Isla H. Myers-Smith’s research while affiliated with University of Edinburgh and other places

Satellite imagery is critical for understanding land-surface change in the rapidly warming Arctic. Since the 1980s, studies have found positive trends in the normalised difference vegetation index (NDVI) derived from satellite imagery over the Arctic—commonly referred to as ‘Arctic greening’ and assumed to represent increased vegetation productivity. However, greening analyses use satellite imagery with pixel sizes ranging from tens to hundreds of metres and do not account for the integration of abiotic phenomena such as snow within vegetation indices. Here, we use high-resolution drone data from one Arctic and one sub-Arctic site to show that fine-scale snow persistence within satellite pixels is associated with both reduced magnitude and delayed timing of annual peak NDVI, the base metric of Arctic greening analyses. We found higher snow persistence within Sentinel-2 pixels is associated with a lower magnitude and later peak NDVI, with a mean difference in NDVI of 0.1 and seven days between high and low snow persistence pixels. These effects were stronger in NASA HLSS30 data, representative of Landsat data commonly used in greening analyses. Our findings indicate that unaccounted changes in fine-scale snow persistence may contribute to Arctic spectral greening and browning trends through either biotic responses of vegetation to snow cover or abiotic integration of snow within the estimated peak NDVI. In order to improve our understanding of Arctic land-surface change, studies should integrate very-high-resolution data to estimate the dynamics of late-season snow within coarser satellite pixels.

Following rapid climate change across the Arctic, tundra plant communities are experiencing extensive compositional shifts. One of the most prevalent changes is the encroachment of boreal species into the tundra (‘borealization’). Borealization has been reported at individual sites, but has not been systematically quantified across the tundra biome. Here, we use a dataset of 1,137 plots at 113 subsites across 32 study areas resurveyed at least once between 1981 and 2023 and encompassing 287 vascular plant species. We i) quantified the borealization of tundra ecosystems as the colonisation and the increase in abundance of boreal specialist and boreal-tundra boundary species, ii) assessed biogeographical, climatic and local drivers of borealization, and iii) identified species contributing most to borealization and their associated traits. Around half of the plots experienced borealization, especially at sites closer to the treeline, at higher elevations (mountains), in warmer and wetter regions, and at sites that had undergone the lowest magnitude of climate change. Boreal species were more likely to expand in Eurasia, and at sites with lower initial abundances of boreal species. Boreal species that colonised more plots were generally short, and more likely to be shrubs and graminoids than forbs. Boreal specialist species colonised three times less frequently than boreal-tundra boundary species, yet abundance changes were similar across groups. These findings indicate that borealization is mainly driven by the spread of already established species in the tundra, and suggest that future changes to Arctic ecosystems might not involve rapid, widespread replacement of Arctic species by boreal species. These observed and future plant community composition changes could affect land-atmosphere interactions, trophic dynamics and local and Indigenous livelihoods.

Arctic ecosystems are experiencing extreme climatic, biotic and physical disturbance events that can cause substantial loss of plant biomass and productivity, sometimes at scales of >1000 km². Collectively known as browning events, these are key contributors to the spatial and temporal complexity of Arctic greening and vegetation dynamics. If we are to properly understand the future of Arctic terrestrial ecosystems, their productivity, and their feedbacks to climate, understanding browning events is essential. Here we bring together understanding of browning events in Arctic ecosystems to compare their impacts and rates of recovery, and likely future changes in frequency and distribution. We also seek commonalities in impacts across these contrasting event types. We find that while browning events can cause high levels of plant damage (up to 100% mortality), ecosystems have substantial capacity for recovery, with biomass largely re-established within five years for many events. We also find that despite the substantial loss of leaf area of dominant species, compensatory mechanisms such as increased productivity of undamaged subordinate species lessen the impacts on carbon sequestration. These commonalities hold true for most climatic and biotic events, but less so for physical events such as fire and abrupt permafrost thaw, due to the greater removal of vegetation. Counterintuitively, some events also provide conditions for greater productivity (greening) in the longer-term, particularly where the disturbance exposes ground for plant colonisation. Finally, we find that projected changes in the causes of browning events currently suggest many types of events will become more frequent, with events of tundra fire and abrupt permafrost thaw expected to be the greatest contributors to future browning due to their severe impacts and occurrence in many Arctic regions. Overall, browning events will have increasingly important consequences for ecosystem structure and function, and for feedback to climate.

