Abstract1. Mammalian herbivores can strongly influence nitrogen (N) cycling and herbivore urine could be a central component of the N cycle in grazed ecosystems. Despite its potential role for ecosystem productivity and functioning, the fate of N derived from urine has rarely been investigated in grazed ecosystems.2. This study explored the fate of 15 N-enriched urea in tundra sites that have been either lightly or intensively grazed by reindeer for more than 50 years. We followed the fate of the 15 N applied to the plant canopy, at 2 weeks and 1 year after tracer addition, in the different ecosystem N pools. 3.15 N-urea was rapidly incorporated in cryptogams and in above-ground parts of vascular plants, while the soil microbial pool and plant roots sequestered only a marginal proportion. Furthermore, the litter layer constituted a large sink for the 15 N-urea, at least in the short term, indicating a high biological activity in the litter layer and high immobilization in the first phases of organic matter decomposition.4. Mosses and lichens still constituted the largest sink for the 15 N-urea 1 year after tracer addition at both levels of grazing intensity demonstrating their large ability to capture and retain N from urine. Despite large fundamental differences in their traits, deciduous and evergreen shrubs were just as efficient as graminoids in taking up the 15 N-urea. The total recovery of 15 N-urea was lower in the intensively grazed sites, suggesting that reindeer reduce ecosystem N retention. Synthesis. The rapid incorporation of the applied | INTRODUCTIONHerbivores play a key role in terrestrial ecosystem by modifying plant and microbial community composition, nutrient cycling and productivity (Bardgett & Wardle, 2003;Frank & Groffman, 1998;McNaughton, 1979). Herbivores influence ecosystem structure and functioning via three main mechanisms: plant defoliation, trampling, and nutrient return in the form of dung and urine (Mikola et al., 2009;Olofsson, 2009). By providing highly decomposable resources rich in labile nutrients, and by stimulating soil microbial activities and thus promoting carbon (C) and nitrogen (N) mineralization, the input of dung and urine can enhance soil nutrient availability and promote plant nutrient acquisition and growth (Bardgett, Keiller, Cook, & Gilburn, 1998;Frank, Inouye, Huntly, Minshall, & Anderson, 1994;Hobbs, 1996;Mikola et al., 2009;Olofsson, 2009;Ruess & McNaughton, 1987;Stark & Kytöviita, 2006). Via enhanced plant productivity, the deposition of dung and urine may at certain conditions create a positive feedback that further enhances the intensity of herbivory (McNaughton, 1979).Although the relative importance of urine and dung may vary depending on forage quality and herbivore type, it has been estimated that half of the N will be often excreted as urine and the other half as dung (Hobbs, 1996;Mosbacher, Kristensen, Michelsen, Stelvig, & Schmidt, 2016). However, urine and dung differ greatly in their chemical composition (Floate, 1970;Hobbs, 1996). While dung is ...
Herbivores play a key role in shaping ecosystem structure and functions by influencing plant and microbial community composition and nutrient cycling. This study investigated the long‐term effects of herbivores on plant resource acquisition. We explored differences in the natural δ15N signatures in plant, microbial and soil N pools, and examined mycorrhizal colonization in two tundra sites that have been either lightly or heavily grazed by reindeer for more than 50 years. The study examined changes in nutrient acquisition in five common tundra plants with contrasting traits and mycorrhiza status; the mycorrhizal dwarf shrubs, Betula nana, Vaccinium myrtillus and Empetrum hermaphroditum; a mycorrhizal grass, Deschampsia flexuosa, and a non‐mycorrhizal sedge, Carex bigelowii. There were large variations in δ15N among coexisting plant species in the lightly grazed sites. This variation was dramatically reduced in the heavily grazed sites. At an individual species level, δ15N was higher in E. hermaphroditum and lower in C. bigelowii in the heavily grazed sites. Mycorrhizal colonization in B. nana and E. hermaphroditum roots were also lower in the heavily grazed sites. The δ15N signatures of the total soil N pool and of the microbial N pools were higher in the heavily grazed sites. Since the strong δ15N differentiation among plant species has been interpreted as a result of plants with different mycorrhizal types using different sources of soil nitrogen, we suggest that the lower variation in δ15N in heavily grazed sites indicates a lower niche differentiation in nitrogen uptake among plants. Reduced mycorrhiza‐mediated nitrogen uptake by some of the species, a shift towards a more mineral nutrition due to higher nutrient turnover, and uptake of labile nitrogen from dung and urine in the heavily grazed sites could all contribute to the changes in plant δ15N. We conclude that herbivores have the potential to influence plant nutrient uptake and provide the first data suggesting that herbivores decrease nutrient partitioning on the basis of chemical N forms among plant species. Reduced niche complementarity among species is potentially important for estimates of the effects of herbivory on plant nutrient availability and species coexistence. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12917/suppinfo is available for this article.
Question:Herbivores exert strong influences on vegetation through activities such as trampling, defoliation, and fertilization. The combined effect of these activities on plant performance may cause dramatic vegetation shifts. Because herbivore pressures and the relative importance of their different activities are not equally distributed across the landscape, it is important to understand their isolated effect. One example of an herbivore-induced vegetation shift is the reindeer-driven transition from a subarctic tundra vegetation dominated by dwarf shrubs into a more productive, graminoid-dominated state. Here, we asked how each of the grazing activities by reindeer separately and combined shape vegetation composition.Location: Nordreisa, Norway. Methods:We used a field experiment over six summers to study the separate and interacting effects of reindeer trampling, defoliation, addition of faeces and removal of moss on tundra heath vegetation, and to identify which of these factors were most important in driving the plant community towards a graminoid-dominated state. Results:The combination of all treatments resulted in the strongest changes in vegetation, but trampling was the single most important factor altering the vegetation composition by reducing the abundance of both evergreen and deciduous dwarf shrubs. In contrast to what was expected, none of our treatments, separate or combined, resulted in an increased abundance of graminoids in 5 years, although such rapid vegetation changes have been observed in the field in similar environmental conditions. Conclusions:Trampling is the key process by which reindeer influence the abundance of functional groups, but only many processes combined result in strong changes in community composition. Moreover, additional factors not included in this experiment, such as urine, may be important in causing a state shift to a graminoid-dominated community.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.