Few studies have quantified pathogen impacts to ecosystem processes, despite the fact that pathogens cause or contribute to regional-scale tree mortality. We measured litterfall mass, litterfall chemistry, and soil nitrogen (N) cycling associated with multiple hosts along a gradient of mortality caused by Phytophthora ramorum, the cause of sudden oak death. In redwood forests, the epidemiological and ecological characteristics of the major overstory species determine disease patterns and the magnitude and nature of ecosystem change. Bay laurel (Umbellularia californica) has high litterfall N (0.992%), greater soil extractable NO3 -N, and transmits infection without suffering mortality. Tanoak (Notholithocarpus densiflorus) has moderate litterfall N (0.723%) and transmits infection while suffering extensive mortality that leads to higher extractable soil NO3 -N. Redwood (Sequoia sempervirens) has relatively low litterfall N (0.519%), does not suffer mortality or transmit the pathogen, but dominates forest biomass. The strongest impact of pathogen-caused mortality was the potential shift in species composition, which will alter litterfall chemistry, patterns and dynamics of litterfall mass, and increase soil NO3 -N availability. Patterns of P. ramorum spread and consequent mortality are closely associated with bay laurel abundances, suggesting this species will drive both disease emergence and subsequent ecosystem function.

Although there is considerable evidence that large mammalian herbivores influence ecosystem-level processes, studies have reported such widely varying results that generalizations have remained elusive. Here, we use an 18-year-old exclosure experiment-stratified across a landscape heterogeneous with respect to soil texture, moisture and herbivore activity-to understand the variable effects of tule elk (Cervus canadensis nannodes), a native reintroduced herbivore, on soil properties along the coast of northern California. Elk significantly increased soil bulk density and created a compacted layer at shallow soil depth, while decreasing infiltration rate and pH. The effects of elk on bulk density, penetration resistance, and pH varied with soil type, being least pronounced in coarse, sandy loams, and greatest in loose sand. The effects of elk on nutrient availability varied along gradients of soil texture and moisture. In coarser soils, elk decreased ammonium availability, but increased it in finer soils. Elk also decreased soil moisture content, in part through their positive effect on bulk density, and this effect was most pronounced in coarser soils. Through decreasing soil moisture content, elk also decreased nitrate availability in coarser soils. At greater levels of elk activity (as measured by dung deposition), the elk effect on bulk density was amplified, and this had a corresponding negative effect on nitrate and phosphate availability. Our study has demonstrated that a better understanding of spatial variation in the effects of herbivores on ecosystems can emerge by evaluating their influences across gradients of soil texture, soil moisture, and herbivore activity. These data enabled us to evaluate several frameworks that have been developed to understand the variable effects of herbivores on ecosystems, which is a significant step in reconciling the many competing ideas put forth to explain the context-dependent effects of large herbivores on grazed ecosystems.

In disturbance-adapted ecosystems, the removal of disturbance can lead to losses of diversity and sometimes irreversible changes in community composition. It is important to identify the thresholds at which these changes can occur, and to understand the reversibility of these shifts. We examined this question in a vernal pool ecosystem that evolved with low to moderate levels of grazing disturbance. In this system, it is not clear whether the negative effects of long-term grazing exclusion are reversible through grazing reintroduction. We compared adjacent vernal pool wetlands in annual Mediterranean grasslands under three grazing management strategies: continuously grazed (100+ years), long-term excluded (40+ years) and 2 years of reintroduced grazing. We also asked whether grazing treatments altered pool characteristics that are likely to influence plant community composition, and how these relationships changed with environmental conditions. Reintroducing grazing to vernal pools led to both increased diversity and native cover, but the effects on native cover were more immediate than on diversity. We identified several biotic and abiotic mechanisms related to this pattern, including changes to competitive dynamics that favour small statured native annuals and increases in hoofprint microdepressions that make soil moisture more available to plants. Synthesis and applications. Our results show that reintroduced grazing at moderate stocking rates can have significant effects on plant communities after just 2 years and can increase native cover more quickly than overall diversity. Our findings suggest that the negative effects of long-term grazing exclusion in vernal pools may be reversible, but that land managers interested in restoring diversity should plan to monitor beyond the first two years of grazing reintroduction.

