Leaf-level nitrogen-use efficiency of canopy and understorey species in a beech forest

1. In a forest stand, canopy and understorey species grow at completely different irradiances and consequently with different carbon and nitrogen availability ratios. We studied how the difference in growth irradiance influenced plant N use in a mature beech forest. 2. We defined leaf-level nitrogen-use efficiency (NUE_L) as the amount of the leaf dry mass produced per unit N taken up by leaves. NUE_L was similar between the canopy species (Fagus crenata) and the understorey species (Lindera umbellata and Magnolia salicifolia). 3. NUE_L was analysed further as the product of two components: leaf-level N productivity (NP_L) and mean residence time of leaf N (MRT_L). The canopy species had significantly larger NP_L and significantly shorter MRT_L than the understorey species. 4. As the photosynthetic capacity was similar among the species, different NP_L between the species was attributable largely to the difference in light conditions to which their leaves were exposed. 5. The difference in MRT_L was not attributable to potential efficiency of N resorption (R_EFF) determined at leaf senescence, but to actual R_EFF, which depended on the amount of green leaf lost before full senescence. The canopy species had significantly smaller actual R_EFF because of strong wind actions in the canopy. 6. Although the canopy species realized higher NP_L by virtue of high irradiance, it had shorter MRT_L due to wind damage to pre-senescent leaves. On the other hand, the canopy species had shorter NP_L under shady conditions, but had longer MRT_L with little wind damage. Interplay of local environmental factors such as light and wind strongly influenced N use by plants in the beech forest.