Insensitivity of Soil Microbial Activity to Temporal Variation in Soil N in Subarctic Tundra: Evidence from Responses to Large Migratory Grazers

Large migratory grazers commonly influence soil processes in tundra ecosystems. However, the extent to which grazing effects are limited to intensive grazing periods associated with migration has not previously been investigated. We analyzed seasonal patterns in soil nitrogen (N), microbial respiration and extracellular enzyme activities (EEAs) in a lightly grazed tundra and a heavily grazed tundra that has been subjected to intensive grazing during reindeer (Rangifer tarandus L.) migration for the past 50 years. We hypothesized that due to the fertilizing effect of the reindeer, microbial respiration and EEAs related to microbial C acquisition should be higher in heavily grazed areas compared to lightly grazed areas and that the effects of grazing should be strongest during reindeer migration. Reindeer migration caused a dramatic peak in soil N availability, but in contrast to our predictions, the effect of grazing was more or less constant over the growing season and the seasonal patterns of microbial activities and microbial N were strikingly uniform between the lightly and heavily grazed areas. Microbial respiration and the EEAs of β-glucosidase, acid-phosphatase, and leucine-aminopeptidase were higher, whereas that of N-acetylglucosamidase was lower in the heavily grazed area. Experimental fertilization had no effect on EEAs related to C acquisition at either level of grazing intensity. Our findings suggest that soil microbial activities were independent of grazing-induced temporal variation in soil N availability. Instead, the effect of grazing on soil microbial activities appeared to be mediated by substrate availability for soil microorganisms. Following a shift in the dominant vegetation in response to grazing from dwarf shrubs to graminoids, the effect of grazing on soil processes is no longer sensitive to temporal grazing patterns; rather, grazers exert a consistent positive effect on the soil microbial potential for soil C decomposition.     

Role of litter decomposition for the increased primary production in areas heavily grazed by reindeer: a litterbag experiment

Heavy grazing and trampling by reindeer increase nutrient cycling and primary production in areas where grasslands have replaced shrub and moss tundra. One way in which herbivores can affect nutrient cycling is through changing the litter decomposition processes. We studied the effect of herbivory on litter decomposition rate by reciprocal transplantation of litter between lightly grazed and heavily grazed areas, using the litterbag technique. We used litter from two of the most common species on the lightly grazed side, Betula nana and Empetrum nigrum , and two of the most common species on the heavily grazed side, Carex bigelowii and Deschampsia flexuosa . We found that herbivory improved litter quality by favouring species with easily decomposed litter. However, herbivory also improved litter quality by increasing the nitrogen content and lowering the C/N ratio of each species. Decomposition rates even correlated with the abundance of the plant category in question. Shrub litter decomposed faster in the lightly grazed area where shrubs were common, and graminoid litter decomposed faster in the heavily grazed area where graminoids were common. These results indicate that the decomposer micro-organisms are adapted to the most common litter types. This study shows that detailed information about the effect of herbivory on litter quality is important to understand differences between the short-term and long-term effects of herbivory on nutrient cycling and primary production.     

Effects of summer grazing by reindeer on composition of vegetation, productivity and nitrogen cycling

In this study, we investigated the effect of reindeer grazing on tundra heath vegetation in northern Norway. Fences, erected 30 yr ago, allowed us to compare winter grazed, lightly summer grazed and heavily summer grazed vegetation at four different sites. At two sites, graminoids dominated the heavily grazed zone completely, while ericoid dwarf shrubs had almost disappeared. In the other two areas, the increase of graminoids was almost significant. At one of the sites where graminoids dominated the heavily grazed area, we also measured plant biomass, primary production and nitrogen cycling. In this site, heavy grazing increased primary production and rate of nitrogen cycling, while moderate grazing decreased primary production. These results were inconsistent with the view that the highest productivity is found at intermediate grazing pressure. These results rather support the hypothesis that intensive grazing can promote a transition of moss-rich heath tundra into productive, graminoid-dominated steppe-like tundra vegetation. Moreover the results suggests that intermittent intensive reindeer grazing can enhance productivity of summer ranges.     

