Douglas A. Frank

CAN PLANTS STIMULATE SOIL MICROBES AND THEIR OWN NUTRIENT SUPPLY? EVIDENCE FROM A GRAZING TOLERANT GRASS

The primary source of mineral nutrients for plants is the decomposition of organic matter by soil microbes. Plants are traditionally viewed as largely passive participants in the decomposition process, incapable of directly affecting rates of decomposition significantly and primarily assimilating nutrients unused by the microbial pool. We performed a 13C pulse-chase experiment on a common grazing tolerant grass, Poa pratensis L., of Yellowstone National Park, to follow carbon flow into the soil rhizosphere and microbial biomass and the associated effects on soil N availability and plant N dynamics. Grazing promoted root exudation of carbon, which was quickly assimilated into a burgeoning microbial population in the rhizosphere of clipped plants. Moreover, these facilitating effects of defoliation on rhizospheric processes positively fed back on soil inorganic N pools, plant N uptake, leaf N content, and photosynthesis. Such findings are the first evidence, to our knowledge, that suggest (1) plants are cap...

Reproductive success, spontaneous embryo abortion, and genetic load in flowering plants

SummaryReproductive success is divided into two phases: preemergent (the number of viable seeds that enter the ambient environment) and postemergent (the percentage of progeny that survive to reproduce). We studied preemergent reproductive success (PERS) in flowering plants by measuring the fruit/flower (Fr/Fl) ratio and the seed/ovule (S/O) ratio in a number of species of outcrossing and inbreeding plants, where PERS=the product of (Fr/Fl) and (S/O). In order to determine the influence of the ambient environment (including resource availability) we studied pairs of outcrossing and inbreeding species occurring in the same habitat. Among outcrossing species PERS averaged about 22%, whereas in inbreeding species the average was approximately 90%. The progeny/zygote (P/Z) ratio was studied in hand-pollinated populations in Epilobium angustifolium (a strongly outcrossing species) from populations in Oregon and Utah, by direct observation of embryogenesis at twoday intervals throughout the course of seed development. The P/Z ratio in both populations averaged near 30%, and the developing embryos showed a surprising array of abnormalities that resulted in embryo death. During early development >95% of the ovules had normally developing globular embryos, but beginning with differentiation (cotyledon formation) about 70% of the original globular embryos aborted during the course of embryogenesis and seed development. The clustering of developmental lethals during peroids of major differentiation events parallels the animal model of development. We found little evidence that PERS was limited by the ambient environment (including resource availability), pollination, or factors associated with the inbreeding habit. Instead, PERS was found to be inextricably linked to outcrossing plants, whose breeding systems promote genetic variability. The high incidence of developmental lethals in E. angustifolium and the resulting low P/Z ratio (ca. 30%) is attributed to genetic load (any lethal mutation or allelic combination) possibly working in combination with developmental selection (interovarian competition among genetically diverse embryos). Examples of maternally controlled, fixed patterns of ovule abortion with respect to position or number are discussed. However, we found no need to employ “female choice” as a hypothesis to explain our results for the extensive, seemingly random patterns of embryo abortion in E. angustifolium and other outcrossing species. A more parsimonious, mechanistic explanation based on genetic load-developmental selection is sufficient to account for the differential survivorship of embryos. Likewise, the traditional concept of a positive growth regulator feedback system based on the number of surviving ovules in an ovary can account for subsequent fruit survivorship.

Ungulate vs. landscape control of soil C and N processes in Grasslands of Yellowstone National Park

Within large grassland ecosystems, climatic and topographic gradients are considered the primary controls of soil processes. Ungulates also can influence soil dynamics; however the relative contribution of large herbivores to controlling grassland soil processes remains largely unknown. In this study, we compared the effects of native migratory ungulates and variable site (“landscape”) conditions, caused by combined climatic and topographic variability, on grassland of the northern winter range of Yellowstone National Park by determining soil C and N dynamics inside and outside 33–37 yr exclosures at seven diverse sites. Sites included hilltop, slope, and slope bottom positions across a climatic gradient and represented among the driest and wettest grasslands on the northern winter range. We performed two experiments: (1) a 12-mo in situ net N mineralization study and (2) a long-term (62-wk) laboratory incubation to measure potential N mineralization and microbial respiration. Results from the in situ exp...

The Ecology of the Earth's Grazing Ecosystems Profound functional similarities exist between the Serengeti and Yellowstone

A s recently as 150 years ago, most of Earths grasslands supported large migratory populations of hoofed herbivores belonging to the Artiodactyla, Perissodactyla, and Proboscidea-that is, ungulates. These herbivores included bison (Bison bison) on the North American plains, saiga antelope (Saiga tatarica) on the Eurasian steppe, wildebeest (Connochaetes taurinus) and zebra (Equus burchelli) on the African savanna, and the ecologically equivalent kangaroos (Macropodidae) on the Australian savanna. As a result of the post-industrial global expansion of cropland and cattle ranching, most grasslands grazed by Pleistocene megaherbivores were eliminated. Today, they are restricted to the worlds few large grassland reserves that protect all seasonal ranges of the animals. In this article, we describe profound functional similarities between two of the most celebrated of these remaining habitats, the Serengeti ecosystem of east Africa and Yellowstone National Park (YNP) of the North American intermountain west, which previously have been considered to

