Multispectral laser scanning, scanning with laser light in two or more wavelengths, opens up new possibilities for mapping large areas of forest with an elevated level of detail. This includes developing maps of tree species, which can facilitate the evaluation of biodiversity, and prove useful in forestry planning. This is something that Eva Lindberg and her research group at SLU (Swedish University of Agricultural Sciences) are working on within Mistra Digital Forest.
– Laser data has been used for more than 20 years to produce estimates of forest variables, such as volume, mean height and stem density. There are already maps with this information, for the whole country. However, what is missing are good tree species maps and also growth maps, says Eva Lindberg, senior lecturer at SLU.
Multispectral laser scanning, which was the type of scanning carried out within the programme in the summer of 2019, generates 3D point clouds in several wavelengths.
– In this way, we can take advantage of colour differences between the tree species, without the signal being disturbed by the colour of the ground, or by shadows, Eva Lindberg explains.
New method for the production of tree species maps
In Mistra Digital Forest, a new method has been developed for producing maps that show tree species. Eva Lindberg has supervised Marian Schimka, a master’s student from Germany, who has worked with a method based on dividing an area into small squares, raster cells. The mean value of the light intensity is calculated within that area.
– In our method, each square covers a small surface unit of 50 by 50 centimeters and the result in each square is a tree species, Eva Lindberg explains and continues:
– The satellite images currently being used for forest mapping have significantly larger pixels, often 10 by 10 meters, which frequently have several tree species in one and the same area. Now, using multispectral laser scanning, it is possible to produce accurate tree species maps.
Small trees – not less important
It’s not just identifying the large trees that’s important, but also the lower layers of vegetation. Being able to see small trees is beneficial from a biodiversity perspective, small trees can act as cover for birds, for example. Also, from an economic perspective, the information in the lower vegetation is valuable for planning clearing and thinning with greater precision.
Langning Huo, postdoctoral student at SLU, is conducting a study on lower vegetation layers and the analysis is almost complete.
– The method Langning Huo uses clusters the points into groups that correspond to tree crowns or shrubs. Then, all the tree crowns in the top crown layer are identified and removed. In this way, you can obtain information about what is further down, Eva Lindberg concludes.
The Holy Grail of variables
Sveaskog has acted as the supervisor, has contributed a study area, and has financed some of the high-resolution scanning in one study area that constitutes a central element in this tree species work. When asked why Sveaskog chose to participate, technology specialist Johan Ekenstedt replies that it is of great importance for forest owners to have good tree species data.
Tree species is something of a holy grail that we really want to discover, because if we don’t know what tree species we have, we don’t know what products we will get either.
In addition to tree species, in order for it to be really great, what’s missing is knowing how many trees there are per hectare, he says.
– If that could be solved, many time-consuming measurements on the ground could be avoided.
With the work done in Mistra Digital Forest, Johan Ekenstedt hopes to produce data as good as – or better than – measurement in the field.
– Our dream is to figure out which scanning method should be used to obtain information about tree species, which method gives the best results and is most cost-effective. It costs a good few millions to scan the forest, he says.
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