Reindeer influence on ecosystem processes in the tundra

Reindeer have been recorded to increase nutrient cycling rate and primary production in studies from fences almost 40 years old that separate areas with different grazing regimes in northern Fennoscandia. To further understand the mechanism behind the effects of herbivores on primary production, we measured the size of the major C and N pools, soil temperature, litter decomposition rate and N mineralization rate in lightly, moderately and heavily grazed areas along two of these fences.
Plant N found in new biomass, indicative of plant N assimilation, was significantly higher in moderately and heavily grazed areas than in lightly grazed areas, which corresponded to a decreased amount of N in old plant parts. The amount of N found in plant litter or organic soil layer did not differ between the grazing treatments. Together with soil N concentrations and litter decomposition rates, soil temperatures were significantly higher in moderately and heavily grazed areas.
We conclude that the changes in soil temperature are important for the litter decomposition rate and thus on the nutrient availability for plant uptake. However, the changes in plant community composition appear to be more important for the altered N pools and thus the enhanced primary production. The results provide some support for the keystone herbivore hypothesis, which states that intensive grazing can promote a transition from moss-rich tundra heath to productive grasslands. Grazing altered N fluxes and pools, but the total N pools were similar in all grazing treatments. Our study thus indicates that grazing can increase the primary production through enhancing the soil nutrient cycling rate, even in a long term perspective on an ecological timescale.     

Linkages between grazing history and herbivore exclusion on decomposition rates in mineral soils of subalpine grasslands

Background & aims

Herbivore-driven changes to soil properties can influence the decomposition rate of organic material and therefore soil carbon cycling within grassland ecosystems. We investigated how aboveground foraging mammalian and invertebrate herbivores affect mineral soil decomposition rates and associated soil properties in two subalpine vegetation types (short-grass and tall-grass) with different grazing histories.

Methods

Using exclosures with differing mesh sizes, we progressively excluded large, medium and small mammals and invertebrates from the two vegetation types in the Swiss National Park (SNP). Mineral soil decomposition rates were assessed using the cotton cloth (standard substrate) method between May and September 2010.

Results

Decomposition displayed strong spatio-temporal variability, best explained by soil temperature. Exclusion of large mammals increased decomposition rates, but further exclusion reduced decomposition rates again in the lightly grazed (tall-grass) vegetation. No difference among treatments was found in the heavily grazed (short-grass) vegetation. Heavily grazed areas had higher decomposition rates than the lightly grazed areas because of higher soil temperatures. Microbial biomass carbon and soil C:N ratio were also linked to spatio-temporal decomposition patterns, but not to grazing history.

Conclusions

Despite altering some of the environmental controls of decomposition, cellulose decomposition rates in the SNP’s subalpine grasslands appear to be mostly resistant to short-term herbivore exclusion.     

Defoliation of a grass is mediated by the positive effect of dung deposition,moss removal and enhanced soil nutrient contents: results from a reindeer grazing simulation experiment

Herbivory is one of the key drivers shaping plant community dynamics. Herbivores can strongly influence plant productivity directly through defoliation and the return of nutrients in the form of dung and urine, but also indirectly by reducing the abundance of neighbouring plants and inducing changes in soil processes. However, the relative importance of these processes is poorly understood. We, therefore, established a common garden experiment to study plant responses to defoliation, dung addition, moss cover, and the soil legacy of reindeer grazing. We used an arctic tundra grazed by reindeer as our study system, and Festuca ovina, a common grazing‐tolerant grass species as the model species. The soil legacy of reindeer grazing had the strongest effect on plants, and resulted in higher growth in soils originating from previously heavily‐grazed sites. Defoliation also had a strong effect and reduced shoot and root growth and nutrient uptake. Plants did not fully compensate for the tissue lost due to defoliation, even when nutrient availability was high. In contrast, defoliation enhanced plant nitrogen concentrations. Dung addition increased plant production, nitrogen concentrations and nutrient uptake, although the effect was fairly small. Mosses also had a positive effect on aboveground plant production as long as the plants were not defoliated. The presence of a thick moss layer reduced plant growth following defoliation. This study demonstrates that grasses, even though they suffer from defoliation, can tolerate high densities of herbivores when all aspects of herbivores on ecosystems are taken into account. Our results further show that the positive effect of herbivores on plant growth via changes in soil properties is essential for plants to cope with a high grazing pressure. The strong effect of the soil legacy of reindeer grazing reveals that herbivores can have long‐lasting effects on plant productivity and ecosystem functioning after grazing has ceased.     

