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1. Successional change following disturbance is a fundamental ecological process that remains central to understanding patterns in plant ecology. Although succession has been studied for well over a century, understanding of the patterns and processes of change is still inadequate, partly because of the dearth of long-term studies. 2. Here, we use, as a model system, a volcanic disturbance that is widespread and of global relevance. We examine succession in old-growth conifer forest understories following burial by tephra (aerially transported volcanic éjecta) in the 1980 eruption of Mount St. Helens. Using four sites with different initial conditions and amounts of disturbance (tephra burial), we sampled plant communities at years 1, 20, and 36 since the eruption using permanent plots representing two treatments: undisturbed tephra and control plots from which tephra was experimentally removed. By using this long-term data, we were able to gauge change through time and differences between tephra-disturbed and control plots. 3. In tephra-impacted plots, cover reached preemption levels by year 36 except for bryophytes at three sites and shrubs at one site. Cover in control plots also increased significantly for all growth forms except bryophytes, remaining above that on tephra plots at the same site in most situations. Sites generally increased in species richness first, and then gradually increased in evenness and Shannon's diversity; after 36 years, differences between control and tephra-impacted plots remained at only one site. Community composition was stable in one site and shifted gradually through time at the two other sites, and differences between plot types persisted at one site. Communities change over 36 years could relate to various endogenic and abiotic factors. 4. Synthesis. These data suggest that, while recovery to the predisturbance status occurs in some cases, recovery is site-specific. Understorey communities can be dynamic, even in old-growth forests, and may still be responding to disturbance over three decades later. Long-term experimental studies are key to understanding succession patterns and generalizations about the importance of initial conditions, disturbance intensity, and the complexity of interactions that determine vegetation change.

Key Points The fact that genetic traits in one species can affect an entire ecosystem has important basic and applied implications that are only now being appreciated. The potential sphere of gene influence can be expanded from the individual and population level (that is, population genetics) to the higher levels included in community and ecosystem genetics. When this is done new fields of study are likely to emerge, such as community and ecosystem genomics. Just as the genotype has a 'traditional' phenotype that is expressed in the individual and population, its expression could also extend to levels higher than the population, to produce community and ecosystem phenotypes. Using traditional population and quantitative genetics methods we show that these community and ecosystem phenotypes are heritable and that some are likely to feed back to affect the fitness of the tree. These combined effects provide a mechanism for exploring the controversial issue of community and ecosystem evolution (that is, the change in genetic interactions between species over time). We show how a mapped trait in a common tree predictably affects the structure and composition of the arthropod community in the canopy of the tree, the microbial community in the soil, the detritivore community in an adjacent stream, the foraging of a mammalian herbivore, and the interactions of different trophic levels. In turn, these interactions predictably affect important ecosystem processes such as litter decomposition and nitrogen mineralization. Future studies will need to evaluate how genetic changes in foundation species — brought about by exotic introductions, genetic engineering and climate change — will result in new community and ecosystem phenotypes. These new phenotypes can affect community structure, biodiversity and fundamental ecosystem processes such as nutrient cycling. Genotypes act not only on individuals but on entire ecological communities. Although it is a complex undertaking, it is possible to extend population and quantitative genetics principles to understanding ecosystem processes, and place them in an evolutionary framework. Can heritable traits in a single species affect an entire ecosystem? Recent studies show that such traits in a common tree have predictable effects on community structure and ecosystem processes. Because these 'community and ecosystem phenotypes' have a genetic basis and are heritable, we can begin to apply the principles of population and quantitative genetics to place the study of complex communities and ecosystems within an evolutionary framework. This framework could allow us to understand, for the first time, the genetic basis of ecosystem processes, and the effect of such phenomena as climate change and introduced transgenic organisms on entire communities.

1. Studies of succession have a long history in ecology, but rigorous tests of general, unifying principles are rare. One barrier to these tests of theory is the paucity of longitudinal studies that span the broad gradients of disturbance severity that characterize large, infrequent disturbances. The cataclysmic eruption of Mount St. Helens (Washington, USA) in 1980 produced a heterogeneous landscape of disturbance conditions, including primary to secondary successional habitats, affording a unique opportunity to explore how rates and patterns of community change relate to disturbance severity, post-eruption site conditions and time. 2. In this novel synthesis, we combined data from three long-term (c. 30-year) studies to compare rates and patterns of community change across three 'zones' representing a gradient of disturbance severity: primary successional blast zone, secondary successional tree blowdown/standing snag zone and secondary successional intact forest canopy/tephra deposit zone. 3. Consistent with theory, rates of change in most community metrics (species composition, species richness, species gain/loss and rank abundance) decreased with time across the disturbance gradient. Surprisingly, rates of change were often greatest at intermediate-severity disturbance and similarly low at high- and low-severity disturbance. There was little evidence of compositional convergence among or within zones, counter to theory. Within zones, rates of change did not differ among 'site types' defined by pre- or post-eruption site characteristics (disturbance history, legacy effects or substrate characteristics). 4. Synthesis. The hump-shaped relationships with disturbance severity runs counter to the theory predicting that community change will be slower during primary than during secondary succession. The similarly low rates of change after high- and low-severity disturbance reflect differing sets of controls: seed limitation and abiotic stress in the blast zone vs. vegetative re-emergence and low light in the tephra zone. Sites subjected to intermediate-severity disturbance were the most dynamic, supporting species with a greater diversity of regenerative traits and seral roles (ruderal, forest and non-forest). Succession in this post-eruption landscape reflects the complex, multifaceted nature of volcanic disturbance (including physical force, heating and burial) and the variety of ways in which biological systems can respond to these disturbance effects. Our results underscore the value of comparative studies of long-term, ecological processes for testing the assumptions and predictions of successional theory.

