Plant Phenols and Nutrients in Relation to Variations in Climate and Rodent Grazing

Jonasson, Sven; Bryant, John; Chapin, F; Anderson, Malte

Secondary plant metabolites such as phenols and tannins sometimes have a repellent effect on herbivores and may reduce consumer pressure on plants. Grazer-induced changes in the concentrations of secondary metabolites have been proposed as an important cause of population cycles among small rodents. To test this hypothesis, we measured several secondary metabolites and nutrients in five species of plants during two rodent cycles on alpine heath in northern Sweden. We also tested whether climatic effects on plant chemistry could generate microtine cycles by altering food quality. Phenol concentrations (Folin-Denis, proanthocyanidin, and vanillin phenols) from 1974 to 1981 correlated strongly with each other, but correlated negatively with concentrations of nitrogen, phosphorus, and potassium. Astringent tannins, calcium, and magnesium showed few significant correlations with the other components. There was no evidence that rodent grazing markedly affected the chemical composition of plants during the same or the subsequent year. Secondary metabolites and nutrients also failed to show any clear relationship to the 4-yr cycles in rodent density, which are probably caused by other factors. Instead, the levels of phenols and nutrients seem to vary according to climatic fluctuations. In warm summers, nutrient concentrations decreased and phenols increased in four of the five plant species examined. A possible explanation is that the increased photosynthate in warm summers was partly allocated to synthesis of phenols and other carbohydrates, causing a simultaneous dilution of nutrients.

Plant Phenols and Nutrients in Relation to Variations in Climate and Rodent Grazing

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