Seasonality and nitrogen supply modify carbon partitioning in understory vegetation of a boreal coniferous forest

Background/Question/Methods

Given the strong coupling between the carbon (C) and nitrogen (N) cycles, there is substantial interest in understanding how N addition affects the way C is cycled in terrestrial ecosystems, especially in N-limited ecosystems. However, most studies in temperate and boreal forests have focused on the effects of N addition on tree growth. By comparison, less is known about the effects of N availability on the cycling of C in understory vegetation despite some evidence that dwarf shrubs, mosses, and lichens play an important role in the forest C balance. In this study, we used an in situ ¹³CO₂ pulse-labelling technique to examine the short-term dynamics of C partitioning in understory vegetation in three boreal Pinus sylvestris forest stands exposed to different rates of N addition. We labelled the understory as a whole and then monitored respiratory ¹³C release and assimilation into different functional groups (i.e., lichens, mosses, and ericaceous dwarf shrubs), as well as the partitioning of ¹³C into different components of ericaceous biomass (leaves, stems, rhizomes, and fine roots). ¹³CO₂ labelling was conducted at two distinct time periods (early vs. late growing season) to provide a seasonal picture of how N addition affects understory C dynamics.

Results/Conclusions

This study reinforces emerging evidence that the response of ecosystem processes to N additions in boreal forests may not be linear or even unidirectional. Moreover, the magnitude and direction of these responses were strongly dependent on the time during the growing season and differed among plant functional groups. In contrast to what has been found in trees, belowground partitioning of recently assimilated C in ericaceous plants was high in the control plots, lower at low rates of N addition, and high again at high rates of N addition. Increasing N addition led to a greater percentage of C being partitioned to ericaceous leaves with a higher turnover, whereas high rates of N addition severely reduced C assimilation in more recalcitrant moss tissues. Addition of N also resulted in a greater percentage of ¹³C being respired back to the atmosphere, especially early in the growing season, and an overall reduction in total understory carbon-use-efficiency. Taken together, our results suggest increasing N additions result in faster C cycling in understory vegetation and highlight the need for further studies aimed at quantifying the partitioning of C to different plant tissues in order to improve the parameterization of process-based forest C models.