Data From: Root Distributions Predict Shrub-Steppe Responses to Precipitation Intensity

Precipitation events are becoming more intense around the world, changing the way water moves through soils and plants. Plant rooting strategies that sustain water uptake under these conditions are likely to become more abundant (e.g., shrub encroachment). Yet, it remains difficult to predict species responses to climate change because we typically do not know where active roots are located or how much water they absorb. Here, we applied a water tracer experiment to describe forb, grass, and shrub root distributions. These measurements were made in 8 m by 8 m field shelters with low or high precipitation intensity. We used tracer uptake data in a soil water flow model to estimate how much water respective plant root tissues absorb over time. In low precipitation intensity plots, deep shrub roots were estimated to absorb the most water (93 mm yr-1) and shrubs had the greatest aboveground cover (27%). Grass root distributions were estimated to absorb an intermediate amount of water (80 mm yr-1) and grasses had intermediate aboveground cover (18%). Forb root distributions were estimated to absorb the least water (79 mm yr-1) and had the least aboveground cover (12%). In high precipitation intensity plots, shrub and forb root distributions changed in ways that increased their water uptake relative to grasses, predicting the increased aboveground growth of shrubs and forbs in these plots. In short, water uptake caused by different rooting distributions predicted plant aboveground cover. Our results suggest that detailed descriptions of active plant root distributions can predict plant growth responses to climate change in arid and semi-arid ecosystems

Introduction, Establishment, and Spread: 50 Years of Invasion Ecology Since Elton

While some may argue that Charles Elton was not a founder but rather a prophet of invasion ecology (Simberloff, this volume), Elton’s 1958 monograph The ecology of invasions by animals and plants (Methuen, London) inspired and informed many of today’s ecologists about the issues and problems of biological invasions. In addition, while many theories Elton proposed in his monograph (e.g., diversityinvasibility) have been questioned and his oversights (e.g., on propagule pressure) have been noted, there is no doubt that he has influenced entire directions of research. Thus, revisiting Elton’s influence on the field of invasion ecology 50 years after his monograph is clearly worthwhile

Root Niche Partitioning among Grasses, Saplings, and Trees Measured Using a Tracer Technique

Niche partitioning of resources by plants is believed to be a fundamental aspect of plant coexistence and biogeochemical cycles; however, measurements of the timing and location of resource use are often lacking because of the difficulties of belowground research. To measure niche partitioning of soil water by grasses, planted saplings, and trees in a mesic savanna (Kruger National Park, South Africa), we injected deuterium oxide into 102,000 points in 15, 154-m2 plots randomly assigned to one of five depths (0–120 cm) and one of three time periods during the 2008/2009 growing season. Grasses, saplings and trees all demonstrated an exponential decline in water uptake early in the season when resources were abundant. Later in the season, when resources were scarce, grasses continued to extract the most water from the shallowest soil depths (5 cm), but saplings and trees shifted water uptake to deeper depths (30–60 cm). Saplings, in particular, rapidly established roots to at least 1 m and used these deep roots to a greater extent than grasses or trees. Helping to resolve contradictory observations of the relative importance of deep and shallow roots, our results showed that grasses, saplings and trees all extract the most water from shallow soils when it is available but that woody plants can rapidly shift water uptake to deeper soils when resources are scarce. Results highlight the importance of temporal changes in water uptake and the problems with inferring spatial and temporal partitioning of soil water uptake from root biomass measurements alone

Chronosequence and Direct Observation Approaches Reveal Complementary Community Dynamics in a Novel Ecosystem

