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Non-native plant invasions are often catalyzed by wildfires and may alter the structure, function, and composition of ecosystems. Of particular concern are non-native annual grasses that promote increased fire frequency and compete with native vegetation for early season moisture. While these effects are well studied in non-forested ecosystems, relatively few studies document invasions of non-native annual grasses in forested ecosystems. We document non-native plant invasion following repeated high-severity wildfire in mixed-conifer/hardwood forests dominated by Douglas-fir (Pseudotsuga menziesii) in the Klamath Mountains of northern California. Despite great concern regarding the loss of conifer dominance and transitions to non-forested states related to recent and projected increases in wildfire activity, there is little known about invasions of non-native plant species following fire in this region. Non-native plant species capable of long-distance dispersal had an average total cover of 26% and were ubiquitous across a systematic, gridded sample of twenty 800 m2 plots. Non-native forbs included multiple species of common post-disturbance invaders (e.g. Cirsium vulgare, Senecio sylvaticus, Lactuca serriola). Non-native annual grasses comprised more of the vegetative cover than non-native forbs (17% vs. 9%), and were dominated by two species, Aira caryophyllea and Vulpia myuros, which reached as high as 80% cover. Our findings indicate that non-native annual grasses are a novel stressor that may accelerate projected loss of conifer dominance in the Klamath Mountains by facilitating fire spread and increasing competition for early season soil moisture. Additional monitoring will be crucial to understanding the impacts of non-native plants as fire activity in this region continues to increase.

Disturbances alter biodiversity via their specific characteristics, including severity and extent in the landscape, which act at different temporal and spatial scales. Biodiversity response to disturbance also depends on the community characteristics and habitat requirements of species. Untangling the mechanistic interplay of these factors has guided disturbance ecology for decades, generating mixed scientific evidence of biodiversity responses to disturbance. Understanding the impact of natural disturbances on biodiversity is increasingly important due to human-induced changes in natural disturbance regimes. In many areas, major natural forest disturbances, such as wildfires, windstorms, and insect outbreaks, are becoming more frequent, intense, severe, and widespread due to climate change and land-use change. Conversely, the suppression of natural disturbances threatens disturbance-dependent biota. Using a meta-analytic approach, we analysed a global data set (with most sampling concentrated in temperate and boreal secondary forests) of species assemblages of 26 taxonomic groups, including plants, animals, and fungi collected from forests affected by wildfires, windstorms, and insect outbreaks. The overall effect of natural disturbances on α-diversity did not differ significantly from zero, but some taxonomic groups responded positively to disturbance, while others tended to respond negatively. Disturbance was beneficial for taxonomic groups preferring conditions associated with open canopies (e.g. hymenopterans and hoverflies), whereas ground-dwelling groups and/or groups typically associated with shady conditions (e.g. epigeic lichens and mycorrhizal fungi) were more likely to be negatively impacted by disturbance. Across all taxonomic groups, the highest α-diversity in disturbed forest patches occurred under moderate disturbance severity, i.e. with approximately 55% of trees killed by disturbance. We further extended our meta-analysis by applying a unified diversity concept based on Hill numbers to estimate α-diversity changes in different taxonomic groups across a gradient of disturbance severity measured at the stand scale and incorporating other disturbance features. We found that disturbance severity negatively affected diversity for Hill number q = 0 but not for q = 1 and q = 2, indicating that diversity–disturbance relationships are shaped by species relative abundances. Our synthesis of α-diversity was extended by a synthesis of disturbance-induced change in species assemblages, and revealed that disturbance changes the β-diversity of multiple taxonomic groups, including some groups that were not affected at the α-diversity level (birds and woody plants). Finally, we used mixed rarefaction/extrapolation to estimate biodiversity change as a function of the proportion of forests that were disturbed, i.e. the disturbance extent measured at the landscape scale. The comparison of intact and naturally disturbed forests revealed that both types of forests provide habitat for unique species assemblages, whereas species diversity in the mixture of disturbed and undisturbed forests peaked at intermediate values of disturbance extent in the simulated landscape. Hence, the relationship between α-diversity and disturbance severity in disturbed forest stands was strikingly similar to the relationship between species richness and disturbance extent in a landscape consisting of both disturbed and undisturbed forest habitats. This result suggests that both moderate disturbance severity and moderate disturbance extent support the highest levels of biodiversity in contemporary forest landscapes., British Ecological Society LRB20/1002, Junta de Andalucia, European Commission B-FQM-366-UGR20, Centro ANID Basal FB210015, Direccion de Investigacion Universidad de La Frontera DIUFRO DI20-0066, Federal Ministry of Education & Research (BMBF), Max Planck Society, INCT Madeiras da Amazonia, Federal Ministry of Education & Research (BMBF) 01LB1001A 01LK1602A, Brazilian Ministry of Science, Technology and Innovation (MCTI/FINEP) 01.11.01248.00, Russian Science Foundation (RSF) 21-14-00227, Ministry of Education, Youth & Sports - Czech Republic Czech Republic Government LTC 20058, Czech Academy of Sciences RVO67985939, UK Research & Innovation (UKRI), Natural Environment Research Council (NERC) GR3/11743, Greek project POL-AEGIS, Program THALES MIS 376737, Projekt DEAL TH 2218/5-1

Early successional wetland habitat is being lost in temperate regions worldwide as a result of changes in disturbance regimes that allow for the establishment and dominance of woody species. This phenomenon is pronounced in fens, which harbor high numbers of special status plant species that require early successional habitat. We investigated the relationship between woody vegetation encroachment and herbaceous species richness within a northern California coastal fen that has been undergoing encroachment by woody vegetation for ca. 80 years. We established 25 transects within the fen and sampled 338 1x1 m plots located at 4-m intervals along transects. At each plot, we recorded the cover of all woody and herbaceous species, as well as litter, thatch, open water, bare soil, and cover of large woody debris. Modeling indicated that woody vegetation cover, height, species richness, litter cover, and distance from the edge of fen were significant predictors of herbaceous species richness. Vegetation sampling indicated that herbaceous species richness declined with increased woody vegetation cover and height and increased litter cover. Canopy closure was found to result in the complete loss of rare plant species and a significant reduction in herbaceous species richness. The results of this study suggest that the re-introduction of disturbance, specifically those that result in the removal of woody vegetation and litter accumulation, is probably essential to maintain herbaceous species diversity and persistence of special status plant species populations in coastal fens.

We compared early seral development between stands subject to single and repeated high-severity wildfire in low-elevation, mixed conifer – hardwood forests in the Klamath Mountains, California, USA. We used a before–after, control–impact (BACI) approach to assess changes in the density of conifer regeneration and the cover of multiple components of vegetation structure (conifers, hardwoods, shrubs, forbs, and graminoids) and compare pathways of seral development between plots that burned once and plots that burned twice. Fifty-three field plots were established 6 years following a high-severity fire in 2004. Nineteen of these plots experienced a second high-severity wildfire 11 years later (2015), and all plots were remeasured in 2016–2017. Conifer regeneration was abundant following the first fire but was greatly reduced by the second fire. Plots that did not reburn increased in conifer, hardwood, and shrub cover, whereas plots that reburned increased in forb cover and decreased in shrub, hardwood, and conifer cover. Despite conifer loss, we found little evidence of shifts to nonforested states following repeated fire due to resilience of resprouting hardwoods. Our results indicate that repeated high-severity fire has the potential to protract early seral development and catalyze transitions from mixed conifer – hardwood forest to hardwood-dominated early seral conditions.

