Anne D. Bjorkman

As the colder regions of the planet warm, species are moving northward and upward from the boreal forest to the tundra biome, a process that has been referred to as borealization. Here, we examine the diverse uses of the term borealization and propose the concept of ‘tundra borealization’ for terrestrial environments to specifically describe shifts and increases in abundance of boreal species into the tundra. We summarize the evidence to date for borealization of plant and animal communities in tundra ecosystems and the different approaches that can be used to quantify borealization. We discuss how land-use change is interacting with climate change, leading to species and community reorganization in colder biomes, and the consequences of borealization for food webs, ecosystem functions and northern livelihoods. Our review brings together the different definitions and lines of evidence for tundra borealization in terrestrial ecosystems to emphasize this important ecological process and rapidly evolving area of research.

As the Arctic experiences continued warming, significant ecosystem changes, such as the northwards migration of woody species, are underway in tundra landscapes throughout the region. Despite these observable shifts, there remains a gap in our understanding of how climate warming impacts the phenology of tundra plants—specifically, the timing of their growth and reproductive cycles—especially across heterogeneous landscapes. Measuring phenology in the Arctic is challenging, requiring observations throughout the growing season and especially early and late in the season—times when field researchers are typically absent from their study sites. While remote observations offer broad coverage across the biome, they lack the detail needed for accurate phenological interpretations and may introduce significant errors. To address this, time‐lapse cameras (phenocams) present a promising solution, enabling simultaneous, individual‐level observations across disparate sites. In this study, we assess and present the precision, accuracy and practicality of monitoring reproductive phenology using repeat photography in tundra ecosystems by comparing satellite imagery, in situ observations and phenocams deployed on Qikiqtaruk—Herschel Island, Yukon Territory, Canada. Our results show that time‐lapse photography is a powerful tool to detect species‐specific phenology of Arctic vegetation, with an accuracy that is similar to in situ observations conducted by park rangers across the growing season, and at a much higher spatial and temporal detail than satellite data. Especially in the remote Arctic the low cost and ease of deployment across disparate sites throughout the whole year make phenocams an important tool for observing vegetation dynamics in a changing Arctic.

Pollination is a key ecological process sustaining biodiversity and food security, yet global patterns of plant–pollinator specialisation have remained unresolved. Using the largest global dataset of quantitative networks (>3,400 networks, >110,000 interactions), we show that the latitudinal specialisation gradient (LSG) exists, but it is non-linear, hemispherically asymmetric, and strongly taxon-dependent. Network-level and pollinator specialisation were lowest in the tropics and peaked at northern mid-latitudes, whereas plants tended to become more specialised toward higher latitudes. Climate consistently outperformed latitude, species richness, and environmental productivity as a predictor of these patterns. Specialisation declined with increasing temperature, rose with moderate rainfall before declining at the wettest sites, and increased with temperature seasonality, but plants and pollinators responded differently to these drivers. Functional groups diverged strongly: ectothermic insects were most specialised in cooler, seasonal climates, while birds showed weaker links to latitude but reduced specialisation in wetter regions. These findings demonstrate that climate, rather than latitude or species richness, structures global variation in specialisation. Because warmer and less seasonal climates promote generalisation, climate change is likely to disrupt the most specialised pollination systems, unevenly across taxa and regions, with important consequences for biodiversity and ecosystem stability.

