Mario E. Biondini

GRAZING INTENSITY AND ECOSYSTEM PROCESSES IN A NORTHERN MIXED-GRASS PRAIRIE, USA

The objective of this study was to evaluate for an 8-yr period the ecosystem-level impacts of no grazing vs. sustained moderate and heavy cattle grazing in terms of: (1) plant species basal cover, density, and composition; (2) aboveground net primary production (ANPP), N content of ANPP (ANPP-N), belowground net primary production (BNPP), and N content of BNPP (BNPP-N); (3) litter and root decomposition and N loss; and (4) soil C, total soil N, and net in situ soil N mineralization. Moderate and heavy grazing treatments were designed to achieve an end-of-the-grazing-season residual vegetation of 50% and 10%, respectively, of the long-term average ANPP of comparable ungrazed sites. The main factor affecting the vegetation response was the increase in precipitation after the drought of 1988; few differences were due to grazing intensity. The total absolute basal cover of grasses increased steadily in all treatments, from an average of 4% during the drought of 1988 to 14% in 1993. Forb density and diversity increased from 51 plants/m2 and 14 species in 1988 to 412 plants/m2 and 36 species in 1995. Grazing, however, increased the relative composition of Poa pratensis and Achillea millefolium, but reduced the relative composition of Bouteloua gracilis and Aster ericoides. ANPP and ANPP-N were correlated with rainfall, but not with grazing intensity. Heavy grazing led to declines in standing dead biomass, litter, peak root biomass and biomass-N, root N concentrations, and in situ net soil N mineralization. There was an increase in root decomposition and N loss with grazing. From this study, we reached the following conclusions about the northern mixed-grass prairie: (1) climatic variations, in particular droughts, control major trends in plant species composition and production, with grazing playing a secondary role; (2) heavy grazing leads to declines in standing dead biomass and biomass-N, litter biomass and biomass-N, peak root biomass and biomass-N, and in situ net soil N mineralization, which may have a significant long-term impact on range condition; and (3) grazing pressures that lead to a removal of 50% of ANPP, however, seem to be sustainable and compatible with the maintenance of range condition.

Secondary successional patterns in a sagebrush (Artemisia tridentata) community as they relate to soil disturbance and soil biological activity

The relationship between secondary succession, soil disturbance, and soil biological activity were studied on a sagebrush community (Artemisia tridentata) in the Piceance Basin of northwestern Colorado, U.S.A. Four levels of disturbance were imposed. I: the vegetation was mechanically removed and as much topsoil as possible was left; 2: the vegetation was mechanically removed and the topsoil scarified to a depth of 30 cm; 3: topsoil and subsoil were removed to a depth of 1 m, mixed and replaced; 4: topsoil and subsoil were removed to a depth of 2 m and replaced in a reverse order. Plant species composition, dehydrogenase and phosphatase enzymatic activity, mycorrhizae infection potentials, and percent organic matter were the variables measured. Treatment 4 drastically altered the pattern of vegetation succession. Treatments 2, 3, and 4 started with Salsola iberica as the dominant species but six years later, 3 and to lesser extent 2 changed in the direction of the species composition of 1, dominated by perennial grasses and perennial forbs. Treatment 4 developed a shrub dominated community. The rate of succession was not decreased by the increased levels of disturbance. Both dehydrogenase enzyme activity and mycorrhizae infection potential (MIP) increased with the change from Salsola iberica to a vegetation dominated by either perennial grasses and forbs or shrubs. The intensity of disturbance in 2, 3, and 4 reduced drastically dehydrogenase activity and MIP, but in six years they recovered to levels comparable to 1. Phosphatase enzyme activity and organic matter were unrelated to species composition but related to treatment and time elapsed. In both cases a significant decrease was observed throughout the six-year period.

Data-dependent permutation techniques for the analysis of ecological data

Two distribution-free permutation techniques are described for the analysis of ecological data. These methods are completely data dependent and provide analyses for the commonly-encountered completely-randomized and randomized-block designs in a multivariate framework. Euclidean distance forms the basis of both techniques, providing consistency with the observed distribution of data in many ecological studies.

Seasonal fire effects on the diversity patterns, spatial distribution and community structure of forbs in the Northern Mixed Prairie, USA

The effects of fire season on forb diversity patterns, density, and composition were determined for a northern Mixed Prairie site, USA. Repeated spring burns (dormant season), summer burns, fall burns (dormant season), and unburned treatments were compared over a 3-yr period characterized by wet and dry moisture conditions. Alpha and beta diversity were highest on unburned and summer burn treatments, while landscape mosaic diversity was highest on fall burns. Forb density was highest on fall and spring burn sites. Nine forb species comprised 82% of total densities and were significantly affected by fire season and year to year variations in moisture. Forb composition for unburned and spring burn treatments was similar, but both treatments were different from the summer burn and fall burn treatments which were similar to each other. Fire alone did not appear to be an intense enough disturbance to initiate drastic changes in the forb component of vegetation patches. Specific fire seasons did appear to either mask or enhance forb structure arising from other disturbance(s). Fire season also affected the scales of forb organization in the landscape. Contrasting spatial characteristics of the forb component of prairie plant communities may provide a diagnostic technique for exposing the interaction of disturbances at different temporal and spatial scales.

