DOI: 10.5061/dryad.9p8cz8whd
title: Circum-Arctic distribution of chemical anti-herbivore compounds arctic shrubs

description: 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
 it varies among and within taxa and functional groups. Using 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.
description: Chemical defense compound concentrations was quantified
 using LC-MS metabolomic analyses. In-vitro digestibility was measured as
 in-vitro organic matter digestibility (IVOMD) by incubating dry leaf
 material in cattel rumen fluid at 38°C for 96 hours an then calculating
 the proportion of digested organic material. Leaf nitrogen and carbin
 content was measured using an elemental analyser interfaced to a continous
 flow isotope ratio mass spectrometer. Leaf chemisty and digestibility were
 all measured using dired and ground leaf material. Resin gland density was
 calculated as number of resin glands per mm<sup>2</sup> on 15
 mm twig segment starting 20 mm from the twig top on current annual growth
 (CAG) twigs. Specific Leaf Area was measured as a proxy (SLA-P) by
 calculating dry leaf area/dry mass. Environmental data was retrieved
 as interpolated grid data with a resolution of 30 arc seconds (~1 km) from
 the Worlclim database and then extracted using GIS software
 ArcMAP.
description: The data file includes metadata and a sheet with data
 explanation, including units for all measures.