A team led by Ying Liu of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research in Potsdam turned to sedimentary ancient DNA, or sedaDNA, extracted from lake sediment cores collected at nine sites across northeastern Siberia and Alaska. Because this region remained largely free of ice sheets during the last glacial period, its lakes preserve an unusually continuous archive of the vegetation that surrounded them. The researchers analysed 369 samples yielding more than 78 million DNA sequence reads, representing 625 plant amplicon sequence variants, all verified against a customised regional reference database. The technique targets the P6 loop of the chloroplast trnL (UAA) intron, a barcode region that offers far finer taxonomic resolution than traditional pollen analysis.

The raw genetic signal told a familiar story of post-glacial change. Before about 14,000 calibrated years before present, during the depths of the last ice age, the plant communities were dominated by herbaceous taxa. Forbs made up roughly 60 percent of total plant abundance, while cushion plants and grass-like graminoids each accounted for less than 10 percent. Woody taxa were scarce, contributing less than 30 percent of the community. Then, as temperatures climbed through the Bølling–Allerød Interstadial between 14,000 and 12,000 years ago, the balance shifted. Willows rose moderately, other shrubs expanded rapidly to peak at 31 percent at the onset of the Holocene around 10,000 years ago, and trees crept up to about 9 percent. Meanwhile, forbs crashed to a minimum of 30 percent and cushion plants and graminoids fell below 5 percent.

Crucially, the team did not stop at documenting which plants lived where. Their central question was how those plants treated one another, and answering it required disentangling genuine biological interactions from the many other forces that make species appear together or apart. Species can co-occur because they favour the same climate, because they disperse similarly, or simply by chance. To isolate the biological signal, the researchers built species distribution models trained on modern occurrence data from the Global Biodiversity Information Facility, using mean annual and mean July temperatures as the key predictors. These models predicted which species the climate alone would allow at each lake through time.

From there, the team applied environmentally constrained null models, a mathematical framework previously used on modern vegetation surveys in the Swiss Alps but never before on palaeoecological data. The approach compares a co-occurrence statistic called the C-score, calculated from the actual sedaDNA records, against the distribution of that statistic in thousands of simulated communities in which species are placed according to climate suitability rather than biology. Species pairs that were significantly more segregated than the climate-only expectation were scored as negative interactions, likely competition, while pairs more aggregated than expected were scored as positive interactions, such as facilitation. Dispersal limitation was ruled out separately by testing whether species pairs had overlapping geographic ranges and aligned distribution centroids. The resulting interaction networks showed a significant overlap of about 26 percent with the Globi database of documented plant interactions, far exceeding what random pairings would produce, which supports the reliability of the inference.

The temporal pattern that emerged is striking. During the glacial period, positive interactions dominated, peaking around the Last Glacial Maximum between 20,000 and 18,000 years ago, when taxa such as Saxifraga oppositifolia, Saxifraga tricuspidata and Dryas were heavily involved in facilitative links. Cushion plants, the classic nurse species of harsh environments, showed a median of roughly 60 percent positive interactions across all time slices, consistent with modern field observations of these mat-forming plants trapping heat, stabilising soil and sheltering seedlings within their crowns. Then, beginning around 14,000 years ago, negative interactions climbed steadily, reaching equilibrium with positive ones at the dawn of the Holocene and becoming predominant thereafter. Woody taxa, including trees and shrubs, were overwhelmingly embedded in negative links, which made up 70 to 95 percent of their connections, exemplified by larch, Larix, in the Holocene networks.

The mechanism behind this behavioural shift appears to lie in plant functional traits. Using trait databases including the Tundra Trait Team, TRY 5.0 and Global Root Trait compilations, the researchers calculated community-weighted averages of plant height and maximum root length across time. As woody taxa encroached, community-weighted height surged from glacial levels to nearly 3 metres between about 16,000 and 14,000 years ago and peaked at approximately 4 metres in the early Holocene, while community-weighted root length climbed to around 0.47 metres and stayed elevated. Taller canopies preempt light and shade out low-growing herbs, potentially causing etiolation or outright exclusion of shade-intolerant species, while longer, long-lived root systems intensify belowground competition for water, nutrients and space, and foster mycorrhizal partnerships that further boost resource uptake.

A structural equation model built from 1,200 observations, spanning 100 resampling iterations across 12 two-thousand-year time slices, confirmed the causal chain. Rising temperature drove the expansion of woody taxa and the associated increase in community height and root length, and it was these trait shifts, modelled as a latent variable, that directly increased the proportion of negative interactions. In other words, climate acted as an external trigger that opened the door for woody expansion, but the internal dynamics of competition then took over and amplified the transformation. The pattern also fits the Stress Gradient Hypothesis, which predicts that facilitation dominates under harsh conditions while competition prevails in benign ones. Intriguingly, some shrubs such as Cassiope tetragona and Empetrum nigrum appear to have switched roles, acting as nurse plants that sheltered neighbours during glacial millennia but becoming competitors once the climate eased, a flexibility that likely helped them persist across both worlds.

The consequences ripple outward through entire communities. During the glacial period, widespread facilitation helped species expand their ranges, kept regional beta diversity low and buffered plants against extinction despite the cold. In the Holocene, intensified competition constrained the establishment of new taxa, suppressed range expansion of residents and contributed to a higher than expected loss of plant taxa, with herbaceous species of short stature and shallow roots especially vulnerable to local extirpation. Yet competition also fostered divergent community assemblies across sites, enhancing regional beta diversity even as local diversity declined. For the present day, the analogy with contemporary Arctic greening is direct: as shrubs and trees advance under rapid warming, species distribution models that consider only climate may badly overestimate the futures of low-competitive native herbs, which could vanish through competitive exclusion before warming ever exceeds their physiological tolerances. The Arctic’s botanical future, this 24,000-year record suggests, will be decided not just by who can tolerate the cold, but by who can win the fight for light and soil.

Subject of Research: Long-term shifts in Arctic plant interactions during post-glacial woody encroachment reconstructed from sedimentary ancient DNA

Article Title: Woody plant encroachment enhanced negative interactions in Arctic plant communities over the last 24,000 years

Article References: Liu, Y., Jia, W., Liu, S., Lisovski, S., Stoof-Leichsenring, K. R., Farkas, L. Z., & Herzschuh, U. (2026). Woody plant encroachment enhanced negative interactions in Arctic plant communities over the last 24,000 years. Nature Communications, 17(1), Article 10083. https://doi.org/10.1038/s41467-026-77801-9

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77801-9

Keywords: Arctic, woody plant encroachment, sedimentary ancient DNA, plant interactions, competition, facilitation, paleoecology, Arctic greening, functional traits, Holocene, biodiversity, Siberia