Effects of reindeer on boreal forest floor vegetation: Does grazing cause vegetation state transitions?

Intensive reindeer grazing has been hypothesized to drive vegetation shifts in the arctic tundra from a low-productive lichen dominated state to a more productive moss dominated state. Although the more productive state can potentially host more herbivores, it may still be less suitable as winter grazing grounds for reindeer, if lichens, the most preferred winter forage, are less abundant. Therefore, such a shift towards mosses may have severe consequences for reindeer husbandry if ground-growing lichens have difficulties to recover. We tested if reindeer cause this type of vegetation state shifts in boreal forest floor vegetation, by comparing plant species composition and major soil processes inside and outside of more than 40-year-old exclosures. Lichen biomass was more than twice as high inside exclosures than in grazed controls and almost 5 times higher than in heavily grazed patches. Contrary to our predictions, net N mineralization and plant production were higher in the exclosures than in the grazed controls. The lack of response of phytometer plants in a common garden bioassay indicated that changed soil moisture may drive effects of reindeer on plant productivity in these dry Pine forest ecosystems.     

Plant responses to fertilization and exclusion of grazers on an arctic tundra heath

This study investigated the impacts of fertilization and grazing by Norwegian lemmings (Lemmus lemmus), grey‐sided voles (Clethrionomys rufocanus), and reindeer (Rangifer tarandus) on a diverse tundra plant community dominated by deciduous shrubs. Four out of eight study areas, having a size of 2500 m² each, were fertilized with a N‐P‐K fertilizer and four areas served as unfertilized controls. Two types of exclosures were used within each study area, one to exclude solely reindeer, and one to exclude both rodents and reindeer. Open, grazed plots served as controls. During 5 years following the fertilization event the changes in vegetation inside and outside the exclosures were monitored using a point frequency method. The densities of rodents on the fertilized and unfertilized areas were investigated by live trapping and by counting nests of overwintering individuals. Reindeer do not graze on the study area during the growing season but migrate through this area in autumn and spring. Fertilization increased the abundance of vascular plants while grazing by reindeer and rodents decreased the abundance of vascular plants significantly on both fertilized and unfertilized areas. Rodents preferred clearly the fertilized areas during winter, decreasing the abundance of Vaccinium myrtillus and Vaccinium vitis‐idaea, while very little grazing occurred during summer. Graminoids showed the strongest positive response to fertilization and dominated the plant community on ungrazed plots, while winter grazing by both reindeer and rodents significantly decreased the abundance of graminoids. Deciduous shrubs (Betula nana, Vaccinium myrtillus) increased slightly but significantly due to fertilization and evergreen dwarf shrubs showed no response to fertilization. However, the use of functional growth forms for predicting the responses of nutrient enrichment and grazing must be questioned, as responses to fertilization as well as preferences by herbivores were shown to be species‐specific rather than uniform within functional groups based on plant growth forms.     

Reindeer grazing and soil microbial processes in two suboceanic and two subcontinental tundra heaths

In oceanic, nutrient-rich Fennoscandian arctic-alpine tundra heaths, grazing by reindeer has been found to increase herbs and graminoids in relation to dwarf shrubs. In continental lichen heaths in the inland with nutrient-poor conditions, however, slowly decomposable dwarf shrubs are favoured by grazing. According to a hypothesis, by favouring easily decomposing plants in nutrient-rich conditions and slowly decomposing plants in nutrient-poor conditions, herbivory enhances soil nutrient cycling in nutrient-rich and retards it in nutrient-poor areas. We tested this hypothesis by comparing the impact of reindeer grazing on soil C and N mineralization between two oceanic and two continental arctic-alpine tundra heaths.
Although soil respiration and microbial metabolic activity were enhanced by grazing in the suboceanic but not in the subcontinental tundra heaths, gross N mineralization rates were higher in the grazed areas in soils from all study sites, indicating that reindeer grazing leads to increased rates of nutrient cycling in both nutrient-poor and nutrient-rich tundra heaths. Thus, in the subcontinental tundra heaths, the increase in soil N concentrations due to mammalian waste products enhances N mineralization rates, even though the organic C quality is not improved by reindeer grazing. There was some site-specific variation in the strength of the reindeer effects on various microbial processes and soil properties, which can be related to spatial variation in grazing intensity and timing, as these factors in turn affect the nutrient sink strength of the vegetation.     

