A study was made of the retention times of N and P in the leaf biomass and their relationship with the retranslocation percentages and the leaf longevities in some woody species in Central Spain. The retention times of both nutrients were strongly related to the nutrient status of each species. These results suggest that a prolonged retention time is a way of increasing nutrient use efficiency in conditions of low nutrient availability. Plants can increase the retention time of nutrients in their leaf biomass by means of an increase in leaf longevity and/or by means of an increase in retranslocation efficiency. However, the effect of the retranslocation efficiency on retention times was almost negligible compared with the effect of leaf longevity. This suggests that an increase in leaf longevity is probably the best adaptation for increasing efficiency in the use of nutrients.

The efficiency of nitrogen recovery before leaf fall in different woody species was studied with respect to the phenology of leaf abscission and to different estimates of nitrogen availability. Among the factors considered, the duration of the abscission period shows the strongest effect on the translocation efficiency. The species with gradual leaf fall exhibit lower percentages of nitrogen recovery, probably due to greater unpredictability of the exact time of abscission. Gradual leaf fall seems to be an adaptation to water stress in regions with an arid or semiarid climate. The species that occupy the more xeric sites thus show lower retranslocation rates. By contrast, the factors related to nitrogen availability show no clear effect on the reabsorption efficiency.

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Nitrogen retranslocation from senescing leaves represents a crucial adaptation by tree species towards a more efficient use of this nutrient. As a result, this part of the nitrogen cycle has received increasing attention in recent years. However, there remain strong discrepancies with respect to the factors responsible for interspecific differences in the efficiency of this process.In the present work the seasonal pattern of leaf growth and the movement of nitrogen in leaves have been studied in a series of Quercus ilex plots with different levels of rainfall and soil quality in centralwestern Spain, as well as in 20 other woody species typical of this area. The percentage of nitrogen retranslocated was estimated from the difference between the maximum mass of nitrogen stored in the leaf biomass and the amount of this nutrient returned annually to the soil through leaf fall.Q. ilex appears as one of the least efficient species in the Mediterranean region in the recovery of nitrogen from senescing leaves (29.7°0 of the maximum pool). Furthermore, the older leaves of Q. ilex do not show the cycles of nitrogen withdrawal during new flushes of shoot growth, such as occurs in Pinus spp. This suggests that older leaves in Q. ilex do not play an important role as nitrogen storage organs.

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