Forests communicate. They send messages to animals like humans, with their color-changing leaves reminding us that the seasons are changing.
Forests play essential roles for individuals, societies and the planet. They help people communicate; we breathe and speak using the oxygen they release. On a global scale, they mitigate rising average temperatures by absorbing planet-heating carbon dioxide for photosynthesis.
But by fragmenting landscapes, expanding industrial agriculture and causing climate change, humans have degraded the planet’s forests, including those in Wisconsin. Exploring how this happened — and understanding what scientists are learning about the ongoing transformation of forests — can provide insights into possible pathways toward a more sustainable future for these treasured ecosystems.
‘Change rather than recovery’
In 2007, researchers including David J. Mladenoff, then a professor of forest ecology at the University of Wisconsin-Madison, published an analysis of how forests in the Upper Midwest had changed since what the authors call the “pre-Euro-American” era.
The study documents how, between approximately 1850 and 1900, logging of unprecedented speed devastated the northern forests of Wisconsin, Minnesota and Michigan. This era gave rise to stories of the mythical clear-cutter Paul Bunyan and the glorification of logging, especially among European settlers in the region known as the Northwoods.
This period of rapid forest degradation and continuing Euro-American colonization caused the dispossession of Native Americans, such as the Ojibwe people, from their ancestral homelands.
Yet Native Americans “are glaringly absent from the tales of loggers and logging,” wrote Kasey Keeler, an assistant professor of American Indian Studies at UW-Madison, and Ryan Hellenbrand, a graduate student.
Keeler and Hellenbrand continue: “The ‘heroic labor’ of logging formed a significant portion of Great Lakes region economies at the turn of the twentieth century, on the heels of, and entangled with, Ojibwe dispossession.”
Meanwhile, forests in the southern portions of Wisconsin were often fragmented by, or converted to, long-term agricultural land. In the mid-20th century, reforestation in parts of the Upper Midwest gave the impression of recovered forest ecosystems — an impression the 2007 study’s authors suggest is distorted.
“Our analysis shows a distinct and rapid trajectory of vegetation change toward historically unprecedented and simplified conditions,” Mladenoff and co-authors write. “Although forests have largely been re-established across northern portions of the region, these forests are on a new trajectory of change rather than recovery toward pre-Euro-American conditions.”
More specifically, Mladenoff and his co-authors show, coniferous trees like white pine and hemlock have declined, while broadleaf deciduous trees like aspen and maple have increasingly dominated since Europeans colonized North America. Overall, forest biodiversity has declined, resulting in what the authors call a “homogenization of the overall composition and structure of the regional forest.”
That’s a problem, according to Richard Lindroth, Vilas Distinguished Achievement Professor of Ecology at University of Wisconsin-Madison.
The Wisconsin Department of Natural Resources (DNR) points out that the state’s growing human population (currently nearing 6 million) and demand for natural resources could spur more land-use changes. This could further fragment and degrade forest ecosystems, ultimately reducing species diversity and with it, an ecosystem’s ability to adapt to rapidly changing circumstances.
Complex climate impacts
The changing climate is making its mark on the forests of the Upper Midwest, affecting everything from the timing of leaf-color change to the collections of species that inhabit forest ecosystems.
Leaves change colors because as the days shorten and hours of sunlight decrease, they stop making chlorophyll, which appears to us as green. As chlorophyll breaks down, other pigments already in the leaves emerge, such as the carotenoids that produce yellow hues and the anthocyanins that appear red.
Climatic factors can affect the timing and speed of leaf color change. This year, the Wisconsin DNR says warmer and drier-than-average autumn conditions in southern Wisconsin have affected leaf color change. As Colleen Matula, a forest ecologist with the Wisconsin DNR, told Wisconsin Public Radio, "Some trees respond to drought stress by skipping the fall color change altogether and the leaves will just turn brown and fall quickly off the tree.”
Additionally, as Wisconsin’s growing season lengthens with warmer average temperatures, some years may bring delayed leaf coloration. A climate-driven increase in the frequency of severe storms may also cause more leaves to drop prematurely. Scholars have shown how varied and hard-to-predict changes to fall leaf coloration in New England can impact “leaf-peeping” turnout and limit fall tourism revenue in communities that have historically benefited from it. Forest-adjacent communities in Wisconsin could soon face the same challenge.
Warming temperatures bring a slew of other impacts to forests. In 2018, a group of researchers from the U.S. Forest Service published a review of the ways in which forests in the Midwest and Northeast are vulnerable to climate change.
Shorter, warmer winters will lead to a decline in forest snowpack, which can harm root systems and reduce plant productivity, according to the Forest Service researchers. An increase in extreme precipitation events — coupled with the fact that more winter precipitation will fall as rain rather than snow — will likely increase agricultural runoff and flooding, changing the composition of forest soils.
Crucially, shifting climatic conditions can allow new species to enter habitats in which they could not previously thrive. For instance, researchers report that warmer winters could expand the range of the southern pine beetle northward, which could pose a threat to pine trees in southern Wisconsin in the coming decades.
