Uptake, demand and internal cycling of nitrogen in saplings of Mediterranean Quercus species.

Nitrogen uptake, nitrogen demand and internal nitrogen cycling were studied to address the question of the importance of nutrient storage in Quercus species with contrasting leaf longevities. We carried out this study at the whole-plant level with young trees (2-4 years old) of three Mediterranean Quercus species: the evergreen Q. ilex, the marcescent/evergreen Q. faginea, and the deciduous Q. pyrenaica. Seasonal dynamics of nitrogen in all compartments of the plant were followed for 3 years. Nitrogen losses were measured through litter production, herbivory and fine root shedding. Nitrogen uptake was estimated using increments of nitrogen plant content plus accumulative nitrogen losses. Nitrogen uptake was limited to a few months during late winter and spring. Before budbreak, acquired nitrogen was stored in old-leaf cohorts of evergreen and woody compartments. After budbreak, Quercus species relied first on soil uptake and second on nitrogen retranslocation to supply new growth requirements. However, in most cases we found a high asynchrony between nitrogen demand by growing tissues and soil supply, which determined a strong nitrogen retranslocation up to 88.4% of the nitrogen demand throughout leaf expansion. Except for the first year after planting, the above- and underground woody fractions provided more nitrogen to the new tissues than the old leaf cohorts. Differences in the benefit of nitrogen withdrawn from senescent and old leaves were not found between species. We conclude that sink/source interaction strength was determined by differences between nitrogen demand and uptake, regulating internal nutrient cycling at the whole plant level.