Herbivores inhibit climate-driven shrub expansion on the tundra

Recent Pan-Arctic shrub expansion has been interpreted as a response to a warmer climate. However, herbivores can also influence the abundance of shrubs in arctic ecosystems. We addressed these alternative explanations by following the changes in plant community composition during the last 10 years in permanent plots inside and outside exclosures with different mesh sizes that exclude either only reindeer or all mammalian herbivores including voles and lemmings. The exclosures were replicated at three forest and tundra sites at four different locations along a climatic gradient (oceanic to continental) in northern Fennoscandia. Since the last 10 years have been exceptionally warm, we could study how warming has influenced the vegetation in different grazing treatments. Our results show that the abundance of the dominant shrub, Betula nana , has increased during the last decade, but that the increase was more pronounced when herbivores were excluded. Reindeer have the largest effect on shrubs in tundra, while voles and lemmings have a larger effect in the forest. The positive relationship between annual mean temperature and shrub growth in the absence of herbivores and the lack of relationships in grazed controls is another indication that shrub abundance is controlled by an interaction between herbivores and climate. In addition to their effects on taller shrubs (>0.3 m), reindeer reduced the abundance of lichens, whereas microtine rodents reduced the abundance of dwarf shrubs (<0.3 m) and mosses. In contrast to short-term responses, competitive interactions between dwarf shrubs and lichens were evident in the long term. These results show that herbivores have to be considered in order to understand how a changing climate will influence tundra ecosystems.     

Multiple Feedbacks Contribute to a Centennial Legacy of Reindeer on Tundra Vegetation

Historical contingency is the impact of past events, like the timing and order of species arrival, on community assembly, and can sometimes result in alternative stable states of ecological communities. Large herbivores, wild and domestic, can cause profound changes in the structure and functioning of plant communities and therefore probably influence historical contingency; however, little empirical data on the stability of such shifts or subsequent drivers of stability are available. We studied the centennial legacy of reindeer (Rangifer tarandus) pressure on arctic tundra vegetation by considering historical milking grounds (HMGs): graminoid- and forb-dominated patches amid shrub-dominated tundra, formed by historical Sami reindeer herding practices that ended approximately 100 years ago. Our results show that the core areas of all studied HMGs remained strikingly stable, being hardly invaded by surrounding shrubs. Soil nitrogen concentrations were comparable to heavily grazed areas. However, the HMGs are slowly being reinvaded by vegetative growth of shrubs at the edges, and the rate of ingrowth increased with higher mineral N availability. Furthermore, our data indicate that several biotic feedbacks contribute to the stability of the HMGs: increased nutrient turnover supporting herbaceous vegetation, strong interspecific competition preventing invasion and herbivore damage to invading shrubs. In particular, voles and lemmings appear to be important, selectively damaging shrubs in the HMGs. We concluded that HMGs provide clear evidence for historical contingency of herbivore effects in arctic ecosystems. We showed that several biotic feedbacks can contribute to subsequent vegetation stability, but their relative importance will vary in time and space.     

Micro-patterns in grassland vegetation created and sustained by sheep-grazing

An initially uniform Holcus lanatus-dominated sward came partly under hay-making and partly under sheep-grazing. Preferential grazing by sheep resulted in grazing at different intensities giving rise to a macro-pattern of various plant communities. Besides this macro-pattern a micro-pattern developed in the grazed area, which was absent under hay-making. In the micro-pattern short, heavily grazed areas alternated with taller, lightly grazed patches, both having the same species composition. The heavily grazed area was characterized by equal amounts of monocots and dicots. The lightly grazed patches were dominated by Agrostis tenuis, and had a large amount of litter which probably causes the absence of mosses. The protein percentage of green material is higher in the heavily grazed areas than in the lightly grazed patches.Sequential charting indicated that the micro-pattern was more or less stable. An interaction between the vegetation micro-pattern and grazing patterns is suggested. Heavy grazing results in forage with a high protein content and hence attracts animals. Light grazing results in forage with a relatively low protein content, animals avoid the area and litter accumulates.Nomenclature follows Heukels & van Ooststroom (1977) Flora van Nederland.Mrs J. O'Brien corrected the English text     

