Ungrazed Treatments Research Articles

Plant species diversity and soil quality in harvested and grazed boreal aspen stands of northeastern British Columbia

Understory vegetation in aspen ( Populus tremuloides Michx.) stands produces up to 50% of all forage on Crown land for domestic livestock grazing in the Peace River area of British Columbia (BC), Canada. The objective of this study was to determine the effects of clearcut harvesting and long-term cattle grazing on plant species diversity and soil quality in aspen cutblocks located about 5 km south of Dawson Creek in the Peace River region of BC. The site was winter harvested from November 1988 to March 1989. Cattle grazing was done for one month per year (early June to early July) to achieve 75% utilization of forage vegetation. The experiment was laid out as a split-plot randomized complete block design with three replications and four transects per plot. Sample collection and vegetation assessments were completed during the summer 2001. Timber harvesting of these aspen stands increased the cover of plant species such as bluejoint reedgrass ( Calamagrostis canadensis), northern bedstraw ( Galium boreale), tall bluebells ( Mertensia paniculata), and sweet coltsfoot ( Petasites frigidus) probably as a result of greater light transmission. The cover of young aspen was increased by timber harvesting by over 15 times, while the cover of young balsam poplar ( Populus balsamifera) increased by over 200 times. Ten years of cattle grazing did not significantly affect the canopy cover of aspen, but increased the total number of plant species relative to the ungrazed treatments because of enhanced colonization of endemic species. The measured soil properties were not significantly affected by either the timber harvest or the long-term cattle grazing. Soil penetration resistance showed that harvesting and cattle grazing did not result in soil compaction. Results showed that with appropriate management the timber and ranching industries are compatible on regenerating aspen stands in the Peace River region.

Cattle treading and phosphorus and sediment loss in overland flow from grazed cropland

This 1-year study investigated the effect of dairy cow treading on soil physical properties and sediment and phosphorus (P) loss via overland flow from pasture and cultivated soil used for wintering dairy cows in southern New Zealand. Treading decreased soil macroporosity and Ksat, and increased overland flow volumes. Treading increased mean suspended sediment concentration in overland flow in the cultivated + trodden treatment (2.6 g/L) compared with ungrazed pasture (0.44 g/L) and ungrazed cultivated (0.98 g/L) treatments over 2 slope positions. Following grazing in the cultivated + trodden treatment, only 25% more sediment was lost in subsequent overland flow events (2.09 and 2.63 g before and after grazing, respectively), and mean total P (TP) losses increased by >250% (from 0.7 to 2.5 mg P). Meanwhile in the cultivated but ungrazed treatment, sediment and TP loss decreased. The increased loss of sediment and P following grazing in the cultivated + trodden treatment was attributed to P from cattle dung, and soil disturbance. Consequently, wintering of animals on cultivated paddocks with forage crops increases the risk of losing much P, especially in particulate form.

Above-ground grazing affects floristic composition and modifies soil trophic interactions

There are few data on the functional inter-relationships between above- and below-ground components of soil ecosystems. Here, we report changes in below-ground soil invertebrate trophic relationships (manifested as alterations in stable isotope natural abundances, δ 13C and δ 15N) that arose in association with the removal of sheep grazing and from the resulting changes in above-ground floristic composition. Consequent to grazing removal, Lolium perenne L. (perennial rye-grass) was replaced as the dominant plant species in ungrazed treatments by Ranunculus repens L. (creeping buttercup), a species with more 13C-enriched foliage. Consequently, all invertebrate functional groups studied, but not whole soil, were more 13C-enriched in ungrazed treatments. Earthworms (detritivore) from grazed treatments were significantly 15N-enriched compared with earthworms from ungrazed treatments. In contrast, slug (herbivore) δ 15N exhibited no treatment effect. Reasons for this are unclear but may be related to the effects of above-ground grazing on the composition of below-ground microbial/microfaunal communities. Omnivores/carnivores (beetles and spiders), were more 15N-enriched than primary producers in the grazed than in the ungrazed treatments (6 vs. 4‰) suggesting a longer below-ground foodchain in the grazed plots. The cessation of fertilizer application had no comparable effects on below-ground trophic relationships.

