Circum-Arctic distribution of chemical anti-herbivore compounds 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
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.