Tree mortality is of considerable concern, but the magnitude and extent of forest canopy decline are relatively unknown in landscapes with high levels of topographic complexity, spatial heterogeneity, and species diversity. We assessed 30 years of canopy decline, including a 5-year period characterized by extreme drought, in one of North America’s most diverse landscapes in the Klamath Mountains of northern California, USA. (1) Characterize tree mortality by species, (2) Quantify temporal and spatial patterns of remotely-sensed canopy decline in relation to climate, (3) Compare canopy decline among vegetation types, topographic settings, and stand structural classes during drought. We characterized tree mortality by species with field data and quantified the role of climate on canopy decline with a 30-year Landsat time series. We assessed and compared the role of topography and stand structure on canopy decline during drought. Most tree mortality and canopy decline occurred at higher elevations in Shasta red fir (Abies magnifica var. shastensis) and subalpine forests. Annual area of canopy decline was positively correlated with summer temperature and minimum vapor pressure deficit but not precipitation. The area of canopy decline was three times greater during the drought. The magnitude of decline was greatest at higher elevations, on more exposed, southwestern slopes, and in stands with old-growth structure. Stands in valleys and low slopes experienced relatively little decline. Our study demonstrates the vulnerability of high elevation, old-growth forests to increasing temperature and suggests the potential for refugia from drought in diverse, heterogeneous landscapes.

There is growing concern that populations of fire-adapted species around the world are at risk of extirpation due to 20th century fire exclusion (i.e. senescence risk). Concurrently, increasing fire activity in many regions is prompting concern that repeated, short-interval fire poses a risk to young stands before trees reach reproductive maturity (i.e. immaturity risk). Conceptual models assessing vulnerability to both risks exist, yet there is a lack of empirically-based studies that assess the effects of anthropogenically-altered fire regimes across the entire range of a fire-adapted species. We used geospatial data on fire occurrence (1900–2015) and a systematic sample of recently re-measured (∼2005–2015) forest inventory plots across the range of a serotinous conifer, knobcone pine (Pinus attenuata), in California and southwestern Oregon, U.S.A. to assess the relative threat of immaturity and senescence risk. We base our assessment on: (1) recent trends in area burned, (2) geographic variability and changes in stand-scale population structure, and (3) patterns of local colonization, extirpation, and population change. The area burned across the species range was lowest during the early and mid-20th century and increased in the early 1980s. Approximately 39% of the range burned between 1984 and 2015 and fire rotation was 81 years for the 25-year period ending in 2015. During this period, approximately 6% of the range of knobcone pine experienced repeated fire, and only 4% of the reburned area (∼0.25% of the entire range) experienced multiple fires at an interval less than 10 years. Most populations of knobcone pine currently exist in a mature or decadent state and we found evidence of recent senescence-related extirpation across approximately 15% of its range. However, we also observed a cumulative expansion of knobcone pine during the last decade, and the rate of colonization (less than half of which was associated with fire) was almost double that of extirpation. Despite high rates of colonization and expansion of knobcone pine in response to recent fires, vulnerability to extirpation varies geographically and remains high in some parts of the species range where wildfire activity has been low. This study provides a rare empirically-based look at the contribution of extirpation and colonization to the range-wide dynamics of a serotinous tree. Our findings underlie the importance of recognizing the different temporal and spatial scales at which contemporary disturbance regimes threaten fire-adapted species, and highlight the potential for alternative pathways of persistence and expansion of serotinous species.

Pickart, A.J.; Maslach, W.R.; Parsons, L.S.; Jules, E.S.; Reynolds, C.M., and Goldsmith, L.M., 2021. Comparing restoration treatments and time intervals to determine the success of invasive species removal at three coastal dune sites in Northern California, U.S.A. Journal of Coastal Research, 37(3), 557–567. Coconut Creek (Florida), ISSN 0749-0208.Dune systems worldwide have been greatly altered by the introduction and spread of invasive species. On the western coast of North America Ammophila arenaria (L.) Link is the most widely introduced species and now represents the dominant vegetation type in the region. In this study, three dune restoration projects in northern California completed between 1997 and 2011 were surveyed, all of which involved the removal of A. arenaria with the aim of increasing native plant cover and richness. Each site underwent a different restoration treatment: manual removal, herbicide application, and the use of heavy equipment to excavate and bury A. arenaria. Species composition and vegetation structure in restored and reference (i.e. uninvaded) areas were recorded at each site. Reference and restored plots differed in species composition at the herbicide-treated site and the heavy-equipment site, but not at the manually treated site. Similarly, plant species richness was similar in restored and reference plots at the manually treated site, but was greater in the reference plots at the other two sites. Vascular plant cover was similar in restored and reference plots at the manually treated site, whereas the herbicide site exhibited higher cover in restored plots and the heavy-equipment site was characterized by lower cover in restored plots. Differences were attributed to a combination of treatment type and time since restoration, with the manually treated plots best meeting goals, but also having the longest time since treatment. Short-term monitoring may fail to identify that a project is on a restoration trajectory and may lead to an inaccurate conclusion of failure to meet goals.

Whitebark pine (Pinus albicaulis) is a long-lived tree found in high-elevation forests of western North America that is declining due to the non-native white pine blister rust (Cronartium ribicola) and climate-driven outbreaks of mountain pine beetle (Dendroctonus ponderosae; MPB). The National Park Service established a monitoring program for whitebark pine in seven parks, including Sequoia & Kings Canyon, Yosemite, Lassen Volcanic, Crater Lake, Mount Rainier, Olympic, and North Cascades National Parks. Using these data, we summarized stand structure, presence of blister rust, and MPB prevalence to provide a baseline for future monitoring. Next, we used a stochastic, size-structured population model to speculate on future trends in the seven national park populations under conditions of increased MPB activity and ongoing blister rust infection observed in Crater Lake. We found that blister rust infected 29 to 54% of whitebark pine in all the parks except the two southernmost, Sequoia & Kings Canyon and Yosemite, where infections rates were 0.3% and 0.2%, respectively. The proportion of dead trees in Sequoia & Kings Canyon and Yosemite was low (0 to 1%), while they ranged from 10 to 43% in the other parks. Model projections suggested an average population decline of 25% in the parks over the next century using Crater Lake conditions, declines which are possible if blister rust continues to spread and climate change results in a significant increase in the frequency or severity of MPB outbreaks. Overall, our study describes conditions at seven western parks and illustrates potential rates of whitebark pine decline if pest outbreaks and/or blister rust infections worsen.