Empirical studies worldwide show that warming has variable effects on plant litter decomposition, leaving the overall impact of climate change on decomposition uncertain. We conducted a meta‐analysis of 109 experimental warming studies across seven continents, using natural and standardised plant material, to assess the overarching effect of warming on litter decomposition and identify potential moderating factors. We determined that at least 5.2° of warming is required for a significant increase in decomposition. Overall, warming did not have a significant effect on decomposition at a global scale. However, we found that warming reduced decomposition in warmer, low‐moisture areas, while it slightly increased decomposition in colder regions, although this increase was not significant. This is particularly relevant given the past decade's global warming trend at higher latitudes where a large proportion of terrestrial carbon is stored. Future changes in vegetation towards plants with lower litter quality, which we show were likely to be more sensitive to warming, could increase carbon release and reduce the amount of organic matter building up in the soil. Our findings highlight how the interplay between warming, environmental conditions, and litter characteristics improves predictions of warming's impact on ecosystem processes, emphasising the importance of considering context‐specific factors.

1. The expansion of woody shrubs, known as shrubification, is one of the most widely observed patterns of vegetation change in the tundra. Yet, we do not know the relative importance of plant plasticity and genetic change in determining shrub responses to warming. Plastic responses to the environment can be rapid, while genetic differentiation is much slower. 2. We established a common garden experiment, using three tundra willow species (two tall willow shrubs: Salix richardsonii, S. pulchra, and one prostrate willow: S. arctica). We transplanted cuttings from southern (alpine, high elevation) and northern (Arctic) source populations to a 5ºC warmer environment in southern boreal Yukon, simulating projected future Arctic conditions. We monitored growth, phenology and functional traits in the common garden over a ten-year period from 2013 to 2023 and measured the same variables in the source populations. 3. The three willow species responded differently to a warmer environment. Southern S. richardsonii shrubs in the common garden grew almost seven times faster than the northern willows of the same species. Neither common garden populations of S. pulchra increased in height, but southern individuals grew wider. S. arctica growth patterns were similar between southern and northern common garden populations. All shrubs in the garden advanced their date of leaf bud burst by approximately one month compared to source populations. All northern willows growing in the garden also advanced senescence timing, resulting in less change to overall growing season length for northern willows. We suggest local adaptation to source population conditions as a likely cause of early senescence and limiting growth of northern willows in the common garden. 4. Synthesis: Our findings suggest longer growing seasons due to the advancement of leaf bud burst but not delayed senescence, and potential for rapid shrub growth as tundra ecosystems continue to warm. However, responses to warming vary by species and population, as we observed varied levels of plasticity for traits, phenology and growth. Local adaptation to past climatic conditions and slow genetic change may limit future shrub growth and determine which shrub species proliferate with future warming.

Satellite imagery is critical for understanding land-surface change in the rapidly warming Arctic. Since the 1980s, studies have found positive trends in the normalised difference vegetation index (NDVI) derived from satellite imagery over the Arctic - commonly referred to as ‘Arctic greening’ and assumed to represent increased vegetation productivity. However, greening analyses use satellite imagery with pixel sizes ranging from tens to hundreds of metres and do not account for the integration of abiotic phenomena such as snow within vegetation indices. Here, we use high resolution drone data from one Arctic and one sub-Arctic site to show that fine-scale snow persistence within satellite pixels is associated with both reduced magnitude and delayed timing of annual peak NDVI, the base metric of Arctic greening analyses. We found higher snow persistence within Sentinel-2 pixels is associated with a lower magnitude and later peak NDVI, with a mean difference in NDVI of 0.088 and seven days between high and low snow persistence pixels. These effects were stronger in NASA HLSS30 data, representative of Landsat data commonly used in greening analyses. Our findings indicate that unaccounted changes in fine-scale snow persistence may contribute to Arctic spectral greening and browning trends through either ecological responses of vegetation to snow cover or abiotic interactions between snow and the estimated peak NDVI. In order to improve our understanding of Arctic land-surface change, studies should integrate very-high-resolution data to estimate the dynamics of late season snow within coarser satellite pixels.

