Richard S. Inouye

Old‐Field Succession on a Minnesota Sand Plain

Vegetation and soils were sampled in 22 old fields ranging in age from 1 to 56 yr since abandonment. Soil nitrogen concentration increased significantly with field age. Vegetation cover, total aboveground plant biomass, and litter cover increased significantly with soil nitrogen. Light penetration to the soil surface was negatively correlated with total plant biomass. Field age and soil nitrogen concentration were used as independent variables in simple regression and partial correlation analyses to determine the relative importance of such time-dependent processes as dispersal vs. the availability of a limiting resource (nitrogen) as predictors of patterns in species richness or the abundance of various plant groups. Species richness per field and within-field heterogeneity in species composition increased with field age. Local species richness decreased with increasing soil nitrogen. Cover of annuals and introduced species decreased with field age and nitrogen; however, annuals contributed an important part of total vegetative cover even in 25-yr-old fields. Cover of perennials and woody species increased with soil nitrogen and field age. Although the fields were bordered by woods, woody species contributed <15?/o cover even in the oldest fields. For several plant groups the relationship between cover and soil nitrogen within individual fields was the opposite of that among all fields. These patterns suggest that while soil nitrogen is an important determinant of local species composition and abundance, dispersal and colonization, which are dependent on field age, determine which species are present in a field.

Effects of predation and competition on survivorship, fecundity, and community structure of desert annuals.

Effects of rodent and ant granivores, watering, and thinning were studied in a diverse community of winter annuals in the Sonoran Desert. Seed predation considerably reduced plant densities, but ants and rodents had qualitatively different effects on the plant community. Ants in- creased species diversity of annuals by differentially harvesting the numerically dominant species, while rodents preyed selectively on species which dominated the community in terms of biomass. Competition among annual plants reduced growth rate, biomass, and fecundity, but had no significant effect on mortality. One species, Euphorbia polycarpa, apparently increased in density when the density of competing Erodium species was reduced by rodent predation.

LANDSCAPE-SCALE CHANGES IN PLANT SPECIES ABUNDANCE AND BIODIVERSITY OF A SAGEBRUSH STEPPE OVER 45 YEARS

Increasing demands on arid and semiarid ecosystems, which comprise one-third of Earths terrestrial environment, create an urgent need to understand their biodiversity, function, and mechanisms of change. Sagebrush (Artemisia) steppe, the largest semiarid vegetation type in North America, is endangered because of losses to agriculture, excessive grazing, and invasive species. Establishment in 1950 of what is now designated as the Idaho National Engineering and Environmental Laboratory (southeastern Idaho, USA) created the largest existing reserve of this extensive vegetation type. We used cover, density, and frequency data for vascular plants sampled on 79 permanent plots nine times during 45 years to (1) assess long-term changes in abundance and distribution of major species and life forms, (2) assess changes in species richness and plot similarity, and (3) test the hypotheses that plant cover and stability of cover are positively associated with species richness and that invasibility is inversely relate...

Pocket gophers (Geomys bursarius), vegetation, and soil nitrogen along a successional sere in east central Minnesota

SummaryPocket gophers (Geomys bursarius: Geomyidae Rodentia) are shown to affect soil resources and thus, indirectly, vegetation. Gophers reduce average soil nitrogen near the surface and increase point-to-point heterogeneity of soil nitrogen by moving nitrogen-poor subsurface soil to the soil surface. Data from 22 old fields at Cedar Creek Natural History Area, Minnesota, USA show correlations of soil nitrogen, vegetation, and gopher mounds that are consistent with this indirect mechanism by which gophers affect local species composition and old field succession.

Convergence and Divergence of Old‐Field Vegetation After 11 yr of Nitrogen Addition

Eleven years of data from experimental nitrogen gradients in three old fields and in native savanna are used to test the hypothesis that similarity of resource availability should influence similarity of plant species composition. Vegetation became less similar on plots that received different amounts of nitrogen. The difference in vegetation between plots was a function of both the difference in added nitrogen and the absolute levels of added nitrogen. Changes in similarity on plots that received the same amount of nitrogen were influenced by initial species composition, the amount of added nitrogen, and the rate of colonization by species not initially present on plots. Replicates of many treatments initially became less similar, only to increase in similarity after 3-6 yr. Data are generally consistent with predictions made in 1988, and clearly illustrate the importance of long- term responses to experimental manipulations.

GRANIVORY IN A DESERT ECOSYSTEM: EXPERIMENTAL EVIDENCE FOR INDIRECT FACILITATION OF ANTS BY RODENTS'

Two major groups of desert granivores, ants and rodents, coexist as permanent residents of local desert habitats in southwestern North America. At our Sonoran Desert study site, both of the major taxa exhibited short-term increase in density when the other taxon was experimentally removed. Over the longer term, density compensation continued at a relatively constant level for rodents in the absence of ants. In contrast, beginning : 2 yr after initiation of experiments, ant populations on rodent removal plots showed a gradual but significant decline relative to densities on control plots. Indirect interactions, mediated through ant and rodent resources, may account for these differences. Removal of harvester ants leads to higher annual plant densities only in small-seeded species. These plants are relatively poor competitors and do not displace the large-seeded annuals, on whose seeds rodents specialize. In contrast, rodent removal leads to a differential increase in large-seeded annuals, which competitively displace the small-seeded resource species of ants. The decline of ant populations on rodent removal plots preceded by several years the first detectable evidence for competitive suppression of small-seeded annuals. Because ants do not excavate buried seed, they probably experienced resource depression before buried seed reserves were exhausted through germination and subsequent competitive inhibition.