Bias in estimates of primary production: An analytical solution

This paper addresses the issue of the effect of random error upon the estimates of above-or belowground net primary production (NPP). We show that ramdom errors in estimates of biomass do not compensate but always result in a positive bias in estimates of NPP. Second, we demonstrate that the larger the number of time intervals considered, the higher is the positive bias or overestimation error. An effect similar to an increase in the number of sampling periods is obtained by increasing the number of components utilized in estimating NPP. These are usually taxonomic or functional (depth, live, recent dead, etc.) components. We calculate the magnitude of the overestimation error as a function of the size of the difference in two sequential estimates of biomass and the variability associated with them. We propose a method that uses this error to correct the estimates of NPP for the positive bias resulting from random errors and to develop confidence intervals for the corrected NPP. We suggest that this method will remove the positive bias from estimates of NPP upon which nutrient budgets, energy flow and trophic webs rely. The concepts presented will help in the design of experiments that use production as a response variable.

Grazing Frequency and Ecosystem Processes in a Northern Mixed Prairie, USA

The objective of this study was to evaluate for a 6-yr period the effects of a twice-over rotation grazing system (ROT) and a season-long grazing system (SL), and compare these effects with long-term grazing exclosures (NG) in terms of (1) species composition and basal cover, (2) aboveground net primary production (ANPP) and aboveground N uptake (ANPP-N), (3) rates of litter and root decomposition and N release, (4) soil N mineralization and immobilization, (5) aboveground C and N flow, and (6) grazing intensity (GI) and animal performance. The study period included the drought of 1988. No major differences were found in ANPP and ANPP-N among treatments, but there were important seasonal variations. An average of 72% of ANPP and >82% of ANPP-N occurred by mid-June. There were no differences among treatments in terms of decomposition and N release rates from litter and root biomass, or in soil N mineralization. Grazing, however, reduced the amount of C and N immobilized in standing dead and litter and the ...

Carbon and nitrogen losses through root exudation by Agropyron cristatum, A smithii and Bouteloua gracilis

Abstract C and N released in root exudates throughout a growing season were estimated in Bouteloua gracilis and Agropyron smithii (dominant species in the shortgrass steppe ecosystem) and A. cristatum (dominant species in a 40-yr-old crested wheatgrass ecosystem). The release of C and N exudate was measured with or without the presence of a rhizosphere microflora. These values were used to estimate the potential contribution of root exudates to the N cycle of both ecosystems. Total C released through root exudates by A. cristatum , A. smithii and vB. gracilis was estimated at 8, 17 and 15%, respectively, of C fixed. The contributions of root exudates to the N cycle were estimated to be 11 g N m −2 in the shortgrass steppe ecosystem and 6 g N m −2 in the crested wheatgrass ecosystem. The presence of rhizosphere microorganisms resulted in a significant increase in root exudates of A. cristatum and A. smithii . A. cristatum and A. smithii plants grown under axenic conditions released 60 and 17%, respectively, of the C released via root exudates by inoculated plants. No differences between inoculated and non-inoculated plants were found in vB. gracilis , which indicated that it may be inherently insensitive to the presence of a rhizosphere microflora. These data suggested that an introduced plant species may be markedly different from native species in the shortgrass steppe in terms of exudate releases. These differences may have a significant influence upon the N cycle dynamics of both ecosystems.

Permutation Techniques Based on Euclidean Analysis Spaces: A New and Powerful Statistical Method for Ecological Research

The statistical procedures that are most widely used in ecological population and community research belong to the family of parametric methods. Embedded in these procedures are assumptions about the normal distribution of the underlying population, homogeneity of variances and linear response patterns. One of the problems encountered in ecological and vegetation studies, however, is that these assumptions are very difficult, if not impossible, to meet. In addition, a very serious shortcoming of the most widely used statistical methods is the lack of congruence between the geometry of the data space, which is for the most part Euclidean, and the analysis space, which in the standard parametric tests and most of the nonparametric tests, is not Euclidean. In ecological and vegetation studies, the combination of a failure to meet model assumptions and a lack of congruence between the geometries of the data space and the analysis space can lead, as shown in this paper, to gross errors in data interpretation and hypothesis testing.

Root morphological plasticity and nitrogen uptake of 59 plant species from the Great Plains grasslands, U.S.A.