Plant-herbivore interactions in a North American mixed-grass prairie

Summary Seasonal dynamics of soil nematodes and root biomass were examined from under western wheatgrass (Agropyron smithii) and little bluestem (Andropogon scoparius) from a heavily grazed prairie dog (Cynomys ludovicianus) colony occupied for 5 to 10 years and an adjacent lightly grazed, uncolonized area in Wind Cave National Park, South Dakota, USA. Nematodes were differentiated into classes of plant-parasitic Tylenchida and Dorylaimida and nonparasitic Dorylamida and Rhabditida. Root-feeding nematodes were generally more numerous from A. smithii than from A. scoparius, while nonparasitic populations were not different in soil from beneath the two plant species. Rhabditida, parasitic Dorylaimida and Tylenchida (from A. scoparius only) were more numerous on the prairie dog colony than from the uncolonized site, but nonparasitic Dorylaimida populations did not differ between the two areas. Mean total (live plus dead) root biomass beneath A. scoparius and A. smithii on the prairie dog colony averaged 71% and 81%, respectively, of values from the uncolonized area. Estimated consumption by root-feeding nematodes averaged 12.6% and 5.8% of annual net root production in the upper 10 cm from the prairie dog colony and uncolonized site, respectively. We conclude that, because of microhabitat modification or reductions in plant resistance to nematodes, heavy grazing by aboveground herbivores apparently facilitates grazing by belowground herbivores. Because heavily grazed plants have less roots than lightly grazed or ungrazed plants, the impact of root-feeding nematodes on primary producers is likely to be greatest in heavily grazed grasslands.     

Native herbivore exerts contrasting effects on fire regime and vegetation structure

Although native herbivores can alter fire regimes by consuming herbaceous vegetation that serves as fine fuel and, less commonly, accumulating fuel as nest material and other structures, simultaneous considerations of contrasting effects of herbivores on fire have scarcely been addressed. We proposed that a colonial rodent, vizcacha (Lagostomus maximus), reduces and increases fire intensity at different stages in its population cycle in the semiarid scrub of Argentina. Specifically, we hypothesized that, when colonies are active, vizcachas create natural fire-breaks through intense grazing, generating over time patches of large unburned shrubs in grazed zones. In contrast, when colonies are abandoned, recovery of fine fuels and previous accumulation of coarse wood on colonies during territorial displays increases fire intensity, creating patches of high shrub mortality. To test these hypotheses, we estimated stem age of the dominant shrub (Larrea divaricata) and measured aboveground biomass in zones actively grazed by vizcachas and in ungrazed zones, and compared densities of live and dead shrubs on abandoned colonies and adjacent zones following fire. In active colonies, age and biomass of shrubs were much greater in grazed than ungrazed zones. In abandoned colonies that had been burnt, density of dead, burned shrubs was higher and density of live shrubs was lower than in adjacent zones. These results support our hypotheses and reveal a new interaction between native herbivores and fire, in which herbivores augment fire intensity by gathering fuel. Our findings indicate that, through opposing effects on fire, native herbivores enhance the heterogeneity of vegetation in woody-dominated ecosystems.     

The role of leaf nitrogen content in determining turtlegrass (Thalassia testudinum) grazing by a generalized herbivore in the northeastern Gulf of Mexico

