J. Hall. Cushman

Although forests serve critical roles maintaining biodiversity and provisioning ecosystem services, many tree species world‐wide have experienced population declines due to climate change and other related factors. One such species is quaking aspen ( Populus tremuloides ), the most widely distributed tree species in North America, which has undergone reductions in survival, growth, and/or recruitment in many parts of its range. To understand the patterns in and drivers of aspen performance across its range in six western US states, we evaluated the importance of 44 climatic, topographic, and forest structure variables in explaining mortality, growth and adult recruitment rates in 263 plots over a 9‐to‐13‐year period. Aspen experienced substantial mortality during this study, with 4% of trees on average dying each year. In addition, aspen performed differently from co‐occurring conifers, with their dominance declining during the study while conifers increased. Mortality rates for aspen increased with higher aspen canopy dieback and dead aspen in the initial sampling and decreased with increased initial dominance of aspen and conifers as well as increased precipitation and reduced aridity. Growth rates of aspen increased with higher temperatures and winter precipitation, in more upland and south‐facing locations, and decreased with warmer and drier conditions relative to the historic baseline, greater initial aspen canopy dieback, higher initial dead aspen and greater initial dominance of aspen and conifers. Increased adult recruitment was associated with higher temperatures and winter precipitation as well as more upland and south‐facing locations, whereas reduced recruitment occurred with higher initial aspen and conifer dominance, greater amounts of initial dead aspen, and hotter and drier conditions relative to the historic baseline. Synthesis . Across the western United States, the health and dominance of aspen declined during a decade‐long study, with annual mortality being three times greater than the recruitment of new adults. We frequently detected long‐lasting effects, where areas that were challenging or optimal for aspen at the outset of our study continued to be so over the next decade. Intra‐ and interspecific competition as well as hotter and drier conditions associated with climate change adversely influenced aspen populations.

Herbivory by wild and domestic ungulates can influence tree recruitment and understory forest communities throughout the world. Herbivore‐driven declines in tree recruitment have been observed for quaking aspen (Populus tremuloides), a foundation species whose health and management is recognized as a critical priority throughout much of its range. Livestock fencing is commonly used to promote aspen regeneration, but its effectiveness is rarely assessed, especially across large spatial scales. Using a livestock‐reduction experiment, we evaluated the effects of ungulate herbivory on aspen in the Great Basin and southern Cascades, an expansive and environmentally heterogeneous region where aspen faces the interacting threats of climate change, conifer encroachment, and herbivory. We found that livestock fencing only reduced the intensity of herbivore browsing on aspen when wild ungulate abundance was low and did not increase stem densities of aspen recruits. Contrary to expectations, wild ungulate abundance was a strong driver of browsing intensity on juvenile aspen within fenced, but not unfenced, aspen stands, and when the abundance of these herbivores was high, browsing intensity in fenced stands exceeded that in unfenced stands. The density of aspen recruits decreased with browsing intensity on juvenile aspen and with the density of both adult aspen and conifers, suggesting that both herbivory and intra‐ and interspecific competition are important drivers of recruitment. Fire history was also an important driver of recruitment, with stands that burned 10–20 years ago having the greatest density of aspen recruits. Finally, in the stand understory, we found that livestock fencing decreased forb cover, increased shrub species richness, and increased the cover of exotic annual grasses, a group dominated by Bromus tectorum. This latter finding suggests that livestock fencing may not be appropriate in areas where controlling the spread of this invader is a priority. In sum, our findings indicate that aspen recruitment is limited by browsing by both wild and domestic ungulates, is mediated by competition with neighboring trees and fire history, and will require management actions beyond livestock fencing, as this approach does not control browsing by wild ungulates.

While large herbivores are critically important components of terrestrial ecosystems and can have pronounced top‐down effects on plants, our understanding of the underlying mechanisms driving these effects remains incomplete. Large herbivores can alter plant growth, reproduction, and abundance through direct effects (predominantly consumption) and through indirect effects via altered interactions with abiotic factors and other species. We know considerably less about these indirect effects than the direct effects. Here, we integrate medium‐ and small‐scale field experiments to investigate how a large vertebrate herbivore, cattle (Bos taurus), affects the aboveground biomass of a dominant forb species, Artemisia scoparia, via diverse direct and indirect pathways in a temperate grassland in northeast China. Although cattle consumed this forb, its biomass increased significantly in response to grazing, due to multiple indirect positive effects that outweighed the direct negative effects of consumption. Cattle preferentially consumed the competing grass Leymus chinensis, and altered Artemisia microhabitats by reducing total plant cover and litter biomass and by increasing the abundance of co‐occurring ant species (e.g., Formica spp. and Lasius spp.). This led to additional indirect positive effects on A. scoparia likely due to (1) increased light availability in understory layers and other limiting resources (e.g., soil nutrients and moisture) caused by removal of competitors and plant litter at the soil surface and (2) the changes in resource availability (e.g., soil nutrients and moisture) associated with ant colonies. Our results show that large herbivores can affect plant growth not only via direct consumption, but also via multiple indirect effects. Focusing on the causes and consequences of herbivore‐induced indirect effects will not only help us to better understand the influence of these animals in ecological systems, but will also lead to more effective land management and conservation practices in the regions they inhabit.

