On the gorgeous afternoon of Sunday October 2, the Arrowhead Native Plant Explorers met at the Cloquet Forestry Center with Dr. Artur Stefanski, a University of Minnesota Research Associate. He took us inside a deer exclosure covering several acres where a variety of research projects are running. For most of our visit Dr. Stefanski discussed one project titled Boreal Forest Warming at an Ecotone in Danger—acronym B4WARMED—which started in 2008.
The study is designed to show us the impacts of climate change on forest ecosystems. Minnesota is an ideal location for the study because it is an “edgy” state, in Dr. Stefanski’s words, sitting at the intersection of prairies, temperate forests, and boreal forests. These ecotones are likely to change more rapidly than other locations.
B4WARMED consists of two sites, with a paired site near Ely. At each site are two plots, one in a recent clearcut and one in older aspen forest. Within each plot are several circular chambers where a variety of tree seedlings have been planted. During the growing season some chambers are heated 2°C above the ambient temperature, others 4°C above, and some left at ambient. The extra heat is added to both the air around the seedlings and to the soil around their roots. These chambers allow us to see how tree seedlings will grow in a warmer climate.
One result is that with 4°C of warming the growing season increased by 14 days on average, depending on tree species. When plants are growing, they assimilate more carbon, but only when there is enough water. Therefore, the warmed seedlings absorb more carbon in the spring and fall, but less during the summer, compared to trees at ambient temperatures. And Dr. Stefanski noted that even though rainfall at Cloquet has increased over the last 100 years, the amount of evaporation has increased even more, leaving less water available to plants.
Boreal tree seedlings, like spruce, fir, aspen, and birch, suffer higher mortality and grow more slowly under warmer conditions, even in the 2°C chambers. Mature trees are not affected as drastically, but the hardship on boreal seedlings will shape the future of the forest. Temperate deciduous trees, like maples and oaks, had neutral or positive responses to warming. Every 3-5 years as the tree seedlings outgrow the chambers, they have been replaced, often with new species, including some invasives. Other investigators have also used the B4WARMED plots to study forest understory plants, soil fungi, and soil carbon flux.
This hidden corner of Cloquet is giving us a glimpse of our future forest, and with recent rates of global warming, Dr. Stefanski noted, the future looms sooner than anticipated.