City trees live fast and short

Urban environment provides for higher growth rates but also greater mortality

Urban trees lead a life under extreme conditions. © Nikada / istock
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Living under extreme conditions: Urban trees live faster, but also shorter than their relatives in the countryside. As a study shows, the urban environment on the one hand leads to faster growth of the trees. At the same time, they are more vulnerable and have a higher mortality rate. In order to fully exploit the positive effect of urban trees, it is important to pay more attention to their health, according to the researchers.

Trees are the green lung of our planet: the plants inhale large quantities of the harmful greenhouse gas carbon dioxide from the atmosphere and produce the oxygen that is so important to us humans. Especially in the city, they also provide other important services. Because evaporation effects can help trees to cool down urban heat islands. In addition, they act as effective dust filters and thus counteract air pollution.

How well this service of the urban trees also does us humans, studies show impressively: The more street green city dwellers have in their immediate environment, the healthier they are and the better they feel. Many municipalities have now recognized this benefit and have significantly intensified their greening efforts in recent years.

City vs. City Forest

However, there is one difficulty: "Cities have been little researched by ecologists so far, " explain Ian Smith and his colleagues from Boston University. Therefore, little is known about the dynamics of the tree-tree ecosystem and which planting strategies achieve the best effects. "Our findings from rural forests are unlikely to be easily transferred to the city."

To find out more about the differences between urban and rural trees, the scientists have now carried out comparative studies in the US state of Massachusetts. They looked at trees in the city of Boston and nearby Harvard Forest in the country: how fast are the plants growing, how long are they living and how big is their potential as carbon storage? display

Turbo growth in the city

The results showed that the trees in the city grew on average much faster an effect that has already been found in previous studies. More concretely, the growth rates of the city trees were almost four times higher than those of their counterparts in the countryside.

At the same time, however, the evaluations revealed that the city trees had a higher mortality rate. According to this, the average mortality rate of the trees in the period from 2006 to 2014 was more than twice that of the forest trees. How fragile a tree was, however, also depended on the species: "The mortality varied greatly depending on the species. However, it was always higher in street trees than in non-urban trees of the same genus, "the researchers report.

"Advantage not fully exploited"

Also in relation to the size of the team noted differences. Thus, both in the city and in rural areas mortality among small trees was particularly high. While the mortality rate of forest trees declined exponentially with increasing size, in the urban environment, however, something else was observed: apart from the very small ones, the larger ones were evidently also the larger ones B besondersume particularly vulnerable.

Overall, it stands out: A tree growing in the city faster than a tree in an intact forest and potentially stores more carbon over the years. "However, this theoretical advantage can not be fully exploited due to the high mortality rate, " say the scientists.

Planting initiatives are not enough

Only planting more trees is therefore not the right approach in the city. It is also about reducing the mortality: "The efforts must be done to help small trees even better in the establishment and strengthened for the health and the maintenance of gr er trees to take care of ", so the team. This is the only way in which the potential of the urban canopy roof, which has not yet been fully exploited, can be exploited in the future. (Plos One, 2019; doi: 10.1371 / journal.pone.0215846)

Source: PLOS

- Daniel Albat