Multiplicity of biochemical factors determining quality of growing birch leaves

Due to rapidly changing physical and biochemical characteristics of growing leaves, correlations between traits of foliage biochemistry and the perfor mance indices of flush feeding herbivores may vary considerably following r elatively minor changes in experimental conditions. We examined the effects of the seasonal and inter-tree variation of a comprehensive array of bioch emical compounds on the success of an early season geometrid, Epirrita autu mnata, feeding on maturing foliage of mountain birch, Betula pubescens ssp. czerepanovii. We monitored the concentrations of individual phenolics, sug ars, total nitrogen, nitrogen of proteins, and nitrogen of soluble compound s, water and acetone-insoluble residue. Simultaneously we recorded larval c onsumption, physiological performance, growth, and pupal mass of E. autumna ta. We found significant phenological changes in almost all leaf traits mea sured. In bioassays with half-grown leaves, leaf gallotannin concentrations showed a nonlinear effect: in trees with high foliar gallotannin concentra tions (over 10 mg g(-1)), physiological performance was strongly reduced by high gallotannin concentrations. In trees with lower gallotannin concentra tions, on the other hand, larval growth was reduced by soluble proanthocyan idins, not gallotannins. Differences between high and low gallotannin trees largely depended on phenology, i.e., on the age of leaves. However, not al l the differences in leaf traits between late (with high gallotannin concen trations at the time of the bioassay) and early flushing trees disappeared with leaf maturation, indicating that there is also phenology-independent v ariance in the tree population. In the full-grown leaves of all the study t rees, low concentrations of water and of nitrogen of proteins (but not nitr ogen of soluble compounds) were the main factors reducing pupal masses of E . autumnata, while neither gallotannin nor proanthocyanidins now played a s ignificant role. The observed change in the factors underlying leaf duality (from gallotannins and proanthocyanidins to nitrogen and water) relate to the activity of the shikimate pathway and the formation of cell walls: gall otannins and proanthocyanidins are both produced in the pathway, and these tannins are assumed to contribute - via binding into cell walls - to tough and durable cell walls. Interestingly, low duality of leaves did not automa tically translate into low foliar consumption (i.e., benefits to the tree). On the trees with young, high gallotannin leaves, larvae actually increase d consumption on low quality foliage. In the group of trees with slightly m ore developed, low gallotannin leaves, the quality of leaves did not clearl y modify amounts consumed. In full-grown leaves, low leaf duality strongly reduced leaf consumption. These results emphasize the strong influence of t ree phenology on the relationships between biochemical compounds and the he rbivore.