Fire exclusion has contributed to substantial basal accumulations of forest floor fuels in many historically fire-prone forests in western North America. These accumulations can contribute to undesired impacts (e.g., elevated tree mortality, more smoke production). Thus, managers interested in limiting some of these impacts require a better understanding of the spatial factors contributing to forest floor fuel variation in long-unburned forests. To address this need, we measured the forest floor mound size and depth of each layer at four distances from the tree (base, half, edge, and beyond) in each cardinal direction from the tree, and a subset of density measures for each layer and position around 25 large sugar pine (Pinus lambertiana) in the Klamath Mountains of California. Based on our study we found: 1) duff depths were greater than litter depths and both decreased with distance from tree; 2) duff densities were typically greater than litter but were highly variable with distance to tree; 3) litter and duff depths were positively correlated with loading; 4) use of a previously developed depth-to-loading equation performed well, but tended to underestimate litter loading and overestimate duff loading; and 5) tree size was the most strongly related to basal accumulations of forest floor fuels. Our results indicate there are consistent tree characteristics that can help predict the accumulation of forest floor fuels. This research can help provide managers with site-specific equations to inform fuel and fire treatments in areas with substantial basal accumulation in forests with large sugar pine.

Intraspecific variation can be an important driver of ecological interactions in species‐rich communities. Predicting the effects of intraspecific variation in different environments, however, remains a major challenge. This is because we often do not quantify both the effects of functional traits on associated communities and the extent to which trait variation is due to genetics (genotype effects) vs. plasticity (environment effects). As a consequence, the relative importance of trait plasticity vs. genetic variation in structuring associated communities remains unclear. We sought to fill this gap by conducting common garden experiments with the plant Salix hookeriana across biotic (ant–aphid interactions) and abiotic (wind exposure) environmental gradients in a coastal dune ecosystem. In each experiment, we simultaneously measured plant traits and species richness of associated above‐ and below‐ground communities. We then used statistical models to quantify the relative importance of trait plasticity vs. genetic variation in structuring communities. Our major finding was that trait plasticity was more important than genetic variation in determining the number of species in associated communities. This result was consistent across different environmental contexts (experimental manipulations of ant–aphid interactions and wind exposure), multiple years, and for above‐ground arthropods and root microbes. This occurred because the traits that had the largest effect on species richness were also the most plastic. Synthesis. These results indicate that trait plasticity can be a dominant driver of above‐ and below‐ground biodiversity.

The Shasta snow-wreath (Neviusia cliftonii Shevock, Ertter & D.W. Taylor; Rosaceae) is a rare shrub, endemic to areas near Shasta Lake in the eastern Klamath Range of northern California, USA. Since discovery in 1992, the number of known populations has increased from 3 to 33. Shasta snow-wreath is a thicket-forming shrub, thought to reproduce primarily vegetatively, where individual stems (ramets) arise from the root system. We provide the first descriptions of demography and site characteristics using data collected in eight populations of Shasta snow-wreath. We established permanently-marked transects and recorded the number of ramets, individual stem heights, and the number of inflorescences in 2011, 2012, and 2013. We also characterized sites by tree canopy, shrub components, fuels, vegetation, and ground cover. Using the number of ramets recorded and the total population area, we estimated the number of ramets in each population (‘ramet population size'). Ramet population size ranged from 716 to 18,641 (mean = 5467), and the average maximum stem height ranged from 50 to 159 cm (mean = 104 cm). Larger ramet population size and taller stems were both associated with less tree cover. The two largest ramet population sizes were found at the two highest elevations and most west-facing sites. The average number of inflorescences per stem over three years ranged from 0.08 to 4.91 (mean = 2.76) and showed an increase with elevation. How Shasta snow-wreath will respond to succession or disturbances is unknown, but the negative relationship of ramet population size and canopy cover indicates that fire may have been important for influencing the population size historically. Continued monitoring of the studied populations, and the addition of more populations to the monitoring program, would be useful for detecting demographic changes and for better understanding the factors that govern Shasta snow-wreath.

Whitebark pine (Pinus albicaulis. Engelm.) is vulnerable to a number of threats including an introduced pathogen (Cronartium ribicola J.C. Fisch.), epidemic levels of native mountain pine beetle (Dendroctonus ponderosae Hopkins), fire suppression, and climate change. To describe the structure of whitebark pine populations in two national parks in the southern Cascades (Crater Lake, Oregon, USA (CRLA) and Lassen Volcanic, California, USA (LAVO) National Parks), we surveyed trees in 30 &times, 50 &times, 50 m plots in both parks. We used these plots to describe the extent of white pine blister rust (the disease caused by Cronartium ribicola), mountain pine beetle occurrence, and to elucidate factors influencing the presence of pests and pathogens, cone production, and canopy kill. In each plot, we recorded data related to tree health, including symptoms of blister rust and mountain pine beetle, and reproductive vigor (cone production). In both parks, encroachment from other species, particularly mountain hemlock (Tsuga mertensiana (Bong.) Carri&egrave, re), was negatively associated with cone production. In CRLA, water stress was a good predictor of blister rust infection and cone production. For CRLA and LAVO, the presence of mountain pine beetle and blister rust was associated with higher canopy kill for whitebark pine. Lastly, we found evidence for a pest-pathogen interaction, mountain pine beetle attack was greater for trees that showed symptoms of blister rust infection in CRLA. Our results indicate that whitebark pine populations in the southern Cascade Range are experiencing moderate levels of blister rust infection compared with other sites across the species range, and that competition from shade-tolerant species may result in an additional threat to whitebark pine in both parks. We present our findings in the context of park management and situate them in range-wide and regional conservation strategies aimed at the protection and restoration of a declining species.

Pathogens and insect pests have become increasingly important drivers of tree mortality in forested ecosystems. Unfortunately, understanding the relative contributions of multiple mortality agents to the population decline of trees is difficult, because it requires frequent measures of tree survival, growth, and recruitment, as well as the incidence of mortality agents. We present a population model of whitebark pine (Pinus albicaulis), a high-elevation tree undergoing rapid decline in western North America. The loss of whitebark pine is thought to be primarily due to an invasive pathogen (white pine blister rust; Cronartium ribicola) and a native insect (mountain pine beetle; Dendroctonus ponderosae). We utilized seven plots in Crater Lake National Park (Oregon, USA) where 1220 trees were surveyed for health and the presence of blister rust and beetle activity annually from 2003–2014, except 2008. We constructed size-based projection matrices for nine years and calculated the deterministic growth rate (λ) using an average matrix and the stochastic growth rate (λs) by simulation for whitebark pine in our study population. We then assessed the roles of blister rust and beetles by calculating λ and λs using matrices in which we removed trees with blister rust and, separately, trees with beetles. We also conducted life-table response experiments (LTRE) to determine which demographic changes contributed most to differences in λ between ambient conditions and the two other scenarios. The model suggests that whitebark pine in our plots are currently declining 1.1% per year (λ = 0.9888, λs = 0.9899). Removing blister rust from the models resulted in almost no increase in growth (λ = 0.9916, λs = 0.9930), while removing beetles resulted in a larger increase in growth (λ = 1.0028, λs = 1.0045). The LTRE demonstrated that reductions in stasis of the three largest size classes due to beetles contributed most to the smaller λ in the ambient condition. Our work demonstrates a method for assessing the relative effects of different mortality agents on declining tree populations, and it shows that the effects of insects and pathogens can be markedly different from one another. In our study, beetle activity significantly reduced tree population growth while a pathogen had minimal effect, thus management actions to stabilize our study population will likely need to include reducing beetle activity.

