Variation in the intensities of above— and belowground competition is proposed to underlie changes in community structure associated with variable fertility and disturbance. Here we report measurements of light, soil nitrogen, and above— and belowground competition in vegetation that had received four combinations of fertilization and disturbance for 3 yr. Two levels of nitrogen addition (none and 17 g°m—2°yr—1) and two of soil disturbance (non and annual tilling to remove all vegetation) were applied in a factorial design to 5 ° 5 m plots (10 replicated) in a 31—yr—old field in southeastern Minnesota. During the 3rd yr of the experiment, community standing crop, light penetration, and soil ammonium and nitrate concentrations were measured every 3—4 wk. Fertilization significantly increased productivity and tilling significantly decreased community biomass at the start of the growing season, indicating that the experimental treatments created variation in stress and disturbance. Transplants of Schizachyrium scoparium, a native perennial grass, were grown with no neighbors, or with the roots of neighbors, or with all neighbors in each combination of fertility and disturbance in order to examine competition intensity. In both undisturbed and tilled plots, belowground competition was the dominant interaction in unfertilized plots, whereas both below— and aboveground competition controlled growth in fertilized plots. The intensity of aboveground competition was greatest in plots with the lowest light penetration and declined significantly with increasing light supply. Similarly, belowground competition was most intense in plots with the lowest nitrogen availability and decreased significantly with increasing availability. The intensities of above— and belowground competition were significantly negatively correlated. The total (above— and belowground) intensity of competition did not vary with fertility, but was significantly reduced by disturbance. Similarly, the effect of neighbor mass on transplant growth was significantly reduced by disturbance.

We tested for variation in the intensity of above— and belowground competition along a 5—yr—old experimental gradient of nitrogen availability in a Minnesota old field. Standing crop increased, species richness decreased, and species composition varied significantly as nitrogen availability increased. Transplants of the three grasses that were dominants at the three levels of the nitrogen gradient (low; Schizachyrium scoparium, intermediate: Poa pratensis, high: Agropyron repens) were grown along the gradient with no neighbors present, with only the roots of neighbors present, or with both the roots and shoots of neighbors present. Resource measurements indicated that the treatment with only neighbor roots present provided at light regime similar to that in which all neighbors had been removed but a nitrogen regime similar to that in which all neighbors were present. At low nitrogen availability, transplants grown with only neighbor roots generally did not differ significantly in biomass or growth rate from those grown with both neighbor roots and shoots, suggesting that the bulk of neighbor effects at low nitrogen were belowground. At high nitrogen availability, plants grown with only neighbor roots generally grew significantly larger than those grown with both roots and shoots of neighbors, but not as large as plants grown with no neighbors, suggesting that both above— and belowground competition occurred. At low nitrogen availability, plants grown with neighbors weighed 3—12% as much as those grown without; at the highest rate of nitrogen addition, plants grown with neighbors weighed 12—58% as much as those grown without, indicating that competition occurred on both poor and rich soils. The intensity of competition, measured as the suppression of transplants by neighbors, did not vary significantly with nitrogen availability. Further, the per—gram effect of neighbors, measured as the slope of the relationship between transplant performance and neighbor biomass, did not vary significantly with nitrogen supply rate. In total, competition was an important force at all points along the experimental productivity gradient, but competition shifted from being mainly belowground in the least productive vegetation to both above— and belowground in fertilized plots.

SummaryAlthough experiments show a positive association between vascular plant and arbuscular mycorrhizal fungal (AMF) species richness, evidence from natural ecosystems is scarce. Furthermore, there is little knowledge about how AMF richness varies with belowground plant richness and biomass.We examined relationships among AMF richness, above-and belowground plant richness, and plant root and shoot biomass in a native North American grassland. Root-colonizing AMF richness and belowground plant richness were detected from the same bulk root samples by 454-sequencing of the AMF SSU rRNA and plant trnL genes.In total we detected 63 AMF taxa. Plant richness was 1.5 times greater belowground than aboveground. AMF richness was significantly positively correlated with plant species richness, and more strongly with below-than aboveground plant richness. Belowground plant richness was positively correlated with belowground plant biomass and total plant biomass, whereas aboveground plant richness was positively correlated only with belowground plant biomass. By contrast, AMF richness was negatively correlated with belowground and total plant biomass.Our results indicate that AMF richness and plant belowground richness are more strongly related with each other and with plant community biomass than with the plant aboveground richness measures that have been almost exclusively considered to date.

