We investigated the role of phenolic and phenolic-related traits of the leaves of
mountain birch (Betula pubescens ssp. czerepanovii) as determinants of their suitability
for the growth of larvae of the geometrid Epirrita autumnata. As parameters of leaf
suitability, we determined the contents of total phenolics, gallotannins, soluble
and cell-wall-bound proanthocyanidins (PAS and PAB, respectively), lignin, protein
precipitation capacity of tannins (PPC), and leaf toughness. In addition, we examined
concentrations of soluble carbohydrates and protein-bound amino acids as background
variables describing the nutritive value of leaves. The correlation of the leaf traits
of our 40 study trees with the tree-specific relative growth rate (RGR) of E. autumnata
showed that the only significant correlation with RGR was that of PAS - the largest
fraction of total phenolics - and even that explained only 15% of the variation in
E. autumnata growth. The nonlinear estimation of the relationship between RGR and
PAS by piecewise linear regression divided the 40 study trees into two groups: (i)
19 trees with good leaves for E. autumnata (RGR ranging from 0.301 to 0.390), and
(ii) 21 trees with poor leaves (RGR ranging from 0.196 to 0.296). The suitability
of leaves within these two groups of trees was determined by different phenolic traits.
Within the good group, the suitability of leaves for larvae was determined by the
PPC of extracts, which strongly correlated with gallotannins, and by the total content
of gallotannins. In contrast, the leaves of poor trees had significantly higher contents
of both PAS and PAB, but leaf toughness correlated only negatively with the RGR of
E. autumnata larvae. We also discuss the causes of variation in the phenolic and phenolic-related
factors that determine the suitability of leaves for E. autumnata larvae in different
groups of trees.