Woody plant expansions are altering ecosystem structure and function, as well as fire regimes, around the globe. Tree‐reduction treatments are widely implemented in expanding woodlands to reduce fuel loads, increase ecological resilience, and improve habitat, but few studies have measured treatment outcomes over long timescales or large geographic areas. The Sagebrush Treatment Evaluation Project (SageSTEP) evaluated the ecological effects of prescribed fire and cut‐and‐leave treatments in sagebrush communities experiencing tree expansion in North American cold desert shrublands. We used 10 yr of data from the SageSTEP network to test how treatments interacted with pre‐treatment tree dominance, soil climate, and time since treatment to affect plant functional groups and dominant species. Non‐sprouting shrub (Artemisiaspp.), sprouting shrub, perennial graminoid, and annual grass responses depended on tree dominance and soil climate, and responses were related to the dominant species' life‐history traits. Sites with warm and dry soils showed increased perennial graminoid but reducedArtemisiashrub cover across the tree dominance gradient after prescribed burning, while sites with cool and moist soils showed favorable post‐burn responses for both functional types, particularly at low to moderate tree dominance. Cut‐and‐leave treatments sustained or increased native perennial plant functional groups and experienced smaller increases in exotic annual plants in both soil climates across the tree dominance gradient. Both treatments reduced biocrust cover. Selecting appropriate tree‐reduction treatments to achieve desired long‐term outcomes requires consideration of dominant species, site environmental conditions, and the degree of woodland expansion. Careful selection of management treatments will reduce the likelihood of undesirable consequences to the ecosystem.

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.

Throughout the world, numerous tree species are reported to be in decline, either due to increased mortality of established trees or reduced recruitment. The situation appears especially acute for oaks, which are dominant features of many landscapes in the northern hemisphere. Although numerous factors have been hypothesized to explain reductions in tree performance, vertebrate herbivores and granivores may serve as important drivers of these changes. Here, using data from 8‐ and 14‐year‐old exclosure experiments, we evaluated the individual and interactive effects of large and small mammalian herbivores on the performance of three widespread oak species in California—coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), and Oregon white oak (Q. garryana). Although impacts varied somewhat by species and experiment, herbivory by black‐tailed deer (Odocoileus hemionus columbianus) reduced the height and survival of juvenile coast live oaks and altered their architecture, as well as reduced the abundance of black oak seedlings, the richness of woody species and the cover of nonoak woody species. Small mammals (Microtus californicus and Peromyscus maniculatus) had even more widespread effects, reducing the abundance of black oak seedlings and the height and cover of all three oak species. We also detected numerous interactions between small mammals and deer, with one herbivore having positive or negative effects on oak abundance and cover when the other herbivore was either present or absent. For example, deer often had negative effects on seedling abundance only when, or even more so when, small mammals were present. In summary, mammalian consumers play crucial roles in limiting oak recruitment by reducing seedling abundance, maintaining trees in stunted states, and preventing them from reaching sapling stages and becoming reproductive. Interactions between large and small mammals can also alter the intensity and direction of their effects on trees.

Understanding the role of native biodiversity in controlling exotic species invasion is a critical goal in ecology. In terrestrial plant communities, most research has focused on the effects of native vascular plants on invasion by exotic vascular plants. However, in many ecosystems, native bryophytes and other non‐vascular plants are common and can affect the establishment, survival, and growth of vascular plants. A more complete picture of how native biodiversity affects exotic plant invasion demands that more studies measure the effects of native bryophytes on exotic vascular plants. Moreover, there is growing realization that the effects of native species on invaders can range from negative to positive and that a complete picture of interactions between native and exotic plants requires measuring interactions in multiple environments. We used both observational and experimental studies to quantify the effects of native bryophytes on vascular plants along a 200‐m environmental gradient in a coastal dune in northern California. We found a positive association between vascular plants and bryophytes across the environmental gradient. Our experiments with two exotic annual grass species showed the effects of bryophytes to be species‐specific and to vary with environmental context. Bryophytes facilitated the survival of one exotic grass species, Vulpia bromoides, at both ends of the environmental gradient. Bryophytes reduced the survival and inflorescence production of the other exotic grass, Bromus diandrus, at one end of the environmental gradient and had no effect at the other end. Our findings provide a test of the effects of native bryophytes on exotic vascular plant invasion and show that these effects can vary dramatically even across local environmental gradients.