Non-native, early-successional plants have been observed to maintain dominance for decades, particularly in semi-arid systems. Here, two approaches were used to detect potentially slow successional patterns in an invaded semi-arid system: chronosequence and direct observation. Plant communities in 25 shrub-steppe sites that represented a 50-year chronosequence of agricultural abandonment were monitored for 13 years. Each site contained a field abandoned from agriculture (ex-arable) and an adjacent never-tilled field. Ex-arable fields were dominated by short-lived, non-native plants. These ‘weedy’ communities had lower species richness, diversity and ground cover, and greater annual and forb cover than communities in never-tilled fields. Never-tilled fields were dominated by long-lived native plants. Across the chronosequence, plant community composition remained unchanged in both ex-arable and never-tilled fields. In contrast, 13 years of direct observation detected directional changes in plant community composition within each field type. Despite within-community changes in both field types during direct observation, there was little evidence that native plants were invading ex-arable fields or that non-native plants were invading never-tilled fields. The more-controlled, direct observation approach was more sensitive to changes in community composition, but the chronosequence approach suggested that these changes are unlikely to manifest over longer time periods, at least in part because of disturbances in the system. Results highlight the long-term consequences of soil disturbance and the difficulty of restoring native perennials in disturbed semi-arid systems

Infection of an invasive frog Eleutherodactylus coqui by the chytrid fungus Batrachochytrium dendrobatidis in Hawaii

The chytrid fungus Batrachochytrium dendrobatidis has contributed to declines and extinctions of amphibians worldwide. B. dendrobatidis is known to infect the frog Eleutherodactylus coqui in its native Puerto Rico. E. coqui was accidentally introduced into Hawaii in the late 1980s, where there are now hundreds of populations. B. dendrobatidis was being considered as a biological control agent for E. coqui because there are no native amphibians in Hawaii. Using a DNA-based assay, we tested 382 E. coqui from Hawaii for B. dendrobatidis and found that 2.4% are already infected. We found infected frogs in four of 10 study sites and on both the islands of Hawaii and Maui. This is the first report of B. dendrobatidis in wild populations in Hawaii. As the range of E. coqui expands, it may become a vector for the transmittance of B. dendrobatidis to geographic areas where B. dendrobatidis does not yet exist

Plant-Soil Feedbacks Provide an Alternative Explanation for Diversity-Productivity Relationships

Plant–soil feedbacks (PSFs) have gained attention for their role in plant community dynamics, but their role in productivity has been overlooked. We developed and tested a biomass-specific, multi-species model to examine the role of PSFs in diversity–productivity relationships. The model predicts a negative relationship between PSFs and overyielding: plants with negative PSFs grow more in communities than in monoculture (i.e. overyield), and plants with positive PSFs grow less in communities than in monoculture (i.e. underyield). This effect is predicted to increase with diversity and saturate at low species richness because the proportion of ‘self-cultivated’ soils rapidly decreases as species are added to a community. Results in a set of glasshouse experiments supported model predictions. We found that PSFs measured in one experiment were negatively correlated with overyielding in three-species plant communities measured in a separate experiment. Furthermore, when parametrized with our experimental PSF data, our model successfully predicted species-level overyielding and underyielding. The model was less effective at predicting community-level overyielding and underyielding, although this appeared to reflect large differences between communities with or without nitrogen-fixing plants. Results provide conceptual and experimental support for the role of PSFs in diversity–productivity relationships

Influence of Pocket Gopher Mounds on Nonnative Plant Establishment in a Shrubsteppe Ecosystem

Soil disturbances across a wide range of spatial scales have been found to promote the establishment of invasive plant species. This study addresses whether mounds built by northern pocket gophers (Thomomys talpoides) in the shrubsteppe environment of north central Washington are facilitating plant invasions into native-dominated fields. Research was conducted in native-dominated plant communities adjacent to ex-arable, nonnative-dominated fields. To determine the effect of mounds on plant growth, we recorded new establishment and persistence of all plant species over 2 growing seasons on 10–19 mound and intermound areas in 10 fields. Nonnative plant establishment was not affected by mounds, but native plant establishment, particularly of the dominant native Pseudoroegneria spicata was lower on mounds than on intermounds. Early in the growing season, mounds had reduced soil moisture, bulk density, soil strength, N mineralization rates, and total N and C concentrations, and similar extractable NO3 – concentrations relative to intermound soils. Our results did not suggest that soil disturbance improved nonnative growth resulting in competitive suppression of natives; rather, our results suggested that low soil moisture and slow N mineralization rates on mounds in this ecosystem present relatively stressful conditions for native plant growth

Winter Wheat Resistant to Increases in Rain and Snow Intensity in a Semi-Arid System