Foliar traits such as specific leaf area (SLA), leaf nitrogen (N), and phosphorus (P) concentrations play important roles in plant economic strategies and ecosystem functioning. Various global maps of these foliar traits have been generated using statistical upscaling approaches based on in-situ trait observations. Here, we intercompare such global upscaled foliar trait maps at 0.5° spatial resolution (six maps for SLA, five for N, three for P), categorize the upscaling approaches used to generate them, and evaluate the maps with trait estimates from a global database of vegetation plots (sPlotOpen). We disentangled the contributions from different plant functional types (PFTs) to the upscaled maps and quantified the impacts of using different plot-level trait metrics on the evaluation with sPlotOpen: community weighted mean (CWM) and top-of-canopy weighted mean (TWM). We found that the global foliar trait maps of SLA and N differ drastically and fall into two groups that are almost uncorrelated (for P only maps from one group were available). The primary factor explaining the differences between these groups is the use of PFT information combined with remote sensing-derived land cover products in one group while the other group mostly relied on environmental predictors alone. The maps that used PFT and corresponding land cover information exhibit considerable similarities in spatial patterns that are strongly driven by land cover. The maps not using PFTs show a lower level of similarity and tend to be strongly driven by individual environmental variables. Upscaled maps of both groups were moderately correlated to sPlotOpen data aggregated to the grid-cell level (R = 0.2-0.6) when processing sPlotOpen in a way that is consistent with the respective trait upscaling approaches, including the plot-level trait metric (CWM or TWM) and the scaling to the grid cells with or without accounting for fractional land cover. The impact of using TWM or CWM was relevant, but considerably smaller than that of the PFT and land cover information. The maps using PFT and land cover information better reproduce the between-PFT trait differences of sPlotOpen data, while the two groups performed similarly in capturing within-PFT trait variation. Our findings highlight the importance of explicitly accounting for within-grid-cell trait variation, which has important implications for applications using existing maps and future upscaling efforts. Remote sensing information has great potential to reduce uncertainties related to scaling from in-situ observations to grid cells and the regression-based mapping steps involved in the upscaling.

The below-ground growing season often extends beyond the above-ground growing season in tundra ecosystems. However, we do not yet know where and when this occurs and whether these phenological asynchronies are driven by variation in local vegetation communities or by spatial variation in microclimate. Here, we combined above- and below-ground plant phenology metrics to compare the relative timings and magnitudes of leaf and root growth and senescence across microclimates and plant communities at five sites across the tundra biome. We observed asynchronous growth between above-ground and below-ground plant tissue, with the below-ground season extending up to 74% beyond the onset of above-ground leaf senescence. Plant community type, rather than microclimate, was a key factor controlling the timing, productivity and growth rates of roots, with graminoid roots exhibiting a distinct ‘pulse’ of growth later into the growing season than shrub roots. Our findings indicate the potential of vegetation change to influence below-ground carbon storage as roots remain active in unfrozen soils for longer as the climate warms. Taken together, increased root growth in soils that remain thawed later into the growing season, in combination with ongoing tundra vegetation change including increased shrubs and graminoids, can act together to alter below-ground productivity and carbon cycling in the tundra biome.

Aim Arctic plants survived the Pleistocene glaciations in unglaciated refugia. The number, ages, and locations of these refugia are often unclear. We use high‐resolution genomic data from present‐day and Little‐Ice‐Age populations of Arctic Bell‐Heather to re‐evaluate the biogeography of this species and determine whether it had multiple independent refugia or a single refugium in Beringia. Location Circumpolar Arctic and Coastal British Columbia (BC) alpine. Taxon Cassiope tetragona L., subspecies saximontana and tetragona , outgroup C. mertensiana (Ericaceae). Methods We built genotyping‐by‐sequencing (GBS) libraries using Cassiope tetragona tissue from 36 Arctic locations, including two ~250‐ to 500‐year‐old populations collected under glacial ice on Ellesmere Island, Canada. We assembled a de novo GBS reference to call variants. Population structure, genetic diversity and demography were inferred from PCA, ADMIXTURE, fastsimcoal2, SplitsTree, and several population genomics statistics. Results Population structure analyses identified 4–5 clusters that align with geographic locations. Nucleotide diversity was highest in Beringia and decreased eastwards across Canada. Demographic coalescent analyses dated the following splits with Alaska: BC subspecies saximontana (5 mya), Russia (~1.4 mya), Europe (>200–600 kya), and Greenland (~60 kya). Northern Canada populations appear to have formed during the current interglacial (7–9 kya). Admixture analyses show genetic variants from Alaska appear more frequently in present‐day than historic plants on Ellesmere Island. Conclusions Population and demographic analyses support BC, Alaska, Russia, Europe and Greenland as all having had independent Pleistocene refugia. Northern Canadian populations appear to be founded during the current interglacial with genetic contributions from Alaska, Europe and Greenland. We found evidence, on Ellesmere Island, for continued recent gene flow in the last 250–500 years. These results suggest that a re‐analysis of other Arctic species with shallow population structure using higher resolution genomic markers and demographic analyses may help reveal deeper structure and other circumpolar glacial refugia.