Canopy and understory additions of nitrogen change the chemical composition, construction cost, and payback time of dominant woody species in an evergreen broadleaved forest.

Simulated nitrogen deposition experiments in forests have mainly used understory nitrogen application, i.e., they failed to consider how canopy interception may alter the effects of nitrogen deposition on forest plants. This study used canopy addition of nitrogen, understory addition of nitrogen, and no-nitrogen addition control to study the effect of nitrogen deposition on the allocation of carbon assimilation products of representative woody species in an evergreen broad-leaved forest. Results showed that the maximum photosynthetic rate (A_sat) of Blastus cochinchinensis (a shrub), Ardisia quinquegona (a small tree), and Schefflera octophylla (a small tree) were significantly higher, but A_sat of Schima superba (a large tree) was significantly lower under canopy addition of nitrogen than under the control. Canopy and understory additions of nitrogen did not change A_sat of Lasianthus chinensis (a shrub). Compared with the control, leaf chemical compositions of these plants were differentially changed by canopy and understory additions of nitrogen. These changes were accompanied by a significant increase in construction cost of A. quinquegona, S. octophylla, and S. superba under canopy addition of nitrogen and of L. chinensis, A. quinquegona, and S. superba under understory addition of nitrogen. The payback time was significantly shorter for B. cochinchinensis, A. quinquegona, and S. octophylla but was significantly longer for S. superba under canopy addition of nitrogen than under the control. In contrast, the payback time was significantly shorter for B. cochinchinensis and A. quinquegona under understory addition of nitrogen than under the control. Correlation analyses showed that the changes in protein and structural carbohydrate contents helped explain the changes in payback time. In summary, nitrogen deposition may increase carbon assimilation and allocation in shrubs and small trees, and large trees may require a longer period to increase carbohydrates, which may help explain the ongoing transformation of evergreen broad-leaved forests into shrublands.