Drought and Grazing: II. Effects on Runoff and Water Quality

Understanding the interacting effects of drought and grazing on runoff, erosion, and nutrient transport is essential for improved rangeland management. Research was conducted at the Fort Keogh Livestock and Range Research Laboratory located near Miles City, Mont. using 12, non-weighing lysimeters for 3 years. During years 1 and 3, no drought treatment was imposed. For year 2, one half of the lysimeters were covered to implement a drought treatment. The 3 grazing treatments were ungrazed, grazed during but not after drought, and grazed during and after drought. Runoff, sediment yield, and an array of nutrients in the runoff water were measured from the lysimeters. First year base line data with no grazing or drought treatments applied indicated no significant differences among lysimeters. Below normal precipitation occurred during year 2, resulting in no runoff from the drought treatment and negated the non-drought control. This prevented a direct assessment of the interaction among the drought and grazing treatments for this year. The drought treatment did produce significant reductions in water, sediment, and nutrient yield. No grazing impact was observed during year 2. The third year with more normal precipitation, there was a trend toward increased runoff, sediment, and nutrient yield from the second year drought treatment lysimeters. In the third year, both grazing treatments showed significantly greater runoff, sediment, and nutrient yield than the ungrazed treatment. Runoff and sediment yield tended to increase from the combination of drought and grazing treatments. The observed increases in runoff and sediment and reduced water quality from the drought and grazing treatments were measured against controls and when compared to the natural variability and water quality standards, they were concluded to be minimal. DOI:10.2458/azu_jrm_v55i3_emmerich

Rehabilitation of degraded Calluna vulgaris (L.) Hull-dominated wet heath by controlled sheep grazing

Many upland areas of the British Isles have seen declines in the area and condition of heather ( Calluna vulgaris)-dominated heathland vegetation. To reverse this decline, management regimes must be designed to rehabilitate areas that have seen this decline. As most of this heathland vegetation is primarily managed by grazing, such management has to determine what stocking levels can maintain the vegetation in a desired state. This paper describes how to reverse this decline through suitable grazing management. A degraded ‘wet-heath’ system, previously grazed at 2.1 sheep ha −1, was subject to a range of grazing treatments over a 5-year period. Treatments varied in intensity (0–1.4 sheep ha −1) and timing (summer only, winter only, or year round) of grazing. Grazing levels were maintained at 2.1 sheep ha −1 outside the fenced areas. Vegetation composition remained stable outside the fenced treatments. All the fenced treatments showed an increase in the relative frequency of the evergreen Calluna vulgaris, with the greatest increase being in the ungrazed treatment, and the least in the year round 1.4 sheep ha −1 treatment. This increase was in line with a reduction in heather utilisation to relatively low and sustainable levels. Other species that benefited from reduced grazing included Carex nigra, Deschampsia flexuosa and to a lesser extent Galium saxatile and Erica tetralix, whereas a range of moss species including Hypnum jutlandicum and Rhytidiadelphus loreus were more frequent at higher grazing levels. Though the recovery of heather was similar in the two seasonally grazed treatments, the vegetation showed different overall trajectories. Winter only grazed allowed a substantial increase in the cover of the deciduous Molinia caerulea, whereas this species was kept in check by summer only grazing. A stocking level of between 0.7 and 1.4 sheep ha −1 appears to be appropriate to maintain and even enhance the cover of heather on degraded wet heath. Complete removal is not necessary. Grazing restricted to the winter period is inappropriate in areas where M. caerulea occurs. Setting appropriate stocking levels to maintain the condition of the vegetation must take into account site conditions, especially the presence of species that can affect the utilisation of heather.