Summary We investigated 59 plant species from the Great Plains grasslands with the following objectives: (1) Determine the ability of individual plant species to: (a) redirect root surface area growth to patches with high N and P concentrations; and (b) alter their root:shoot ratio in response to a non-uniform distribution of soil N and P. (2) Determine how a patchy distribution of soil N and P within a plants rooting zone affects biomass, total root surface area, N uptake, %N in plant tissue, and N uptake per unit of root surface area. Results showed a diversity of responses with some important common patterns. (1) Root responses (defined as total RSA, allocation of RSA, and R:S ratio) were twice as prevalent as whole plant responses (defined as total N uptake, %N in plant tissue, and plant biomass). (2) The percentage of species with the ability to allocate their RSA to nutrient rich patches was 9 times higher in forbs than in grasses, but there were proportionally more grasses than forbs that increased their R:S ratios in response to nutrient patchiness. (4) The proportions of late successional forbs that responded to nutrient patchiness was higher than that of mid successional ones, but the size of the response was substantially larger in the latter. (4) We found a very weak coupling between root plasticity and plant performance. (5) Our results tend to suggest that: (a) most plants have sufficient plasticity in root system development to track the scale of soil nutrient heterogeneity and thus show similar performance regardless of the degree of nutrient patchiness; and (b) the benefits of root plasticity may be more critical for subdominant species as a general adaptation to compete for soil nutrients in mixed plant communities regardless of the extent of soil nutrient heterogeneity. Wir haben 59 Pflanzenarten der Prarie in den Great Plains mit folgenden Zielen untersucht: (1) Feststellung der Fahigkeiten der einzelnen Pflanzenart in Bezug auf: (a) die Steuerung des Wurzelwachstums in den oberflachennahen Bodenschichten in Bereiche mit hohem N- and P-Gehalt; und (b) Veranderung des Verhaltnisses Wurzelmasse: Pflanzenmasse als Reaktion auf eine uneinheitliche Verteilung des N und P im Boden. (2) Festzustellen, wie die Verteilung der verschiedenen Zonen des N- und P-Gehaltes innerhalb der Wurzelzone einer Pflanze sich auf die Biomasse, die gesamte durchwurzelte Flache, die N-Aufnahme, den N-Gehalt (%) in der Pflanzenmembran und die N-Aufnahme pro Einheit der Wurzeloberflache auswirkt. Die Ergebnisse zeigten eine Vielfalt an Reaktionen mit einigen wichtigen allgemeinen Mustern. (1) Wurzelreaktionen (definiert als Total RSA, Lokalisierung der RSA und R:S ratio) waren doppelt so ausgepragt wie die Reaktionen der gesamten Pflanze (definiert als Gesamt N-Aufnahme, N-Gehalt (%) in der Pflanzenmembran und Biomasse der Pflanze). (2) Die Prozentzahl der Arten mit der Fahigkeit, ihre RSA in nahrstoffreiche Bodenzonen zu dirigieren war bei Blattpflanzen neunmal hoher als bei Grasern, aber es waren propotional mehr Graser als Blattpflanzen, die ihr R:S ratio als Reaktion auf die Nahrstoffverteilung veranderten. (3) Die Reaktion derjenigen Blattpflanzen, die sich erst spater ansiedeln war groser als die der Mittelansiedler, wobei die Anzahl der Reaktionen unter den letzteren uberwog. (4) Wir stellten eine sehr schwache Verknupfung zwischen Wurzelflexibilitat und der Leistung der Pflanze fest. (5) Unsere Ergebnisse lassen folgende Schlusse zu: (a) die meisten Pflanzen haben eine ausreichende Flexibilitat in ihrer Wurzelsystementwicklung um Nahrstoffuneinheitlichkeiten im Boden zu begegnen und zeigen deshalb unveranderte Leistungsfahigkeit unabhangig von der Nahrstoffverteilung; und (b) der Nutzen der Wurzelflexibilitat kann fur nichtdominante Arten entscheidender sein, da in gemischten Pflanzenpopulationen generell die einzelnen Arten in Konkurrenz um die Nahrstoff stehen unabhangig von der inhomogenen Nuhrstoffverteilung.

Rhizosphere microorganism effects on soluble amino acids, sugars and organic acids in the root zone ofAgropyron cristatum, A. smithii andBouteloua gracilis

Three axenic and rhizosphere microorganism-inoculated shortgrass steppe plant species were evaluated for possible differences in residual organic carbon and nitrogen present as sugars, organic acids and amino acids. IntroducedAgropyron cristatum was compared toA. smithii andBouteloua gracilis, which are dominant species in the native shortgrass steppe. These plants, grown for 90 days in root growth chambers, showed differences in residual organic carbon and nitrogen per gram of root, and rhizosphere microbe presence resulted in additional changes in these compounds. The root biomass ofB. gracilis was significantly increased with microbes present. TheAgropyron species had significantly lower amino acid levels with microbes present, while under the same conditions, theB. gracilis showed significant decreases in residual sugars. Based on the amino acids, sugars and organic acids, the C/N ratio of the sterileA. cristatum was higher than forB. gracilis. Rhizosphere microbe presence did not result in changes in these C/N ratios. These results suggest thatA. cristatum, with microbes present, will have lower levels of amino acids present, whileB. gracilis, with a lower C/N ratio, will have sugars used to a greater extent by the rhizosphere microbes. This resulted in the higher levels of residual soluble organic C and N in the rhizosphere ofB. gracilis, in comparison with the introducedA. cristatum. These differences may be critical in influencing the course of nutrient accumulation and plant competition in short-grass steppe communities, and in understanding basic aspects of plant-rhizosphere microorganism interactions.