Take a road trip from Wisconsin up through Northern Minnesota. Head through the Superior National Forest, right up to the Canadian border. Gaze out your window and watch as maple-oak forests gradually give way to aspen, spruce and birch. This changing view occurs because Northern Minnesota sits at the transition between temperate forests in the southern US and boreal forests to the north. It makes the region an ideal place to track the impact of a warming climate.
Scientists have hypothesised that, as global temperatures rise, plant species of both temperate and boreal forests will move northwards to escape warmer conditions. To test this hypothesis, researchers with the B4WARMED study have been artificially heating 18 small forest plots in the transition zone of Northern Minnesota ever since 2012. Using covers and heaters, the team increased air and soil temperatures by 1.9C and 3.5C respectively, and excluded 42–45 per cent of mean average rainfall. The long-term experiment is now yielding valuable insights into how our changing climate will affect the structure and ecosystems of boreal and temperate forests.
First, the team demonstrated that boreal species, such as balsam fir and white spruce, are less able to emerge and survive under warmer conditions. Overall, the seedling species richness in the temperate–boreal forest transition was reduced by warming. The researchers suggest that, unless the temperate species can compensate for the loss of the boreal species, the overall biodiversity of these forests is likely to decline.
Second, to explain why boreal forest species struggle under warmer conditions, the team measured the amount of photosynthesis taking place. Their results show that, as soils dry up in warmer conditions, boreal species reduce their photosynthetic rate, reducing the amount of organic matter available for growth.
This research is important because temperate and boreal forests contain 46 per cent of all trees on earth. Of the global carbon stock stored by forests, boreals contain 32 per cent, temperates 14 per cent. In the coming years, boreals will experience extreme increases in air and soil temperatures. An increase of 1.5°C has already been measured, with a rise of between 4°C and 6°C projected in the next century.
To dig even deeper into the results, the team then looked below the trees. ‘There’s been a lot of research on aboveground responses to climate change. We are also interested in how these changes cascade belowground and influence the species in the soil,’ says Christopher Fernandez, soil ecologist of the B4WARMED study. Tree species form remarkable underground partnerships with ‘mycorrhizal’ fungi in the soil: the fungi supply trees with nutrients such as phosphorous and nitrogen in exchange for carbon, which the trees produce through photosynthesis. The fungi then use the supply of carbon to form extensive underground networks, locking carbon into the soil. ‘Mycorrhizal associations are arguably the most important symbioses in terrestrial ecosystems because of their importance for plant productivity,’ says Fernandez.
Under the warmer conditions of the study, the diversity of the mycorrhizal fungi associated with boreal forest species fell. Worryingly, the types of fungi shifted from those that build extensive underground networks, to ‘weedier’ varieties that cause the network’s connectivity to break down. The researchers hypothesise that, due to the restricted ability of boreal forests to produce carbon through photosynthesis under warmer conditions, the fungi associating with the roots shift to species that demand less carbon. The warming, drier climate could therefore disrupt the vital networks of fungi underground, further depriving boreal trees of nutrients. ‘Mycorrhizal networks are so important for plant nutrition. We think that this will ultimately affect seedling recruitment in boreal forests, which depend on the nutrient flow from established mycorrhizal networks,’ says Fernandez.