Drought and grazing: II. Effects on runoff and water quality

Understanding the interacting effects of drought and grazing on runoff, erosion, and nutrient transport is essential for improved rangeland management. Research was conducted at the Fort Keogh Livestock and Range Research Laboratory located near Miles City, Mont. using 12, non-weighing lysimeters for 3 years. During years 1 and 3, no drought treatment was imposed. For year 2, one half of the lysimeters were covered to implement a drought treatment. The 3 grazing treatments were ungrazed, grazed during but not after drought, and grazed during and after drought. Runoff, sediment yield, and an array of nutrients in the runoff water were measured from the lysimeters. First year base line data with no grazing or drought treatments applied indicated no significant differences among lysimeters. Below normal precipitation occurred during year 2, resulting in no runoff from the drought treatment and negated the "non-drought" control. This prevented a direct assessment of the interaction among the drought and grazing treatments for this year. The drought treatment did produce significant reductions in water, sediment, and nutrient yield. No grazing impact was observed during year 2. The third year with more normal precipitation, there was a trend toward increased runoff, sediment, and nutrient yield from the second year drought treatment lysimeters. In the third year, both grazing treatments showed significantly greater runoff, sediment, and nutrient yield than the ungrazed treatment. Runoff and sediment yield tended to increase from the combination of drought and grazing treatments. The observed increases in runoff and sediment and reduced water quality from the drought and grazing treatments were measured against controls and when compared to the natural variability and water quality standards, they were concluded to be minimal.

LONG–TERM GRAZING DENSITY IMPACTS ON SOIL COMPACTION

Livestock activity can modify soil properties by degrading soil structure, increasing soil bulk density, and reducinginfiltration rate. Impacts of grazing on soil compaction were studied on rangeland in the tallgrass prairie region ofOklahoma. Rotational grazing treatments included low (12.5 cows/ha), medium (25 cows/ha), and high (50 cows/ha) stockingdensities with ungrazed pastures used as control sites. Soil compaction was assessed 10 years after grazing treatments wereinstalled using penetration resistance, bulk density, and rate of water infiltration. Longterm livestock grazing increasedresistance to penetration and bulk density values, but only in the upper 10 cm. The surface infiltration rate decreased from28.5 cm/day for the ungrazed treatment to 7 cm/day under light grazing, 5 cm/day under moderate stocking, and 5 cm/dayfor the heavily grazed treatment. Longterm grazing impacts were restricted to the soil surface, thus increasing the potentialsoil loss due to erosion.The reduced rainfall infiltration caused by compaction suggests that less water is available for plantuse.

Succession during the re‐creation of a flood‐meadow 1985‐1999

Abstract. The study site, Somerford Mead, is located on the river Thames floodplain and was a species‐rich flood‐meadow in the 1950s. In the 1960s and 1970s it was subjected to intensive grassland management with regular NPK additions and occasional herbicide treatment. In 1981 Somerford Mead was ploughed for the first time and converted to arable land. Seeds of an Alopecuruspratensis‐Sanguisorba officinalis flood‐meadow community (MG4; Rodwell 1992) were sown onto prepared soil in the autumn of 1986, and botanical records were made from 1985 to 1999. From 1989 to 1999, three replicates of three treatments: cow‐grazing, sheep‐grazing and no‐grazing were introduced after hay‐cutting. Analysis successfully separated the establishment phase from the experimental phase and showed a significant difference between the grazed and ungrazed treatments. Abiotic and biotic factors which might contribute to successional trends are discussed. A convoluted pattern for each treatment could be attributed in part to intrinsic‘cycles’of perennial hemicryptophytes behaving as short‐lived species and in part to the percentage frequency of many species which was reduced in 1990 and 1995/1996, years of drought. After the initial inoculation of MG4 seed and the disappearance of arable therophytes, recruitment of new species was very slow. Coefficients for Somerford Mead matched against MG4 (Rodwell 1992) produced an equilibrium within three years. It subsequently fluctuated over a 10‐yr period well below the level of Oxey Mead, the donor site.Land managers should ensure that their proposed site has the right soils and hydrology for MG4 grassland and that traditional management of hay‐cutting and aftermath grazing is practised. Only one cut a year in July could lead to a reduction in percentage frequency of most species except Arrhenatherum elatius.

Effects of cattle treading and natural amelioration on soil physical properties and pasture under dairy farming in Southland, New Zealand

The effects of current dairy cow grazing practice, reduced levels of grazing, and stock exclusion on soil physical properties and pasture dry matter production were investigated under dairy farming in Southland. Current grazing practice involves rotational grazing with dairy cows from September to May each year, with no grazing during winter. For the reduced grazing treatments, cattle were excluded during the 3rd, or combined 3rd, 4th, and 5th grazing cycles, or for half‐day grazing intervals to reduce grazing intensity. Macroporosity increased by 70% in the ungrazed treatment compared with current grazing practice (control) within four months of dairy cow exclusion. Air permeability was increased by over two orders of magnitude 18 months after trial commencement, and saturated hydraulic conductivity increased by 200% to the 10‐cm soil depth. Macroporosity, air permeability, and hydraulic conductivity for the reduced grazing treatments were intermediate between the control and ungrazed treatments. Relationships between macroporosity and pasture relative yield are presented. At 97% relative pasture yield, the level of macroporosity was 11.5–11.7% (v/v) in three silt loams.

