No effect of snow on shrub xylem traits: Insights from a snow-manipulation experiment on Disko Island, Greenland.

Widespread shrubification across the Arctic has been generally attributed to increasing air temperatures, but responses vary across species and sites. Wood structures related to the plant hydraulic architecture may respond to local environmental conditions and potentially impact shrub growth, but these relationships remain understudied. Using methods of dendroanatomy, we analysed shrub ring width (RW) and xylem anatomical traits of 80 individuals of Salix glauca L. and Betula nana L. at a snow manipulation experiment in Western Greenland. We assessed how their responses differed between treatments (increased versus ambient snow depth) and soil moisture regimes (wet and dry). Despite an increase in snow depth due to snow fences (28-39 %), neither RW nor anatomical traits in either species showed significant responses to this increase. In contrast, irrespective of the snow treatment, the xylem specific hydraulic conductivity (Ks) and earlywood vessel size (LA95) for the study period were larger in S. glauca (p < 0.1, p < 0.01) and B. nana (p < 0.01, p < 0.001) at the wet than the dry site, while both species had larger vessel groups at the dry than the wet site (p < 0.01). RW of B. nana was higher at the wet site (p < 0.01), but no differences were observed for S. glauca. Additionally, B. nana Ks and LA95 showed different trends over the study period, with decreases observed at the dry site (p < 0.001), while for other responses no difference was observed. Our results indicate that, taking into account ontogenetic and allometric trends, hydraulic related xylem traits of both species, along with B. nana growth, were influenced by soil moisture. These findings suggest that soil moisture regime, but not snow cover, may determine xylem responses to future climate change and thus add to the heterogeneity of Arctic shrub dynamics, though more long-term species- and site- specific studies are needed.

Influences of summer warming and nutrient availability on Salix glauca L. growth in Greenland along an ice to sea gradient.

The combined effects of climate change and nutrient availability on Arctic vegetation growth are poorly understood. Archaeological sites in the Arctic could represent unique nutrient hotspots for studying the long-term effect of nutrient enrichment. In this study, we analysed a time-series of ring widths of Salix glauca L. collected at nine archaeological sites and in their natural surroundings along a climate gradient in the Nuuk fjord region, Southwest Greenland, stretching from the edge of the Greenlandic Ice Sheet in the east to the open sea in the west. We assessed the temperature-growth relationship for the last four decades distinguishing between soils with past anthropogenic nutrient enrichment (PANE) and without (controls). Along the East-West gradient, the inner fjord sites showed a stronger temperature signal compared to the outermost ones. Individuals growing in PANE soils had wider ring widths than individuals growing in the control soils and a stronger climate-growth relation, especially in the inner fjord sites. Thereby, the individuals growing on the archaeological sites seem to have benefited more from the climate warming in recent decades. Our results suggest that higher nutrient availability due to past human activities plays a role in Arctic vegetation growth and should be considered when assessing both the future impact of plants on archaeological sites and the general greening in landscapes with contrasting nutrient availability.

Chemical signature of Eurois occulta L. outbreaks in the xylem cell wall of Salix glauca L. in Greenland.

Insect defoliations are a major natural disturbance in high-latitude ecosystems and are expected to increase in frequency and severity due to current climatic change. Defoliations cause severe reductions in biomass and carbon investments that affect the functioning and productivity of tundra ecosystems. Here we combined dendro-anatomical analysis with chemical imaging to investigate the direct and lagged effects of insect outbreaks on carbon investment. We analysed the content of lignin vs. holocellulose, i.e. unspecified carbohydrates in xylem samples of Salix glauca L. collected at Iffiartarfik, Nuuk fjord, Greenland, featuring two outbreak events of the moth Eurois occulta L. Cross sections of the growth rings corresponding to both outbreaks ±3 years were analysed using confocal Raman imaging to identify possible chemical signatures related to insect defoliation on fibres, vessels, and ray parenchyma cells and to get insight into species-specific defence responses. Outbreak years with narrower rings and thinner fibre cell walls are accompanied by a change in the content of cell-wall polymers but not their underlying chemistry. Indeed, during the outbreaks the ratio between lignin and carbohydrates significantly increased in fibre but not vessel cell walls due to an increase in lignin content coupled with a reduced content of carbohydrates. Parenchyma cell walls and cell corners did not show any significant changes in the cell-wall biopolymer content. The selective adjustment of the cell-wall composition of fibres but not vessels under stressful conditions could be related to the plants priority to maintain an efficient hydraulic system rather than mechanical support. However, the higher lignin content of fibre cell walls formed during the outbreak events could increase mechanical stiffness to the thin walls by optimizing the available resources. Chemical analysis of xylem traits with Raman imaging is a promising approach to highlight hidden effects of defoliation otherwise overlooked with classical dendroecological methods.