Daniel S. Maynard

Mapping tree density at a global scale

The global extent and distribution of forest trees is central to our understanding of the terrestrial biosphere. We provide the first spatially continuous map of forest tree density at a global scale. This map reveals that the global number of trees is approximately 3.04 trillion, an order of magnitude higher than the previous estimate. Of these trees, approximately 1.30 trillion exist in tropical and subtropical forests, with 0.74 trillion in boreal regions and 0.66 trillion in temperate regions. Biome-level trends in tree density demonstrate the importance of climate and topography in controlling local tree densities at finer scales, as well as the overwhelming effect of humans across most of the world. Based on our projected tree densities, we estimate that over 15 billion trees are cut down each year, and the global number of trees has fallen by approximately 46% since the start of human civilization.

Untangling the fungal niche: the trait-based approach

Fungi are prominent components of most terrestrial ecosystems, both in terms of biomass and ecosystem functioning, but the hyper-diverse nature of most communities has obscured the search for unifying principles governing community organization. In particular, unlike plants and animals, observational studies provide little evidence for the existence of niche processes in structuring fungal communities at broad spatial scales. This limits our capacity to predict how communities, and their functioning, vary across landscapes. We outline how a shift in focus, from taxonomy toward functional traits, might prove to be valuable in the search for general patterns in fungal ecology. We build on theoretical advances in plant and animal ecology to provide an empirical framework for a trait-based approach in fungal community ecology. Drawing upon specific characteristics of the fungal system, we highlight the significance of drought stress and combat in structuring free-living fungal communities. We propose a conceptual model to formalize how trade-offs between stress-tolerance and combative dominance are likely to organize communities across environmental gradients. Given that the survival of a fungus in a given environment is contingent on its ability to tolerate antagonistic competitors, measuring variation in combat trait expression along environmental gradients provides a means of elucidating realized, from fundamental niche spaces. We conclude that, using a trait-based understanding of how niche processes structure fungal communities across time and space, we can ultimately link communities with ecosystem functioning. Our trait-based framework highlights fundamental uncertainties that require testing in the fungal system, given their potential to uncover general mechanisms in fungal ecology.

Understanding the dominant controls on litter decomposition

Summary 1. Litter decomposition is a biogeochemical process fundamental to element cycling within ecosystems, influencing plant productivity, species composition and carbon storage. 2. Climate has long been considered the primary broad-scale control on litter decomposition rates, yet recent work suggests that plant litter traits may predominate. Both decomposition paradigms, however, rely on inferences from cross-biome litter decomposition studies that analyse site-level means. 3. We re-analyse data from a classical cross-biome study to demonstrate that previous research may falsely inflate the regulatory role of climate on decomposition and mask the influence of unmeasured local-scale factors. 4. Using the re-analysis as a platform, we advocate experimental designs of litter decomposition studies that involve high within-site replication, measurements of regulatory factors and processes at the same local spatial grain, analysis of individual observations and biome-scale gradients. 5. Synthesis. We question the assumption that climate is the predominant regulator of decomposition rates at broad spatial scales. We propose a framework for a new generation of studies focused on factoring local-scale variation into the measurement and analysis of soil processes across broad scales. Such efforts may suggest a revised decomposition paradigm and ultimately improve confidence in the structure, parameter estimates and hence projections of biogeochemical models.