Motivation: Here, we make available a second version of the BioTIME database, which compiles records of abundance estimates for species in sample events of ecological assemblages through time. The updated version expands version 1.0 of the database by doubling the number of studies and includes substantial additional curation to the taxonomic accuracy of the records, as well as the metadata. Moreover, we now provide an R package (BioTIMEr) to facilitate use of the database.Main Types of Variables Included: The database is composed of one main data table containing the abundance records and 11 metadata tables. The data are organised in a hierarchy of scales where 11,989,233 records are nested in 1,603,067 sample events, from 553,253 sampling locations, which are nested in 708 studies. A study is defined as a sampling methodology applied to an assemblage for a minimum of 2 years.Spatial Location and Grain: Sampling locations in BioTIME are distributed across the planet, including marine, terrestrial and freshwater realms. Spatial grain size and extent vary across studies depending on sampling methodology. We recommend gridding of sampling locations into areas of consistent size.Time Period and Grain: The earliest time series in BioTIME start in 1874, and the most recent records are from 2023. Temporal grain and duration vary across studies. We recommend doing sample‐level rarefaction to ensure consistent sampling effort through time before calculating any diversity metric.Major Taxa and Level of Measurement: The database includes any eukaryotic taxa, with a combined total of 56,400 taxa.Software Format: csv and. SQL.

The Arctic is warming four times faster than the global average¹ and plant communities are responding through shifts in species abundance, composition and distribution^2–4. However, the direction and magnitude of local changes in plant diversity in the Arctic have not been quantified. Using a compilation of 42,234 records of 490 vascular plant species from 2,174 plots across the Arctic, here we quantified temporal changes in species richness and composition through repeat surveys between 1981 and 2022. We also identified the geographical, climatic and biotic drivers behind these changes. We found greater species richness at lower latitudes and warmer sites, but no indication that, on average, species richness had changed directionally over time. However, species turnover was widespread, with 59% of plots gaining and/or losing species. Proportions of species gains and losses were greater where temperatures had increased the most. Shrub expansion, particularly of erect shrubs, was associated with greater species losses and decreasing species richness. Despite changes in plant composition, Arctic plant communities did not become more similar to each other, suggesting no biotic homogenization so far. Overall, Arctic plant communities changed in richness and composition in different directions, with temperature and plant–plant interactions emerging as the main drivers of change. Our findings demonstrate how climate and biotic drivers can act in concert to alter plant composition, which could precede future biodiversity changes that are likely to affect ecosystem function, wildlife habitats and the livelihoods of Arctic peoples^5,6.

Summary: The relationship between polyploid formation, triploid fitness and plant reproduction has been studied for over a century, and uniparental reproduction has long been recognized to play a crucial role in polyploid establishment. Yet, we lack a synthesized framework of how polyploid establishment is expected to be influenced by different reproductive modes among angiosperms. Here, we provide new perspectives on how uniparental reproduction, pollination ecology, triploid fitness and assortative mating can impact minority cytotype exclusion (MCE) and, thereby, the likelihood of polyploid establishment. We review the current state of knowledge of the reproductive mechanisms that impact polyploid establishment and discuss often overlooked aspects of these processes, such as the influence of pollinator communities on rates of self‐pollination. We propose a framework for considering how variation in reproductive strategies and pollinator communities can impact the ability of a polyploid to overcome MCE. Finally, we propose links between patterns of variation in uniparental reproduction across plant communities and observed patterns in the distribution and abundance of polyploids.

Climate change is commonly assumed to induce species’ range shifts toward the poles. Yet, other environmental changes may affect the geographical distribution of species in unexpected ways. Here, we quantify multidecadal shifts in the distribution of European forest plants and link these shifts to key drivers of forest biodiversity change: climate change, atmospheric deposition (nitrogen and sulfur), and forest canopy dynamics. Surprisingly, westward distribution shifts were 2.6 times more likely than northward ones. Not climate change, but nitrogen-mediated colonization events, possibly facilitated by the recovery from past acidifying deposition, best explain westward movements. Biodiversity redistribution patterns appear complex and are more likely driven by the interplay among several environmental changes than due to the exclusive effects of climate change alone.