Summary Fire-prone woodlands and savannas world-wide face management challenges resulting from fire exclusion and subsequent encroachment of fire-sensitive trees. In the Pacific Northwest (USA), Quercus garryana oak woodlands and savannas are threatened by encroachment from the native conifer Pseudotsuga menziesii in the absence of fire. In the Bald Hills of Redwood National Park (California, USA), prescribed fire and conifer removal have been used to restore encroached woodlands. We examined the effects of encroachment and restoration on understorey vegetation, comparing four treatments: prescribed fire, prescribed fire and conifer removal, conifer removal, and encroached (control). Treatments including prescribed fire had the greatest native species richness. These two treatments also had the greatest non-native species richness, at both the site level and the treatment level. Woodlands treated with conifer removal and no prescribed fire were intermediate in species richness and diversity compared to burned treatments and encroached woodlands. Encroached woodlands had diminished richness and diversity compared to all restoration treatments. Non-metric multidimensional scaling (NMS) ordination demonstrated that conifer basal area, conifer litter and fine wood were associated with low species richness and diversity and that elevation and thatch were associated with higher species richness and diversity. Indicator species analysis identified that most native species and non-native species were associated with burned woodlands that were never encroached. Synthesis and applications. Our results suggest that both prescribed fire and conifer removal have benefits for understorey plant communities, increasing species richness, diversity and cover in oak woodlands and shifting understorey communities from forest-associated species to more woodland-associated species. Restoration of remnant Quercus garryana oak woodlands is complicated by the persistence and abundance of non-native herbaceous plants.

Disturbances such as wildfire play a major role in the diversity, structure and composition of plant communities, however, little is known about the differential impacts of fire across landscapes that vary in characteristics such as soil nutrients and site productivity. Theory predicts that productivity can mediate the impacts of fire for reasons related to broad ecological processes and differential selective forces. For instance, ecosystems with lower site productivity are less limited by space and light and consequently experience less pronounced changes in these resources following a disturbance. Moreover, resource availability related to disturbance and productivity can affect the proportion of plants with competitive versus stress‐tolerant life history strategies. In this study, we utilized a model system for testing predictions about productivity and disturbance that included a mixed conifer forest across a gradient of edaphically harsh, ultramafic “serpentine” soils and “nonserpentine” soils in the northern Sierra Nevada (California, USA). We predicted that the magnitude of fire effects on plant diversity from a 2008 wildfire would be positively related to productivity (higher on nonserpentine soils) and that these factors would interact as environmental filters driving post‐fire species assemblage. In summer 2013 we established 90 vegetation plots in burned areas and 40 plots outside the fire perimeter as a proxy for pre‐fire conditions. We found a unimodal relationship between species diversity and fire severity (peaking at low/moderate severity), and mild evidence post‐fire changes were more pronounced on nonserpentine soils. In contrast, we found strong evidence that productivity and fire severity interact as drivers of species composition and functional traits with a higher proportion of resprouting shrubs on nonserpentine soils and, contrary to our prediction, more invaders on serpentine soils. We hypothesize that differences in biomass between serpentine and nonserpentine forests were not substantial enough to elicit a differential diversity response, possibly deriving from a weaker serpentine syndrome in this region that has been previously noted. Our study reveals that differences in productivity can mediate the outcome of disturbances in ways that cannot be detected through standard community diversity metrics, and that consideration of life history trait variation is necessary.

Host-plant genetic variation affects the diversity and composition of associated above and belowground communities. Most evidence supporting this view is derived from studies within a single common garden, thereby constraining the range of biotic and abiotic environmental conditions that might directly or indirectly (via phenotypic plasticity) affect communities. If natural variability in the environment renders host-plant genetic effects on associated communities unimportant, then studying the community-level consequences of genetic variation may not be warranted. We addressed this knowledge gap by planting a series of common gardens consisting of 10 different clones (genotypes) of the willow Salix hookeriana in a coastal dune ecosystem and manipulated natural variation in ant-aphid interactions (biotic) and wind exposure (abiotic) in two separate experiments. We then quantified the responses of associated species assemblages both above (foliar arthropods) and belowground (rhizosphere fungi and bacteria). In addition, we quantified plant phenotypic responses (plant growth, leaf quality, and root quality) to tease apart the effects of genetic variation, phenotypic plasticity, and direct environmental effects on associated communities. In the ant-aphid experiment, we found that willow genotype explained more variation in foliar arthropod communities than aphid additions and proximity to aphid-tending ant mounds. However, aphid additions modified willow genetic effects on arthropod community composition by attracting other aphid species to certain willow genotypes. In the wind experiment, wind exposure explained more variation than willow genotype in structuring communities of foliar arthropods and rhizosphere bacteria. Still, willow genotype had strong effect sizes on several community properties of arthropods and fungi, indicating that host-plant genetic variation remains important. Across both experiments, genetic variation in plant traits was more important than phenotypic plasticity in structuring associated communities. The relative importance of genetic variation vs. direct environmental effects though depended on the type of environmental gradient (G > E-aphid, but E-wind > G). Taken together, our results suggest that host-plant genetic variation is an important driver of above and belowground biodiversity, despite natural variation in the biotic and abiotic environment.

Summary Port Orford cedar (Chamaecyparis lawsoniana) is endemic to northern California and southwestern Oregon and is considered a foundation species that plays critical roles in riparian areas and on nutrient-poor soils. Since 1952, a non-native, pathogenic oomycete (Phytophthora lateralis) has been spreading throughout the range of the cedar. Most spread occurs by vehicles carrying infested soil along gravel roads primarily used for timber harvest. In a previous study conducted in 1998 and 1999, Port Orford cedar and P. lateralis were censused in a 37-km2 study area and dendrochronology was used to reconstruct the history of pathogen invasion. That work, which represents the only detailed analysis of spread rates for P. lateralis, showed that the first successful invasion into the study area took place in 1977 and that 43% of the susceptible host sites (stream crossings) were infested by 1999. In the work presented here, all sites that were uninfested in 1999 were re-censused in 2012, extending the historical reconstruction of P. lateralis spread to 35 years. Two new infestations were initiated between 1999 and 2012, suggesting that the rate of spread of P. lateralis has slowed greatly. Between 1980 and 1989, the average number of new site infestations was 1.8 infestations per year, while between 1990 and 1999 the average was 0.4 infestations per year and between 2000 and 2009 the average was 0.2 infestations per year. Several potential explanations for the reduced number of new infestations are discussed.