Stability increases with diversity in plant communities: empirical evidence from the 1988 Yellowstone drought

The idea that the species diversity of ecological communities contributes to stability is among ecologys most venerable hypotheses, but there are few data on how those properties are associated in nature. We document for the first time that stability of plant community species composition increases with diversity. Resistance to drought-induced species composition change and plant community diversity were positively related in grasslands of Yellowstone National Park. Community diversity was due principally to the spatial heterogeneity of the community, pattern diversity, thus suggesting pattern diversity as a potential factor involved in ecological stability

EFFECTS OF MIGRATORY GRAZERS ON SPATIAL HETEROGENEITY OF SOIL NITROGEN PROPERTIES IN A GRASSLAND ECOSYSTEM

Large herbivores are known to respond to spatial heterogeneity in the distribution of forage resources, but how important herbivores are in creating those spatial patterns, how their influence may be scale dependent, and how herbivore-induced patterns affect ecosystem processes remain unclear. We examined the effects of native migratory ungulates on the spatial distribution of soil nitrogen and N-mineralization potential at scales ranging from 0.1–30 m in grasslands of Yellowstone National Park using geostatistical analyses of soils collected inside and outside long-term (36+-yr) exclosures. At small spatial scales (0.1–2 m), ungrazed grasslands showed a high degree of patchiness in the distribution of soil N and N-mineralization potential (94% and 77% of sample variation explained by small-scale patchiness, respectively). For both variables, patches occurred at a consistent mean size of ∼40 cm. In contrast, grazed grassland exhibited minimal spatial structure in the distribution of soil N and N-mineralization rates (<24% of variation for both variables spatially dependent) and no consistent patch size at a scale of 0.1–2 m. In grazed grassland, most variation was extremely fine grained, occurring across distances <10 cm. The high degree of fine-grained heterogeneity in grazed grassland was associated with greater plant diversity in small (20 × 20 cm) patches. Recycling of nitrogen through dung and urine is clearly important for maintaining long-term nitrogen balance of the system, but we estimated that only 2.5% of the grazed grassland sampled was recently affected by urine. Results conflict with predictions of increased heterogeneity in grazed communities based primarily on the deposition of dung and urine in discrete patches and suggest that the dominant mechanism(s) by which grazers alter N cycling in this ecosystem operates through the plant community. We hypothesize that grazers promote fine-scale heterogeneity by diversifying plant species effects on soils and/or increasing the spatial variability in plant litter inputs. At larger spatial scales (5–30 m), large herbivores altered the distribution of soil N across a topographic gradient. In a grazed community, soil N properties were associated with the topographic gradient and exhibited increasing variance among sampling points separated by increasing distances from 5 to 30 m. Ungrazed grassland exhibited no spatial structure in soil N distribution and no correlation with topography. Collectively, our data show that grazers influence the distribution of soil N properties at every spatial scale from individual plants to landscapes.

FEEDBACKS BETWEEN SOIL NUTRIENTS AND LARGE HERBIVORES IN A MANAGED SAVANNA ECOSYSTEM

Small-scale fertilization experiments have shown that soil nutrients limit plant productivity in many semiarid grasslands and savannas, but linkages among nutrients, grasses, and grazers are rarely studied in an ecosystem context. We used hectare-scale heterogeneity in soil nutrients created by cattle management practices within a geologically homogeneous savanna to examine relationships among soil nitrogen and phosphorus, above- ground net primary production (ANPP), grass nutrient content, and a mixed community of native and domestic herbivores on central Kenyan rangeland. Increasing soil N and P content was consistently associated with increasing plant productivity and rainfall use efficiency in wet, dry, and drought years. A fertilization experiment and analyses of grass N:P ratios across sites indicated that N is the primary limiting nutrient on nutrient-rich glades, whereas N and P co-limit productivity on nutrient-poor bushland sites. Variation in ANPP among patches within the landscape was linearly correlated with consumption rates of large her- bivores. Grazing pressure was consistently high ( .60% of ANPP) at all but one site in a dry year (1999), and was greater in nutrient-rich glades (73 6 4% of ANPP) than in nutrient- poor bushland sites (43 6 7% of ANPP) in a wet year (2001). Grasses of nutrient-rich sites contained sufficient P concentrations to meet requirements for pregnant and lactating un- gulates, whereas grasses in nutrient-poor swards were P deficient. Even though native and domestic herbivores selectively used and intensively grazed nutrient-rich sites, productivity on these sites remained high throughout the study. Analyses of nitrogen budgets for nutrient- rich and nutrient-poor sites showed that large herbivores themselves caused a net N input to the former and a net N loss from the latter. Thus, large herbivores not only respond to heterogeneity in soil and plant nutrients across the landscape, but also play a role in maintaining the N-enriched status of highly productive and intensively grazed sites.

