{"id":2646,"date":"2025-10-01T12:13:31","date_gmt":"2025-10-01T11:13:31","guid":{"rendered":"https:\/\/research.reading.ac.uk\/lemontree\/?p=2646"},"modified":"2025-10-01T15:01:39","modified_gmt":"2025-10-01T14:01:39","slug":"trading-nitrogen-for-water","status":"publish","type":"post","link":"https:\/\/research.reading.ac.uk\/lemontree\/trading-nitrogen-for-water\/","title":{"rendered":"Trading Nitrogen for Water: What Sugar Maples Teach Us About Photosynthetic Economics"},"content":{"rendered":"

How do trees decide how to use the resources available to them? Just like any economy, plants face trade-offs. They must balance investments of nutrients and water to maintain photosynthesis.<\/p>\n

A new study<\/a> led by LEMONTREE\u2019s Evan Perkowski (Texas Tech University) study has provided rare experimental evidence for this balancing act. Working with sugar maple (Acer saccharum<\/em>), a dominant tree species of northeastern U.S. forests, they showed that trees maintained steady rates of photosynthesis across a nitrogen fertilisation gradient by using nitrogen less efficiently in exchange for conserving water. In other words, when nitrogen was plentiful, trees \u201cspent\u201d it more freely, while saving water.<\/p>\n

These findings provide strong support for predictions from photosynthetic least-cost theory, the Eco Evolutionary Optimality framework that explains how plants optimise resource use to achieve photosynthesis at the lowest combined cost. And they have important implications for understanding how forests will respond to global changes in nitrogen deposition, soil acidification, and climate.<\/p>\n

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Study site<\/h2>\n

The study took place in summer 2019 at three mature forest stands near Ithaca, New York. These late-successional mixed deciduous forests are dominated by sugar maple, which accounts for nearly half (43%) of total aboveground biomass. Other important species include American beech, white ash, red maple, and red oak.<\/p>\n

The soils are channery silt loam Inceptisols (young soils with little layering), and the climate is cool-temperate: the region receives nearly 1,000 mm of precipitation annually, with an average temperature of just under 8\u00b0C. Long-term monitoring shows large declines in nitrogen and sulfur deposition since the early 2000s, thanks to U.S. Clean Air Act regulations. Nitrogen inputs, however, remain high enough to influence forest nutrient dynamics\u2014making these sites an ideal living laboratory for studying how trees balance nutrient and water use.<\/p>\n

To test least-cost predictions, the team used a nine-year nitrogen-by-pH field manipulation experiment. By carefully adjusting nitrogen availability and soil acidity across plots, they could observe how sugar maple leaves responded in terms of photosynthetic traits, nitrogen allocation, and water use efficiency.<\/p>\n

Fieldwork in these tall forests came with some challenges. Collecting sunlit leaves from the upper canopy required an ingenious technique: using a slingshot with a string and weighted ball to snag high branches. As Evan notes, it was both an effective and rather amusing way to gather samples (see photo of collaborator David Frey putting the method to work).<\/p>\n

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Left: Co-author David Frey (Cornell University) collecting leaf samples. Right: taking leaf physiology measurements in the field.<\/figcaption><\/figure>\n

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What the trees revealed<\/h2>\n

The results aligned well with least-cost theory. Across the nitrogen treatments, sugar maples:<\/p>\n