PREMISE OF THE STUDY Serpentine soils have attracted the attention of evolutionary biologists for decades because of their high number of rare and endemic taxa, though less is known about the ecological factors that govern the diversity and composition of serpentine communities. Theory suggests that vegetation on these low-productivity soils will be relatively resilient to fire, the most common natural disturbance in serpentine systems. METHODS We studied the recovery of vegetation in Darlingtonia fens, a unique habitat dominated by herbaceous perennials, from a major fire that burned ∼202,000 ha in California and Oregon's Klamath Mountains in 2002. We established permanent plots in eight unburned and eight burned fens in 2003 and recorded percent cover of vascular plant species. We re-sampled plots each year through 2007. KEY RESULTS Burned fens had less plant cover than unburned fens for 2 yr after the fire. Average species density was ∼10% lower in burned fens 1 yr after the fire but ∼4-8% higher for the next 4 yr. Burned fens exhibited greater evenness but not until 4 yr after the fire. Differences in community composition were detected between the two fen types, but species ranks were similar, and species neither were added to nor removed from the burned assemblages. CONCLUSIONS Burning of Darlingtonia fens has detectable, albeit modest, effects on serpentine communities. Because fens have little or no canopy cover, fire has little influence on light availability in this system. This relatively small resource change, combined with high soil moisture and well-developed underground organs of fen plants, produces a highly resilient assemblage.

Patterns of species occurrence and abundance are infl uenced by abiotic factors and biotic interactions, but these factors are diffi cult to disentangle without experimental manipulations. In this study, we used observational and experimental approaches to investigate the role of temperature and interspecifi c competition in controlling the structure of groundforaging ant communities in forests of the Siskiyou Mountains of southwestern Oregon. To assess the potential role of competition, we fi rst used null model analyses to ask whether species partition temporal and/or spatial environments. To understand how thermal tolerances infl uence the structure of communities, we conducted a laboratory experiment to estimate the maximum thermal tolerance of workers and a fi eld experiment in which we added shaded microhabitats and monitored the response of foragers. Finally, to evaluate the roles of temperature and interspecifi c competition in the fi eld, we simultaneously manipulated shading and the presence of a dominant competitor (Formica moki). Th e foraging activity of species broadly overlapped during the diurnal range of temperatures. Species co-occurrence patterns varied across the diurnal temperature range: species were spatially segregated at bait stations at low temperatures, but co-occurred randomly at high temperatures. Th e decreased abundance of the co-occurring thermophilic Temnothorax nevadensis in shaded plots was a direct eff ect of shading and not an indirect eff ect of competitive interactions. Th ermal tolerance predicted the response of ant species to the shading experiment: species with the lowest tolerances to high temperatures showed the greatest increase in abundance in the shaded plots. Moreover, species with more similar thermal tolerance values segregated more frequently on baits than did species that diff ered in their thermal tolerances. Collectively, our results suggest that thermal tolerances of ants may mediate competitive eff ects in habitats that experience strong diurnal temperature fl uctuations. A major goal of ecology is to identify the processes that con trol patterns of community structure. At large spatial scales, historical processes determine properties of the regional species pool (Ricklefs 2004), which in turn infl uence the composition of local communities (Webb et al. 2002, Wiens and Donoghue 2004). At smaller spatial scales, both abiotic fi ltering (pH, temperature, water availability, etc.), and biotic interactions (interspecifi c competition, parasites, herbivory, etc.) may operate to determine which species occur, and cooccur, together within the local community (Weiher and Keddy 1999, Cavender-Bares et al. 2009).

The large-scale conversion of old forests to tree plantations has made it increasingly important to understand how understory vegetation responds to such landscape changes. For instance, in some forest types a reduction in understory richness and cover is thought to result from the development of canopy closure in plantations, although there is a paucity of empirical data demonstrating this relationship. We used a 420-year forest chronosequence as a case study to assess the relationship between stand age, tree canopy cover and understory vascular plant richness and composition in the Siskiyou Mountains of Oregon. The chronosequence consisted of six young managed (age 7–44) and nine older unmanaged (age 90–427) stands. All stands were similar in underlying geology, slope, elevation, and aspect. We found a non-linear relationship between stand age and richness, in which richness was highest in the youngest stands, reached a low in mid-aged stands (∼55 years), then increased in the oldest stands. We also found that percent tree canopy cover was correlated with total understory cover, richness, diversity, and species composition. In general, young stands were characterized by high shrub and graminoid cover and old stands were characterized by an abundant herb layer. Our work suggests that a major component of our study landscape is currently entering the forest stage (canopy closure) characterized by low levels of vascular plant species richness and cover. We use our results to discuss the potential effects of future forest management on understory plants.

Theory predicts that intraspecific genetic variation can increase the complexity of an ecological network. To date, however, we are lacking empirical knowledge of the extent to which genetic variation determines the assembly of ecological networks, as well as how the gain or loss of genetic variation will affect network structure. To address this knowledge gap, we used a common garden experiment to quantify the extent to which heritable trait variation in a host plant determines the assembly of its associated insect food web (network of trophic interactions). We then used a resampling procedure to simulate the additive effects of genetic variation on overall food-web complexity. We found that trait variation among host-plant genotypes was associated with resistance to insect herbivores, which indirectly affected interactions between herbivores and their insect parasitoids. Direct and indirect genetic effects resulted in distinct compositions of trophic interactions associated with each host-plant genotype. Moreover, our simulations suggest that food-web complexity would increase by 20% over the range of genetic variation in the experimental population of host plants. Taken together, our results indicate that intraspecific genetic variation can play a key role in structuring ecological networks, which may in turn affect network persistence.

Aim A major endeavour of community ecology is documenting non-random patterns in the composition and body size of coexisting species, and inferring the processes, or assembly rules, that may have given rise to the observed patterns. Such assembly rules include species sorting resulting from interspecific competition, aggregation at patchily distributed resources, and co-evolutionary dynamics. However, for any given taxon, relatively little is known about how these patterns and processes change through time and vary with habitat type, disturbance history, and spatial scale. Here, we tested for non-random patterns of species co-occurrence and body size in assemblages of ground-foraging ants and asked whether those patterns varied with habitat type, disturbance history, and spatial scale. Location Burned and unburned forests and fens in the Siskiyou Mountains of southern Oregon and northern California, USA. Methods We describe ground-foraging ant assemblages sampled over two years in two discrete habitat types, namely Darlingtonia fens and upland forests. Half of these sites had been subject to a large-scale, discrete disturbance ‐ a major fire ‐ in the year prior to our first sample. We used null model analyses to compare observed species co-occurrence patterns and body-size distributions in these assemblages with randomly generated assemblages unstructured by competition both within (i.e. at a local spatial scale) and among (i.e. at a regional scale) sites. Results At local spatial scales, species co-occurrence patterns and body-size ratios did not differ from randomness. At regional scales, co-occurrence patterns were random or aggregated, and there was evidence for constant body-size ratios of forest ants. Although these patterns varied between habitats and years, they did not differ between burned and unburned sites. Main conclusions Our results suggest that the operation of assembly rules depends on spatial scale and habitat type, but that it was not affected by disturbance history from fire.

