Paper Reviewed

Li, L., Manning, W. and Wang, X. 2019. Elevated CO 2 increases root mass and leaf nitrogen resorption in red maple (Acer rubrum L.). Forests 10: 420; doi:10.3390/f10050420.

Working with nine continuously stirred tank reactor chambers located inside a glass greenhouse at the University of Massachusetts, Li et al. (2019) recently exposed two-year-old red maple seedlings (Acer rubrum) to three different concentrations of atmospheric CO 2 over the course of one full growing season. The main objective of their experiment was to examine the leaf nitrogen (N) content, N resorption efficiency and biomass allocation responses of this deciduous tree species that is widespread across eastern and central North America. The importance of studying N resorption efficiency (NRE), which is defined as the process by which N is mobilized from senescent leaves and transported to other plant parts, was to determine if this process is enhanced under elevated CO 2 so as to help counteract leaf N decline that is often observed in tree leaves at high levels of CO 2 .

The results of the authors' study are summarized in the figure below. Panel (a) illustrates the total dry mass produced at the end of the growing season at the three different CO 2 treatments, which shows that the trees in the 800 ppm treatment experienced a 19.2% growth enhancement compared to the ambient (400 ppm) CO 2 treatment. However, no biomass difference was found between trees growing in the ambient and 600 ppm CO 2 treatments.

Li et al. also report that the N content of senescent leaves declined by 27.5% and 28.1% (relative to ambient CO 2 ) in the 600 and 800 ppm CO 2 treatments, respectively. In addition, elevated CO 2 enhanced NRE by 46.2% and 50.3% at 600 and 800 ppm CO 2 , again respectively. The significance of this latter observation, according to the authors is that "higher NRE of senescent leaves makes [it] possible [for] more nitrogen transfer to other plant organs for early growth [the] next year." And thus, thanks to elevated CO 2 , NRE is becoming "an increasingly important N source" for red maple and likely other trees and plants.



Figure 1. Total dry mass (Panel a) and nitrogen resorption efficiency (NRE, Panel b) of red maple seedlings grown under three different atmospheric CO 2 treatments (A400, A600 and A800 correspond to 400, 600 and 800 ppm CO 2 , respectively) at the end of one growing season. Source: Adapted from Li et al. (2019).