Cities – home of non-native trees
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.
Non-native trees in Alpine space: to use or not to use?
For four hundred years we have been bringing various non-native tree species into the Alpine region. In the forests and especially in the urban environment, where there are the most non-native tree species. In the ALPTREES project, at least 526 different were recorded through data collection. The number of non-native trees in the cities and forests of the Alpine region has increased in recent decades. Partly at the expense of globalization, which accelerates the exchange of goods and services, and partly at the expense of climate change. The latter have made the Alpine region less friendly to some native tree species and more friendly to certain alien species, which are becoming increasingly powerful and even invasive in some cases.
Why do we bring non-native tree species into our alpine environment when we have a large number of native tree species? One of the reasons is certainly their beauty, or the difference with which we want to beautify cities, parks and gardens. Another reason is more economic. In the desire for faster growth, higher yields or other, better technical properties of wood, we have introduced various tree species from other continents into our forests. From then on, the development of each tree species proceeded in three directions: Some tree species did not survive the new environment, others stabilized, reproduced normally, but did not develop the invasive potential of the third option: these tree species spread excessively, displacing natural tree species and other vegetation.
Basically, people do not like anything that has an invasive sign. We feel like something is threatening us. Of course, when the first black locust was brought to France in the 17th century, no one could have guessed that it would become a tree species that is now one of the five most common non-native tree species in the Alps. Although it has proven to be a very useful tree species, it is very invasive and in some places difficult to control.
From past experience, it is easier to predict which tree species will develop their invasive character: These are primarily species that grow rapidly and reproduce asexually (vegetatively) in addition to sexually, light seed species, and species that have a combination of both, such as Paulownia tomentosa. If such species are also modest in nutrients and water, then we are in for a completely invasive species. New tree species of this caliber should be avoided in the forest.
Often colleagues who deal professionally with non-native tree species are asked: what do we think about non-native tree species? Is it necessary to introduce new tree species into the forest, to promote already established ones, or to avoid or even remove them? The answer is not simple, and because of the varying characteristics of non-native tree species, there is no easy answer. However, there are some principles to consider when managing non-native tree species: Native tree species should be given priority in forests. However, due to climate change, particularly rising temperatures and changing precipitation conditions and droughts, native tree species may not fully adapt to new conditions in certain habitats and forest sites. In these cases, we may consider incorporating non-native tree species. However, integration should be gradual and small in proportion. In any case, we must make an effort to learn about new non-native tree species and their ecology, and the tree composition of forests should be based on several basic species.
In addition to native tree species, the urban environment is quite a suitable environment for beautiful and hardy non-native tree species. Especially for those that can tolerate the difficult site conditions in the city. However, the use of invasive tree species should be avoided.
What are NNT and what is the situation regarding NNT in Alpine space?
Non-native trees (NNT) also known as “non-indigenous”, “alien”, “introduced”, “allochthonous” or “exotic” trees, refer to tree species, breeds or hybrids outside of area of natural origin, whose presence there is as a result of human activity, due to intentional or accidental introduction. Depending on the time of introduction, they can be separated into Archaeophytes and Neophytes. Archaeophytes include NNT introduced prior to the year 1492, while neophytes include NNT introduced after 1492, when Christopher Columbus arrived in the New World and the Columbian Exchange began. Not all NNT are automatically invasive. (Potentially) Invasive trees refer to NNT whose introduction, establishment and/or spread pose potential or actual risks to the native biodiversity, ecosystem functioning, or socio-economy including human health. So far about 5% of reported NNT have been considered (potentially) invasive in one or several regions of the Alpine Space.
In Work package 1 of our project, we performed a study aimed to perform an inventory of NNT growing in Alpine Space forests and cities, analysing their diversity, distribution, and geographic origin. You can find full report here. The results have shown that at least 526 NNT are present in the Alpine Space. The number of NNT growing in urban areas (352) is much higher compared to woodlands (13). This low number, however, is due to the fact that species in urban areas also include those in parks, cemeteries or even urban forests, where typical forest tree species are often found. The most common NNT in cities of Alpine space are Aesculus hippocastanum (horse chestnut), Robinia pseudoacacia (black locust), and Gleditsia triacanthos (honey locust). Switzerland and Austria had the highest proportion of NNT whilst Slovenia had the lowest.
Most of the reported NNT (90%) have their natural distribution range outside of the European geographical area. The largest number of these species were introduced from Asia (248) followed by North America (180), but the ones that are considered potentially invasive, are mostly originating from North America. The largest number of NNT, which are native to parts of Europe, were introduced from southern or south-east Europe, such as from the Balkans, southern Italy, or from the entire Mediterranean region. From our data we also found out that 35% of the NNT are being cultivated for landscape and gardening purposes, 18% are used as ornamental tree, for example, in parks or arboreta, and 15% are being valued for their potential to sequester carbon. Other commonly mentioned benefits include fuel wood, shading, soil erosion control and timber production.