Nitrogen fertilization has differential effects on N allocation and lignin in two Populus species with contrasting ecology

Black cottonwood (BC, Populus trichocarpa) and hybrid aspen (HA, P. tremula 9 tremuloides) differ in their ecology of being adapted to wet and drier conditions as riparian and early successional forest species, respec- tively. We tested the hypothesis that these ecological dif- ferences were reflected in higher nitrogen (N) use efficiency in HA than in BC and that HA would allocate more resources belowground than BC in the presence of high and low N availability. We expected that responses of wood properties to elevated N would be more pronounced in the species with higher wood formation in response to N supply. HA showed higher belowground biomass parti- tioning than BC in the presence of low (0.2 mM) and high (2 mM) N supply, but in contrast to our expectation whole- plant nitrogen use efficiency and the stem carbon-to- nitrogen balance were lower than in BC. In response to elevated N, HA exhibited stronger stimulation of biomass production than BC, especially of the stem, which showed significant increases in biomass and volume but decreases in density. Lignification, especially the production of guaiacyl (G)-compared to syringyl (S)-lignin, was delayed in HA compared with BC wood. Since G lignin leads to stronger crosslinking than S lignin, its delayed formation may have enabled stronger expansion and higher volume increment of HA than of BC stems. Our results suggest that BC, a poplar species adapted to fluctuating N supply, is less responsive to differences in N availability than aspen that occurs in low N environments.