Soil water properties in a recently established forest as affected by grazing in a semiarid valley

In arid and semiarid areas, soil water deficit is a major constraint to vegetation establishment (Zhang 1992). While climatic conditions can be a constraint to soil water supply, other factors can be extremely important in determining forest survival. One of the key factors leading to tree death is livestock grazing in the early growth stage of forest establishment (Li 1995). Braunack & Walker (1985) found that after 16 years without grazing, the surface soil properties of a semiarid woodland still showed evidence of prior damage by grazing sheep. Infiltration rates can still increase many years after grazing has ceased (Gifford & Hawkins 1978).Greenwood et al. (1998) reported that there were significant increases in unsaturated hydraulic conductivity in the ungrazed treatments compared with the grazed controls after 2.5 years without livestock. The purpose of this study was to quantify the changes in soil water properties in a recently established forest as affected by grazing in a semiarid climate.

Western Ragweed Effects on Herbaceous Standing Crop in Great Plains Grasslands

Western ragweed [Ambrosia psilostachya DC. ], a major forb species in mixed and tallgrass prairies, is considered to have little value for cattle grazing but is an important food item for bobwhite quail [Colinus virgini anus ]. While often thought to be a strong increaser with grazing pressure, information on the actual relationship between western ragweed and grasses is contradictory. Our objectives were to 1) determine the effect of western ragweed on grass standing crop, and 2) determine the effect of vegetation type and grazing on survival and shoot morphology of western ragweed. Western ragweed did not appear to reduce grass standing crop. Instead, standing crop (40 to 620 kg ha - 1 ) and density (6 to 41 shoots m - 2 ) of western ragweed were positively related to grass and grassforb standing crop in mixed prairie. Standing crop of western ragweed was not related to grass standing crop in tallgrass prairie. Competitive thresholds for western ragweed in mixed and tallgrass prairies appear to be above the levels observed in this study. Density of western ragweed shoots decreased over the growing season under both grazed and ungrazed treatments. Survival of western ragweed shoots from June to September was greater in mixed prairie (81%) than in tallgrass prairie (63%) and was greater in ungrazed (76%) than grazed plots (68%). Western ragweed shoots weighed less per unit of height in tallgrass prairie. Western ragweed shoots in ungrazed plots were taller than shoots in grazed plots but weighed less per unit of height. These differences in shoot morphology are consistent with increased competition for light in tallgrass prairie and in ungrazed sites. Western ragweed may not directly reduce grass standing crop but, rather, increase only when grasses are reduced by other stresses such as improper grazing.

Aboveground dry-matter allocation in ungrazed and grazed stands of Indigofera spinosa in the arid zone of Turkana, Kenya

The dwarf shrub Indigofera spinosa, indigenous to arid and semi-arid rangelands of northeastern Africa, is an important food source for livestock. Proper management of the shrub requires improved understanding of the effects of grazing and climatic variability on aboveground dry-matter allocation. Between 1986 and 1990, we compared the temporal variability of aboveground dry-matter allocation to different plant biomass compartments. We also compared dry-matter transfers between components; total live biomass to litter, standing dead to litter and live biomass to standing dead between continuously grazed and an ungrazed treatments. Partitioning of combined total dry-matter production among different structural organs (called allocation ratio) is influenced by phenological changes, episodic rainfall and her-bivory. Dry-matter production in the grazed treatment responded more markedly to episodic rainfall events more than in the ungrazed treatment. Exclusion of grazers failed to improve the relative growth rate (RGR) of shrub biomass, while grazing improved it. RGR declined in the ungrazed treatment following the accumulation of standing dead dry-matter, while m the grazed treatment it declined following the shedding of leaves. The shrub allocated more to total live biomass than to standing dead. Greater reduction of total live allocation ratio in the grazed than in the ungrazed treatment occurred during a dry year. The ungrazed treatment had higher standing dead allocation ratio than did the grazed treatment. Plants transferred more dry-matter from total live biomass compartment to litter, than from standing dead to or from total live biomass to standing dead independent of treatment. The rates of transfer were higher in the ungrazed than in the grazed treatment. The results suggest that I spinosa has evolved to respond to climatic variability and grazmgbyallocating dry allocating dry-matter differently between various compartments.