Biotic interactions mediate soil microbial feedbacks to climate change

Significance The land carbon–climate feedback is incorporated into the earth system models that inform current Intergovernmental Panel on Climate Change projections. This feedback is driven by increases in soil microbial decomposition and carbon loss from soils under global change scenarios. The present study shows how trophic interactions in soil can mediate microbial responses to combined global change factors. As soil nitrogen deposition increases, the limitations on fungal growth are alleviated, stimulating total enzyme activity and decomposition rates. However, this process also affects the grazing activity of soil invertebrates. In the absence of nutrient limitation, top-down control by grazing isopods emerges as a dominant control, limiting any increases in fungal activity and carbon cycling. Decomposition of organic material by soil microbes generates an annual global release of 50–75 Pg carbon to the atmosphere, ∼7.5–9 times that of anthropogenic emissions worldwide. This process is sensitive to global change factors, which can drive carbon cycle–climate feedbacks with the potential to enhance atmospheric warming. Although the effects of interacting global change factors on soil microbial activity have been a widespread ecological focus, the regulatory effects of interspecific interactions are rarely considered in climate feedback studies. We explore the potential of soil animals to mediate microbial responses to warming and nitrogen enrichment within a long-term, field-based global change study. The combination of global change factors alleviated the bottom-up limitations on fungal growth, stimulating enzyme production and decomposition rates in the absence of soil animals. However, increased fungal biomass also stimulated consumption rates by soil invertebrates, restoring microbial process rates to levels observed under ambient conditions. Our results support the contemporary theory that top-down control in soil food webs is apparent only in the absence of bottom-up limitation. As such, when global change factors alleviate the bottom-up limitations on microbial activity, top-down control becomes an increasingly important regulatory force with the capacity to dampen the strength of positive carbon cycle–climate feedbacks.

Predicting the responsiveness of soil biodiversity to deforestation: a cross‐biome study

The consequences of deforestation for aboveground biodiversity have been a scientific and political concern for decades. In contrast, despite being a dominant component of biodiversity that is essential to the functioning of ecosystems, the responses of belowground biodiversity to forest removal have received less attention. Single-site studies suggest that soil microbes can be highly responsive to forest removal, but responses are highly variable, with negligible effects in some regions. Using high throughput sequencing, we characterize the effects of deforestation on microbial communities across multiple biomes and explore what determines the vulnerability of microbial communities to this vegetative change. We reveal consistent directional trends in the microbial community response, yet the magnitude of this vegetation effect varied between sites, and was explained strongly by soil texture. In sandy sites, the difference in vegetation type caused shifts in a suite of edaphic characteristics, driving substantial differences in microbial community composition. In contrast, fine-textured soil buffered microbes against these effects and there were minimal differences between communities in forest and grassland soil. These microbial community changes were associated with distinct changes in the microbial catabolic profile, placing community changes in an ecosystem functioning context. The universal nature of these patterns allows us to predict where deforestation will have the strongest effects on soil biodiversity, and how these effects could be mitigated.

Temperate forest termites: ecology, biogeography, and ecosystem impacts

1. Wood decomposition in temperate forests is dominated by termites, fungi, and some species of ants and beetles. Outside of urban areas, temperate termite ecology is largely unknown, particularly when compared to tropical termites and other temperate organisms in the functional guild of wood‐decomposing animals.

Diversity begets diversity in competition for space

Competition can profoundly affect biodiversity patterns by determining whether similar species are likely to coexist. When species compete directly for space, competitive ability differences should theoretically promote trait and phylogenetic clustering, provided that niche differences are otherwise minimal. Yet many sessile communities exhibit high biodiversity despite minimal reliance on niche differentiation. A potential explanation is that intransitive competition (‘rock–paper–scissors’ competition) not only promotes species richness but also fosters coexistence among highly dissimilar species with different competitive strategies. Here, we test this hypothesis using a combination of empirical and analytical approaches. In an experimental system comprising 37 wood-decay basidiomycete fungi grown in nutrient-rich agar media, pairwise displacement was maximized when species had widely different competitive traits and divergent evolutionary histories. However, when these interactions were embedded in models of species-rich communities, high levels of intransitivity ultimately overwhelmed the pairwise relationships, allowing the weakest and most dissimilar species to survive. In line with theoretical expectations, these multispecies assemblages exhibited reduced functional and phylogenetic diversity, yet the smallest losses were likewise observed in species-rich communities. By demonstrating that species richness can act as a self-reinforcing buffer against competitive exclusion, these results contribute to our understanding of how biodiversity is maintained in natural systems.