The recovery of native understory plant communities after timber harvest has received a great amount of attention worldwide. However, most of these studies have ignored the potentially significant effects that differences in habitat across forest development may have on the species–area relationship, even though sampling efficacy among forest developmental stages may differ markedly as a result of equal sampling within stages. We examined vascular plant community structure in coastal redwood forests of northern California (USA) in forest patches in each of four forest stages that develop with forest age: Initiation, Closure, Mature, and Old-growth. We also used a set of traditional and novel approaches to assess sampling efficacy and guide our sampling design. We initially sampled 75 circular plots (0.04 ha) in stands within each forest stage across a 1,347 km2 area. The species–area relationship suggested we had adequately sampled all stages after the first season of data collection, but non-parametric richness estimators (Incidence-based Coverage Estimator or ICE) and Michaelis–Menten means (MMMeans) rarefaction curves, indicated inadequate sampling of the older stages. Thus, we added four more samples within the Old-growth forest stage, establishing a more equitable sample of all stages. Our full data set demonstrated significant differences between plant communities in previously harvested forests (‘managed’ = Initiation, Closure, and Mature) and ‘unmanaged’ forests (Old-growth). Though several understory species were present in all stages, a suite of Old-growth indicator and restricted species suggest that recovery of some species may take long periods in managed forests. Interestingly, a separate suite of common understory species were lacking in Old-growth, and species richness in this stage was lower than in the mature managed forests. We discuss how this departure from the prediction of peak richness in Old-growth may relate to fire suppression in Old-growth forests. Lastly, analysis of our initial, smaller sampling of plots suggested that Old-growth plots were the least species rich. However, additional sampling of the Old-growth stage demonstrated that Old-growth plots were as rich as Closure plots. This has important implications for all studies concerned with measuring recovery after forest disturbances; stage-dependent habitat-mediated differences that affect sampling efficacy may not be captured by traditional sampling methods.

Although ants perform many critical functions in forested ecosystems, little is known about how they respond to timber harvesting, especially in temperate systems. We examined ground-foraging ant communities and 11 forest characteristics in temperate conifer forests of southwestern Oregon, USA that ranged in age from 5 to 427 years. Seven forest characteristics were related to stand age and were summarized using principal components analysis (PCA). In this case study, species richness was nearly three times higher in young clearcuts compared to closed-canopy and old stands, and worker number was highest in young stands, lowest in closed-canopy stands, and intermediate in old stands. Using stepwise multiple regression, we found that both species richness and worker number were significantly related to PCA axis 1, which represented the environmental changes that accompany stand development, and canopy variability. Though species richness was high in recently logged stands, our study suggests that, in this system, the ant community does not resemble those found in mature forests until over 100 years following disturbance. Because ants modify their environment and perform ecosystem functions like seed dispersal, the alteration of ant communities may cascade through other parts of ecosystems.

1. Community genetics research has posited a genetic basis to the assembly of ecological communities. For arthropod herbivores in particular, there is strong support that genetic variation in host plants is a key factor shaping their diversity and composition. However, the specific plant phenotypes underlying herbivore responses remain poorly explored for most systems. 2. We address this knowledge gap by examining the influence of both genetic and phenotypic variation in a dominant host-plant species, Salix hookeriana, on its associated arthropod herbivore community in a common garden experiment. Specifically, we surveyed herbivore responses among five different arthropod feeding guilds to 26 distinct S. hookeriana genotypes. Moreover, we quantified the heritability of a suite of plant traits that determine leaf quality (e.g. phenolic compounds, trichomes, specific leaf area, C : N) and whole-plant architecture, to identify which traits best accounted for herbivore community responses to S. hookeriana genotype. 3. We found that total herbivore abundance and community composition differed considerably among S. hookeriana genotypes, with strong and independent responses of several species and feeding guilds driving these patterns. We also found that leaf phenolic chemistry displayed extensive heritable variation, whereas leaf physiology and plant architecture tended to be less heritable. Of these traits, herbivore responses were primarily associated with leaf phenolics and plant architecture; however, different herbivore species and feeding guilds were associated with different sets of traits. Despite our thorough trait survey, plant genotype remained a significant predictor of herbivore responses in most trait association analyses, suggesting that unmeasured host-plant characteristics and/or interspecific interactions were also contributing factors. 4. Taken together, our results support that the genetic basis of herbivore community assembly occurs through a suite of plant traits for different herbivore species and feeding guilds. Still, identifying these phenotypic mechanisms requires measuring a broad range of plant traits and likely further consideration of how these traits affect interspecific interactions. Fil: Barbour, Matthew A.. University Of British Columbia; Canadá Fil: Rodriguez Cabal, Mariano Alberto. University Of British Columbia; Canadá. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Patagonia Norte. Instituto de Investigación en Biodiversidad y Medioambiente; Argentina. Universidad Nacional del Comahue; Argentina Fil: Wu, Elizabeth T.. Humboldt State University; Estados Unidos Fil: Julkunen Tiitto, Riitta. University of Eastern Finland; Finlandia Fil: Ritland, Carol E.. University Of British Columbia; Canadá Fil: Miscampbell, Allyson E.. University Of British Columbia; Canadá Fil: Jules, Erik S.. Humboldt State University; Estados Unidos Fil: Crutsinger, Gregory M.. University Of British Columbia; Canadá

We investigated the effects of the abiotic environment, plant community composition and disturbance by fire on ant assemblages in two distinct habitat types in the Siskiyou Mountains in northern California and southern Oregon, USA. Sampling over 2 years in burned and unburned Darlingtonia fens and their adjacent upland forests, we found that the effects of disturbance by fire depended on habitat type. In forests, fire intensity predicted richness in ant assemblages in both years after the fire, and plant community composition predicted richness 2 years after the fire. No factors were associated with richness in the species-poor fen ant assemblages. Speciesspecific responses to both habitat type and disturbance by fire were idiosyncratic. Assemblage composition depended on habitat type, but not disturbance by fire, and the composition of each assemblage between years was more dissimilar in burned than unburned sites.

We investigated the long-term recovery of Trillium ovatum (Liliaceae) following clear-cut logging by assessing demographic trends across a forest chronosequence that ranged in age from 2 to 424 years. In each of 20 sites across the chronosequence, we used 50 50 m plots to assess the population size, age and stage structure, seed production, and the spatial distribution of seedlings relative to reproductive plants. We found that Trillium populations were significantly reduced in younger sites and a significant positive relationship of population size and stand age, suggesting that population recovery following standreplacement disturbances may take centuries. Slow recovery rates of Trillium can partly be explained by short seed dispersal distances. We found 67% of new Trillium recruits within only one meter of potential parent plants. Despite the general trend of slow recovery, a few recently disturbed populations showed fast population growth. In these cases, high rates of post-disturbance recruitment were related to the number of individuals surviving the disturbance event. We found that local, within-site seed production can most likely mitigate limited dispersal abilities of Trillium. The rate of Trillium recovery following disturbance, therefore, depends on the number of plants that have persisted through the disturbance event and serve as within-site seed sources. Our study suggests that the nature of the stand-replacement disturbance (i.e., magnitude and intensity) may be critical in determining the recovery of understory plants in managed forests. # 2005 Elsevier B.V. All rights reserved.

