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Climate warming is severely affecting high-latitude regions. In the Arctic tundra, it may lead to enhanced soil nutrient availability and interact with simultaneous changes in grazing pressure. It is presently unknown how these concurrently occurring global change drivers affect the root-associated fungal communities, particularly mycorrhizal fungi, and whether changes coincide with shifts in plant mycorrhizal types. We investigated changes in root-associated fungal communities and mycorrhizal types of the plant community in a 10-yr factorial experiment with warming, fertilisation and grazing exclusion in a Finnish tundra grassland. The strongest determinant of the root-associated fungal community was fertilisation, which consistently increased potential plant pathogen abundance and had contrasting effects on the different mycorrhizal fungal types, contingent on other treatments. Plant mycorrhizal types went through pronounced shifts, with warming favouring ecto- and ericoid mycorrhiza but not under fertilisation and grazing exclusion. Combination of all treatments resulted in dominance by arbuscular mycorrhizal plants. However, shifts in plant mycorrhizal types vs fungi were mostly but not always aligned in their magnitude and direction. Our results show that our ability to predict shifts in symbiotic and antagonistic fungal communities depend on simultaneous consideration of multiple global change factors that jointly alter plant and fungal communities., (© 2024 The Authors. New Phytologist © 2024 New Phytologist Foundation.)

Climate change is leading to species redistributions. In the tundra biome, shrubs are generally expanding, but not all tundra shrub species will benefit from warming. Winner and loser species, and the characteristics that may determine success or failure, have not yet been fully identified. Here, we investigate whether past abundance changes, current range sizes and projected range shifts derived from species distribution models are related to plant trait values and intraspecific trait variation. We combined 17,921 trait records with observed past and modelled future distributions from 62 tundra shrub species across three continents. We found that species with greater variation in seed mass and specific leaf area had larger projected range shifts, and projected winner species had greater seed mass values. However, trait values and variation were not consistently related to current and projected ranges, nor to past abundance change. Overall, our findings indicate that abundance change and range shifts will not lead to directional modifications in shrub trait composition, since winner and loser species share relatively similar trait spaces., (© 2023. The Author(s).)

Variation in intraspecific traits is one important mechanism that can allow plant species to respond to global changes. Understanding plant trait responses to environmental changes such as grazing patterns, nutrient enrichment and climate warming is, thus, essential for predicting the composition of future plant communities. We measured traits of eight common tundra species in a fully factorial field experiment with mammalian herbivore exclusion, fertilization, and passive warming, and assessed how trait responsiveness to the treatments was associated with abundance changes in those treatments. Herbivory exhibited the strongest impact on traits. Exclusion of herbivores increased vegetative plant height by 50% and specific leaf area (SLA) by 19%, and decreased foliar C:N by 11%; fertilization and warming also increased height and SLA but to a smaller extent. Herbivory also modulated intraspecific height, SLA and foliar C:N responses to fertilization and warming, and these interactions were species-specific. Furthermore, herbivory affected how trait change translated into relative abundance change: increased height under warming and fertilization was more positively related to abundance change inside fences than in grazed plots. Our findings highlight the key role of mammalian herbivory when assessing intraspecific trait change in tundra and its consequences for plant performance under global changes., (© 2020 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

The tundra is warming more rapidly than any other biome on Earth, and the potential ramifications are far-reaching because of global feedback effects between vegetation and climate. A better understanding of how environmental factors shape plant structure and function is crucial for predicting the consequences of environmental change for ecosystem functioning. Here we explore the biome-wide relationships between temperature, moisture and seven key plant functional traits both across space and over three decades of warming at 117 tundra locations. Spatial temperature-trait relationships were generally strong but soil moisture had a marked influence on the strength and direction of these relationships, highlighting the potentially important influence of changes in water availability on future trait shifts in tundra plant communities. Community height increased with warming across all sites over the past three decades, but other traits lagged far behind predicted rates of change. Our findings highlight the challenge of using space-for-time substitution to predict the functional consequences of future warming and suggest that functions that are tied closely to plant height will experience the most rapid change. They also reveal the strength with which environmental factors shape biotic communities at the coldest extremes of the planet and will help to improve projections of functional changes in tundra ecosystems with climate warming.

Climate warming is altering the diversity of plant communities but it remains unknown which species will be lost or gained under warming, especially considering interactions with other factors such as herbivory and nutrient availability. Here, we experimentally test effects of warming, mammalian herbivory and fertilization on tundra species richness and investigate how plant functional traits affect losses and gains. We show that herbivory reverses the impact of warming on diversity: in the presence of herbivores warming increases species richness through higher species gains and lower losses, while in the absence of herbivores warming causes higher species losses and thus decreases species richness. Herbivores promote gains of short-statured species under warming, while herbivore removal and fertilization increase losses of short-statured and resource-conservative species through light limitation. Our results demonstrate that both rarity and traits forecast species losses and gains, and mammalian herbivores are essential for preventing trait-dependent extinctions and mitigate diversity loss under warming and eutrophication.Warming can reduce plant diversity but it is unclear which species will be lost or gained under interacting global changes. Kaarlejärvi et al. manipulate temperature, herbivory and nutrients in a tundra system and find that herbivory maintains diversity under warming by reducing species losses and promoting gains.

