Plant responses to selective grazing by bison: interactions between light,herbivory and water stress

Two abundant tallgrass prairie forb species, Ambrosia psilostachya and Vernonia baldwinii, are commonly found intact in patches where the grasses have been selectively grazed by bison. Microclimatic patterns and physiological responses of these forbs were measured in grazed and ungrazed patches. These experiments demonstrated that bison herbivory indirectly enhanced water availability and productivity of forbs growing in grazed patches. This was due primarily to the reduction in transpiring grass leaf area in grazed patches and an increase in light availability. In grazed patches, incident light at forb mid-canopy height was 53% greater than ungrazed sites at midseason and soil temperatures were always warmer (e.g., 10°C at 5 cm), perhaps enabling forbs to initiate growth earlier in the spring. Enhanced leaf xylem pressure potential and stomatal conductance in plants in grazed areas were most evident when water availability was low (i.e., late in the growing season and over short-term dry periods characteristic of the tallgrass prairie environment). Relative to individuals in ungrazed areas, end-of-season biomass of A. psilostachya was 40% greater and reproductive biomass and head number of V. baldwinii was 45% and 40% greater, respectively, in plants in grazed patches. A favorable growing environment maintained in grazed patches during periods of water limitation enhances carbon gain in forbs leading to increased biomass and potential fitness.     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.     

Grazing history effects on above- and below-ground litter decomposition and nutrient cycling in two co-occurring grasses

Large herbivores may alter carbon and nutrient cycling in soil by changing above- and below-ground litter decomposition dynamics. Grazing effects may reflect changes in plant allocation patterns, and thus litter quality, or the site conditions for decomposition, but the relative roles of these broad mechanisms have rarely been tested. We examined plant and soil mediated effects of grazing history on litter mass loss and nutrient release in two grazing-tolerant grasses, Lolium multiflorum and Paspalum dilatatum, in a humid pampa grassland, Argentina. Shoot and root litters produced in a common garden by conspecific plants collected from grazed and ungrazed sites were incubated under both grazing conditions. We found that grazing history effects on litter decomposition were stronger for shoot than for root material. Root mass loss was neither affected by litter origin nor incubation site, although roots from the grazed origin immobilised more nutrients. Plants from the grazed site produced shoots with higher cell soluble contents and lower lignin:N ratios. Grazing effects mediated by shoot litter origin depended on the species, and were less apparent than incubation site effects. Lolium shoots from the grazed site decomposed and released nutrients faster, whereas Paspalum shoots from the grazed site retained more nutrient than their respective counterparts from the ungrazed site. Such divergent, species-specific dynamics did not translate into consistent differences in soil mineral N beneath decomposing litters. Indeed, shoot mass loss and nutrient release were generally faster in the grazed grassland, where soil N availability was higher. Our results show that grazing influenced nutrient cycling by modifying litter breakdown within species as well as the soil environment for decomposition. They also indicate that grazing effects on decomposition are likely to involve aerial litter pools rather than the more recalcitrant root compartment.     

Sawdust Addition Reduces the Productivity of Nitrogen‐Enriched Mountain Grasslands

Anthropogenic nutrient enrichment of mountain grasslands has boosted grasses and fast‐growing unpalatable plants at the expense of slow‐growing species, resulting in a significant loss in biodiversity. A potential tool to reduce nutrient availability and aboveground productivity without destroying the perennial vegetation is carbon (C) addition. However, little is known about its suitability under severe climatic conditions. Here, we report the results of a 3‐year field study assessing the effects of sawdust addition on soil nutrients, aboveground productivity, and vegetational composition of 10 grazed and ungrazed mountain grasslands. Of particular interest was the effect of C addition on grasses and on the tall unpalatable weed Veratrum album. After 3 years, soil pH, ammonium, and plant‐available phosphorus were not altered by sawdust application, and nitrate concentrations were marginally higher in treatment plots. However, the biomass of grasses and forbs (without V. album) was 20–25% lower in sawdust‐amended plots, whereas the biomass of V. album was marginally higher. Sawdust addition reduced the cover of grasses but did not affect evenness, vegetation diversity, or plant species richness, although species richness generally increased with decreasing biomass at our sites. Our results suggest that sawdust addition is a potent tool to reduce within a relatively short time the aboveground productivity and grass cover in both grazed and ungrazed mountain grasslands as long as they are not dominated by tall unpalatable weeds. The technique has the advantage that it preserves the topsoil and the perennial soil seed bank.     

