Kristofer R. Covey

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.

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.

Dissecting the Effects of Diameter on Wood Decay Emphasizes the Importance of Cross-Stem Conductivity in Fraxinus americana

Pest outbreaks are driving tree dieback and major influxes of deadwood into forest ecosystems. Understanding how pulses of deadwood impact the climate system requires understanding which factors influence greenhouse gas production during wood decay. Recent analyses identify stem diameter as an important control, but report effects that vary in magnitude and direction. This complexity may reflect interacting effects of soil contact, geometry and variable tissue properties. To dissect these effects, we implemented a three-way factorial experiment in Fraxinus americana, (white ash), an iconic North American species threatened by an invasive beetle. Soil contact accelerated decay rates by an order of magnitude with an effect that varied with stem diameter, not bark presence. After experimentally controlling surface area-to-volume ratio, half-buried wide stems decayed more slowly than half-buried narrow stems but more quickly than the aggregate decay rate of buried and suspended stems. These results closely matched variation in moisture content within and among samples, suggesting that limited vertical conduction of soil moisture through deadwood mediates the effect of stem diameter on wood decay. Soil contact also influenced greenhouse gas concentrations reinforcing recent evidence that deadwood acts as a source for CO2 and CH4 while acting as a sink for N2O. Our results suggest that managing tree species affected by pest outbreaks, including F. americana, for biomass salvage and greenhouse gas mitigation requires understanding traits that mediate wood permeability and diffusivity to soil moisture and greenhouse gases.

Spatially-explicit models of global tree density

Remote sensing and geographic analysis of woody vegetation provide means of evaluating the distribution of natural resources, patterns of biodiversity and ecosystem structure, and socio-economic drivers of resource utilization. While these methods bring geographic datasets with global coverage into our day-to-day analytic spheres, many of the studies that rely on these strategies do not capitalize on the extensive collection of existing field data. We present the methods and maps associated with the first spatially-explicit models of global tree density, which relied on over 420,000 forest inventory field plots from around the world. This research is the result of a collaborative effort engaging over 20 scientists and institutions, and capitalizes on an array of analytical strategies. Our spatial data products offer precise estimates of the number of trees at global and biome scales, but should not be used for local-level estimation. At larger scales, these datasets can contribute valuable insight into resource management, ecological modelling efforts, and the quantification of ecosystem services.

The Physiological Ecology of Carbon Science in Forest Stands

In order to better understand the ways in which future forests will change and be changed by shifting climates, it is necessary to understand the underlying drivers of forest development and the ways these drivers are affected by changes in atmospheric carbon dioxide concentrations, temperature, precipitation, and nutrient levels. Successional forces lead to somewhat predictable changes in forest stands throughout the world. These changes can lead to corresponding shifts in the dynamics of carbon uptake, storage, and release. Many studies have attempted to elucidate the effects of changing climate conditions on forest ecosystem dynamics; however, the complexity of forest systems, long time horizons, and high costs associated with large-scale research have limited the ability of scientists to make reliable predictions about future changes in forest carbon flux at the global scale. Free Air Carbon Exchange (FACE) experiments are suggesting that forest net primary productivity, and thus carbon uptake, usually increases when atmospheric carbon dioxide levels increase, likely due to factors such as increased nitrogen use efficiency and competitive advantages of shade tolerant species. Experiments dealing with drought and temperature change are providing evidence that water availability, may be the most important factor driving forest carbon dynamics. Forest ecosystem experiments, such as FACE programs, have not been operating long enough to predict long term responses of forest ecosystems to increases in carbon dioxide.

Reply to Veresoglou: Overdependence on “significance” testing in biology

In PNAS, we explore the effects of interacting global change factors on the functioning of decomposer communities and show how biotic interactions influence the strength of soil carbon feedbacks to climate change (1). Veresoglou (2) highlights that the highly interactive nature of our multifactor experiment can increase the likelihood of type I errors (i.e., “false positives”), an effect that he refers to as “P hacking.” We appreciate this perspective because it provides a platform to discuss what we believe is a critical topic in biology: an overdependence on significant P values.

Species associations overwhelm abiotic conditions to dictate the structure and function of wood-decay fungal communities

Environmental conditions exert strong controls on the activity of saprotrophic microbes, yet abiotic factors often fail to adequately predict wood decomposition rates across broad spatial scales. Given that species interactions can have significant positive and negative effects on wood-decay fungal activity, one possibility is that biotic processes serve as the primary controls on community function, with abiotic controls emerging only after species associations are accounted for. Here we explore this hypothesis in a factorial field warming- and nitrogen-addition experiment by examining relationships among wood decomposition rates, fungal activity, and fungal community structure. We show that functional outcomes and community structure are largely unrelated to abiotic conditions, with microsite and plot-level abiotic variables explaining at most 19% of the total variability in decomposition and fungal activity, and 2% of the variability in richness and evenness. In contrast, taxonomic richness, evenness, and species associations (i.e., co-occurrence patterns) exhibited strong relationships with community function, accounting for 52% of the variation in decomposition rates and 73% in fungal activity. A greater proportion of positive vs. negative species associations in a community was linked to strong declines in decomposition rates and richness. Evenness emerged as a key mediator between richness and function, with highly even communities exhibiting a positive richness-function relationship and uneven communities exhibiting a negative or null response. These results suggest that community-assembly processes and species interactions are important controls on the function of wood-decay fungal communities, ultimately overwhelming substantial differences in abiotic conditions.

Corrigendum: Mapping tree density at a global scale

This corrects the article DOI: 10.1038/nature14967