Effects of [CO 2] and nitrogen on morphological and biomass traits of white birch ( Betula papyrifera) seedlings

Abstract

To investigate the interactive effects of CO 2 concentration ([CO 2]) and nitrogen supply on the growth and biomass of boreal trees, white birch seedlings ( Betula papyrifera) were grown under ambient (360 μmol mol −1) and elevated [CO 2] (720 μmol mol −1) with five nitrogen supply regimes (10, 80, 150, 220, and 290 μmol mol −1) in greenhouses. After 90 days of treatment, seedling height, root-collar diameter, biomass of different organs, leaf N concentration, and specific leaf area (SLA) were measured. Significant interactive effects of [CO 2] and N supply were found on height, root-collar diameter, leaf biomass, stem biomass and total biomass, stem mass ratio (SMR), and root mass ratio (RMR), but not on root mass, leaf mass ratio (LMR), leaf to root ratio (LRR), or leaf N concentration. The CO 2 elevation generally increased all the growth and biomass parameters and the increases were generally greater at higher levels of N supply or higher leaf N concentration. However, the CO 2 elevation significantly reduced SLA (13.4%) and mass-based leaf N concentration but did not affect area-based leaf N concentration. Increases in N supply generally increased the growth and biomass parameters, but the relationships were generally curvilinear. Based on a second order polynomial model, the optimal leaf N concentration was 1.33 g m −2 for height growth under ambient [CO 2] and 1.52 g m −2 under doubled [CO 2]; 1.48 g m −2 for diameter under ambient [CO 2] and 1.64 g m −2 under doubled [CO 2]; 1.29 g m −2 for stem biomass under ambient [CO 2] and 1.43 g m −2 under doubled [CO 2]. The general trend is that the optimal leaf N was higher at doubled than ambient [CO 2]. However, [CO 2] did not affect the optimal leaf N for leaf and total biomass. The CO 2 elevation significantly increased RMR and SMR but decreased LMR and LRR. LMR increased and RMR decreased with the increasing N supply. SMR increased with increase N supply up to 80 μmol mol −1 and then leveled off (under elevated [CO 2]) or stated to decline (under ambient [CO 2]) with further increases in N supply. The results suggest that the CO 2 elevation increased biomass accumulation, particularly stem biomass and at higher N supply. The results also suggest that while modest N fertilization will increase seedling growth and biomass accumulation, excessive application of N may not stimulate further growth or even result in growth decline.