Differential photosynthetic performance of three Mediterranean shrubs under grazing by domestic goats

The impact of grazing by domestic goats, Capra hircus, on the photochemical apparatus of three co-ocurring Mediterranean shrubs, Erica scoparia, Halimium halimifolium, and Myrtus communis was evaluated. Seasonal course of gas exchange, chlorophyll fluorescence and photosynthetic pigment concentrations were measured in the field in grazed and ungrazed plants. Net photosynthetic rate was higher in grazed plants of E. scoparia and H. halimifolium in May, while there were not significant differences in M. communis. Photosynthetic enhancement in grazed plants of E. scoparia could be explained largely by higher stomatal conductance. On the other hand, the lack of differences in stomatal conductance between grazed and ungrazed plants of H. halimifolium could indicate that carboxylation efficiency, and ribulose-1,5-bisphosphate (RuBP) regeneration may have been enhanced by grazing. Overall grazing has little effect on the photochemical (PSII) apparatus, however grazed plants of M. communis showed chronic photoinhibition in the short term. Finally, seasonal variations recorded on photosynthesis, photochemical efficiency and pigment concentrations may be a physiological consequence of environmental factors, such as summer drought and competition for light, rather than an adaptation to grazing.     

长白山林线主要木本植物叶片养分的季节动态及回收效率

植物叶片养分含量的季节动态和回收效率对植被生态系统的养分循环和植物生长策略具有重要意义。以长白山高山林线上分布的3种主要木本植物——岳桦(Betula ermanii),牛皮杜鹃(Rhododendron aureum)和笃斯越橘(Vaccinium uliginosum)为研究对象,通过测定叶片中N、P、K、Ca、Mg、Fe等6种养分元素含量,分析在林线处植物叶片养分含量的季节动态及其与土壤养分含量的关系。结果表明岳桦和笃斯越橘叶片中养分元素的季节动态基本一致,即:N、P、K含量在生长季内逐渐降低,而Ca的含量逐渐增加;Mg在生长季旺盛期最低,而Fe含量却最高。牛皮杜鹃作为常绿灌木,叶片养分的季节动态与其余两种植物明显不同,表现为在生长季初期6种养分元素含量最低。岳桦和笃斯越橘植物叶片中N、P、K、Fe都有一定程度的回收,但笃斯越橘叶片的养分回收率更高,反映了笃斯越橘更能适应相对贫瘠的环境。3种林线植物叶片中养分含量与土壤养分并不存在显著的相关性,说明长白山林线上土壤中养分的分布没有对林线上3种主要的木本植物的生长和分布产生直接的影响。     

Soil and plant water relations in a crested wheatgrass pasture: response to spring grazing by cattle

Summary Few field studies have attempted to relate effects of actual livestock grazing on soil and plant water status. The present study was initiated to determine the effects of periodic defoliations by cattle during spring on soil moisture and plant water status in a crested wheatgrass (Agropyron cristatum (L.) Gaertn. and A. desertorum (Fisch. ex Link) Schult.) pasture in central Utah. Soil moisture in the top 130 cm of the soil profile was depleted more rapidly in ungrazed plots than in grazed plots during spring and early summer. Soil moisture depletion was more rapid in grazed plots in one paddock after 1 July due to differential regrowth, but there was no difference in soil water depletion between plots in another paddock during the same period. This difference in soil water depletion between paddocks was related to a difference in date of grazing. Although more water had been extracted from the 60 cm to 130 cm depths in ungrazed plots by late September, cumulative soil moisture depletion over the entire 193 cm profile was similar in grazed and ungrazed plots. Prior to 1 July, grazing had no effect on predawn leaf water potentials as estimated by a pressure chamber technique; however, after 1 July, predawn leaf water potentials were lower for ungrazed plants. Midday leaf water potentials were lower for grazed plants before 1 July, but did not differ between grazed and ungrazed plants after 1 July. A 4- to 8-day difference in date of defoliation did not affect either predawn or midday leaf water potentials. The observed differences in water use patterns during spring and early-summer may be important in influencing growth and competitive interactions in crested wheatgrass communities that are subject to grazing by domestic livestock.     