Agroecosystems are on both the receiving and contributing ends of increasingly demanding climatic and environmental conditions. Maintaining productive systems under resource scarcity and multiplicative stresses requires precise monitoring and systems-scale planning. By incorporating ecological resilience into agroecosystems research we can gain valuable insight into agroecosystem identity, change, responsivity, and performance under stress, but only if we move away from resilience as a mere touchstone concept. Using the productivity, stability, resistance, and recovery of system processes as a basic framework for resilience monitoring, we propose quantitative research approaches to tackle the continuing lack of biophysical, field-scale indicators needed to lend insight into dynamic resilience variables and mechanisms. We emphasize the importance of considering productive functions, sources of system regulation and disturbance, and cross-scale interactions when applying resilience theory to agroecosystems. Agroecosystem resilience research requires understanding of multiple scales and speeds of influence both above and below the focal scale. When these considerations are addressed, resilience theory can add tangible value to agroecosystems research, both for the purposes of monitoring current systems and of planning future systems that can reconcile productivity and sustainability goals.

Disturbance often increases local-scale (α) diversity by suppressing dominant competitors. However, widespread disturbances may also reduce biotic heterogeneity (β diversity) by making the identities and abundances of species more similar among patches. Landscape-scale (γ) diversity may also decline if disturbance-sensitive species are lost. California's vernal pool plant communities are species rich, in part because of two scales of β diversity: (1) within pools, as species composition changes with depth (referred to here as vertical β diversity), and (2) between pools, in response to dispersal limitation and variation in pool attributes (referred to here as horizontal β diversity). We asked how grazing by livestock, a common management practice, affects vernal pool plant diversity at multiple hierarchical spatial scales. In terms of abundance-weighted diversity, grazing increased α both within local pool habitat zones and at the whole-pool scale, as well as γ at the pasture scale without influencing horizontal or vertical β diversity. In terms of species richness, increases in α diversity within habitat zones and within whole pools led to small decreases in horizontal β diversity as species occupancy increased. This had a dampened effect on species richness at the γ (pasture) scale without any loss of disturbance-sensitive species. We conclude that grazing increases species richness and evenness (α) by reducing competitive dominance, without large disruptions to the critical spatial heterogeneity (β) that generates high landscape-level diversity (γ).

Mediterranean-type ecosystems are increasingly threatened by climate change and exotic annual species, jeopardizing the native communities and their global biodiversity. In these systems, soil nitrogen (N) limits net primary production, and its availability can be influenced by both of these stressors. To understand the interactive effects of droughts and exotic herbaceous species on soil N, we monitored the temporal variability of soil inorganic N, net N mineralization, net nitrification, and NO3- leaching under native- and exotic-dominated stands exposed to rainfall manipulation plots in a Mediterranean-type shrub-dominated community. Increasing drought severity resulted in the accumulation of soil NH4+ and NO3-, with a more pronounced increase in exotic-dominated plots. Increased net N mineralization and net nitrification and reduced leaching losses were observed as mechanisms of inorganic N accumulation. In comparison to soils under native plants, soils under exotic plants had enhanced leaching losses upon soil rewetting. We propose that distinct traits of exotic annual herbaceous species associated with higher N inputs, faster turnover, and reduced temporal uptake determine the changes in N cycling in response to droughts. Severe droughts and exotic plants may produce a larger, more vulnerable pool of N that is prone to losses while providing a competitive advantage to promote exotic growth in these N-limited ecosystems.

Introductions of mammalian herbivores to remote islands without predators provide a natural experiment to ask how temporal and spatial variation in herbivory intensity alter feedbacks between plant and soil processes. We investigated ecosystem effects resulting from introductions of Rangifer tarandus (hereafter "Rangifer") to native mammalian predator- and herbivore-free islands in the Aleutian archipelago of Alaska. We hypothesized that the maritime tundra of these islands would experience either: (1) accelerated ecosystem processes mediated by positive feedbacks between increased graminoid production and rapid nitrogen cycling; or (2) decelerated processes mediated by herbivory that stimulated shrub domination and lowered soil fertility We measured summer plant and soil properties across three islands representing a chronosequence of elapsed time post-Rangifer introduction (Atka: ∼100 yr; Adak: ∼50; Kagalaska: ∼0), with distinct stages of irruptive population dynamics of Rangifer nested within each island (Atka: irruption, K-overshoot, decline, K-re-equilibration; Adak: irruption, K-overshoot; Kagalaska: initial introduction). We also measured Rangifer spatial use within islands (indexed by pellet group counts) to determine how ecosystem processes responded to spatial variation in herbivory. Vegetation community response to herbivory varied with temporal and spatial scale. When comparing temporal effects using the island chronosequence, increased time since herbivore introduction led to more graminoids and fewer dwarf-shrubs, lichens, and mosses. Slow-growing Cladonia lichens that are highly preferred winter forage were decimated on both long-term Rangifer-occupied islands. In addition, linear relations between more concentrated Rangifer spatial use and reductions in graminoid and forb biomass within islands added spatial heterogeneity to long-term patterns identified by the chronosequence. These results support, in part, the hypothesis that Rangifer population persistence on islands is facilitated by successful exploitation of graminoid biomass as winter forage after palatable lichens are decimated. However, the shift from shrubs to graminoids was expected to enhance rates of nitrogen cycling yet rates of net N-mineralization, NH+4 pools, and soil δ15N declined markedly along the chronosequence and were weakly associated with spatial use within islands. Overall plant and soil patterns were disrupted but responded differently to intermediate (50 yr) and long-term (100 yr) herbivory, and were correlated with distinct stages of irruptive population dynamics. Copyright:

