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When you look out across a snowy winter landscape, it might seem like nature is taking a long nap. Yet, beneath the surface, tiny organisms are working hard. They are consuming dead plant material and other organic matter left over from the previous year. These soil microorganisms act as Earth's recyclers. They release nutrients that will serve as fertilizer once grasses and other plants wake up with the spring snowmelt.
Key among these workers are arbuscular mycorrhizal fungi. These fungi are found in over 75% of plant species around the planet. They grow like webs inside plant roots. In this hidden space, they provide up to 50% of a plant's water and nutrient supply. In exchange, the plants give the fungi carbon, which the fungi use to grow and reproduce. This relationship is essential for healthy ecosystems.
In winter, the snowpack insulates mycorrhizal fungi and other microorganisms like a blanket, allowing them to continue to decompose soil organic matter, even when air temperatures above the snow are well below freezing. However, when rain washes out the snowpack or a healthy snowpack doesn’t form, water in the soil can later freeze – as can mycorrhizal fungi.
In a new study in the Rocky Mountain grasslands, scientists dug into plots of land that had been warmed by 2 degrees Celsius (3.6 Fahrenheit) for three decades. The researchers used suspended heaters to mimic the air temperatures the area is likely to see by the end of this century. These heaters were led by ecologist John Harte and his team. Above ground, the plots changed significantly over that time. The land shifted from being predominantly grassland to becoming more like desert shrublands. Under the surface, the scientists found something else alarming. There were noticeably fewer beneficial mycorrhizal fungi. Without these fungi, plants were less able to acquire nutrients or protect themselves from environmental stressors like freezing temperatures and drought. These changes represent a major shift in the ecosystem. On a wide scale, this could reverberate through the food web. As the grasses and forbs, such as wildflowers, that cattle and wildlife rely on decline, they are replaced by a more desertlike environment.
Warmer winters and a changing snowpack can affect the growth of plants and fungi in a few important ways. One of the first signs of changing winters is when the timing of activities for plants, fungi, and animals that rely on one another gets out of sync. For example, a mountain of evidence from around the world has documented how early snowmelt can lead to flowers blooming before pollinators arrive. This mismatch means pollinators cannot find food, and flowers cannot be pollinated.
Timing also matters greatly for plants that rely on mycorrhizal fungi. Their growth must overlap for the partnership to work. Since plants are cued to start growing by light in addition to temperature, whereas underground microorganisms are cued mainly by temperature and nutrient availability, warmer winters may cause microorganisms to become active well before their plant counterparts. This creates a gap where the fungi are working but the plants are not there to benefit.
At our research site, in a subalpine meadow in Colorado, we also initiated an early snowmelt experiment in April 2023 that advanced snowmelt in five large plots by about two weeks.
Early snowmelt can also lead to a significant loss of nutrients from the soil. When microorganisms decompose organic matter in warmer soils, nutrients accumulate in the air and water pockets between soil particles. These nutrients are then available for mycorrhizal fungi to transfer to plants. While mycorrhizal fungi transfer nutrients to the plant, other fungi are primarily decomposers that keep the nutrients for themselves. This balance usually keeps nutrients in the soil where plants need them.
However, if rain falls on the snow or the snow melts early, before plants are active, the nutrients can leach from the soil into lakes and streams. The effect is similar to fertilizer runoff from farm fields. The excess nutrients fuel algae growth, which can create low-oxygen dead zones in the water. At the same time, plants in the field have fewer nutrients available to them. This loss weakens the plants and makes them more vulnerable to stress.
This kind of nutrient leaching has happened in a variety of ecosystems with warming winters and rain-on-snow events, ranging from mountain grasslands in Colorado to temperate forests in New England and the Midwest.
Under all of these scenarios—a timing mismatch, more rain causing nutrients to leach out, or frozen soil—warmer winters are leading to less spring growth. The foundation of the ecosystem is shifting. However, ecosystems are often resilient. Organisms could acclimate to lower nutrient concentrations or shift their ranges to more favorable conditions. How plants and mycorrhizal fungi both adapt will determine how this hidden world adjusts to changing winters. Scientists are watching closely to see if the relationship can survive the pressure.
So, the next time rain on snow or a snow drought delays your outdoor winter plans, remember that it is more than a hassle for humans. It is affecting that hidden world below, with potentially long-term effects. The snow is not just a backdrop for winter sports; it is a critical shield for the life beneath it. Without it, the delicate balance between plants and fungi begins to unravel, changing the landscape from grasslands to scrub and altering the food web for everyone.