regimes).

Biological soil crusts (BSCs) were sampled by habitat types within and adjacent to the Orchard Combat Training Center (OCTC) in southwest Idaho, U.S.A. Plots consisting of a 34.7 m radius circle, approximately equal to one acre or 0.38 hectares were sampled. We focused on five native vascular plant-dominated current habitat types within the OCTC, including: 1) Wyoming sagebrush, 2) saltbush, 3) rabbitbrush, 4) winterfat, and 5) Sandberg bluegrass. We describe how BSC cover and species richness varied with habitat types in the study area. We recorded the relative abundance of BSCs and vascular plant species and collected voucher specimens for each BSC. The biodiversity of each BSC in these arid habitat types was much greater than many ecologists have assumed. We found a total of 68 species of BSC across all 17 plots. BSC cover differed significantly across the different habitat types. BSC cover was significantly higher in sagebrush and saltbush as compared with Poa, rabbitbrush and winterfat habitat types. Overall, there was substantially more BSC richness (17–47 species) than vascular plant richness (4– 13 species), and BSC richness was positively related to vascular plant richness (R2=0.18, p=0.041). On average, each additional plant species was associated with 1.36 additional BSC species. BSC communities also varied across the habitat types with Buellia punctata as a significant indicator species for sagebrush, Toninia sedifolia for saltbush, and Cladonia pocillum for winterfat. Several BSC species were associated with 2 or 3 habitat types; for example, Cladonia fimbriata, Diploschistes muscorum, Leptogium lichenoides, Massalongia carnosa, Riccia sorocarpa and Trapeliopsis steppica were most common in the sagebrush, Poa, and rabbitbrush habitats. In contrast, Caloplaca tominii, Endocarpon loscosii, Placidium squamulosum and Psora tuckermanii were most common in winterfat and saltbush habitats.

Intensive forest management (IFM) promises to help satisfy increasing global demand for wood but may come at the cost of local reductions to forest biodiversity. IFM often reduces early serai plant diversity as a result of efforts to eliminate plant competition with crop trees. If diversity is a function of bottom-up drivers, theory predicts that specialists at lower trophic levels (e.g., insect herbivores) should be particularly sensitive to reductions in plant diversity. We conducted a stand-level experiment to test bottom-up controls on moth community structure, as mediated by degrees of forest management intensity. Using a dataset of 12,003 moths representing 316 moth species, moth richness decreased only slightly, if at all, as herbicide intensity increased (P = 0.062); the moderate treatment, which is most commonly applied in the northwestern USA, was estimated to have 4.72 (±2.14 SE, P = 0.039) fewer species than the control. Structural equation modeling revealed strong support for an effect of herbicide on plant abundance, which influenced plant species richness and subsequently moth species richness. Moth species richness was associated with plant species richness and followed a power law function (z = 0.42, P = 0.006), which is surprisingly consistent with a recent largescale experiment in agricultural systems, and provides support for bottom-up drivers of moth community structure. Moth abundance was not influenced by the direct effects of silvicultural herbicide treatments. Site-level effects and variation in pre-harvest vegetation communities resulted in residual broadleaf and herbaceous vegetation in even the most intensive treatment. Even at low densities, these residual deciduous and herbaceous plants supported higher than expected moth abundance and richness. We conclude that forest management practices that retain early seral vegetation diversity are the most likely to conserve moth communities.

Recovery of ecological communities following distur-bance is a central theme of ecology and may be mediated by priority effects, in which the species or individuals that first arrive at a site alter the biotic or abiotic environment in ways that affect the establishment and growth of species or individuals that arrive at a later time. Priority effects are common, influencing species distributions and how ecosystems function (Connell and Slatyer 1977; Werner et al. 2016). Ubiquitous in arid and semi-arid regions worldwide, biological soil crusts (Figure 1) contribute to a large number of ecosystem functions including reducing erosion, increasing water infiltration into the soil, cycling nutrients, and influencing vascular plant establishment (Bowker et al. 2011). Lichens within these soil crusts also provide habitat for rich microbial communities (Wedin et al. 2015), as do species of parasitic fungi and others that live off dead or decaying material (Honegger 2012). Biological soil crusts are sensitive to disturbance, but the recovery of complex lichen-dominated crusts is poorly understood (Belnap and Eldridge 2003; Read et al. 2016). To date, research into how priority effects influence the post-disturbance recovery of soil crust communities has been very limited, despite their ubiquity and importance in numerous ecosystem processes.

Biological soil crusts are ecosystem engineers in arid and semi-arid habitats; they affect soil chemistry, stability, and vegetation. Their ecosystem functions may vary depending on species composition; however, lichen species diversity is poorly known in the Pacific Northwestern drylands of North America. We sampled 59 random and 20 intuitive plots throughout central and eastern Oregon identifying 99 lichen taxa, 33 of which occurred in only one plot and seven of which were new to Oregon (Acarospora obpallens, A. terricola, Catapyrenium psoromoides, Placidium fingens, P. pilosellum, P. yoshimurae and Psora luridella). We compile records from herbaria and other studies to evaluate the rarity of observed species and potentially rare species known from nearby locations. We conclude that 37 species are likely rare or uncommon in our study area. Many of these appear to be associated with calcareous substrates. We model occurrences in relation to climate and soil variables for four uncommon lichen species: Acarospora schleicheri, Fuscopannaria cyanolepra, Rhizocarpon diploschistidina, and Texosporium sancti-jacobi. Based on climate and soil variables, we map regions of Oregon that may support new populations of these species and overlay habitats unsuitable for biotic crusts due to development and agriculture. These species, except Fuscopannaria cyanolepra, are strongly associated with the fine soils along the Columbia and Treasure Valleys that are most intensively used for agriculture. We anticipate that our summaries will further the understanding of lichen component of biological soil crust communities in eastern Oregon and suggest focal species for future conservation efforts.

The distribution of sample units in multivariate species space typically departs strongly from the multivariate normal distribution. Instead of forming a hyperellipse in species space, the sample points tend to lie along high-dimensional edges of the space. This dust bunny distribution is seen in most ecological community datasets. The practical consequences of the distribution to the analysis of community data are well known and severe, but no one has demonstrated how population processes generate these problems. We evaluate potential causes of dust bunny distributions by simulating a large number of non-equilibrial communities under varying conditions, verifying that they resemble real data, then analyzing the relationship between the intensity of the dust bunny distribution in these datasets and the population and environmental parameters that gave rise to them. All community datasets, both simulated and real, departed strongly from multivariate normal and lognormal distributions. Four parameters influenced intensity of dust bunnies: time since community-replacing disturbance, number of environmental factors, dispersal limitation, and niche width. Samples measured soon after community-replacing disturbance had strong dust bunny distributions. Near-equilibrial communities sampled from a narrow range in environments lead to only weak dust bunnies. Community samples taken across multiple simultaneous strong environmental gradients are likely to show strong dust bunnies, regardless of the successional state, niche width of the component species, and degree of dispersal limitation. Dust bunny intensity depends not only on population processes and disturbance, but also on the properties of the sample, such as sample unit area or volume.