A test of the hierarchical model of litter decomposition

Our basic understanding of plant litter decomposition informs the assumptions underlying widely applied soil biogeochemical models, including those embedded in Earth system models. Confidence in projected carbon cycle–climate feedbacks therefore depends on accurate knowledge about the controls regulating the rate at which plant biomass is decomposed into products such as CO2. Here we test underlying assumptions of the dominant conceptual model of litter decomposition. The model posits that a primary control on the rate of decomposition at regional to global scales is climate (temperature and moisture), with the controlling effects of decomposers negligible at such broad spatial scales. Using a regional-scale litter decomposition experiment at six sites spanning from northern Sweden to southern France—and capturing both within and among site variation in putative controls—we find that contrary to predictions from the hierarchical model, decomposer (microbial) biomass strongly regulates decomposition at regional scales. Furthermore, the size of the microbial biomass dictates the absolute change in decomposition rates with changing climate variables. Our findings suggest the need for revision of the hierarchical model, with decomposers acting as both local- and broad-scale controls on litter decomposition rates, necessitating their explicit consideration in global biogeochemical models.Accurate understanding of plant litter decomposition is vital to inform Earth system modelling. Here the dominant hierarchical model for plant litter decomposition is found to be wanting, and revisions are suggested.

Fungal interactions reduce carbon use efficiency

The efficiency by which fungi decompose organic matter contributes to the amount of carbon that is retained in biomass vs. lost to the atmosphere as respiration. This carbon use efficiency (CUE) is affected by various abiotic conditions, including temperature and nutrient availability. Theoretically, the physiological costs of interspecific interactions should likewise alter CUE, yet the magnitude of these costs is untested. Here we conduct a microcosm experiment to quantify how interactions among wood-decay basidiomycete fungi alter growth, respiration and CUE across a temperature and nitrogen gradient. We show that species interactions induced consistent declines in CUE, regardless of abiotic conditions. Multispecies communities exhibited reductions in CUE of up to 25% relative to individual CUE, with this biotic effect being greater than the observed variation attributable to abiotic conditions. Our results suggest that the extent to which fungal-mediated carbon fluxes respond to environmental change may be influenced strongly by species interactions.

Competitive network determines the direction of the diversity–function relationship

Significance Diverse communities typically have higher functional potential (e.g., biomass production) because species use different resources and respond to different environmental cues. Yet, in highly competitive communities, individuals often grow less efficiently together due to intense competition for shared resources. Here, we show that the structure of the competitive network (i.e., who beats who) ultimately determines whether an increase in diversity leads to an increase or a decrease in functioning. The diversity–function relationship depended both on whether communities exhibited “rock–paper–scissors” (i.e., intransitive) dynamics, and whether they comprised strong versus weak competitors. These results highlight that knowledge of the competitive network may be integral for predicting when losses in biodiversity will have positive or negative effects on ecosystem function. The structure of the competitive network is an important driver of biodiversity and coexistence in natural communities. In addition to determining which species survive, the nature and intensity of competitive interactions within the network also affect the growth, productivity, and abundances of those individuals that persist. As such, the competitive network structure may likewise play an important role in determining community-level functioning by capturing the net costs of competition. Here, using an experimental system comprising 18 wood decay basidiomycete fungi, we test this possibility by quantifying the links among competitive network structure, species diversity, and community function. We show that species diversity alone has negligible impacts on community functioning, but that diversity interacts with two key properties of the competitive network—competitive intransitivity and average competitive ability—to ultimately shape biomass production, respiration, and carbon use efficiency. Most notably, highly intransitive communities comprising weak competitors exhibited a positive diversity–function relationship, whereas weakly intransitive communities comprising strong competitors exhibited a negative relationship. These findings demonstrate that competitive network structure can be an important determinant of community-level functioning, capturing a gradient from weakly to strongly competitive communities. Our research suggests that the competitive network may therefore act as a unifying link between diversity and function, providing key insight as to how and when losses in biodiversity will impact ecosystem function.