Urban Trees: Rapid Growth but Reduced Long-Term Ecological Resilience

The city warms trees like an open greenhouse, with 3 to 10 °C more in some dense urban centers

The team led by Hans Pretzsch at the Technical University of Munich compared nearly 1,400 trees across ten metropolises and their surroundings. The researchers read the tree rings as one would read an annual record etched into the trunk.

The result places urban trees in an environment that is less hostile than it may appear. At a given age, city subjects are on average taller than their rural neighbors, with about 25 % difference at 50 years and still nearly 20 % at 100 years.

The main explanation lies in the urban heat island. This term refers to the excess heat retained by mineral soils, façades, asphalt, and human activities. For the tree, a few extra mild weeks resemble a shop that stays open longer each day.

CO2 and streetlights extend the green season, by almost 24 days in some observations each year

Heat alone is not enough to tell the whole story. In cities, carbon dioxide concentrates near traffic, heating systems, and human activities. Photosynthesis makes sugars from light, water, and CO2. This feedstock supports growth.

Trees do not respond only to the thermometer. A study published in 2025 in Nature Cities followed 428 northern-hemisphere cities between 2014 and 2020. It links nocturnal artificial light to phenology, the biological calendar of leaves.

In these observations, urban greening begins on average 12.6 days earlier and the season lasts 11.2 days longer than in rural surroundings. Street lamps act as a day-length extension, especially when the tree should receive the signal to rest.

Growing fast does not necessarily mean storing carbon sustainably, as urban mortality really changes the balance

The Boston study by Ian A. Smith and colleagues shows the flip side of the phenomenon. They compared street trees with stands in Harvard Forest. In Boston, the average diameter growth was almost four times faster than in the nearby forest.

But this advance comes with higher mortality. The average rate for street trees exceeded 3% per year, versus about 1.4% in the studied forest. Compacted roots, impervious soils, and mechanical injuries then reduce durable storage.

In Paris, planting many trees is not enough if the roots lack living soil, water, and enduring space deep enough

The City of Paris has an objective of planting 170,000 trees between 2020 and 2026 as part of its Tree Plan. Its monitoring dashboard notes 129,542 plantings since November 2020. Mayor Anne Hidalgo defends this strategy against heat waves.

Urban forests give a visible shape to this policy, but the decisive point lies beneath the paving. A permeable soil allows water, air, and roots to circulate. To cool a street, the tree needs a usable volume of soil, not a decorative pit.