Premise: Numerous studies have found a positive association between dioecy and polyploidy; however, this association presents a theoretical conflict: While polyploids are predicted to benefit from self‐reproduction for successful establishment, dioecious species cannot self‐reproduce. We propose a theoretical framework to resolve this apparent conflict. We hypothesize that the inability of dioecious species to self‐reproduce hinders their establishment as polyploids. We therefore expect that genera with many dioecious species have fewer polyploids, leading to a negative association between polyploidy and dioecy across genera.Methods: We used three publicly available databases to determine ploidy and sexual systems for 131 genera and 546 species. We quantified (1) the relationship between the frequency of polyploid species and the frequency of dioecious species across genera, and (2) the proportion of polyploids with hermaphroditism and dioecy across species, adjusting for phylogenetic history.Results: Across genera, we found a negative relationship between the proportion of polyploids and the proportion of dioecious species, a consistent trend across clades. Across all species, we found that sexual system (dioecious or not) was not associated with polyploidy.Conclusions: Polyploids are rare in genera in which the majority of species are dioecious, consistent with the theory that self‐reproduction favors polyploid establishment. The low frequency of polyploidy among dioecious species indicates the association is not as widespread as previously suggested. Our findings are consistent with previous studies identifying a positive relationship between the two traits, but only if polyploidy promotes a transition to dioecy, and not the reverse.

Background: Northern ecosystems are strongly influenced by herbivores that differ in their impacts on the ecosystem. Yet the role of herbivore diversity in shaping the structure and functioning of tundra ecosystems has been overlooked. With climate and land-use changes causing rapid shifts in Arctic species assemblages, a better understanding of the consequences of herbivore diversity changes for tundra ecosystem functioning is urgently needed. This systematic review synthesizes available evidence on the effects of herbivore diversity on different processes, functions, and properties of tundra ecosystems. Methods: Following a published protocol, our systematic review combined primary field studies retrieved from bibliographic databases, search engines and specialist websites that compared tundra ecosystem responses to different levels of vertebrate and invertebrate herbivore diversity. We used the number of functional groups of herbivores (i.e., functional group richness) as a measure of the diversity of the herbivore assemblage. We screened titles, abstracts, and full texts of studies using pre-defined eligibility criteria. We critically appraised the validity of the studies, tested the influence of different moderators, and conducted sensitivity analyses. Quantitative synthesis (i.e., calculation of effect sizes) was performed for ecosystem responses reported by at least five articles and meta-regressions including the effects of potential modifiers for those reported by at least 10 articles. Review findings: The literature searches retrieved 5944 articles. After screening titles, abstracts, and full texts, 201 articles including 3713 studies (i.e., individual comparisons) were deemed relevant for the systematic review, with 2844 of these studies included in quantitative syntheses. The available evidence base on the effects of herbivore diversity on tundra ecosystems is concentrated around well-established research locations and focuses mainly on the impacts of vertebrate herbivores on vegetation. Overall, greater herbivore diversity led to increased abundance of feeding marks by herbivores and soil temperature, and to reduced total abundance of plants, graminoids, forbs, and litter, plant leaf size, plant height, and moss depth, but the effects of herbivore diversity were difficult to tease apart from those of excluding vertebrate herbivores. The effects of different functional groups of herbivores on graminoid and lichen abundance compensated each other, leading to no net effects when herbivore effects were combined. In turn, smaller herbivores and large-bodied herbivores only reduced plant height when occurring together but not when occurring separately. Greater herbivore diversity increased plant diversity in graminoid tundra but not in other habitat types. Conclusions: This systematic review underscores the importance of herbivore diversity in shaping the structure and function of Arctic ecosystems, with different functional groups of herbivores exerting additive or compensatory effects that can be modulated by environmental conditions. Still, many challenges remain to fully understand the complex impacts of herbivore diversity on tundra ecosystems. Future studies should explicitly address the role of herbivore diversity beyond presence-absence, targeting a broader range of ecosystem responses and explicitly including invertebrate herbivores. A better understanding of the role of herbivore diversity will enhance our ability to predict whether and where shifts in herbivore assemblages might mitigate or further amplify the impacts of environmental change on Arctic ecosystems.