Circum-Arctic distribution of chemical anti-herbivore compounds suggests biome-wide trade-off in defence strategies in Arctic shrubs

Spatial variation in plant chemical defence towards herbivores can help us understand variation in herbivore top-down control of shrubs in the Arctic and possibly also shrub responses to global warming. Less defended, non-resinous shrubs could be more influenced by herbivores than more defended, resinous shrubs. However, sparse field measurements limit our current understanding of how much of the circum-Arctic variation in defence compounds is explained by taxa or defence functional groups (resinous/non-resinous). We measured circum-Arctic chemical defence and leaf digestibility in resinous (Betula glandulosa, B. nana ssp. exilis) and non-resinous (B. nana ssp. nana, B. pumila) shrub birches to see how they vary among and within taxa and functional groups. Using liquid chromatography-mass spectrometry (LC-MS) metabolomic analyses and in vitro leaf digestibility via incubation in cattle rumen fluid, we analysed defence composition and leaf digestibility in 128 samples from 44 tundra locations. We found biogeographical patterns in anti-herbivore defence where mean leaf triterpene concentrations and twig resin gland density were greater in resinous taxa and mean concentrations of condensing tannins were greater in non-resinous taxa. This indicates a biome-wide trade-off between triterpene- or tannin-dominated defences. However, we also found variations in chemical defence composition and resin gland density both within and among functional groups (resinous/non-resinous) and taxa, suggesting these categorisations only partly predict chemical herbivore defence. Complex tannins were the only defence compounds negatively related to in vitro digestibility, identifying this previously neglected tannin group as having a potential key role in birch anti-herbivore defence. We conclude that circum-Arctic variation in birch anti-herbivore defence can be partly derived from biogeographical distributions of birch taxa, although our detailed mapping of plant defence provides more information on this variation and can be used for better predictions of herbivore effects on Arctic vegetation.Peer reviewe

Evidences of cyclic Eurois occulta outbreaks in West Greenland based on shrub-ring anatomy and remote sensing

Insect outbreaks are among the major disturbances in high latitude ecosystems. Such outbreaks cause severe canopy defoliation and reduce the vegetation biomass and C investments, with potential consequences for species composition, functioning and productivity of tundra ecosystems. Outbreaks are expected to increase in severity and frequency in the future due to climate changes. Despite their importance, up to now only few studies tried a retrospective reconstruction of past outbreaks, and none has investigated their effect on shrub anatomical structure, e.g., cell wall thickness. In this research, we use a dendro-anatomical approach combined with remotely sensed data to assess and reconstruct past outbreaks of the moth Eurois occulta in West Greenland. We additionally quantify changes in annual growth and C investment for the host species Salix glauca L. We analysed Salix glauca L. samples collected along the Nuuk fjord (7 sites, 136 samples) to identify outbreak events and quantify inter-annual variation in conduit diameters and wall thickness. Time series of Landsat images were used to detect NDVI deviations caused by reductions in the photosynthetic activity in the area. Wood samples were successfully crossdated and 7 chronologies were established spanning more than 50 years. We clearly identified three distinctive pointer years of reduced annual growth (1997, 2003 and 2010), where wood-anatomical traits showed either a significant reduction in cell-wall thickness or no variation in vessel size. This implies that under defoliations Salix glauca L. undergoes an adjustment in the xylem traits aimed to maintain the hydraulic structure but with a detrimental effect on fiber cell walls. This multi-proxy approach allowed us to distinguish between abiotic (climate) and biotic (the moth) drivers of narrow ring formation

Tundra Trait Team : A database of plant traits spanning the tundra biome

Motivation The Tundra Trait Team (TTT) database includes field-based measurements of key traits related to plant form and function at multiple sites across the tundra biome. This dataset can be used to address theoretical questions about plant strategy and trade-offs, trait-environment relationships and environmental filtering, and trait variation across spatial scales, to validate satellite data, and to inform Earth system model parameters. Main types of variable contained Spatial location and grain The database contains 91,970 measurements of 18 plant traits. The most frequently measured traits (> 1,000 observations each) include plant height, leaf area, specific leaf area, leaf fresh and dry mass, leaf dry matter content, leaf nitrogen, carbon and phosphorus content, leaf C:N and N:P, seed mass, and stem specific density. Measurements were collected in tundra habitats in both the Northern and Southern Hemispheres, including Arctic sites in Alaska, Canada, Greenland, Fennoscandia and Siberia, alpine sites in the European Alps, Colorado Rockies, Caucasus, Ural Mountains, Pyrenees, Australian Alps, and Central Otago Mountains (New Zealand), and sub-Antarctic Marion Island. More than 99% of observations are georeferenced. Time period and grain Major taxa and level of measurement All data were collected between 1964 and 2018. A small number of sites have repeated trait measurements at two or more time periods. Trait measurements were made on 978 terrestrial vascular plant species growing in tundra habitats. Most observations are on individuals (86%), while the remainder represent plot or site means or maximums per species. Software format csv file and GitHub repository with data cleaning scripts in R; contribution to TRY plant trait database (www.try-db.org) to be included in the next version release.Peer reviewe