Alien Trees & Filling the Knowledge Gap

recent report jointly published by WWF, Sabima, Friends of the Earth Norway and the Norwegian Botanical Society showed that alien tree species are one of the largest threats to native tree species, even inside protected areas. The news even reached Norwegian news outlet NRK. But why are alien trees a problem? Isn’t a tree, well, just a tree? As guest blogger Tanja Petersen explains, not quite.

Alien species can outcompete native species and affect the local ecosystems severely. For example, fir trees can completely overshadow the forest floor, leaving it unsuitable for native forest floor species, which would originally have occurred in the area. This is one of the reasons why we need to deal with alien tree species. Limiting the spread of alien, invasive species is even incorporated into national law, and on international level as well.

Estimates on shaky grounds

We’ve previously talked about the Norwegian Blacklist, and how this list categorizes alien species according to invasion potential and ecological effects. But to give a good estimate of these factors, it is crucial to know where the alien is, where it came from, and how much it has multiplied, and herein lies the issue. Most alien species are not very well reported in available databases, which makes the foundation on which these estimates are made rather shaky – an example is an article from 2013 in Norsk Botanisk Forening’s journal Blyttia. The total population of Swiss stone pine in Norway was estimated to be around 1500 individuals, yet when a systematic registration and mapping was initiated, they counted 1379 individuals in an area of approximately 0.05 km2 in Trondheim.

Filling in the gaps

Obviously, there is a knowledge gap that needs to be filled. You can aid in this by registering your own species observations through services like Artsobservasjoner and GBIF. Not just the rare, exciting species, but also the invasive trees and other escaped garden plants that are mentioned here on the blog.

Filling the knowledge gap is also where the scientific work of the NTNU University Museum joins the party. When the employees of the museum carry out ecological surveys and register presence and/or abundance of plant species, these data sets are digitized and published so that they will be available for assessments in the future, and available to anyone who could be interested. The museum also joins in on teaching new students and taking them into the field for this kind of work.


Professor Gunnar Austrheim from the NTNU University Museum pointing out the first Swiss stone pine of the day, teaching the students how to tell it apart from the native Scots pine (Pinus sylvestris). From left to right: Kjirsten Coleman, Khumanand Dhunganam, Gunnar Austrheim, Lukas Tietgen, Hu Ri, Sarah Yawson and Vilde Haukenes (Photo Credit: Tanja Petersen, NTNU University Museum, CC BY-SA 4.0.).

Students to the rescue

As a part of a theme on invasive plant species, Master’s students from the course “BI3036 Plant Ecology” helped out by registering individual trees of a few selected alien species: the Swiss stone pine (Pinus cembra) and various species of larch (Larix spp.).

The days’ fieldwork took place in Lade, and the students were blessed with sunshine and warm temperatures – more than what can generally be expected from a Tuesday in April.

In total, the students managed to register and measure 94 individual trees in total, ranging from 12 cm to more than 15 meters in height. This work demanded a lot of hard work – especially getting through the dense cover of branches from the larches, and climbing the cliffs when the paths they were to follow, demanded that they climbed rather steep cliffs. Several times, we feared that we might have lost a student or two to the wilderness!

You can read more about Tanja’s research at the NTNU University Museum here.

Title image by Tanja Petersen, NTNU University Museum, CC BY-SA 4.0.

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