Jeffrey R. Smith

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.

The effective use of undergraduates to staff large introductory CS courses

In the past few years many schools have tried to simultaneously achieve the following goals in their introductory CS courses:Allow more students to enroll Improve the quality of education Keep spending at current levels Everyone has discovered that the first two goals are difficult to achieve in the presence of the third. This paper presents a model that has evolved over the last five years at Stanford University where all three goals have been accomplished by replacing graduate student TAs with undergraduate section leaders.

Potential impact of Halyomorpha halys (Hemiptera: Pentatomidae) on grape production in the finger lakes region of New York.

Abstract New York State is the leading grape producer in the eastern US and third leading grape producer in the US. The generalist feeding invasive species Halyomorpha halys (Stål) is currently present in NY, although at relatively low densities. Our aim in this study was to determine how H. halys will affect grape production in NY if it becomes well established. We measured the impact of density, sex, and life stage of H. halys on Concord and Chardonnay grapevines by enclosing insects on a single grape cluster using a fine mesh bag. The insects remained caged on the clusters for 2 wk during and after the period of fruit set, after which damaged and undamaged berries were enumerated nondestructively. At this point, we found a strong positive correlation between density and both number and percentage of berries damaged for both nymphs and adults. In late summer, at harvest time, clusters were removed from the vines, damaged and undamaged berries were once again counted, and berries were weighed. Cluster weight for both Chardonnay and Concord cultivars decreased with increased density of adults. Furthermore, adult females were found to have a greater effect than adult males. In contrast to adult feeding, nymphs were found to have little impact on cluster weight. There was relatively little incidence of disease as a result of H. halys feeding. These results show that H. halys is potentially an economic threat to the grape industry in cool-climate regions, but only at high densities currently not observed.

Phototaxis, Host Cues, and Host-Plant Finding in a Monophagous Weevil, Rhinoncomimus latipes

Rhinoncomimus latipes is a monophagous weevil used as a biological control agent for Persicaria perfoliata in the eastern United States. Density of adult R. latipes and resulting feeding damage has been shown to be higher in the sun than in the shade. This study aimed to determine whether phototaxis, sensitivity to enhanced host cues from healthier sun-grown plants, or a combination is driving this behavior by the weevil. A series of greenhouse choice tests between various combinations of plant and light conditions showed that R. latipes is positively phototactic, responsive to host cues, and preferentially attracted to sun-grown plants over shade-grown plants. From our experiments, we hypothesize two phases of dispersal and host finding in R. latipes. The initial stage is controlled primarily by phototaxis, whereas the later stage is controlled jointly by host cues and light conditions.

Predator community composition is linked to soil carbon retention across a human land use gradient

Soil carbon (C) storage is a major component of the carbon cycle. Consensus holds that soil C uptake and storage is regulated by plant-microbe-soil interactions. However, the contribution of animals in aboveground food webs to this process has been overlooked. Using insights from prior long-term experimentation in an old-field ecosystem and mathematical modeling, we predicted that the amount of soil C retention within a field should increase with the proportion of active hunting predators comprising the aboveground community of active hunting and sit-and-wait predators. This comes about because predators with different hunting modes have different cascading effects on plants. Our test of the prediction revealed that the composition of the arthropod predator community and associated cascading effects on the plant community explained 41% of variation in soil C retention among 15 old fields across a human land use gradient. We also evaluated the potential for several other candidate factors to explain variation in soil C retention among fields, independent of among-field variation in the predator community. These included live plant biomass, insect herbivore community composition, soil arthropod decomposer community composition, degree of land use development around the fields, field age, and soil texture. None of these candidate variables significantly explained soil C retention among the fields. The study offers a generalizable understanding of the pathways through which arthropod predator community composition can contribute to old-field ecosystem carbon storage. This insight helps support ongoing efforts to understand and manage the effects of anthropogenic land use change on soil C storage.

Variable Seed Viability of Mile-a-Minute Weed (Devil's Tearthumb, Persicaria perfoliata)

Abstract Mile-a-minute weed or devils tearthumb (Polygonum perfoliatum, syn.  =  Persicaria perfoliata) is an invasive annual vine in the Mid-Atlantic and Northeastern United States that reproduces solely through seeds. Our study aimed to identify how mile-a-minute seed viability is affected by time of year and the maturity of the fruit surrounding the achene. Full-sized immature (green) and mature (blue) fruits were collected from five field sites every 2 wk over a 3 mo period, and seed viability was assessed using a triphenyl tetrazolium chloride (TZ) assay. At the onset of seed production in mid-August, 35% of seeds from immature fruits were viable. This percentage increased steadily, peaking at 84% in late September before declining at some sites around the time of the first frost. In contrast nearly all seeds with mature fruits (96%) were viable at all collection dates. Thus land managers who apply physical or chemical control methods for mile-a-minute weed should do so before the onset of any seed production and not simply before fruit maturation. If it is necessary to apply control methods after fruit set, it should be done as early in the season as possible. Nomenclature: Mile-a-minute weed; devils tearthumb; Persicaria perfoliata (L.) H. Gross; Polygonum perfoliatum L., POPE10. Management Implications: This study highlights two important implications for managing the invasive vine mile-a-minute weed (devils tearthumb) using either chemical or physical control methods. First, mile-a-minute seeds surrounded by immature fruits were shown to be viable to some extent throughout the entire season of seed production. Therefore, physical and chemical controls should be applied before any full-sized immature (green) fruits are produced and not just before mature (blue) fruits are present. Second, the viability of seeds in green fruits increased throughout the season, peaking before the first frost. This indicates that if it is necessary to apply physical or chemical control methods during the fruiting period, these methods should be applied as early in the season as possible, when immature fruits are less likely to contain viable seeds.

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.

A global test of ecoregions

A foundational paradigm in biological and Earth sciences is that our planet is divided into distinct ecoregions and biomes demarking unique assemblages of species. This notion has profoundly influenced scientific research and environmental policy. Given recent advances in technology and data availability, however, we are now poised to ask whether ecoregions meaningfully delimit biological communities. Using over 200 million observations of plants, animals and fungi we show compelling evidence that ecoregions delineate terrestrial biodiversity patterns. We achieve this by testing two competing hypotheses: the sharp-transition hypothesis, positing that ecoregion borders divide differentiated biotic communities; and the gradual-transition hypothesis, proposing instead that species turnover is continuous and largely independent of ecoregion borders. We find strong support for the sharp-transition hypothesis across all taxa, although adherence to ecoregion boundaries varies across taxa. Although plant and vertebrate species are tightly linked to sharp ecoregion boundaries, arthropods and fungi show weaker affiliations to this set of ecoregion borders. Our results highlight the essential value of ecological data for setting conservation priorities and reinforce the importance of protecting habitats across as many ecoregions as possible. Specifically, we conclude that ecoregion-based conservation planning can guide investments that simultaneously protect species-, community- and ecosystem-level biodiversity, key for securing Earth’s life support systems into the future.Data from more than 200 million observations of plants, animals and fungi provide support for the concept that terrestrial biodiversity patterns reflect distinct ecoregions.

Corrigendum: Mapping tree density at a global scale

This corrects the article DOI: 10.1038/nature14967