Lindroth, who studies insects and their interactions with tree species, said increasing average winter temperatures may allow for more outbreaks of the invasive insect formerly known as the “gypsy moth” (The species is now undergoing a name change due to the term’s offensiveness to the Romani people). The moths can be devastating to forests, especially aspen and oak trees, but struggle to survive extreme cold spells that are declining in regularity as the climate warms.
“Severe cold snaps cause high mortality of overwintering eggs, and as those cold snaps become less and less frequent, we have the potential for movement of this insect species further and further north,” Lindroth said of the moths.
In addition, invasive jumping earthworms, which have proliferated at the UW Arboretum, seem to thrive under conditions of wetter soil. Their presence in local forests could have wide-ranging implications, including disrupting native soils by inhibiting underground mycelial fungi from connecting one tree to another, obstructing internal communication within the forest.
The deer that feed on forest vegetation also thrive in warmer winter temperatures, according to the Wisconsin Initiative on Climate Change Impacts (WICCI). In a recent report, authors from WICCI and other environmental organizations laid out the risks that a growing deer population presents.
“Deer browsing pressure may limit the ability of forests to respond to climate change, because species like oaks, pines, and red maple are browsed so heavily,” the report states.
A 2014 report from the U.S. Department of Agriculture (USDA) predicts that climate change will lead to a decrease in suitable habitat for several tree species in the forests of Michigan and Wisconsin, including balsam fir, black spruce, black ash, quaking aspen and paper birch. Species projected to increase the size of their habitats in the region include American beech, red mulberry and bur oak.
“There will likely be long-term changes in the composition of forests because some of the trees are going to respond better than others,” Lindroth said.
To summarize, climate change can unpredictably tweak the timing of leaf-color change that guides us from fall to winter, creates wild fluctuations between wet and dry conditions, and brings a growing assortment of invasive plants and animals to forests where they often face little to no competition for resources.
A recent article in Science uses Germany as a case study for how climate impacts on forests can build on one another, in a “deadly chain reaction.” A 2018 drought, coupled with extreme heat, deprived Germany’s spruces of water, preventing them from producing the resin that protects them from bark beetles. The beetles took advantage, devouring the leaves of entire forests, “turning them from green to ghostly gray,” writes journalist Gabriel Popkin.
Invasive species have repeatedly entered ecosystems through international trade and transport, in often avoidable circumstances. Ecosystems may be better prepared to adapt to a changing climate if governments can become “more attuned to the distribution of potentially harmful and invasive species,” Lindroth said. “That could help a lot.”
Up to this point, “the United States has not committed the resources necessary to effectively control the transcontinental shipment of items,” Lindroth said.
Satellites and cutting-edge remote-sensing technologies offer hope for monitoring and assessing ecosystems more accurately and at larger scales than ground-based surveys can. With these technologies, scientists and policymakers will be able to flexibly respond to ongoing deforestation and ecosystem degradation – and aim to reduce or reverse harmful consequences – by measuring biodiversity from space.
A fierce debate among scientists is ongoing about whether humans need to proactively help vulnerable tree species migrate faster than they otherwise could as climatic conditions change. Some researchers argue this human help could save some species from extinction. The process, known as “assisted migration,” is an example of climate adaptation in which humans intervene with natural processes in order to solve problems they caused in the first place.
This recurring theme is explored in the book Under A White Sky: The Nature of the Future by Elizabeth Kolbert: “We face a no-analog predicament,” Kolbert writes. “If there is to be an answer to the problem of control, it’s going to be more control.”
The promise of biodiversity
There are signs of hope for Wisconsin’s forests. For instance, the Conservation Fund recently bought about 700,000 acres of the Pelican River Forest, an area in Oneida County that was (until it was bought) the largest unprotected forest in the state. This bodes well for water quality in local communities near the forest and for the ecosystem’s ability to retain biodiversity in the face of climate change.
Wisconsin contains a wide range of biomes, or different communities of flora and fauna, from grasslands and savannas to an array of forest types. That ecosystem diversity gives the state potential for rich species diversity, which promotes resilience in the face of change.
“Wisconsin as a state has unusually rich biological diversity because we’re at the historical borders of these major biomes, the prairies and the Northern forests,” Lindroth said.
But thwarting invasive species, assisting trees on their journeys into new habitats, and ultimately promoting rich biological diversity, are challenging and complicated tasks.
“Ecology is not rocket science. It’s way harder,” Lindroth said.
And there is more at stake in protecting forest biodiversity than the condition of the forest.
“There is an emotional component to rich biological diversity that people can connect to,” Lindroth said. “There is this wonder and awe component. It enriches our lives.”
In the world of ecology, where every point has a counterpoint, nearly every tendency an exception, rules are difficult to come by. But one key pattern that is well-documented is the “species-area relationship,” which describes how, as the amount of land in a habitat increases, the number of species it can support increases as part of an exponential function.
In other words, adding even relatively small amounts of land to a forest habitat can significantly increase its ability to support greater biological diversity. Thus, preserving forest land preserves forest biodiversity.
And it is worth preserving. After all, biodiversity represents “the assembly of life that took a billion years to evolve,” as the late biologist E.O. Wilson wrote. “It has eaten the storms – folded them into its genes – and created the world that created us. It holds the world steady.”