Riparian ecosystems support mosaics of terrestrial and aquatic plant species that enhance regional biodiversity and provide important ecosystem services to humans. Species composition and the distribution of functional traits - traits that define species in terms of their ecological roles - within riparian plant communities are rapidly changing in response to various global change drivers. Here, we present a conceptual framework illustrating how changes in dependent wildlife communities and ecosystem processes can be predicted by examining shifts in riparian plant functional trait diversity and redundancy (overlap). Three widespread examples of altered riparian plant composition are: shifts in the dominance of deciduous and coniferous species; increases in drought-tolerant species; and the increasing global distribution of plantation and crop species. Changes in the diversity and distribution of critical plant functional traits influence terrestrial and aquatic food webs, organic matter production and processing, nutrient cycling, water quality, and water availability. Effective conservation efforts and riparian ecosystems management require matching of plant functional trait diversity and redundancy with tolerance to environmental changes in all biomes.

Using two genetic approaches and seven different plant systems, we present findings from a meta-analysis examining the strength of the effects of plant genetic introgression and genotypic diversity across individual, community and ecosystem levels with the goal of synthesizing the patterns to date. We found that (i) the strength of plant genetic effects can be quite high; however, the overall strength of genetic effects on most response variables declined as the levels of organization increased. (ii) Plant genetic effects varied such that introgression had a greater impact on individual phenotypes than extended effects on arthropods or microbes/fungi. By contrast, the greatest effects of genotypic diversity were on arthropods. (iii) Plant genetic effects were greater on above-ground versus below-ground processes, but there was no difference between terrestrial and aquatic environments. (iv) The strength of the effects of intraspecific genotypic diversity tended to be weaker than interspecific genetic introgression. (v) Although genetic effects generally decline across levels of organization, in some cases they do not, suggesting that specific organisms and/or processes may respond more than others to underlying genetic variation. Because patterns in the overall impacts of introgression and genotypic diversity were generally consistent across diverse study systems and consistent with theoretical expectations, these results provide generality for understanding the extended consequences of plant genetic variation across levels of organization, with evolutionary implications.

Determining whether organisms can undergo adaptive evolution at a pace commensurate with contemporary climate change is critical to understanding and predicting the consequences of such change. Hybrid introgression is a mechanism of rapid evolution by which species may adapt to climatic shifts. Here, we examine variation in growth and survival in a long-term common garden experiment with a foundation tree species to determine if introgression is enhancing climate change resilience. Two naturally hybridizing tree species, low elevation Populus fremontii and high elevation Populus angustifolia , and hybrid and backcross genotypes were planted in a low elevation, warm common garden. We show that P. angustifolia and backcross trees are vulnerable to warming, and their survival is related to climate and transfer distance (proxies for climate change). Increased odds of survival are associated with genetic introgression, as indicated by RFLP genetic markers. Thus, for these long-lived foundation trees, hybrid introgression is associated with increased resistance to selection pressures in warmer, drier climates. These data highlight the importance of evolutionary patterns and processes in shaping ecosystem responses to climate change. If adaptive introgression through hybrid zones is common, hybrid-specific conservation policies and restoration should be reconsidered in the context of global change. A long-term common garden experiment shows that hybrid introgression is associated with enhanced survival and increased resistance to selection pressures in warmer, drier climates for riparian cottonwood trees.

