To further test the effects of leaf litter accumulation on the emergence and establishment of sugar maple seedlings, the forest floor mass was experimentally manipulated at one of the four Michigan sugar maple test sites.

Sierra Patterson, who conducted the study for her master’s thesis at the University of Michigan School of Natural Resources and Environment, is the lead author of the article published online Dec. 8 in the Journal of Applied Ecology.

Patterson explained the process:

The researchers created three small (1 meter by 0.5 meter) subplots alongside each of the ambient nitrogen-deposition plots. Three more subplots were established alongside each of the simulated nitrogen-deposition plots.

In each of the subplots, all the leaf litter was removed down to the soil, and 120 sugar maple seeds were evenly distributed on the soil. Then the leaf cover was replaced. Half of each subplot served as a control treatment; the original forest floor litter mass was replaced over the seeds.

In the other half of the subplots alongside the ambient nitrogen-deposition plots, extra leaves were added (2,579 grams per square meter) to simulate leaf cover at a nitrogen-supplemented treatment. Subplots next to the nitrogen deposition plots received a thin layer of leaves (1,708 grams per square meter), similar to the leaf cover found around the no-nitrogen-added treatment.

In the control portion of each subplot, five times as many seedlings established in the ambient nitrogen treatment versus the simulated nitrogen treatment. Increasing forest floor mass in the ambient nitrogen-deposition subplots to equal that of the simulated nitrogen-deposition treatment resulted in a 65 percent decline in seedling establishment.

In contrast, when forest floor mass under simulated nitrogen deposition was decreased to match that seen under ambient nitrogen deposition conditions, seedling establishment increased by 80 percent.

“There could have been other reasons why we had higher densities of sugar maples in areas where there was ambient nitrogen. So we isolated that one variable – forest floor accumulation – and demonstrated that it’s what’s responsible for this difference in seedling establishment,”said Patterson, who conducted the study with Donald Zak, a professor at the U-M School of Natural Resources and Environment.