Herbivore phenology can predict response to changes in plant quality by livestock grazing

Livestock grazing can have a strong impact on herbivore abundance, distribution and community. However, not all species of herbivores respond the same way to livestock grazing, and we still have a poor understanding of the underlying mechanisms driving these differential responses. Here, we investigate the effect of light intensity cattle grazing on the abundance of two grasshoppers (Euchorthippus cheui and E. unicolor) that co-occur in the same grasslands and feed on the same food plant (the dominant grass Leymus chinensis). The two grasshopper species differ in phenology so that their peak abundances are separated into early- and late-growing seasons. We used an exclosure experiment to monitor grasshopper abundance and food quality in the field under grazed and ungrazed conditions, and performed feeding trials to examine grasshopper preference for grazed or ungrazed food plants in the laboratory. We found that the nitrogen content of L. chinensis leaves continuously declined in the ungrazed areas, but was significantly enhanced by cattle grazing over the growing season. Cattle grazing facilitated the early-season grasshopper E. cheui, whereas it suppressed the late-season grasshopper E. unicolor. Moreover, feeding trials showed that E. cheui preferred L. chinensis from grazed plots, while E. unicolor preferred the leaves from ungrazed plots. We conclude that livestock grazing has opposite effects on the two grasshopper species, and that these effects may be driven by grazing-induced changes in plant nutrient content and the unique nutritional niches of the grasshoppers. These results suggest that insects that belong to the same guild can have opposite nutrient requirements, related to their distinct phenologies, and that this can ultimately affect their response to cattle grazing. Our results show that phenology may link insect physiological needs to local resource availabilities, and should be given more attention in future work on interactions between large herbivores and insects.     

Effects of lengthened growing season and soil warming on the phenology and physiology of Polygonum bistorta

The phenological and physiological responses of arctic tundra plant species are key to predicting their survival in a warmer climate. One of the consequences of a warmer climate in the Arctic will be a longer growing season. We examined the effects of lengthened growing season and soil warming on the widely distributed forb, Polygonum bistorta L. Three treatments were established near Toolik Lake, Alaska in 1995 and 1996: extended season, extended season with soil warming, and an unmanipulated control. The season was extended by removing the snow load in the spring and keeping the treatments free of snow in the autumn. The spring snow removal extended the snow‐free period over that of controls by 8 d in 1995 and 24 d in 1996. As a result, the number of accumulated soil thaw days and consequently the depth of soil thaw increased on the treatment plots. Polygonum bistorta responded to the treatments by becoming active earlier and senescing earlier, resulting in a growth period of similar duration to that of the controls. Leaf size and leaf number were unaffected by the treatments, as were leaf photosynthetic assimilation rates and nutrient concentrations. The results indicate that internal constraints limit the response of this species to lengthened growing season, suggesting that it is a determinant or periodic species. With climate warming, this periodic growth will put P. bistorta at a competitive disadvantage relative to plants that can respond to lengthened growing season.     

The influence of Acacia tortilis (Forssk.) Subsp. raddiana (Savi) and livestock grazing on grass species composition, yield and soil nutrients in arid environments of South Tunisia

The objectives of the study, conducted during the 2003/2004 growing season in the National Park of Bou Hedma (South Tunisia), were to quantify the effects of the single-woody species Acacia tortilis subsp. raddiana on grass species composition, on total plant cover, on density of perennial species, on dry matter (DM) yield and on soil nutrients at lightly and heavily grazed sites. In each study site, two subhabitats were distinguished, i.e. under tree canopies and open grasslands. In the lightly grazed site, the nutrient status of soil (organic matter, total N, extractable P, K⁺, Ca²⁺, Na⁺, Mg²⁺) under Acacia raddiana canopy, was found to be significantly higher (p<0.05) than under the open grassland. In the same way, total plant cover (p<0.05), density of perennial species (p<0.01) and DM yield (p<0.01) were significantly higher under tree canopies in the lightly grazed site. Heavy grazing proved to exert a strong overriding effect over the positive influences of the woody plants. For most studied parameters, a non-significant difference was recorded between canopied and uncanopied subhabitats. Some palatable species were frequently found under trees. In the heavily grazed site, these species are being replaced by less desirable species. This emphasizes the importance of conservation stocking rates and proper pasture management.     