In shallow marine environments the variability in grazing on seagrasses has been hypothesized to be controlled, in part, by the nutritive quality (i.e., nitrogen content) of their leaves. The few existing studies of the relationship between leaf nitrogen content and seagrass grazing have all found a positive relationship between leaf nitrogen content and preference by selective vertebrate grazers (i.e., the bucktooth parrotfish, green sea turtles, and dugongs). However, most marine herbivores (both vertebrate and invertebrate) are thought to be extreme generalists with broad diets of variable nutritive quality (e.g., detritus, living plants, and animals), suggesting the currently held view on the role leaf nutrient content in explaining the variability of seagrass grazing is an oversimplification.In this study, we evaluated how leaf nitrogen content influenced grazing on turtlegrass by a generalist invertebrate herbivore (the pink sea urchin Lytechinus variegatus) in the northeastern Gulf of Mexico. Using a short-term laboratory test and a longer-term field experiment, we tested the hypothesis that leaf nitrogen content controls sea-urchin grazing on seagrass leaves. We hypothesized that if poor nutritive value of seagrasses is responsible for reduced rates of feeding, then increasing leaf nitrogen concentrations should lead to increased rates of seagrass consumption by sea urchins.In the field experiment, we significantly enriched seagrass leaf nitrogen concentrations (some 10-20% depending on month) in experimental plots with a commercial fertilizer and we manipulated grazing intensity by enclosing adult sea urchins at densities that bracketed the range of average densities observed in the region (i.e., 0, 10 and 20 individuals/m(2)). Comparisons of changes in aboveground seagrass production and biomass showed no evidence that sea urchins grazed significantly more in treatments where leaf nitrogen was enriched. Because the statistical power of our test to detect such differences was low and aboveground seagrass production varied significantly among treatments, we also used a mass balance equation to estimate sea urchin consumption of nitrogen-enriched and unenriched leaves. This showed that sea urchins compensated for low nitrogen levels in our unenriched treatments by eating more leaves than in treatments where leaf nitrogen was elevated. Using a laboratory test, we also found that sea urchins ate less nitrogen-enriched seagrass than unenriched seagrass. In combination, these results show that, in contrast to findings reported for vertebrate herbivores, sea urchins feed at higher rates when offered seagrass leaves of lower leaf nitrogen content, and that low levels of leaf nitrogen are not always an effective defense against herbivores.     

Long- and short-term effects of reindeer grazing on tundra wetland vegetation

We studied long-term (50 years) and short-term (4 years) effects of summer grazing of reindeer on subarctic tundra wetland vegetation. The long-term effects of summer grazing were studied by comparing vegetation on Finnish and Norwegian sides of the fence line separating reindeer grazing regimes. The Finnish side was intensively grazed and trampled throughout the year, whereas the Norwegian side was grazed in winter. Experimental fences were erected to examine short-term effects of grazing exclusion. Both in the long- and short-term, summer grazing decreased the height of Salix lapponum whereas the short-term effects on willow cover were less clear than the long-term effects. In contrast, Carex spp. benefited from grazing. Long-term grazing had little effect on total bryophyte cover. Grazing had negligible effects on the nutrient content of leaves of S. lapponum and Eriophorum angustifolium. We conclude that tundra wetlands can withstand moderately high grazing pressure sustained over several decades.     

A hierarchical model for analysing the stability of vegetation patterns created by grazing in temperate pastures

Questions: Does vegetation structure display any stability over the grazing season and in two successive years, and is there any correlation between the stability of these spatial patterns and local sward composition? Location: An upland grassland in the French Massif Central. Method: The mosaic of short and tall vegetation stands considered as grazed and ungrazed patches respectively is modeled as the realization of a Boolean process. This method does not require any arbitrarily set sward‐height thresholds to discriminate between grazed and ungrazed areas, or the use of additional variables such as defoliation indexes. The model was validated by comparing empirical and simulated sward‐height distributions and semi‐variograms. Results: The model discriminated between grazed and ungrazed patches at both a fine (1 m²) and a larger (500 m²) scale. Selective grazing on legumes and forbs and avoidance of reproductive grass could partly explain the stability of fine‐scale grazing patterns in lightly grazed plots. In these plots, the model revealed an inter‐annual stability of large‐scale grazing patterns at the time peak biomass occurred. At the end of the grazing season, lightly grazed plots showed fluctuating patch boundaries while heavily grazed plots showed a certain degree of patch stability. Conclusion: The model presented here reveals that selective grazing at the bite scale could lead to the creation of relatively stable patches within the pasture. Locally maintaining short cover heights would result in divergent within‐plot vegetation dynamics, and thus favor the functional diversity of vegetation.     