Tree-ring data from Chamaecyparis lawsoniana (A. Murr.) Parl. (Port Orford cedar; Cupressaceae) were used to create a standardized chronology, assess the local, limiting factors on radial growth, and investigate the extent of a unique climatic event. We produced a 580-year tree-ring chronology (A.D. 1420 to 2000) from a large number of cedars (n = 1537) sampled in one 37 km2 area in the Siskiyou Mountains of southwestern Oregon and northern California. This chronology represents an area with few long-term climatic studies and a species with no dendrochronological data. We found radial growth to be positively correlated with year-round soil moisture conditions, specifically with cool, wet conditions in summer and warm, wet conditions in winter. The year 1739 stood out as a climatic pointer year with the smallest ring width index for the entire chronology and anatomically distinctive damage to the latewood of 1738 and earlywood of 1739. This pointer year was consistently identified across watershed...

Most studies of alpine vegetation communities focus on the variation occurring across large scales, in which diverse environmental habitats are assessed across the landscape and detailed plant associations are described. However, community patterns occurring at more intricate scales are easily concealed by such a broad perspective. To complement large-scale studies, we assessed smaller-scale patterns occurring at the extremes of moisture and exposure gradients in an alpine environment. We studied the variation in plant community composition among and within seven ridges extending down two rocky metamorphic peaks in Yosemite National Park. Species cover and environmental factors (proximity to a summit, elevation, aspect, slope, and substrate size) were sampled in 281 2 × 2-meter plots. Variation among ridges was primarily determined by differences in aspect. Within the ridges, although the physical environment in which we sampled was seemingly homogenous, ordinations identified two distinct vegeta...

Cascading ecological effects of anthropogenic habitat fragmentation have been studied primarily in extreme cases (e.g., the isolation of habitat fragments in a novel habitat matrix such as suburban developments, reservoirs, or agricultural fields), with less attention to more subtle and widespread cases, such as habitat fragmentation due to timber harvest. Few studies have used rigorous demographic data to demonstrate the direct and indirect effects of habitat fragmentation. We trapped deer mice (Peromyscus maniculatus) at five sites over two years in southwest Oregon, USA, and used multi-state capture- recapture models to estimate deer mouse survival and movement in clearcuts, forest-frag- ment edges, forest-fragment interiors, and contiguous forests. We also estimated deer mouse densities in fragmented and unfragmented forests and combined deer mouse demographic studies with trillium (Trillium ovatum) seed predation trials to link deer mouse changes to reduced trillium recruitment previously observed at the same study sites. Mouse survival was highest in clearcuts, intermediate in forest fragments, and lowest in unfragmented (control) forests. Mouse movement among clearcuts, forest edges, and forest interiors was common over short time intervals. Collectively, demographic rates led to mouse densities that were 3-4 times higher at forest-fragment sites than at unfragmented sites. Trillium seeds were ;3 times more likely to be depredated in areas of elevated relative mouse abundance than in areas of lower relative abundance. Forest fragmentation has favored mouse populations, resulting in increased seed predation that may decrease recruitment rates and increase local extinction risks for trillium.

Understanding biological invasions requires information on the history of spatial spread, as well as measures of landscape and biotic features that control habitat invasibility. Because invasive species often spread quickly over large areas, attaining these two sets of information simultaneously is uncommon. We studied the spread of a fatal nonnative root pathogen, Phytophthora lateralis, across a heterogeneous landscape of its host, Port Orford cedar (Chamaecyparis lawsoniana). Within our 37-km2 study area in southwestern Oregon and northwest California, Port Orford cedar populations are generally restricted to riparian zones along creeks. The pathogen is spread between watersheds in two ways: (1) by spore-infested material being dislodged from vehicles, and (2) by animals or people moving infested mud (i.e., via foot traffic). Using dendrochronological techniques, we determined the date of infection for dead cedars and reconstructed spread history across our study area from 1977 to 1999. Twenty-six of t...

Studies of plants in fragmented habitats have focused on single ecological processes, such as seed production or seed dispersal, that may be altered by increased fragmentation and that operate as the mecha- nism(s) that increase extinction probability. We examined a suite of potential mechanisms to explain demo- graphic shifts toward extinction in populations of Trillium ovatum , a long-lived herbaceous perennial found in the understory of western North American conifer forests. Past work has shown that populations of T. ova- tum found within z 65 m of forest-clearcut edges in southwestern Oregon have had almost no new recruit- ment since the edges were formed. We hypothesized that changes in abiotic conditions and biotic interactions present along edges are responsible for reduced recruitment. In eight populations in eight separate fragments, we evaluated the relationship of distance of the populations to the forest edge with respect to six processes: flowering phenology (timing), seed production, pollination- and resource-limitation of seed set, seed dis- persal, seed predation, and germination. Those factors that showed a significant relationship with edge dis- tance were then compared with recruitment of younger age classes. Two processes were significantly different near edges and were highly correlated with decreased recruitment: decreased seed production due to changes in pollination and increased seed predation by rodents. Our study (in conjunction with previous studies) sug- gests that several ecological processes show no significant relationship with edge distance and can be elimi- nated as possible mechanisms of reduced recruitment: flowering phenology, resource-limitation of seed set, seed dispersal, germination, herbivory, and survivorship of established plants. Edges influence some but not all components of a plant's life history. Thus, determining shifts in only one part of a life history will be inad- equate for testing the prediction of increasing extinction probabilities in fragmented landscapes. Future stud- ies should include enough information to conduct comprehensive analyses, such as matrix projections and sensitivity analyses.

I studied the influence of forest fragmentation on an understory herb, Trillium ovatum, in the Siskiyou Mountains of Oregon, where logging practices over the past 35 yr have created a mosaic of fragments surrounded by clearcuts and tree plantations. The age of trillium plants can be estimated by counting the annual constrictions on their rhizomes. Based on data collected by Whittaker in 1949 (i.e., pre-fragmentation) and a survey I conducted in 1995, I estimated that the process of clearcutting and subsequent conifer planting results in the mortality of almost all trillium (∼97.6%). In general, the remaining plants are not recruiting new individuals, even in sites clearcut 30 yr ago. Thus, trillium is restricted to smaller amounts of remnant, uncut forest. My study also demonstrated that populations in forest remnants that were within ∼65 m of forest-clearcut edges have had almost no recruitment of young plants since the time of the adjacent clearcutting, while forest interior populations contained higher recruitment levels. Projections based on these recruitment estimates indicated that edge populations will decline in size and interior populations will not decline. This study provides the first evidence of demographic changes in plant populations resulting from habitat fragmentation, and it offers evidence for the mechanisms responsible for such demographic changes.

