International research team proves systematic branch movements using an autonomous laser scanning system and tracking algorithm
Dead branches of trees are by no means static but clearly move in the course of the day. Geoinformatics researchers at Heidelberg University tracked these movements in a study conducted in cooperation with the Finnish Geospatial Research Institute (FGI). With the help of an autonomously operating laser scanning system, the scientists mapped a specific forest plot at regular intervals. Using a specially developed tracking algorithm, they were able to quantify precisely even small branch movements, which are driven by changes in humidity. Their approach opens up new opportunities for automated monitoring, which could make it possible to assess tree health based on characteristic movement patterns.
Trees appear static at first sight but are constantly adapting to influences from their environment and continually moving their branches in response. A systematic movement of dead tree branches had long been suspected but had not yet been quantitatively proven under real forest conditions. The 3D Geospatial Data Processing (3DGeo) research group at Heidelberg University’s Institute of Geography, led by Prof. Dr. Bernhard Höfle, has now explored this question in international cooperation with the FGI. Using terrestrial laser scanning, the scientists succeeded in automatically measuring the suspected movement in detail every hour over several days, and in making it visible by means of the tracking algorithm.
To explain what drives these movements, the scientists modeled the anatomical and mechanical properties of tree branches. They found that relative humidity is one of the key factors: as it changes, the branch wood absorbs or releases moisture, causing the wood to expand or contract. “This change in length does not occur uniformly across the branch’s cross-section. Depending on the tree species, the upper and lower parts expand at differing rates. As a result, even dead branches can move significantly up or down as humidity fluctuates during the day,” explains Vanessa Guthier, a research assistant with the 3DGeo research group. The analyses show that all branches studied – here of 16 conifers and a deciduous tree in a boreal forest in Hyytiälä in Finland – followed a clear, systematic pattern of daily movement.
Vanessa Guthier explains that the dead conifer branches moved downward during the day and upward at night, while the dead birch branch they studied exhibited the opposite pattern. The tips of the branches moved up to 43 centimeters in height within a few days. Their movement was closely connected with what is called the wood equilibrium moisture content, which is calculated from relative humidity and air temperature. The branch movements occurred with a time lag, on average, of approximately three and a half hours after the changes in moisture content, which were modeled by the scientists. “Our results support the assumption that the daily movements of dead branches are primarily due to the hygroscopic properties of the wood, i.e., that they are caused by the absorption and release of moisture in wood,” Vanessa Guthier says.
According to the Heidelberg geoinformatics experts, the precise quantification shows that the movement signatures of living and dead branches clearly differ from one another. Analyzing these patterns with the help of the tracking algorithm and the autonomous laser scanning system would ultimately make it possible to identify dead branches of several trees at the same time without physical contact. “This research approach offers great opportunities for tree monitoring in forests and cities, enabling automated tracking of tree health and detecting a decline in tree vitality early on,” Prof. Höfle underlines.
The work was funded by the German Research Foundation, the Research Council of Finland, and the Tulanet Postdoctoral Programme for Research Institutes in Finland. The findings from the research cooperation between Heidelberg University’s Institute of Geography and the Finnish Geospatial Research Institute have been published in the journal “Agricultural and Forest Meteorology”.