We present evidence that the heritable genetic variation within individual species, especially dominant and keystone species, has community and ecosystem consequences. These consequences represent extended phenotypes, i.e., the effects of genes at levels higher than the population. Using diverse examples from microbes to vertebrates, we demonstrate that the extended phenotype can be traced from the individuals possessing the trait, to the community, and to ecosystem processes such as leaf litter decomposition and N mineralization. In our development of a community genetics perspective, we focus on intraspecific genetic variation because the extended phenotypes of these genes can be passed from one generation to the next, which provides a mechanism for heritability. In support of this view, common-garden experiments using synthetic crosses of a dominant tree show that their progeny tend to support arthropod communities that resemble those of their parents. We also argue that the combined interactions of extended phenotypes contribute to the among-community variance in the traits of individuals within communities. The genetic factors underlying this among-community variance in trait expression, particularly those involving genetic interactions among species, constitute community heritability. These findings have diverse implications. (1) They provide a genetic framework for understanding community structure and ecosystem processes. The effects of extended phenotypes at these higher levels need not be diffuse; they may be direct or may act in relatively few steps, which enhances our ability to detect and predict their effects. (2) From a conservation perspective, we introduce the concept of the minimum viable interacting population (MVIP), which represents the size of a population needed to maintain genetic diversity at levels required by other interacting species in the community. (3) Genotype X environment interactions in dominant and keystone species can shift extended phenotypes to have unexpected consequences at community and ecosystem levels, an issue that is especially important as it relates to global change. (4) Documenting community heritability justifies a community genetics perspective and is an essential first step in demonstrating community evolution. (5) Community genetics requires and promotes an integrative approach, from genes to ecosystems, that is necessary for the marriage of ecology and genetics. Few studies span from genes to ecosystems, but such integration is probably essential for understanding the natural world.

While individual tree genotypes are known to differ in their impacts on local soil development, the spatial genetic influence of surrounding neighboring trees is largely unknown. We examine the hypothesis that fine root dynamics of a focal tree is based on the genetics of the focal tree as well as the genetics of neighbor trees that together define litter inputs to soils of the focal tree. We used a common garden environment with clonal replicates of individual tree genotypes to analyze fine root production, turnover and allocation with respect to modeled neighborhood: 1) foliar mass, 2) foliar condensed tannins (CT), 3) genetic identity of trees, and 4) genetic dissimilarity of neighbors. In support of our central hypothesis, we found that the presence of genetically dissimilar trees and high leaf CT contributions to the soil predicted increased fine root production. In fact, the modeled effects of neighbors accounted for ~90% of the explanatory weight of all models predicting root production. Nevertheless, the ultimate fate of those roots in soil (turnover) and the balance of fine roots relative to aboveground tree mass were still more predictable based on the genetics of the individual focal trees (explaining 99% of the variation accounted for by models). Our data provide support for a method allowing a comparison of the relative effects of individuals versus spatial neighborhood effects on soils in a genetic context. Such comparisons are important for placing plant-soil feedbacks in a genetic and evolutionary framework since neighbors can decouple feedbacks between an individual and the surrounding environment.

Soil carbon dioxide (CO2) efflux is a major component of terrestrial carbon (C) cycles; yet, the demonstration of covariation between overstory tree genetic-based traits and soil C flux remains a major frontier in understanding biological controls over soil C. Here, we used a common garden with two native tree species, Populus fremontii and P. angustifolia, and their naturally occurring hybrids to test the predictability of belowground C fluxes on the basis of taxonomic identity and genetic marker composition of replicated clones of individual genotypes. Three patterns emerged: soil CO2 efflux and ratios of belowground flux to aboveground productivity differ by as much as 50–150% as a result of differences in clone identity and cross type; on the basis of Mantel tests of molecular marker matrices, we found that c. 30% of this variation was genetically based, in which genetically similar trees support more similar soil CO2 efflux under their canopies than do genetically dissimilar trees; and the patterns detected in an experimental garden match observations in the wild, and seem to be unrelated to measured abiotic factors. Our findings suggest that the genetic makeup of the plants growing on soil has a significant influence on the release of C from soils to the atmosphere.

Several processes bury plants, but sediment can also be subsequently removed, often by delayed erosion. Thus, the ability to survive multiple years of burial and to respond when released are important to vegetation changes and population dynamics. We experimentally evaluated the effects of delayed removal of tephra (aerially transported volcanic ejecta) in an old-growth forest understory near Mount St. Helens, using 1-m2 plots assigned to three treatments: tephra removed 4 months after deposition (50 plots), tephra removed 28 months after deposition (the delayed erosion treatment, 50 plots), and undisturbed, natural tephra (100 plots). Prior to tephra removal, species density, cover, shoot density, and shoot size in the delayed erosion treatment were all similar to values in natural plots and significantly less than values in plots cleared initially, indicating that 24 months of additional burial adversely affected understory plants. However, all attributes eventually approached pre-eruption values for shrubs and herbs, indicating that erosion greatly facilitated vegetation recovery. Responses varied substantially among species and growth forms. Overall, our experimental results indicate that some plants of most species can respond effectively after release from burial of at least three growing seasons. In addition, the delay of erosion retards ecosystem recovery relative to early erosion, facilitates recovery relative to no erosion, and modifies the trajectory of post-disturbance vegetation change.