Semiarid ecosystems such as grasslands are characterized by high temporal variability in abiotic factors, which has led to suggestions that management actions may be more effective in some years than others. Here we examine this hypothesis in the context of grassland restoration, which faces two major obstacles: the contingency of native grass establishment on unpredictable precipitation, and competition from introduced species. We established replicated restoration experiments over three years at two sites in the northern Great Plains in order to examine the extent to which the success of several restoration strategies varied between sites and among years. We worked in 50‐yr‐old stands of crested wheatgrass (Agropyron cristatum), an introduced perennial grass that has been planted on >10 × 106 ha in western North America. Establishment of native grasses was highly contingent on local conditions, varying fourfold among years and threefold between sites. Survivorship also varied greatly and increased significantly with summer precipitation. No consistent differences were found between drilling and broadcasting in their effects on establishment, but survivorship was nearly threefold higher in broadcast plots. Plots without seed added, or with native hay added, had almost no seedlings of native grasses. In contrast, broadcasting the residue remaining after cleaning seeds from native hay produced the highest seedling densities of any treatment. Competition from A. cristatum was significantly and consistently reduced through annual application of a generalist herbicide (glyphosate), which increased native grass establishment and survivorship and the richness and total cover of native species. Herbicide decreased standing crop and increased soil moisture and available nitrogen. A. cristatum was controlled without suppressing native vegetation, both by spraying in early spring, which selectively killed the cool‐season A. cristatum, and by application with a wick, which selectively killed the taller A. cristatum. A. cristatum persisted over four years, however, in spite of annual herbicide application. A. cristatum cover in control plots increased significantly with summer precipitation. In summary, broadcasting and drilling differed little in their effects on establishment, but broadcasting increased survivorship and will allow the emergence of plant‐induced heterogeneity. Competition from introduced species can be reduced but not eliminated by continuing herbicide application. Lastly, the positive relationships between precipitation and both A. cristatum and native seedling survivorship suggest that management should focus on controlling A. cristatum during dry years and on introducing native species during wet years. Corresponding Editor: I. C. Burke.

We tested the prediction that plant species that grow in undisturbed, nutrient-rich habitats tend to have higher competitive abilities than those found in disturbed or nutrient-poor habitats. The distributions of seven species (Eriocaulon septangulare, Rhynchosporafusca, Hypericum ellipticum, Juncus pelocarpus, Lysimachia terrestris, Dulichium arundinaceum, and Drosera intermedia) were measured along a gradient of exposure to wave action on the shore of Axe Lake, Ontario. The exposure gradient incorporates disturbance, through the removal of plant biomass, and stress, through the creation of a gradient in sediment organic content, nutrient concentrations, and fine particle sizes. Species distributions on the exposure gradient were quantified by determining the mean sediment organic content of the quadrats containing each species. Competitive abilities were measured as relative increase in dry mass per plant, in a field experiment in which species were grown together in all pairwise combinations (N = 10 replicates). Species had significantly heterogeneous competitive abilities (P < .01). Species found on exposed, nutrient-poor shores (e.g., E. septangulare) had low competitive abilities, while those growing on sheltered, nutrient-rich shores (e.g., D. arundinaceum) had high competitive abilities. Competitive ability was significantly correlated with mean position on the exposure gradient.

No abstract

Drylands cover 40% of the global terrestrial surface and provide important ecosystem services. While drylands as a whole are expected to increase in extent and aridity in coming decades, temperature and precipitation forecasts vary by latitude and geographic region suggesting different trajectories for tropical, subtropical, and temperate drylands. Uncertainty in the future of tropical and subtropical drylands is well constrained, whereas soil moisture and ecological droughts, which drive vegetation productivity and composition, remain poorly understood in temperate drylands. Here we show that, over the twenty first century, temperate drylands may contract by a third, primarily converting to subtropical drylands, and that deep soil layers could be increasingly dry during the growing season. These changes imply major shifts in vegetation and ecosystem service delivery. Our results illustrate the importance of appropriate drought measures and, as a global study that focuses on temperate drylands, highlight a distinct fate for these highly populated areas.

Variation in plant species richness has been described using only aboveground vegetation. The species richness of roots and rhizomes has never been compared with aboveground richness in natural plant communities. We made direct comparisons of grassland plant richness in identical volumes (0.1 × 0.1 × 0.1 m) above and below the soil surface, using conventional species identification to measure aboveground richness and 454 sequencing of the chloroplast trnL(UAA) intron to measure belowground richness. We described above- and belowground richness at multiple spatial scales (from a neighbourhood scale of centimetres to a community scale of hundreds of metres), and related variation in richness to soil fertility. Tests using reference material indicated that 454 sequencing captured patterns of species composition and abundance with acceptable accuracy. At neighbourhood scales, belowground richness was up to two times greater than aboveground richness. The relationship between above- and belowground richness was significantly different from linear: beyond a certain level of belowground richness, aboveground richness did not increase further. Belowground richness also exceeded that of aboveground at the community scale, indicating that some species are temporarily dormant and absent aboveground. Similar to other grassland studies, aboveground richness declined with increasing soil fertility; in contrast, the number of species found only belowground increased significantly with fertility. These results indicate that conventional aboveground studies of plant richness may overlook many coexisting species, and that belowground richness becomes relatively more important in conditions where aboveground richness decreases. Measuring plant belowground richness can considerably alter perceptions of biodiversity and its responses to natural and anthropogenic factors.