Compost amendment to grassland is a novel strategy proposed and demonstrated to increase ecosystem carbon (C) storage. However, the effects of compost applications on biomass production and plant diversity are not well known. We assessed impacts of a one-time compost amendment over 4 yr on plant dynamics in two grazed grassland ecosystems in California: a coastal prairie and valley grassland. The valley grassland was dominated by exotic annual grasses and had significantly lower species diversity than the coastal prairie, which consisted of a mix of perennial and annual grasses and forbs. We observed large and persistent increases in aboveground biomass. Over the 4 study years, aboveground biomass from compostamended plots increased by 76% ± 21% at the valley grassland and 41% ± 21% at the coastal prairie, compared with controls. Plant N content was also greater from compost-amended plots. There were no major shifts in species richness or abundance at either grassland site. Overall, plant communities at both grasslands were relatively resistant to the compost addition, but responses of some individual species were observed. Notably at the valley grassland, the abundance of forbs decreased slightly, while the abundance of grasses increased. The abundance of two noxious weeds did not change as a result of the compost amendment. Our results suggest that a single application of composted organic matter used to sequester C provided cobenefits to grassland plant dynamics. The compost amendments produced sustained increases in plant productivity and forage production as well as plant N content without greatly affecting the diversity of these exotic annual grasslands.

Global and regional environmental changes often co-occur, creating complex gradients of disturbance on the landscape. Soil microbial communities are an important component of ecosystem response to environmental change, yet little is known about how microbial structure and function respond to multiple disturbances, or whether multiple environmental changes lead to unanticipated interactive effects. Our study used experimental semi-arid grassland plots in a Mediterranean-climate to determine how soil microbial communities in a seasonally variable ecosystem respond to one, two, or three simultaneous environmental changes: exotic plant invasion, plant invasion + vegetation clipping (to simulate common management practices like mowing or livestock grazing), plant invasion + nitrogen (N) fertilization, and plant invasion + clipping + N fertilization. We examined microbial community structure 5-6 years after plot establishment via sequencing of >1 million 16S rRNA genes. Abiotic soil properties (soil moisture, temperature, pH, and inorganic N) and microbial functioning (nitrification and denitrification potentials) were also measured and showed treatment-induced shifts, including altered NO(-) 3 availability, temperature, and nitrification potential. Despite these changes, bacterial and archaeal communities showed little variation in composition and diversity across treatments. Even communities in plots exposed to three interacting environmental changes were similar to those in restored native grassland plots. Historical exposure to large seasonal and inter-annual variations in key soil properties, in addition to prior site cultivation, may select for a functionally plastic or largely dormant microbial community, resulting in a microbial community that is structurally robust to single and multiple environmental changes.

Livestock grazing in North American rangelands has the capacity both to promote and control the spread of undesirable plant species. Within California annual grasslands, desirable forage peaks in spring and supports considerable livestock grazing. However, spring grazing appears to promote the invasion and spread of two late-season and unpalatable non-native annual grasses, Aegilops triuncialis and Elymus caput-medusae. We tested the hypothesis that grazing reduces the leaf area and water use of early-spring annuals, thus increasing residual soil water availability for the late-season species. We used grazing-exclosure experiments to examine the interactive effects of simulated grazing (i.e., clipping) and competition on soil moisture availability, and on physiological, phenological, and demographic responses. When compared to unclipped controls, spring clipping significantly increased late-season volumetric soil moisture by 13–24% in the top 7 cm of soil, and 8–11% in the top 20 cm of soil (p < 0.05, all sites), which supported significantly higher rates of stomatal conductance (73–100% increase) in both late-season invading species (p < 0.01, all sites). Flowering was significantly delayed in clipped plots for both invader species suggesting these species experienced a longer growing period (p < 0.0001 in all cases). In competition plots, the effects of clipping on the demographic response depended on neighborhood composition. When invaders were grown together, no significant effect of clipping on survival or reproduction was detected in either invader. However, when growing in mixtures with early-spring forage annuals or native species, clipping increased survival and reproductive output in late-season invader species by 3-fold. We suggest that strategies for arresting or reversing the dominance of these late-season invasive annuals must recognize the influence of current biomass management strategies on late-season resource availability.