Large mammalian herbivores are well known to shape the structure and function of ecosystems world‐wide, and these effects can in turn cascade through systems to indirectly influence other animal species. A wealth of studies has explored the effects of large mammals on arthropods, but to date they have reported such widely varying results that generalizations have been elusive. Three factors are likely drivers of this variability: the widely varying life‐history characteristics of different arthropod groups, the highly variable landscapes that mammalian herbivores commonly inhabit and temporal variation in environmental conditions. Here, we use an 18‐year‐old exclosure experiment stratified across three distinct coastal prairie habitats in northern California to address the effects of a reintroduced mammalian herbivore, tule elk (Cervus canadensis nannodes) on the composition, richness and abundance of ground‐dwelling arthropods over two years with very different precipitation regimes. We found that elk shifted the composition of arthropod communities, increasing the abundance of ants, beetles, spiders and mites, decreasing the abundance of woodlice and bristletails in some but not all habitats types, and having no effect on the abundance of bugs, crickets and springtails. Elk also increased richness and changed the composition of ant genera and beetle morpho‐species. Interestingly, the effects of elk on arthropod composition, richness and abundance varied little between years, despite very different precipitation levels, biomass accumulation and thatch height. Elk reduced shrub cover, above‐ground herbaceous biomass and thatch height and increased soil compaction, and these changes predicted the abundance and richness of arthropods, although taxonomic groups varied in their responses, presumably due to differences in environmental requirements. Synthesis. Our research highlights the importance of using long‐term experiments to assess the cascading effects of large herbivores on the composition of grounddwelling arthropod communities and to identify the mechanisms that indirectly shape arthropod responses to herbivores among variable habitats and years in order to develop a greater understanding of the variable responses of arthropods to large mammalian herbivores.

Wild large herbivores are declining worldwide. Despite extensive use of exclosure experiments to investigate herbivore impacts, there is little consensus on the effects of wild large herbivores on ecosystem function. Of the ecosystem functions likely impacted, we reviewed the five most‐studied in exclosure experiments: ecosystem resilience/resistance to disturbance, nutrient cycling, carbon cycling, plant regeneration, and primary productivity. Experimental data on large wild herbivores' effects on ecosystem functions were predominately derived from temperate grasslands (50% grasslands, 75% temperate zones). Additionally, data were from experiments that may not be of adequate size (median size 400 m²despite excluding all experiments below 25 m²) or duration (median duration 6 years) to capture ecosystem‐scale responses to these low‐density and wide‐ranging taxa. Wild herbivore removal frequently impacted ecosystem functions; for example, net carbon uptake increased by three times in some instances. However, the magnitude and direction of effects, even within a single function, were highly variable. A focus on carbon cycling highlighted challenges in interpreting effects on a single function. While the effect of large herbivore exclusion on carbon cycling was slightly positive when its components (e.g. pools vs. fluxes of carbon) were aggregated, effects on individual components were variable and sometimes opposed. Given modern declines in large wild herbivores, it is critical to understand their effects on ecosystem function. However, this synthesis highlights strong variability in direction, magnitude, and modifiers of these effects. Some variation is likely due to disparity in what components are used to describe a given function. For example, for the carbon cycle we identified eight distinctly meaningful components, which are not easily combined yet are potentially misrepresentative of the larger cycle when considered alone. However, much of the observed difference in responses likely reflects real ecological variability across complex systems. To move towards a general predictive framework we must identify where variation in effect is due to methodological differences and where due to ecosystem context. Two critical steps forward are (a) additional quantitative synthetic analyses of large herbivores' effects on individual functions, and (b) improved, increased systematic exclosure research focusing on effects of large herbivores' exclusion on functions. A freePlain Language Summarycan be found within the Supporting Information of this article.

Multiple consumer species frequently co‐occur in the same landscape and, through effects on surrounding environments, can interact in direct and indirect ways. These interactions can vary in occurrence and importance, and focusing on this variation is critical for understanding the dynamics of interactions among consumers. Large mammalian herbivores are important engineers of ecosystems worldwide, have substantial impacts on vegetation, and can indirectly affect small‐mammal populations. However, the degree to which such indirect effects vary within the same system has received minimal attention. We used a 16‐year‐old exclosure experiment, stratified across a heterogeneous landscape, to evaluate the importance of context‐dependent interactions between tule elk ( Cervus canadensis nannodes ) and small mammals (deer mice [ Peromyscus maniculatus ], meadow voles [ Microtus californicus ], and harvest mice [ Reithrodontymys megalotis ]) in a coastal grassland in California. Effects of elk on voles varied among habitats and seasons: In open grasslands, elk reduced vole numbers during fall 2013 but not summer 2014; in Lupinus ‐dominated grasslands, elk reduced vole numbers during summer 2014 but not fall 2013; and in Baccharis ‐dominated grasslands, elk had no effect on vole numbers in either season. Effects of elk on the two mice species also varied among habitats and seasons, but often in different ways from voles and each other. In fall 2013, elk decreased mice abundances in Lupinus ‐dominated grasslands, but not in Baccharis ‐dominated or open grasslands. In summer 2014, elk decreased the abundance of harvest mice consistently across habitat types. In contrast, elk increased deer‐mice numbers in open grasslands but not other habitats. Within the same heterogenous study system, the influence of elk on small mammals was strongly context‐dependent, varying among habitats, mammal species, and seasons. We hypothesize that such variability is common in nature and that failure to consider it may yield inaccurate findings and limit our understanding of interactions among co‐occurring consumers.