As the atmosphere warms, precipitation events have been predicted and observed to become fewer and larger. Changes in precipitation patterns can have large effects on dryland agricultural production, but experimental tests on the effects of changing precipitation intensity are limited. Over 3 years, we tested the effects of increased precipitation intensity on winter wheat (Triticum aestivum L.; Promontory variety) in a temperate dryland agricultural system that was on a rotation of crop and fallow years. We used 11 (2.1 × 2.5 m) shelters to collect and redeposit rain and snow as larger, more intense events. Total precipitation was the same in all plots, but event sizes in each plot varied from 1 to 18 mm. Treatments increased soil water availability, but winter wheat biomass and grain yield did not differ among treatments. Similarly, other measured plant growth responses, including vegetation greenness, leaf area index, canopy temperature, photochemical efficiency, root area, and new root growth, did not differ among treatments. Results indicate that at least in the semiarid climate and silt loam soils studied here, anticipated increases in precipitation intensity are unlikely to affect winter wheat production negatively. Further, increased precipitation intensity may mitigate water stress caused by increasing temperatures and encourage the use of wheat varieties that utilize deeper, later season soil water

Soil type more than precipitation determines fine-root abundance in savannas of Kruger National Park, South Africa

Aims Our aim was to examine how soil type and precipitation affect fine-root abundance in savanna ecosystems across Kruger National Park (KNP), South Africa. Methods Fine-root distributions were measured in four sites that represent the natural factorial combination of soil types (basalt-derived clay or granite-derived sand) and precipitation regimes [wet (~750 mm mean annual precipitation) or dry (~500 mm mean annual precipitation)] that occur in KNP. Root area and biomass (at soil depths of 0–75 cm) were estimated from measurements of root number, length and width in images from minirhizotron tubes at each site. Measurements were made during one mid-season sampling during three subsequent years. Results Fine-root area was more than twice as large in clay (2.3 ± 0.0 mm2 cm−2) than sand (0.8 ± 0.3 mm2 cm−2) sites but did not differ between wet and dry sites. Root number, length and width, used to derive area, showed similar patterns to fine-root area. Fine-root biomass estimated from these values was 5.5 ± 0.6 Mg ha−1 in clay sites and 2.2 ± 0.9 Mg ha−1 in sand sites. Conclusions Across the four sites, a change from sand to clay soils had a greater effect on fine-root abundance and distributions than a 50% increase in precipitation from dry to wet sites. Results highlight the importance of soil properties on root dynamics and carbon pools in the region

Frogs (Coqui Frogs, Greenhouse Frogs, Cuban Tree Frogs, and Cane Toads)

Amphibians are perhaps most well known for their highly threatened status, which often masks appreciation for the great numbers of species that are widespread global invaders (Kraus 2009). Both purposeful and accidental introductions of amphibians have occurred worldwide. Motivations for purposeful amphibian introductions include their use as biocontrol agents and culinary ambitions (Storer 1925; Kraus 2009). However, there are an increasing number of amphibians that are being accidentally introduced and becoming widespread (Kraus 2009). These introductions are in some ways more disconcerting because they may be the most difficult to prevent in the future. There are 19 nonnative amphibians that have become successfully established in 28 of the 50 U.S. states (Figure 9.1; Kraus 2009). The most successful non-native amphibian is the bullfrog (Lithobates catesbeianus), which has become established in 19 states outside of its native range on the eastern side of the United States, followed by the Cuban greenhouse frog (Eleutherodactylus planirostris), which has established itself in six states, and five frog species, including the Puerto Rican coqui (E. coqui), which are now established in three states outside of their native range (Figure 9.1; Kraus 2009). The state with the most nonnative frogs is California with eight species, followed by Hawaii with six, and Florida and Arizona with four (Table 9.1; Kraus 2009). Many nonnative amphibians in the United States, particularly in the western United States, are from other parts of the United States, namely, east of the Mississippi River. However, there are also many nonnative amphibians with tropical or subtropical origins that are primarily successful in tropical and subtropical states, such as Florida and Hawaii, and territories, such as Guam