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

Abstract

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