The Ecology of Plants, Large Mammalian Herbivores, and Drought in Yellowstone National Park

The purpose of this study was to examine the effect of abundant native large herbivores on ecosystem function of a spatially and temporally heterogeneous temperate grassland. Net aboveground primary production (ANPP), large herbivore consumption (C), and dung deposition (D), an index of nutrient flow from herbivores to the soil, were measured in grassland and shrub-grassland habitat on winter, transitional, and summer range used by herds of elk (Cervus elaphus) and bison (Bison bison) in northern Yellowstone National Park. Temporary exclosures (5-7 per site) were moved every 4 wk during the snow-free season to determine ANPP and C. Data were collected during 1988, a year of drought and unusually high elk and bison population levels, and 1989, a climatically near- average year, with dramatically fewer elk and bison. All three processes, ANPP, C, and D, varied widely among sites: ANPP range: 16-589 g/m2, C range: 0-306 g/m2, and D range: 0-68 g/m2. An average of 45% of ANPP was consumed by herbivores. Production and consumption, and consumption and dung de- position were positively correlated across all sites. In addition, sites were grazed when plants were growing. There was a 19% reduction in ANPP from 1988 to 1989, likely caused by death or injury to plants during the 1988 drought. Drought also appeared to be partially responsible for reductions in elk and bison from 1988 to 1989, which were coincident with declines in C and D. Results indicate direct effects and suggest indirect effects of a single-season drought on grassland function that will persist for several years after the event.

Evidence for the promotion of aboveground grassland production by native large herbivores in Yellowstone National Park

We examined the effect of native large herbivores on aboveground primary production of nonforested habitat in Yellowstone National Park, Wyoming. Productivity of vegetation grazed by elk (Cervus elaphus) and bison (Bison bison) was compared with that of ungrazed (permanently fenced) vegetation at four sites. Two methods were used that, we believed, would provide the most accurate measurements under the different grazing regimes encountered in the study. Production of ungrazed vegetation in permanent exclosures (10×10 m or 15×15 m, 3 per site) and that of vegetation that was grazed only in the winter was taken as peak standing crop. Production of vegetation grazed during the growing season was the sum of significant increments (P<0.05) in standing crop inside temporary exclosures (1.5×1.5 m, 6 per site) moved every four weeks to account for herbivory.Aboveground productivity of grazed vegetation was .47% higher than that of ungrazed vegetation across sites (P<0.0003). This result could be explained by either a methodological or grazer effect. We believe it was the latter. Results from a computer simulation showed that sequential sampling with temporary exclosures resulted in a slight underestimation of production, suggesting that the reported differences between treatments were conservative. We suggest that stimulation of aboveground production by ungulates may be, in part, due to the migratory behavior of native ungulates that track young, high quality forage as it shifts spatially across the Yellowstone ecosystem.

Ungulate stimulation of nitrogen cycling and retention in Yellowstone Park grasslands

Abstract We studied how ungulates and a large variation in site conditions influenced grassland nitrogen (N) dynamics in Yellowstone National Park. In contrast to most grassland N studies that have examined one or two soil N processes, we investigated four rates, net N mineralization, nitrification, denitrification, and inorganic N leaching, at seven paired sites inside and outside long-term (33+ year) exclosures. Our focus was how N fluxes were related to one another among highly variable grasslands and how grazers influenced those relationships. In addition, we examined variation in soil δ15N among grasslands and the relationships between soil 15N abundance and N processes. Previously, ungulates were reported to facilitate net N mineralization across variable Yellowstone grasslands and denitrification at mesic sites. In this study, we found that herbivores also promoted nitrification among diverse grasslands. Furthermore, net N mineralization, nitrification, and denitrification (kg N ha–1 year–1, each variable) were postively and linearly related to one another among all grasslands (grazed and fenced), and grazers reduced the nitrification/net N mineralization and denitrification/net N mineralization ratios, indicating that ungulates inhibited the proportion of available NH4+ that was nitrified and denitrified. There was no relationship between net N mineralization or nitrification with leaching (indexed by inorganic N adsorbed to resin buried at the bottom of rooting zones) and leaching was unaffected by grazers. Soil δ15N was positively and linearly related to in situ net N mineralization and nitrification in ungrazed grasslands; however, there was no relationship between isotopic composition of N and those rates among grazed grasslands. The results suggested that grazers simultaneously increased N availability (stimulated net N mineralization and nitrification per unit area) and N conservation (reduced N loss from the soil per unit net N mineralization) in Yellowstone grasslands. Grazers promoted N retention by stimulating microbial productivity, probably caused by herbivores promoting labile soil C. Process-level evidence for N retention by grazers was supported by soil δ15N data. Grazed grassland with high rates of N cycling had substantially lower soil δ15N relative to values expected for ungrazed grassland with comparable net N mineralization and nitrification rates. These soil 15N results suggest that ungulates inhibited N loss at those sites. Such documented evidence for consumer control of N availability to plants, microbial productivity, and N retention in Yellowstone Park is further testimony for the widespread regulation of grassland processes by large herbivores.