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.     

Grazers and soil moisture determine the fate of added 15NH4 + in Yellowstone grasslands

The influence of ungulate grazers on nutrient cycling and ecosystem productivity in grasslands has been shown to differ with moisture, nutrient availability, and feedbacks between above- and belowground activities. We examined the movement of nitrogen (N), applied as (¹⁵NH₄)₂SO₄, through both dry and mesic sites in the northern range of Yellowstone National Park to test the hypothesis that plants were more able to acquire added N in grazed relative to ungrazed sites. Previous studies showed enhanced N mineralization in grazed areas, and detritus removal by grazers was predicted to enhance early-season plant growth. Thirteen months after tracer addition, there were no differences in plant ¹⁵N as a function of grazing, but historically ungrazed sites retained more ¹⁵N in accumulated litter than at grazed sites. This result demonstrated the importance of detritus in regulating redistribution of incoming N and the role of grazers in this process. Site moisture status influenced ¹⁵N recovery in all pools—soils, microbial biomass, and plants—and greater plant ¹⁵N acquisition occurred in roots at dry relative to mesic sites. Understanding how grazers influence nutrient cycling at the landscape scale requires further investigation of interactions among soil moisture, plant production, litter accumulation, grazing intensity, and belowground processes.     

Top-down impact through a bottom-up mechanism: the effect of limpet grazing on growth,productivity and carbon allocation of Zostera marina L. (eelgrass)

The unusual appearance of a commensal eelgrass limpet [Tectura depicta (Berry)] from southern California at high density (up to 10 shoot^–1) has coincided with the catastrophic decline of a subtidal Zostera marina L. meadow in Monterey Bay, California. Some commensal limpets graze the chloroplast-rich epidermis of eelgrass leaves, but were not known to affect seagrass growth or productivity. We evaluated the effect on eelgrass productivity of grazing by limpets maintained at natural densities (8±2 shoot^–1) in a natural light mesocosm for 45 days. Growth rates, carbon reserves, root proliferation and net photosynthesis of grazed plants were 50–80% below those of ungrazed plants, but biomass-specific respiration was unaffected. The daily period of irradiance-saturated photosynthesis (H _sat) needed to maintain positive carbon balance in grazed plants approached 13.5 h, compared with 5–6 h for ungrazed plants. The amount of carbon allocated to roots of ungrazed plants was 800% higher than for grazed plants. By grazing the chlorophyll-rich epidermis, T. depicta induced carbon limitation in eelgrass growing in an other-wise light-replete environment. Continued northward movement of T. depicta, may have significant impacts on eelgrass production and population dynamics in the northeast Pacific, even thought this limpet consumes very little plant biomass. This interaction is a dramatic example of top-down control (grazing/predation) of eelgrass productivity and survival operating via a bottom-up mechanism (photosynthesis limitation).     

Spatial and temporal heterogeneity in soil nutrient supply measured usingin situ ion-exchange resin bags

Summary Data are presented which illustrate the range of ion values obtained from soil solutions eluted fromin situ ion exchange resin bags in grazed and ungrazed grassland soils sampled in the summer and early autumn. Overall, higher levels of cations were being supplied in both the grazed and ungrazed plots in the autumn compared with during the summer. Variation in ion levels reflected spatial heterogeneity in ion supply in these soils. This variation was correlated with the distribution and abundance of the dominant plants and soil surface microtopography. The use ofin situ ion exchange resin bags allow an understanding of short-term temporal and spatial heterogeneity in ion supply.     

Performance of two Picea abies (L.) Karst. stands at different stages of decline

Summary The annual replacement of tillers of Agropyron desertorum (Fisch. ex Link) Schult., a grazing-tolerant, Eurasian tussock grass, was examined in the field following cattle grazing. Heavy grazing before internode (culm) elongation seldom affected tiller replacement. Heavy grazing during or after internode elongation, which elevates apical meristems, increased overwinter mortality of fall-produced tillers and reduced the number and heights of these replacement tillers. Unexpectedly, tussocks grazed twice within the spring growing season tended to have lower overwinter tiller mortality, greater tiller replacement, and larger replacement tillers than tussocks grazed only once in late spring. These responses of twice-grazed tussocks, however, were still less than those of ungrazed tussocks or tussocks grazed moderately in early spring. The presence of ungrazed tillers on partially grazed tussoks did not increase the replacement of associated grazed tillers relative to tillers on uniformly grazed plants. This result indicates that resource sharing among tillers, if present, is short-lived or ecologically unimportant in this species. Although A. desertorum is considered grazing-tolerant, tiller replacement on heavily grazed tussocks, particularly those grazed during or after internode elongation when apical meristems were removed, was usually inadequate for tussock maintenance. These observations at the tiller (ramet) level of organization in individual tussocks (genet) may explain the often noted reduction in stand (population) longevity with consistent heavy grazing.     

