Spotlight on the forest floor

Why do plants grow where they grow was one of the first question I had when I started being interested in botany. Furthermore, I was puzzled why some places harbour more species than others, although site conditions are similar. In forests, the amount of light reaching the forest floor is often proposed as having a high influence on plant community compositions. In order to test that, I conduct a large experiment to investigate that on a small scale.

We installed 320 so called PAR sensors. PAR is the abbreviation for “photo catalytic active radiation” and is the range of visible light that is used by plants to conduct photosynthesis and produce sugar. The sensors are connected to data loggers (see figure 1, inside the white boxes) were designed, built and programmed by members of the Geobotany lab of the University of Freiburg. This allows us to reduce costs and have full control of the recorded data. In addition, it keeps the loggers flexible and they can easily be adapted for other (new) projects.

Inside the forest, twelve sensors are placed in one row at 80cm intervals. The vegetation in the area around the sensors (40cm by 40cm quadrats) is mapped in high detail. The range of light availability over time can explain why plants with differing light demands occur in close proximity.

As usual not everything went smooth. A problem we encountered during our first runs were rodents because apparently they like to nibble on the wires connecting the sensors with the data loggers. To avoid that, corrugated tubes were placed around the connecting wires (see figure 2).

Currently, the sensors are still outside and take one recording of the light intensity every three minutes. They started at the end of April/early May 2018 and will be left outside until October 2018 to cover one full growing season. Afterwards the sensors and loggers will be recovered and data will be read out. Data sets will be large (approximately 8MB per CSV file) and will hopefully provide a detailed overview of the light conditions at the recorded place. This allows us to calculate accurate light niches for the recorded plant communities. These niches might be a more precise approach to characterise the light preferences of plants than Ellenberg’s parameters. In the mid 70s Heinz Ellenberg developed a sophisticated system to classify plants in Central Europe based on their ecological requirements. Among several abiotic factors, he classified light preferences in nine classes, ranging from preferring deep shade to preferring full light in open spaces.

by Jan Helbach (B2)