Seasonal variation in the nutritional value of woody plants along a natural gradient in Eastern Africa

Several hypotheses relate a negative relationship between foliar concentration of phenolic compounds and nitrogen to physiological processes such as leaf development, seasonal variation in allocation priorities, nutrient, light and water related growth limitation, as well as herbivore attack. We sampled four common deciduous woody species of central Tanzania monthly during the growing season to assess changes in this relation and their nutritional value to ruminants. We found a negative relationship between leaf N and phenolic compounds within and among species and sites that weakens during the course of the growing season that was consistent for total phenolics, but not for condensed tannins. Leaf N concentration decreased throughout the season, its withdrawal being positively related with leaf N at first sampling date. Secondary compounds concentration showed no consistent seasonal trend. Concentrations of leaf N and phenolics were correlated with ¹³C discrimination in the two shrub species and with soil P in the two tree species. Digestibility was positively correlated with foliar N and negatively correlated with secondary compounds. We conclude that phenolic compounds may serve as reliable clues for selecting foliage rich in N at site and species level only during the first months of the growing season.