Interspecific defoliation responses of trees depend on sites of winter nitrogen storage

1. To determine if the response of trees to herbivory is related to their ability to store nitrogen, saplings of Pinus sylvestris L., Betula pendula Roth. and Sorbus aucuparia L. were clipped, either when dormant or at spring bud burst, to remove half the previous year's shoot growth. The impact of clipping on N remobilization and uptake was quantified in relation to their growth responses. 2. Pinus sylvestris stored N during the winter in needles grown the previous summer, whereas B. pendula and S. aucuparia used their woody roots and older stems. Therefore, N remobilization was unaffected by clipping the deciduous species, but was reduced by half in the evergreen. 3. For both P sylvestris and B. pendula, root uptake contributed N for leaf growth immediately after bud burst, concurrently with remobilization. Sorbus aucuparia had remobilized half the N from storage before any N was taken up by the roots. 4. Pinus sylvestris, which has a fixed pattern of growth, produced a lower total needle mass when clipped, but individual needles were heavier. Nitrogen uptake during summer was reduced by 26 and 44% for winter- and spring-clipped saplings, respectively. Both deciduous species showed compensatory leaf growth such that, by the end of the summer, leaf mass (and B. pendula leaf area) were unaffected by clipping. Nitrogen uptake by both deciduous species was unaffected by clipping. 5. We conclude that the site of N storage during winter is a crucial factor in determining the response of a sapling to herbivory, as interactions with the growth patterns of the three species can lead to different responses to damage. We suggest that for evergreen trees, the production of antiherbivory compounds serves primarily to protect N stored in their foliage, rather than the leaves themselves.