Many observations suggest that morphological evolution occurs slowly in bryophytes, and this has been suggested to reflect low genetic diversity within species. Isozyme studies, however, stand in apparent contrast and have shown that bryophytes can contain high levels of genetic variability within and among populations. In light of this conflict, we tested the potential of the moss, Ceratodon purpureus, to undergo adaptive change (i.e., ecotypic differentiation) in response to soils that have been contaminated with high levels of metals for 90 years by measuring gametophytic growth and reproductive expression under experimental conditions. Variation in protonemal growth in sterile culture indicates that plants from one population growing on contaminated soil near a smelter are significantly more tolerant of zinc, cadmium, and lead than plants from uncontaminated sites. Results from a common garden experiment, in which plants were grown on soil from the smelter site, indicate that plants from near the smelter are significantly more tolerant of contaminated soils than plants from uncontaminated sites for vegetative growth. The same experiment suggests that plants from the smelter site are also more tolerant in terms of gametangial production (although we could not test this statistically). Our results demonstrate that C. purpureus has been able to undergo relatively rapid evolution in response to strong selective pressures.

Behaviorally mediated trophic cascades (BMTCs) occur when the fear of predation among herbivores enhances plant productivity. Based primarily on systems involving small-bodied predators, BMTCs have been proposed as both strong and ubiquitous in natural ecosystems. Recently, however, synthetic work has suggested that the existence of BMTCs may be mediated by predator hunting mode, whereby passive (sit-and-wait) predators have much stronger effects than active (coursing) predators. One BMTC that has been proposed for a wide-ranging active predator system involves the reintroduction of wolves (Canis lupus) to Yellowstone National Park, USA, which is thought to be leading to a recovery of trembling aspen (Populus tremuloides) by causing elk (Cervus elaphus) to avoid foraging in risky areas. Although this BMTC has been generally accepted and highly popularized, it has never been adequately tested. We assessed whether wolves influence aspen by obtaining detailed demographic data on aspen stands using tree rings and by monitoring browsing levels in experimental elk exclosures arrayed across a gradient of predation risk for three years. Our study demonstrates that the historical failure of aspen to regenerate varied widely among stands (last recruitment year ranged from 1892 to 1956), and our data do not indicate an abrupt cessation of recruitment. This pattern of recruitment failure appears more consistent with a gradual increase in elk numbers rather than a rapid behavioral shift in elk foraging following wolf extirpation. In addition, our estimates of relative survivorship of young browsable aspen indicate that aspen are not currently recovering in Yellowstone, even in the presence of a large wolf population. Finally, in an experimental test of the BMTC hypothesis we found that the impacts of elk browsing on aspen demography are not diminished in sites where elk are at higher risk of predation by wolves. These findings suggest the need to further evaluate how trophic cascades are mediated by predator-prey life history and ecological context.

Theoretical study of invasion dynamics has suggested that spatial heterogeneity should strongly influence the rate and extent of spreading organisms. However, empirical support for this prediction is scant, and the importance of understanding heterogeneity for real-world systems has remained ambiguous. This study quantified the influence of host and environmental heterogeneity on the dynamics of a 19-year disease invasion by the exotic and fatal pathogen, Phytophthora lateralis, within a stream population of its host tree, Port Orford cedar (Chamaecyparis lawsoniana). Using dendrochronology, we reconstructed the invasion history along a 1350-m length of infected stream, which serves as the only route of pathogen dispersal. Contrary to theoretical predictions, the temporal progression of the disease invasion was not related to a host's downstream spatial position, but instead was determined by two sources of heterogeneity: host size and proximity to the stream channel. These sources of heterogeneity influenced both the epidemic and endemic dynamics of this pathogen invasion. This analysis provides empirical support for the influence of heterogeneity on the invasion dynamics of a commercially important forest pathogen and highlights the need to incorporate such natural variability into both invasion theory and methods aimed at controlling future spread.

Invasive pathogens are becoming increasingly important in forested ecosystems, yet they are often difficult to study because of their rapid transmission. The rate and extent of pathogen spread are thought to be partially controlled by variation in host characteristics, such as when host size and location influence susceptibility. Few host-pathogen systems, however, have been used to test this prediction. We used Port Orford cedar (Chamaecyparis lawsoniana), a foundation tree species in riparian areas of California and Oregon (USA), and the invasive oomycete Phytophthora lateralis to assess pathogen impacts and the role of host characteristics on invasion. Across three streams that had been infected for 13–18 years by P. lateralis, we mapped 2241 trees and determined whether they had been infected using dendrochronology. The infection probability of trees was governed by host size (diameter at breast height [DBH]) and geomorphic position (e.g., active channel, stream bank, floodplain, etc.) similarly across streams. For instance, only 23% of trees

Plant species and communities often reflect historic fire regimes via ecological and evolutionary responses to recurrent fires. Plant communities of the southeastern USA experience a wide array of fire regimes, perhaps nowhere more marked than the juxtaposition of fire-prone uplands and adjacent mesic ravines along Florida's Apalachicola River. The ravines contain many endemic and disjunct species, most notably the endangered endemic conifer Torreya taxifolia. A rapid decline in T. taxifolia over the past 60 years has been associated with widespread replacement by other tree species. To understand the changes accompanying the shift in ravine composition, we compared leaf litter flammability of nine historic and contemporary species. We measured maximum flame height, flame duration, smoldering duration, mass loss, absorptive capacity, and drying rate. Ordination and perMANOVA suggest the nine species segregated into three distinct groups: the fire-impeding T. taxifolia and Taxus floridana; an intermediate group of three deciduous angiosperms; and a mixed cluster of four flammable species. Results suggest T. taxifolia and T. floridana were fire-impeding species in these communities, while contemporary dominants burn similarly to the upslope pyric species. The increasing presence of fire-facilitating species may portend a shifting fire regime that further imperils T. taxifolia and other rare species in the formerly fire-safe ravines.

Gametophytic plants from six populations of the moss Ceratodon purpureus (Hedw.) Brid. were grown on substrates with varied degrees of heavy-metal contamination in order to assess the effects of metal pollution on growth, leaf size, and formation of archegonia and antheridia. Sex ratios and the overall levels of sexual expression varied greatly among samples from natural pop- ulations. Sex ratios in four populations were biased in favor offemales, one in favor of males, and one contained asexual gametophores only. Experimental substrates had a significant effect on the overall levels of sexual expression but not on the relative frequencies of male versus female stems. Innate differences in leaf size among populations were maintained in a common garden but leaf size was also affected by substrate. Patterns ofphenotypic plasticity characterizing the six populations were indistinguishable. Plants from several populations that originated in uncontaminated habitats grew as well on mine soil as plants from a mine-site population, suggesting an absence of ecotypic