Chronic additions of simulated atmospheric N deposition (3 g NO3--N m-2 y-1) since 1994 have elicited a number of responses in Michigan sugar maple forests, including enhanced aboveground growth and greatly reduced survival of seedlings. Changes in light availability could play a role in both responses, but total stand leaf area index (LAI), calculated from leaf litter mass and specific leaf area, was not altered by the N-deposition treatment. However, visual observations in the spring suggested that rates of canopy development differed between the N-deposition and control treatments, even though final canopy LAI was not affected. If true, this could affect both seedling survival and annual canopy carbon gain. To quantitatively assess this possibility, canopy transmittance of photosynthetically active radiation (PAR) was measured approximately biweekly during the period of rapid canopy development in late May and June, and less frequently thereafter, during both the 2014 and 2015 growing seasons. On each sampling date, PAR transmittance was determined at 36 evenly spaced grid locations on each of three control and three N-deposition plots (30 m x 30 m) at the study site. The PAR transmittance values were then used to calculate LAI for each plot using the Beer-Lambert law.
In both years, PAR transmittance was less in the N-deposition treatment than in the control from late May through June, as canopy leaf area developed more rapidly. Significantly greater LAI existed for the N-deposition treatment during this period, with values averaging 0.7 m2 m-2 more than those in the control. Later in the growing season, treatment effects on canopy leaf area and light penetration did not exist. Light availability at the forest floor prior to full canopy leaf-out has been shown to contribute a large proportion annual carbon gain in sugar maple seedlings. Thus more rapid LAI development and reduced early season light availability for the N-deposition treatment could contribute to the observed reductions in sugar maple seedling survival at the study site. More rapid development of full leaf area also has implications for growing season canopy C assimilation and may play a role in enhanced aboveground woody biomass production for the N-deposition treatment.