退化草地恢复过程中土壤氮素状况以及与植被地上绿色生物量形成关系的研究

 研究了在不同放牧率下形成的不同退化阶段的草地各形态氮素（全氮、硝态氮、铵态氮、无机氮和微生物氮）的变化情况,同时也研究了植被地上绿色生物量与各形态氮素季节变化的同步性关系。土壤全氮含量相对稳定,随草地植被状况和植物生长时期变化不大,说明土壤总氮库有相当的弹性。土壤硝态氮（NO－3-N）、铵态氮（NH+4-N）、无机氮（IN）和微生物氮（Micro-N）季节变化明显。土壤Micro-N和NO-3-N含量随植物生长逐渐降低,到植物枯黄期含量又回复到较高的水平;土壤NH+4-N含量随植物生长有逐渐升高的趋势;IN则随着植物的生长出现低-高-低-高的特点,且与植被地上绿色生物量呈显著负相关（R=-0.247, p<0.01）。在放牧条件下草原植物优先利用NO－3-N,NO－3-N与植被地上绿色生物量有显著的负相关性,是形成草原植被地上绿色生物量的有效性氮素。Micro-N能解释土壤IN 22.3%的变异（R2=0.223, p<0.01）,Micro-N是土壤无机氮的重要来源。土壤NH+4-N与Micro-N呈显著负相关（R=-0.222, p<0.01）,说明土壤微生物对土壤NH+4-N有偏好吸收。总体上,不同形态的氮素在各土壤层次间差异显著,随土壤层次的加深含量逐步降低。连续放牧11年恢复两年后,各氮素组分对放牧压力消除的响应并不一致。土壤全氮含量与停止放牧前相比变化差异不显著;而Micro-N对放牧压力消失的响应在不同处理下整个生长季的结果比较一致,即以前过度和中度放牧处理的Micro-N含量较高,无牧和轻牧含量较低;IN、NH+4-N和NO－3-N变化比较复杂,在不同放牧恢复处理上结果并不一致。总的来看,以前中度和过度放牧的IN、NH+4-N和NO－3-N含量较高,存在潜在损失的可能。经过两年的恢复,植被地上绿色生物量（8月）过牧处理与无牧处理差异不显著。     

The effects of grazers on the performance of individuals and populations of scarlet gilia,Ipomopsis aggregata

Summary I. aggregata exhibits considerable powers of regrowth following removal of its primary shoot by herbivores, but we found no evidence of overcompensation (i.e. of significantly higher plant performance where plants were exposed to ungulate herbivory) in a comparison between individuals on grazed and ungrazed sides of exclosure fences, in a comparison between artificially clipped and control plants in one population in the Okanagan National Forest, or in comparisons between grazed and ungrazed plants in 14 natural populations. We tested whether ungulate grazing affects the population size of Ipomopsis aggregata by comparing populations inside and outside deer exclosures at 7 sites in the Western United States. We found consistent, highly significant differences in plant population density on the grazed and ungrazed sides of these exlosure fences. Plant density was a modal 25-fold higher on the protected side of the fence, suggesting that exposure to ungulate grazing increases plant death rates at some stage in the life cycle. Our results show that the presence of ungulate grazers leads to a substantial decrease in plant density despite the fact that grazing on young bolting shoots has very little influence on fruit production. Since this decrease in population density is not correlated with a decrease in the fecundity of individuals, it must instead be due to other direct and indirect effects of ungulate grazers.     

