Edward F. Redente

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.

Effects of nitrogen limitation on species replacement dynamics during early secondary succession on a semiarid sagebrush site

SummaryA soil nitrogen (N) availability gradient was induced on a disturbed sagebrush site in northwestern Colorado by fertilizing with nitrogen (high available N), applying sucrose (low available N), and applying neither nitrogen nor sucrose (control). Species composition was studied for 3 years. At the end of the study, N concentration of aboveground tissue of 3 major species was determined. The rate of species replacement was most rapid on plots receiving the sucrose treatment and was slowest on plots receiving the N treatment. Early-seral dominats had greater tissue N concentrations when availability of the resource was high but lower tissue N concentrations when available soil N became limited. Midseral dominants displayed the opposite pattern. These results suggest that the supply of available soil N, and therefore the dynamics of N incorporation in perennial plant tissue, is a primary mechanism in controlling the rate of secondary succession within this semiarid ecosystem.

Nitrogen and Phosphorus Effects on Secondary Succession Dynamics on a Semi‐Arid Sagebrush Site

A sagebrush steppe community in northwestern Colorado was disturbed in 1984 and subjected to annual applications of nitrogen and phosphorus, and successional responses were studied over a 5-yr period. Phosphorus was not found to be significant but nitrogen did significantly affect succession for all years except the first. Three seral groups developed on the non-fertilized plots, the first two dominated by annuals and lasting 3 yr, the third transitional and dominated by perennials. The addition of N altered this successional pattern by allowing annuals to remain as site dominants through the 5th yr. Results of this study suggests that dominance of a site by annuals in early stages of secondary succession is related to high nutrient availability.

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.

Response of early and late semiarid seral species to nitrogen and phosphorus gradients

Above- and below-ground biomass production, nitrogen (N) and phosphorus (P) tissue concentrations, and root: shoot ratios were examined for five species that are characteristic of a semiarid successional sequence under controlled greenhouse conditions. In two simultaneous experiments, seedlings of one forb, two grass, and two shrub species important in a sagebrush successional sere, were subjected to seven levels of N and P. Results of the experiments suggest distinct differences in nutrient response patterns between early and late seral species. Early seral species produced more biomass but had lower tissue nutrient concentrations than late seral species. As N and P availabilities decreased, late seral species displayed characteristics indicative of increasing competitive advantage over those of early seral species. Root: shoot ratios of the five species primarily reflected patterns related to lifeform, but with some early and late seral characteristics. Results from this study 1) confirm that nutrient use pattern, nutrient availability, and seral position relationships characteristic of mesic ecosystems hold equally true for semiarid systems, and 2) suggest that nutrients are important organizing factors in semiarid ecosystems.

INTERACTIONS BETWEEN VESICULAR-ARBUSCULAR MYCORRHIZA AND Rhizobium AND THEIR EFFECT ON SWEETVETCH GROWTH

Sweetvetch (Hedysarum boreale Nutt.) is a legume native to the intermountain West that grows on foothills and intermediate mountain elevations. It has many attributes that are desirable in plant species used in revegetating disturbed lands and improving deteriorated rangelands in the West. We studied the effect and interactions of Glomus fasciculatus (Thaxter sensu Gerdemann) Gerdemann and Trappe and Rhizobium sp. on sweetvetch growth to determine the dependence of sweetvetch on these symbionts. Plants were grown under controlled conditions in sterilized soil or sterilized soil infested with G. fasciculatus, Rhizobium, or a combination of the symbionts. Following a 12-week growth period, above- and belowground plant production, percent mycorrhizal infection, nitrogen and phosphorus content, and rate of nitrogen fixation were quantified. Plants inoculated with either a combination of mycorrhizal fungi and rhizobia or mycorrhizal fungi alone grew taller and produced more aboveground plant material than plants infected with rhizobia alone or control plants. The presence of rhizobia increased the nitrogen content of the aboveground foliage, and inoculation with mycorrhizal fungi increased the phosphorus content. Mycorrhizal fungi and rhizobia also were found to be synergistic with respect to nitrogen fixation rates and mycorrhizal infection levels. The results point to a need for having appropriate inocula for normal plant growth and nitrogen fixation when growing sweetvetch on disturbed lands.

Observations on biomass dynamics of a crested wheatgrass and native shortgrass ecosystem in southern Wyoming.

Above- and belowground net primary production (ANPP and BNPP) were compared between a 30-year-old crested wheatgrass site and an adjacent native shortgrass prairie. ANPP was estimated using successive harvests in May, June, July, and October 1985. BNPP was estimated using soil cores to a depth of 100 cm at the same time that aboveground harvests were made. ANPP was significantly greater in the crested wheatgrass site compared to the native site, but belowground and total net primary production were not different. The native shortgrass system, however, had greater live root biomass early in the growing season. The crested wheatgrass system had a high accumulation of aboveground dead material at the start of the growing season, which was followed by a significant decline in June and an increase in July and October. The native shortgrass system, however, had significantly lower accumulations of aboveground dead material. Approximately 92% of the fixed carbon in the native site was allocated belowground, while crested wheatgrass allocated about 85% of its fixed carbon belowground.

Succession patterns following soil disturbance in a sagebrush steppe community

SummaryA study was begun in 1976 to measure succession patterns following soil disturbance within a sagebrush community in northwestern Colorado. The principal hypothesis was that type of disturbance affects the direction of succession, resulting in different plant communities over time. Successional dynamics were studied through 1988. Four types of soil disturbance resulted in 3 early seral communities: one dominated by grasses, one by annuals, and one intermediate. The annual-dominated communities were opportunistic on these sites, lasting 3–5 years and not determining the direction in which succession proceeded following their replacement. Twelve years after disturbance, 3 communities (one grass-dominated, one shrub-dominated, and one intermediate) occupied the site, the characteristics of which were functions of type of initial soil disturbance. For the period of time covered by this study (12 years), degree of disturbance was found to affect the direction of succession, resulting in different plant communities over time. There were, however, successional characteristics toward the end of the study that suggest that over a longer time period, succession might progress to a single community regardless of type of disturbance.

Plant community dynamics in a semi-arid ecosystem in relation to nutrient addition following a major disturbance.

Effects of annual additions of mineral N and P (100 kg ha−1) on plant species composition and annual aboveground net primary production (ANPP) were investigated during the first three years following disturbance in a semi-arid ecosystem. Additions of N reduced richness of perennial plant species during years 2 and 3, while P reduced the number of perennial species only in year 3. From year 1 to year 2, annual and biennial species richness declined in all treatments while ANPP of annual species increased greatly. Added N increased ANPP of annual species while it decreased ANPP of most perennial species relative to the unfertilized control treatment. Community similarities were higher for the control and native vegetation than for other pairs of treatments using both species presence and plant production data. Nitrogen additions have retarded but not completely arrested secondary succession in this system.