Phytochemical diversity
 
The allelochemical composition of willows is dominated by phenolics (Orians et al., 1996). While phenolics of willows can show environmentally induced variation, due to light and soil nutrient concentrations (Bryant et al., 1987), a high heritability and an underlying genetic base have been documented (Orians et al., 1996). The genetically determined phenolic composition and residual environmentally-induced variability are likely to result in much phytochemical diversity between populations, in addition to variation between species and hybrids (e.g. Palo, 1984; Keinanen et al., 1999). This in turn is hypothesised to confer diversity in ecosystem processes via the broad range of allelochemic processes in which secondary metabolites are involved. These include defense against vertebrate and invertebrate herbivores, or pathogens such as rust fungi (Rosenthal & Janzen, 1979; Feeny, 1991; Hakulinen et al., 1999). Other effects include phago-stimulation or stimulation to oviposition of insects (Kolehmainen et al., 1994, 1995). Variations in the effectiveness of defences against insects and pathogens between parents and F1 hybrids, and between the sexes have also been hypothesised, but evidence is equivocal (Hjalten et al., 2000; Ahmann, 1997).

We tested the hypothesis that concentration and ratio of potentially allelochemic secondary metabolites, particularly the phenolic glycosides, vary within and between sub-arctic willow populations and species. The value of this variation as a taxonomic signal was compared to that derived from molecular markers.