Herbivory and nutrient limitation can increase the resistance of temperature-limited systems to invasions under climate warming. We imported seeds of lowland species to tundra under factorial treatments of warming, fertilization, herbivore exclusion and biomass removal. We show that warming alone had little impact on lowland species, while exclusion of native herbivores and relaxation of nutrient limitation greatly benefitted them. In contrast, warming alone benefitted resident tundra species and increased species richness; however, these were canceled by negative effects of herbivore exclusion and fertilization. Dominance of lowland species was associated with low cover of tundra species and resulted in decreased species richness. Our results highlight the critical role of biotic and abiotic filters unrelated to temperature in protecting tundra under warmer climate. While scarcity of soil nutrients and native herbivores act as important agents of resistance to invasions by lowland species, they concurrently promote overall species coexistence. However, when these biotic and abiotic resistances are relaxed, invasion of lowland species can lead to decreased abundance of resident tundra species and diminished diversity., (© 2016 John Wiley & Sons Ltd.)

Climate change is resulting in a rapid expansion of shrubs in the Arctic. This expansion has been shown to be reinforced by positive feedbacks, and it could thus set the ecosystem on a trajectory toward an alternate, more productive regime. Herbivores, on the other hand, are known to counteract the effects of simultaneous climate warming on shrub biomass. However, little is known about the impact of herbivores on resilience of these ecosystems, that is, the capacity of a system to absorb disturbance and still remain in the same regime, retaining the same function, structure, and feedbacks. Here, we investigated how herbivores affect resilience of shrub-dominated systems to warming by studying the change of shrub biomass after a cessation of long-term experimental warming in a forest-tundra ecotone. As predicted, warming increased the biomass of shrubs, and in the absence of herbivores, shrub biomass in tundra continued to increase 4 years after cessation of the artificial warming, indicating that positive effects of warming on plant growth may persist even over a subsequent colder period. Herbivores contributed to the resilience of these systems by returning them back to the original low-biomass regime in both forest and tundra habitats. These results support the prediction that higher shrub biomass triggers positive feedbacks on soil processes and microclimate, which enable maintaining the rapid shrub growth even in colder climates. Furthermore, the results show that in our system, herbivores facilitate the resilience of shrub-dominated ecosystems to climate warming., (© 2015 John Wiley & Sons Ltd.)

Seed limitation can narrow down the number of coexisting plant species, limit plant community productivity, and also constrain community responses to changing environmental and biotic conditions. In a 10‐year full‐factorial experiment of seed addition, fertilisation, warming and herbivore exclusion, we tested how seed addition alters community richness and biomass, and how its effects depend on seed origin and biotic and abiotic context. We found that seed addition increased species richness in all treatments, and increased plant community biomass depending on nutrient addition and warming. Novel species, originally absent from the communities, increased biomass the most, especially in fertilised plots and in the absence of herbivores, while adding seeds of local species did not affect biomass. Our results show that seed limitation constrains both community richness and biomass, and highlight the importance of considering trophic interactions and soil nutrients when assessing novel species immigrations and their effects on community biomass. [ABSTRACT FROM AUTHOR]

Warm-adapted low elevation plants are expected to exhibit considerable range shifts to higher altitudes and latitudes as a result of climate warming and increased nutrient loads. However, empirical studies show that the magnitude and direction of plant responses are highly species- and site-specific, suggesting that several additional drivers interact with warmer climate., We experimentally tested the interactive effects of climate warming, mammalian herbivory and soil fertility on low elevation plants. Seedlings of three warm-adapted lowland forbs ( Epilobium angustifolium, Silene dioica and Solidago virgaurea) were transplanted to an open tundra site with native mountain tundra vegetation, and the effects of full factorial combinations of herbivore exclosures, warming and fertilization on transplant survival, growth and flowering were studied for two growing seasons. We also investigated the response of native vegetation biomass to the same treatments and compared it with the responses of transplanted lowland forbs., Effects of both warming and fertilization on the transplanted lowland forbs strongly hinged on herbivore exclusion, resulting in 2-13-fold increase in biomass in warmed and fertilized plots without herbivores compared with warmed and fertilized plots with herbivores present, the magnitude depending on the species. While warm-adapted transplants benefited from warming, the native tundra plant community biomass did not respond to warming treatment., Our results show that grazing limits the growth of transplants under warmer and more productive conditions, indicating that the expansion of lowland plant species to higher altitudes with warming may be hampered by mammalian herbivory. Furthermore, our results also suggest that migration of warm-adapted species into lightly grazed high-altitude tundra ecosystems might transform these communities to be more responsive to warmer climate and nutrient loads. Studies that do not consider species' upward shifts from lower altitudes might thus have underestimated vegetation responses to global warming, as well as the potential of herbivory to influence these responses. [ABSTRACT FROM AUTHOR]