On a crisp autumn morning in New Hampshire’s White Mountains, a hush settles over the canopy of sugar maple (Acer saccharum) and American beech (Fagus grandifolia). Some leaves glow yellow and orange in the bright sun, while others have already curled at the edges, preparing their quiet descent. Scientists at the Hubbard Brook Experimental Forest have long recognized that this annual display of color conceals a remarkable balancing act. As leaves age, trees withdraw precious nutrients—especially nitrogen—from their foliage and shuttle them back into woody tissues, ensuring they have the raw materials needed for next spring’s flush of growth.
In the paper, “Foliar resorption of beech and maple along an elevation gradient in a northern hardwood forest,” Timothy J. Fahey, Natalie L. Cleavitt, Pamela H. Templer, Peter M. Groffman, Amey S. Bailey, Stephen B. Caron, and Geoffrey Wilson explore how these two iconic hardwoods manage their nutrient economics under changing conditions. From warmer, low-elevation sites to the cool ridges at nearly 800 meters, the authors trace an elegant but often overlooked physiological dance. As they write, “Foliar N resorption is a principal nutrient conservation mechanism in terrestrial vegetation that could be sensitive to ongoing changes in climate and atmospheric nitrogen (N) deposition.”
Although it might seem counterintuitive, soil at higher elevations in Hubbard Brook actually holds more available nitrate than one might expect. Nevertheless, the authors found that colder upper slopes do not necessarily lead to heightened leaf nitrogen withdrawal. “Foliar N resorption proficiency (NRP) increased significantly at lower elevations for both sugar maple and American beech,” the authors state, suggesting that warmer conditions may allow for a more extended or more efficient period of resorption. Indeed, when autumn nights remain relatively mild, trees have extra time to break down proteins and shuttle amino acids back into their perennial tissues, gleaning as much nitrogen as possible.
What emerges is a nuanced portrait of how sugar maple and American beech approach this resource puzzle. Beech, the paper reports, is especially adept at stripping nitrogen from its leaves. Yet interestingly, maple leaves, while withdrawing nitrogen efficiently, also end up with particularly low nitrogen in the final litter. “Both species also exhibited strong correlations between resorption efficiency of N and C, but resorption of both elements was much greater for beech than sugar maple,” say the authors. Sugar maple’s leaf litter, in turn, often has a higher carbon-to-nitrogen ratio—meaning it decomposes more slowly, keeping nitrogen in the ecosystem under tighter wraps. It is a small reminder that even in a single patch of forest, different species can adopt different strategies to thrive.
Such subtle differences reverberate through the forest floor and beyond. In an era marked by declining atmospheric nitrogen inputs—an outcome of environmental regulations that have reduced air pollution—trees’ ability to conserve what they have becomes all the more significant. “We anticipate that with climate warming and decreasing N inputs, northern hardwood forests can be expected to exhibit stronger N conservation via foliar resorption,” the authors write, highlighting how internal recycling of nitrogen might become a critical bulwark against nutrient scarcity. Extended growing seasons and delayed frosts could grant these trees extra days of relatively pleasant weather in which to shuttle amino acids back into their trunks and roots before winter forces the leaves to drop.
What is perhaps most striking about Hubbard Brook’s “natural laboratory” is that it offers a sneak peek into future scenarios. Warm lower elevations can serve as analogs for the temperatures that upper slopes may experience decades from now. “Resorption is a complex yet ordered process in which macromolecules (e.g., proteins) are broken down enzymatically into soluble forms (e.g., amino acids) and transported via phloem from ephemeral tissues (e.g., foliage) to storage pools in perennial tissues,” the authors explain. Under warming conditions, the finely-tuned coordination of these enzymes might operate for a bit longer each fall, allowing trees to reclaim more nitrogen.
The team underscores that while sugar maple and beech share habitats, they’re not identical in their nutrient-scavenging approaches. Beech, for instance, has typically shown greater nitrogen resorption efficiency, a finding that could set the stage for shifts in relative abundance of these species as the climate warms. Yet the story remains far from simple, because so many factors—from precipitation patterns to pest outbreaks—can upset even the best-laid strategies in a forest’s nutrient dance.
Ultimately, the changing climate at Hubbard Brook is not just a matter of whether a sunny afternoon in October feels a bit warmer; it is about whether beech and maple can seize that warmth to secure the nitrogen they need. “Nitrogen resorption proficiency (NRP) increased significantly at lower elevations,” the authors conclude, driving home the point that a warming world may nudge these trees toward ever-more conservative nutrient recycling. In one sense, the trees are thrifty: they’ve learned to bank their resources for the harder times ahead. In another sense, they remind us of nature’s breathtaking complexity—how a single forest can show us, leaf by falling leaf, exactly what it means to adapt.
Timothy J. Fahey, Natalie L. Cleavitt, Pamela H. Templer, Peter M. Groffman, Amey S. Bailey, Stephen B. Caron, Geoffrey Wilson, Foliar resorption of beech and maple along an elevation gradient in a northern hardwood forest, Forest Ecosystems, Volume 13, 2025, 100304, ISSN 2197-5620, https://doi.org/10.1016/j.fecs.2025.100304.