Western ragweed effects on herbaceous standing crop in Great Plains grasslands

Western ragweed [Ambrosia psilostachya DC. ], a major forb species in mixed and tallgrass prairies, is considered to have little value for cattle grazing but is an important food item for bobwhite quail [Colinus virginianus]. While often thought to be a strong increaser with grazing pressure, information on the actual relationship between western ragweed and grasses is contradictory. Our objectives were to 1) determine the effect of western ragweed on grass standing crop, and 2) determine the effect of vegetation type and grazing on survival and shoot morphology of western ragweed. Western ragweed did not appear to reduce grass standing crop. Instead, standing crop (40 to 620 kg ha-1) and density (6 to 41 shoots m-2) of western ragweed were positively related to grass and grass-forb standing crop in mixed prairie. Standing crop of western ragweed was not related to grass standing crop in tallgrass prairie. Competitive thresholds for western ragweed in mixed and tallgrass prairies appear to be above the levels observed in this study. Density of western ragweed shoots decreased over the growing season under both grazed and ungrazed treatments. Survival of western ragweed shoots from June to September was greater in mixed prairie (81%) than in tallgrass prairie (63%) and was greater in ungrazed (76%) than grazed plots (68%). Western ragweed shoots weighed less per unit of height in tall grassprairie. Western ragweed shoots in ungrazed plots were taller than shoots in grazed plots but weighed less per unit of height. These differences in shoot morphology are consistent with increased competition for light in tallgrass prairie and in ungrazed sites. Western ragweed may not directly reduce grass standing crop but, rather, increase only when grasses are reduced by other stresses such as improper grazing.

Scaling effects of grazing in a semi‐arid grassland

Abstract. Previous studies have demonstrated relationships between spatial scale and spatial pattern and developed general hypotheses of scaling effects. Few studies, however, have examined the interactive relationship between scale and pattern‐driving processes such as grazing. The goal of this study is to evaluate scale‐dependent patterns across three spatial scales for three grazing intensities over 45 yr and to identify some mechanisms that may be associated with scale related differences. Correlation analysis and analysis of the coefficients of variation indicate that the relationships between units are dependent upon spatial scale and treatment. Across all grazing treatments, the relationship between units of the same scale becomes stronger as the spatial scale is increased. However, the rate of increase in the correlation coefficient is different for each treatment. The coefficient of variation responded inversely across scales with the greatest variation between small‐scale units and little difference between the intermediate‐ and large scales. In addition to different relationships between units at each scale, differences in heterogeneity within treatments over time is illustrated by the relationship between small‐scale units within each treatment and their associated larger scale units. The strongest relationship occurred in the heavily grazed treatments where correlation coefficients of small‐scale units with intermediate‐ and large‐scale units were ca. 0.60, indicating similar dynamics across scales. For the moderately grazed and ungrazed treatments this relationship varied from 0.40 to 0.47.Results from this study suggest that grazing alters scaling effects. Variability between small‐scale units was greatest in the ungrazed treatment which had greater heterogeneity and less predictability than grazed treatments because of the influence of grazing on plant morphology, demography and composition. At the intermediate scale, relationships between units were fairly similar with the least variation occurring in the moderately grazed treatment. Alternatively, variation between large‐scale units was greatest in the moderately grazed treatment because of the relationship between rest cycles, weather patterns, and patch grazing. Therefore, grazing can have a positive, a negative, or no influence on heterogeneity between units depending upon the scale of observation. Evaluation of long‐term dynamics across these treatments at the same small spatial scale results in different variances within each treatment which may violate assumptions of some statistical and experimental designs. Therefore, evaluations of temporal dynamics should consider scale relative to the relationship between plant size, density and longevity (relative scale).