The biogeochemistry of a north-temperate grassland with native ungulates: Nitrogen dynamics in Yellowstone National Park

Nutrient dynamics of large grassland ecosystems possessing abundant migratory grazers are poorly understood. We examined N cycling on the northern winter range of Yellowstone National Park, home for large herds of free-roaming elk (Cervus elaphus) and bison (Bison bison). Plant and soil N, net N mineralization, and the deposition of ungulate fecal-N were measured at five sites, a ridgetop, mid-slope bench, steep slope, valley-bottom bench, and riparian area, within a watershed from May, 1991 to April, 1992.Results indicated similarities between biogeochemical properties of Yellowstone grassland and other grassland ecosystems: (1) landscape position and soil water affected nutrient dynamics, (2) annual mineralization was positively related to soil N content, and (3) the proportion of soil N mineralized during the year was negatively related to soil C/N.Grazers were a particularly important component of the N budget of this grassland. Estimated rates of N flow from ungulates to the soil ranged from 8.1 to 45.6 kg/ha/yr at the sites (average = 27.0 kg/ha/yr), approximately 4.5 times the amount of N in senescent plants. Rates of nitrogen mineralization for Yellowstone northern range grassland were higher than those measured in other temperate grassland ecosystems, possibly due to grazers promoting N cycling in Yellowstone.

Density-dependent germination response by seeds of desert annuals

SummaryField observations and experiments indicate that the presence of seedlings at high densities inhibits subsequent germination of desert annuals. Since plants growing at high densities face severe competition for limited resources, this response by seeds is interpreted as an adaptation to avoid an unfavorable competitive climate where growth and survivorship are likely to be low.

Soil nitrogen availability, plant luxury consumption, and herbivory by white-tailed deer

Abstract. N is often cited as a limiting factor for sapling growth in northeastern USA forests. However, under conditions of elevated soil N, seedlings and saplings of some tree species exhibit luxury consumption of N, leading to elevated tissue N concentration. While this pool of plant N may have benefits for saplings if light levels change, it may also increase the risk of herbivory by white-tailed deer (Odocoileus virginianus L.). We conducted a field fertilization experiment to test the hypothesis that saplings increase stem tissue N when soil N availability is elevated. We fertilized saplings of nine tree species under closed and open canopies. Two of the nine species, yellow birch (Betula alleghaniensis) and black cherry (Prunus serotina), had significant increases in radial growth when fertilized under high light. Six of the nine species showed increased stem N concentration under either low light or high light conditions. Under low light conditions, black cherry and sugar maple (Acer saccharum) had significantly higher concentrations of stem N when fertilized. Regardless of fertilization treatment, yellow birch had significantly greater stem N in saplings under low light conditions when compared with saplings found in high light conditions. Under high light conditions, fertilization resulted in increased stem N in saplings of white ash (Fraxinus americana) and eastern hemlock (Tsuga canadensis). There was a trend for saplings of red maple (Acer rubrum) and white ash to show elevated stem N concentration when fertilized under low light. Red oak (Quercus rubra), American beech (Fagus grandifolia) and white pine (Pinus strobus) did not show any evidence of luxury consumption. We compared browse frequency of fertilized and unfertilized saplings within similar forest types. Browse frequency was consistently higher on fertilized saplings. When averaged across all species, however, the difference between treatments was not significant. In contrast, we found significantly higher browse rates for fertilized saplings that had been previously identified as luxury N consumers. Our results indicate that increased soil N availability has positive effects on at least some of the species under high light, but, has potentially negative indirect effects on a larger group of species under both high and low light due to a higher risk of herbivory.

CONVERGENCE AND DIVERGENCE OF OLD-FIELD PLANT COMMUNITIES ALONG EXPERIMENTAL NITROGEN GRADIENTS'

Data from experiments in three old fields were analyzed using similarity indices and population trajectories to determine if similarity of resource supply rates in- fluenced the similarity of plant community composition. In two of three fields, similarity indices indicated that experimental plots having different resource supply rates diverged in species composition with time. There was some evidence of convergence both within and between fields on plots with similar resource supply rates; however, divergence was also common, especially at high rates of nutrient supply. Much of the divergence indicated by similarity indices can be explained by plot-to-plot differences in the initial abundances of three dominant species: Agropyron repens, Poa pratensis, and Schizachyrium scoparium. High nitrogen treatments favored Agropyron repens if it was present, but led to major increases in P. pratensis or S. scoparium if A. repens was absent. The trajectories through time of these three species suggest that plots will converge at high nitrogen levels.