Cities – home of non-native trees
by Anica Simčič <This email address is being protected from spambots. You need JavaScript enabled to view it.>, Slovenian Forestry Institute

Cities around the world are getting bigger and bigger. Once small, today much bigger and forecasts predict that they will become even bigger in the future. This phenomenon is due to the fact that more and more people are moving to cities from near and far rural areas and from all over the world. When they move, they bring their belongings with them, their culture, their food, their animals and plants, but unfortunately also diseases and pathogens. Tourism and migrations are affecting also remote areas, where only few people used to live. This is happening in all regions and also to Alpine space. The European Alps are among the most densely populated mountain regions in the world. People build holiday houses almost everywhere, and by having a house they also want to decorate the surroundings with beautiful trees and plants from all over the world. This is one of the most convenient ways for new trees to invade new habitats. Some of the newcomers will be welcomed into the new area, others will be not. As long as these plants stay within the confines of the garden fences, they can also be a wonderful ornament, but if they escape beyond the boundaries, they can begin to out-compete native species and cause problems.

But not everything is black and white when it comes to non-native trees in urban areas of the Alpine space. Trees of all origins improve the quality of the urban environment by improving air quality, reducing noise, providing habitats for wildlife, cooling the air through evapotranspiration providing shade, and enhancing biodiversity. However, climate change (higher temperatures, heat waves, drought, weather disasters) made cities even more harsh condition for living (for both people and  plants) and native trees sometimes can’t adapt to new conditions. In this case, new species are introduced into urban areas.

Non-native tree species can be part of the solution to adapt to changing climatic conditions. This can be achieved with green urban infrastructure such as parks, green spaces, gardens, green roofs and walls, and roadside greening. It is clear how important trees are and will be in the future for coping with and adapting to difficult climate changes.

Non-native tree species are also valuable for their aesthetic value, because they usually look more exotic and extraordinary. In the ALPTREES project, we found more than 352 non-native tree species growing within the Alpine space cities. Some of them (tree of heaven (Ailanthus altissima), princess tree (Paulownia tomentosa), box elder (Acer negundo)) already showed their invasive character and are now under special observation and management. Sometimes they must be removed from the environment and sometimes an urban area (or other area where this tree species is not native) can no longer be a home to this tree species. It is very important how we choose tree species for urban sites. There are several factors to consider for successful growth and optimal life span, and some questions need to be answered before introducing them to new area:

• Is the climate suitable for selected tree species?
• Where in city will tree be planted? How much space will it have?
• Will it be alone or in a group (road tree, park tree, garden)?
• Did it show invasive potential anywhere yet?
• Does it cause any allergies?

Nonnative trees are one way to successfully adapt to changing climate, but they must be chosen wisely.