Restorative crops for the amelioration of degraded soil conditions in New Zealand

The effects of restorative crops on the amelioration of a degraded soil were investigated in a 6-year field experiment. Treatments included perennial pastures, annual pastures, and arable crops. Improvements in some aspects of chemical, biological, and physical fertility were related to the amount of herbage dry matter returned to the soil and root production. Beneficial effects associated with returned organic matter were partly negated by the degradative effect of tillage. Treatments that returned most organic material to the soil showed the greatest increase in aggregate stability and supported the largest earthworm populations, especially without annual tillage. Differences between treatments in soil organic C content were not generally significant until the sixth year. In contrast, differences between treatments in microbial biomass C were apparent by the third year. Compaction by sheep during grazing appeared to result in a loss of soil macroporosity. In the sixth year, soil macroporosity was greatest in the annually cultivated, ungrazed treatments. The grazed perennial ryegrass and ryegrass/white clover treatments were the most effective in ameliorating degraded soil conditions. The rate of soil amelioration declined with depth, and was mainly confined to the top 10 cm of soil. The rate of amelioration was relatively slow, with, for example, 3 years needed for most of the increase in aggregate stability at 0–5 cm depth.

Songbird Community Composition and Nesting Success in Grazed and Ungrazed Pinyon-Juniper Woodlands

Livestock grazing is a dominant land use of pinyon-juniper habitats in the western United States, yet the effects of grazing on breeding bird communities in this habitat have been poorly studied. We compared habitat structure, songbird abundance, and nesting productivity within pinyon-juniper woodlands on an actively grazed site and a site experiencing long-term relief from livestock grazing in northeastem New Mexico. From 1992 to 1995, we performed vegetation sampling, conducted songbird point counts, and located and monitored nests on 8 35-ha study plots. Four of these plots experienced moderate cattle grazing and 4 were ungrazed since 1973. We found no differences in habitat or vegetation features between grazed and ungrazed plots. Bird communities were similar, with only 1 of the 11 species we tested more abundant on the ungrazed treatment (western scrub-jay; Aphelocoma californicus). We detected no differences in nesting success or cause-specific rates of nest failure for 7 common bird species (P < 0.05), and we detected no differences in brown-headed cowbird (Molothrus ater) parasitism rates for the major hosts between grazed and ungrazed areas. Greater than 75% of the nests of the solitary vireo (Vireo solitarius), western tanager (Piranga ludoviciana), and blue-gray gnatcatcher (Polioptila caerulea) were parasitized on both treatments. These high parasitism rates may be the result of high densities of local cowbirds because of abundant feeding sites (i.e., livestock), the high mobility of cowbirds, and the close proximity of ungrazed plots to grazed areas (all <4 km). Our results suggest that 20 years of relief from grazing had little influence on the habitat structure or bird species composition of the pinyon-juniper woodlands on our study site. However, livestock grazing has indirectly affected the nesting success of some songbird species via the influence of grazing on cowbird abundance. Our findings highlight the need for studies that incorporate nest monitoring and landscape-scale approaches to better understand the relation between cowbirds, livestock, and songbirds, and the time required for recovery from grazing effects.

Changes to soil physical properties after grazing exclusion

Abstract. The potential for degraded physical properties of soil to regenerate naturally after exclusion of grazing animals was examined at a long‐term stocking rate trial near Armidale, New South Wales, Australia. Unsaturated hydraulic conductivity was measured before grazing was excluded, and after 7 months and 2.5 years’ grazing exclusion. These data were compared with controls grazed at 10,15 and 20 sheep/ha. After 2.5 years, there were significant increases in unsaturated hydraulic conductivity at 5 and 15 mm tension in the ungrazed treatments compared with the grazed controls. The unsaturated hydraulic conductivities and bulk density of surface soils under pasture which had been ungrazed for 2.5 years were comparable to those where the pasture had been ungrazed for 27 years. We speculate that the natural amelioration of soil physical properties in these soils was due to biological activity and wetting and drying cycles, in the absence of the compactive effect of animal treading.

