Effects of resource manipulation on the correlation between total phenolics and astringency in Douglas-fir

The correlation between total phenolics and tannin content within a species is often considered to be suitably strong to allow researchers to assume, with some degree of confidence, that levels of one will approximately parallel the other. However, the manipulation of resource availability could lead to disproportionate changes in total phenolics and tannins and/or in the specific monomers of which these fractions are composed, thus altering the correlation between these components. In order to test this hypothesis, we examined the correlation between foliar levels of total phenolics (as measured by the ferric chloride assay) and tannins (as measured by an astringency assay) in Douglas-fir (Pseudotsuga menziesii Mirb. Franco) before and after the manipulation of nitrogen and water availability. Prior to manipulation of resources, the correlation between total phenolics and tannins was strong and highly significant (r²=0.869;P < 0.001). This correlation was considerably weaker and not statistically significant following resource manipulation (r² = 0.392; 0.20 <P < 0.50). These results demonstrate that manipulation of resource availability can alter the correlation between total phenolics and tannins in intraspecific comparisons. The causes underlying the observed degradation in the correlation between these measures (whether qualitative, quantitative, or both) are unknown and require further investigation.