For 25 consecutive summers, a team from Michigan State University has walked the same transects through Manistee National Forest in northwestern Lower Michigan, crouching down to count and identify tree seedlings no taller than a few inches. The result is a dataset of 188,821 individual seedlings from 10 common tree species, collected across 12 sites spread over a 370-square-mile area.
The work, published in Global Change Biology Communications, is starting to show which young trees can handle a warming, drying climate and which cannot. That question matters enormously in a state where forests cover half the land, support more than 90,000 jobs, and add $27 billion to the economy each year.
The study was led by forestry professor Richard Kobe, with co-author Bailey McNichol, a postdoctoral scholar in forestry and MSU's Ecology, Evolution, and Behavior program. Average temperatures in the Great Lakes region have already risen by 2.3 degrees Fahrenheit since the 1950s, and projections call for an increase of 6 to 11 degrees Fahrenheit by the end of the century. Shifting precipitation patterns are changing conditions on the forest floor at the same time.
Researchers focused on seedlings less than one year old because that stage is when trees are most sensitive to environmental stress. Seedlings have shallower roots than established trees, making them more exposed to warming and drying conditions near the soil surface.
"This is also the stage when trees are most at risk from diseases and grazing deer," McNichol said.
The seedling stage sits at a particular crossroads in a tree's life.
"It's the point in a tree's life when it has passed the first series of gauntlets — it's made it through germination and it's starting to sprout — but it still has a long way to go," McNichol said.
Many seedlings never reach maturity. Some are lost to drought, some to deer, some to disease. But those that do survive eventually grow into the canopy trees that define what a forest looks like and what it can do, from filtering water and air to preventing erosion to providing timber and wildlife habitat.
At each site, researchers walked a set transect and counted every seedling under a year old that they found. Identification required close attention. Maple seedlings could often be spotted by their jagged leaf edges or sunken veins. Others required more skill to distinguish.
By comparing seedling counts across years and sites, the team can see how different species respond to the climate conditions in any given year. Over time, that information points toward which species are likely to increase their presence in Michigan forests and which may thin out as conditions shift.
The research does not project a single fixed future for the forest. But 25 years of data across nearly 190,000 individual seedlings gives scientists a clearer picture than they have had before of how the next generation of trees is taking shape beneath the current canopy.
