What was done
For two full growing seasons the authors studied the interactive effects of elevated atmospheric CO2 concentration (658 ppm vs. the ambient concentration of 383 ppm), nitrogen (N) deposition (ambient and ambient + 30 kg N ha^-1 year^-1), and light availability (limited and saturated) on leaf photosynthesis, growth and survival of understory seedlings of six different hardwood tree species - paper birch (Betula papyrifera), quaking aspen (Populus tremuloides), sugar maple (Acer saccharum), American beech (Fagus grandifolia), eastern white pine (Pinus strobus) and black cherry (Prunus serotina) - inside open-top chambers located within a 90-year-old N-limited northern hardwood forest in northern Lower Michigan, USA.

What was learned
Over the course of the two-year study, the 72% increase in the air's CO2 concentration increased light-limited photosynthesis in the six different species by an average of 47%, while it increased light-saturated photosynthesis by fully 60%. With respect to survival, at low N-availability seedling survival rates were similar in the ambient and elevated CO2 treatments at 57% ± 5% and 55% ± 4%, respectively; but, as the researchers report, "for plants grown with high N availability, those grown in ambient CO2 demonstrated 78 ± 4% survival, and those grown in elevated CO2 exhibited the greatest survival rate of all of the treatment combinations with an 85 ± 2% survival rate."

What it means
First of all, Sefcik et al. write that "N deposition may alleviate some photosynthetic acclimation [i.e., down regulation] to long-term CO2 enrichment in N-limited understory seedlings." Second, and most importantly - as well as very bluntly but absolutely correctly stated - they say that "increasing CO2 and N deposition from fossil fuel combustion [our italics] can directly impact seedling physiology and survivorship," quite obviously for the better.