Below‐ground biomass and productivity of a grazed site and a neighbouring ungrazed exclosure in a grassland in central Argentina

Abstract We estimated the below‐ground net plant productivity (BNPP) of different biomass components in an intensively and continuously 45‐ha grazed site and in a neighbouring exclosure ungrazed for 16 years for a natural mountain grassland in central Argentina. We measured approximately twice as much dead below‐ground biomass in the grazed site as in the ungrazed site, with a strong concentration of total below‐ground biomass towards the upper 10 cm of the soil layer in both sites. The main contribution to total live biomass was accounted for by very fine (<0.5 mm) and fine roots (0.5–1.0 mm) both at the grazed (79%) and at the ungrazed (81%) sites. We measured more dead biomass for almost all root components, more live biomass of rhizomes, tap roots and bulbs, and less live biomass of thicker roots (>1 mm) in the grazed site. The seasonal variation of total live below‐ground biomass mainly reflected climate, with the growing season being limited to the warmer and wetter portion of the year, but such variation was higher in the grazed site. Using different methods of estimation of BNPP, we estimated maximum values of 1241 and 723 g m^?2 year^?1 for the grazed and ungrazed sites, respectively. We estimated that very fine root productivity was almost twice as high at the grazed site as at the ungrazed one, despite the fact that both sites had similar total live biomass, and root turnover rate was twofold at the grazed site.     

Utilisation of Wadden Sea salt marshes by geese in relation to livestock grazing

To arctic breeding geese, the salt marshes of the International Wadden Sea are important spring staging areas. Many of these marshes have always been grazed with livestock (mainly cattle and sheep). To evaluate the influence of livestock grazing on composition and structure of salt-marsh communities and its consequences for habitat use by geese, a total of 17 pairs of grazed and ungrazed marshes were visited both in April and May 1999, and the accumulated grazing pressure by geese was estimated using dropping counts. Observed grazing pressure was related to management status and to relevant vegetation parameters.The intensity of livestock grazing influences the vegetation on the marsh. Salt marshes that are not grazed by livestock are characterised by stands with a taller canopy, a lower cover of grasses preferred by geese, and a higher cover of plants that are not preferred.Overall goose-dropping densities are significantly lower in ungrazed marshes compared to marshes grazed by livestock. Some ungrazed marshes had comparatively high goose grazing pressure, and these were all natural marshes on a sandy soil, or artificial mainland marshes with a recent history of intensive livestock grazing. Goose grazing is associated with a short canopy. The plant communities with short canopy, dominated by Agrostis stolonifera, Festuca rubra and Puccinellia maritima, together account for 85% of all goose droppings in our data.The sites that were not visited by geese differed very little from those that were visited, in the parameters we measured. This might indicate that there was no shortage of available habitat for spring staging geese in the Wadden Sea, in the study period.     

Impact of reindeer grazing on ground-dwelling Carabidae and Curculionidae assemblages in Lapland

Reindeer Rangifer tarandus L. grazing shapes forest vegetation, microclimate, and soil respiration in Lapland, especially due to grazing on lichens (Cladina). We studied how these changes and their magnitude affect ground‐dwelling species of beetle families Carabidae (predators) and Curculionidae (herbivores), by using pitfall traps to collect invertebrates from pairs of grazed and ungrazed study plots over a wide range of site types. Changes in abundance, composition, richness and diversity of beetle assemblage were tested in relation to magnitude of the impacts on vegetation. The species compositions of Carabidae and Curculionidae differed between grazed and ungrazed plots in all sites. The relative difference between grazed and ungrazed plots in the number of individuals increased linearly with the impact of reindeer on vegetation cover. Carabid beetles, as a family, were more common in grazed plots in all sites. Curculionid beetles were more common in ungrazed plots in the birch dominated sites. This difference was mainly due to the species that feeds on deciduous leaves. In the pine dominated sites with high Cladina cover and more changes in ground vegetation, the number of curculionids feeding on conifers was higher in grazed plots. Species richness and diversity (H’) of both families were higher in grazed plots. Of the total 27 species, 11 were found only in grazed plots, while not a single species was found only in ungrazed plots. The relative difference between plots in diversity and evennes (H’/H'max) had humped response to the difference in Cladina cover. The diversity values were greater in grazed plots at the intermediate levels of grazing impact, and only in sites with very low or extremely high Cladina cover difference was the diversity higher in ungrazed plots. The response of beetle diversity resembled the hypotheses suggested for the relationship between grazing and vegetation diversity: greatest positive effect at intermediate grazing intensity and negative effects at unproductive sites.