The influence of soil depth on plant species response to grazing within a semi-arid savanna

Grassland patches within a semi-arid savanna were evaluated over 45-years for (1) local temporal dynamics of basal area for five dominant grass species within long-term heavily grazed and ungrazed treatments, (2) the influence of soil depth (resource availability) on vegetation dynamics, and (3) the applicability of community-level grazing response groups over fine-scale patterns of soil heterogeneity. Temporal patterns in species composition and basal area were dependent upon soil depth. In the heavy grazed treatment, Hilaria belangeri dominated deep soils while Erioneuron pilosum and Bouteloua trifida were restricted to shallow soils. In the ungrazed treatment, removal of grazing resulted in successional changes that were significantly different across soil depths. After 45 years without grazing, Eriochloa sericea was most abundant on deep soils while Bouteloua curtipendula was more abundant on intermediate and shallow soils. Community-level functional groups that are based on grazing were not appropriate when multiple pattern-driving variables were considered across multiple scales indicating that functional groups should only be applied to certain processes at specific scales. Within the ungrazed treatments, variable soil depths have resulted in a shifting mosaic in time and space where early- and late-successional species co-exist continuously but spatially separated within the community. In the heavily grazed treatment, species are somewhat spatially arranged by soil depths, but much of the inherent heterogeneity is eliminated and species composition is dominated by the three grazing-resistant short-grasses. Broad scale successional changes may appear linear and predictable while at finer scales, the same changes may be described as non-linear and dependent upon soil depth resulting in thresholds that are partially explained by weather patterns, seed bank limitations and competitive inhibitions.

Long-term vegetation dynamics mediated by herbivores, weather and fire in aJuniperus-Quercussavanna

Long-term (45-yr) basal area dynamics of dominant graminoid species were analyzed across three grazing inten- sity treatments (heavily grazed, moderately grazed and ungrazed) at the Texas A&M University Agricultural Re- search Station on the Edwards Plateau, Texas. Grazing inten- sity was identified as the primary influence on long-term variations in species composition. Periodic weather events, including a severe drought (1951-1956), had little direct influ- ence on composition dynamics. However, the drought inter- acted with grazing intensity in the heavily grazed treatment to exacerbate directional changes caused by grazing intensity. Species response to grazing was individualistic and noisy. Three response groups were identified. Taller, more produc- tive mid-grasses were most abundant under moderate or no grazing. Short grasses were most abundant under heavy graz- ing. Intermediate species were most abundant under moderate grazing and opportunistic to weather patterns. Graminoid diver- sity increased with the removal or reduction of grazing inten- sity. The moderately and ungrazed treatments appeared most resistant to short-term weather fluctuations, while the heavily grazed treatment demonstrated significant resilience when graz- ing intensity was reduced after over 110 yr of overgrazing. Identification of a 'climax' state is difficult. Significant directional change, which took nearly 20 yr, appears to con- tinue in the ungrazed treatment after 45 yr of succession. The observed, relatively linear patterns of perennial grass compo- sition within the herbaceous patches of this savanna were generally explained by traditional Clementsian succession. However, when dynamics of the herbaceous community are combined with the woody component of this savanna, the frequency and intensity of fire becomes more important. Across the landscape, successional changes follow several pathways. When vegetation change is influenced by several factors, a multi-scale model is necessary to demonstrate interactions and feedbacks and accurately describe successional patterns. Ab- sence of fires, with or without grazing, leads ultimately to a Juniperus/Quercus woodland with grazing intensity primarily influencing the fuel load and hence fire intensity.

Impacts of Rotational Grazing on Mixed Prairie Soils and Vegetation

In this study the impact of a rotation grazing system on the soil and vegetation of a Stipa-Bouteloua-Agropyron community in the mixed prairie ecoregion was compared with the ungrazed treatment in exclosures. At a low stocking rate, grazing had no effect on the vegetation but did alter soil quality. Grazing pressure was so light in the rotational grazing treatment that recovery of productivity, as measured by standing crop and litter, was not significantly different from the ungrazed treatment. Conversely, the species distribution was unchanged but was indicative of a lower seral state for this mixed prairie. The effect of grazing on this community was indirect, possibly by altering the microenvironment. The relationships observed among forage production, soil chemistry, and species composition raise questions on the importance of any one variable expressing range condition on the mixed prairie.