Leaf nutrient balance helps explain earlier fall color in US forests

Leaf nutrient balance helps explain earlier fall color in US forests Sadie Harley Scientific Editor Robert Egan Senior Editor For devout leaf peepers, timing is everything. A new study led by researchers from Duke University's Nicholas School of the Environment shows that leaf nutrient levels affect the onset of fall color. The findings, published in Nature Communications, could improve leaf drop forecasts and carbon cycle tracking. Starting roughly in September in the United States, deciduous trees—species that seasonally drop their leaves—stop making the green pigment chlorophyll, used in photosynthesis. As other pigments become visible, leaves shift to shades of turmeric and crimson before dropping off. By mid-November, most deciduous trees are bare. That process, called senescence, is a tree's way of conserving energy for winter. As days shorten, less sunlight is available to leaves to convert carbon dioxide from the air into life-sustaining sugars. Photosynthesis, which is energy-intensive, basically becomes more trouble than it's worth for several months. Although trees in a given locale start changing color at roughly the same time each year, environmental factors such as temperature and rainfall can affect autumn leaf senescence. For example, more rain during autumn can extend how long leaves cling to trees. Nutrient balance shapes autumn timing Nutrients in leaves also play a role in senescence. For instance, the elements nitrogen, phosphorus and potassium regulate critical leaf processes, including chlorophyll production and the transport and use of sugars. Those same nutrients are also important components of fertilizers used on crops. Their release into the environment could disrupt the balance of nutrients in leaves and therefore alter the timing of senescence, according to Tong Qiu, assistant professor of ecology at the Nicholas School, who oversaw the new study. However, measuring nutrient levels in leaves has traditionally been labor-intensive and expensive, inhibiting evidence-based research on autumn senescence. "Usually, measuring nutrients requires people to collect leaves and go back to the lab and do an experiment to quantify their concentrations," Qiu says. Reading canopy nutrients from reflected light To work around those obstacles, Qiu and a multi-institutional team turned to the National Ecological Observatory Network, or NEON. NEON assembles troves of ecological data collected by remote-sensing instruments, such as satellites and airplanes, and by researchers in the field. The data cover "a diverse range of ecosystems, from boreal tundra to tropical rainforest and dry forest, and to eastern deciduous forest and western conifers," says Hanshi Chen, a Ph.D. candidate at the Nicholas School and the study's first author. The team first chose 25 U.S. forests representing different species compositions and locations. Using NEON, they assembled remote-sensing data on hundreds of different wavelengths of light reflected and absorbed by the forests. From that spectral data, they derived information about nutrient content in tree canopies. Next, the researchers used statistical models to determine how ratios of nitrogen, phosphorus and potassium affected senescence in each forest. Nutrient ratios point to different outcomes They found that higher ratios of nitrogen to phosphorus—that is, more nitrogen, less phosphorus—were associated with earlier senescence across 20 forests, regardless of climate, elevation, species composition and tree canopy structure. By contrast, canopies with a high ratio of nitrogen to potassium were slower to drop leaves. "There seems to be a sweet spot where nitrogen and phosphorus are in balance. If that gets crossed, trees will start changing color earlier," Qiu says. Further analysis indicated that higher ratios of nitrogen to phosphorus cause stress to leaves, likely causing them to shut down photosynthesis and drop sooner. Earlier leaf loss reduces carbon uptake What do these findings mean? Put simply, an imbalance between nitrogen and phosphorus "is bad for vegetation," Qiu says. "If you have earlier senescence, it means that carbon sequestration through photosynthesis has been reduced." The findings could have implications for agribusinesses such as orchards, according to Qiu. A productive growing season is generally tied to later senescence. As time goes by, the release of nitrogen into the environment from human activities will likely increase, Qiu says, so growers "might need to apply phosphorus to balance out the nitrogen-to-phosphorus ratio." Publication details Hanshi Chen et al., Canopy nutrient balance predicts autumn senescence across U.S. forests, Nature Communications (2026). DOI: 10.1038/s41467-026-78891-1 Journal information: Nature Communications Provided by Duke University