Non-parallel changes in soil microbial carbon and nitrogen dynamics due to reindeer grazing in northern boreal forests

Reindeer grazing has a considerable influence on mineralization processes in northern Fennoscandian boreal forests, but the mechanisms underlying the observed differences between grazed and ungrazed areas are not well understood. We studied the below-ground impacts of reindeer grazing by comparing the carbon and nitrogen mineralization rates inside and outside long-term fenced reindeer exclosure areas in five oligotrophic, lichen-dominated and five mesotrophic, dwarf-shrub dominated forests. The soil C mineralization rates and microbial metabolic activity (qCO₂) were significantly lower in the grazed than the ungrazed areas in both oligotrophic and mesotrophic forests. The reductions occurred irrespective of the impact on soil moisture. We conclude that reindeer grazing causes a reduction in the supply of labile C substrates to microbes, resulting in reduced organic matter decomposition rates through changes in the activity of the microbial biomass. Simultaneously, grazing had no consistent effect on the microbial N dynamics, but the impact ranged from no change to increased or decreased in N mineralization rates at the different study sites. The impact of grazing on the N mineralization potential thus seems to be site-specific and uncoupled from the impact of grazing on soil C mineralization. Reciprocal transplant incubations showed no interactions between N mineralization rates and the reindeer-mediated impact on the soil microclimate. We suggest that plant root damage due to trampling by reindeer may be an important mechanism for the deceleration of soil C cycling. In some cases, however, the impact of grazing on the soil active N pool may be strong enough to outweigh the reduction in soil organic matter decomposition, and by these means uncouple soil N dynamics from soil C quality.     

Herbivory and ramet performance in the clonal herb Trientalis europaea L.

1 The intensity of leaf damage caused by invertebrate herbivores and grazing by vertebrates and their effects on clonal growth, survival and reproduction were examined in a population of the forest herb Trientalis europaea during 4 years.
2 Levels of herbivory were low and varied between years in the studied population. Larger ramets were exposed to a greater risk of both grazing and leaf defoliation. Ramets that suffered leaf damage in 1 year experienced an increased probability of leaf damage in succeeding years. The probability of suffering herbivore attack was independent among the ramets belonging to the same clonal fragment, suggesting that clonal propagation might operate as a risk-spreading strategy in this species.
3 Leaf damage did not affect any measure of plant performance, probably due to the low amounts of leaf area removed by invertebrate herbivores. In contrast, vertebrate grazing affected all phases of the pseudo-annual life cycle of T. europaea . Grazing prevented flowering and fruiting, increased ramet mortality during summer and decreased tuber production. Furthermore, grazed ramets produced shorter stolons and smaller tubers, which in turn had a lower winter survival and produced smaller ramets in the following growing season. The large impact of grazing was due to the consumption of the whole of the single shoot of ramets of T. europaea . Although regrowth was possible, secondary shoots were significantly smaller and assimilation was delayed.
4 Tubers originating from grazed ramets were placed shallower than tubers from ungrazed ramets. This could be due to an alteration in the growth pattern of stolons of the grazed ramets.     

Influence of Grazing on Soil Seed Banks Determines the Restoration Potential of Aboveground Vegetation in a Semi‐arid Savanna of Ethiopia

Species composition, number of emerging seedlings, species diversity and functional group of the soil seed banks, and the influence of grazing on the similarity between the soil seed banks and aboveground vegetation, were studied in 2008 and 2009 in a semi‐arid savanna of Ethiopia. We tested whether the availability of persistent seeds in the soil could drive the transition from a degraded system under heavy grazing to healthy vegetation with ample perennial grasses. A total of 77 species emerged from the soil seed bank samples: 21 annual grasses, 12 perennial grasses, 4 herbaceous legumes, 39 forbs, and 1 woody species. Perennial grass species dominated the lightly grazed sites, whereas the heavily grazed sites were dominated by annual forbs. Heavy grazing reduced the number of seeds that can germinate in the seed bank. Species richness in the seed bank was, however, not affected by grazing. With increasing soil depth, the seed density and its species richness declined. There was a higher similarity in species composition between the soil seed bank and aboveground vegetation at the lightly grazed sites compared with the heavily grazed sites. The mean similarity between the seed banks and aboveground vegetation was relatively low, indicating the effect of heavy grazing. Moreover, seeds of perennial grasses were less abundant in the soil seed banks under heavy grazing. We concluded that restoration of grass and woody species from the soil seed banks in the heavily grazed areas could not be successful in semi‐arid savannas of Ethiopia.