Innovative satellite remote sensing method enables more accurate forest ecosystem models

To evaluate the development of a forest’s ecosystem, forest floor vegetation needs to be monitored consistently. Currently, this kind of data is missing. In the beginning of the year, Jan Pisek, an associate professor with the University of Tartu, Tartu Observatory, published an article in the scientific journal Biogeosciences with his colleagues. The researchers successfully combined past efforts of measuring forest floor parameters with multi-angle satellite remote sensing.

To validate satellite measurements, Pisek and colleagues used data provided by the new ICOS (Integrated Carbon Observation System) research infrastructure (RI), also proving that the usefulness of ICOS RI goes beyond its designated tasks of measuring carbon exchange and greenhouse gases in the environment.

Forest floor vegetation often excluded from carbon models

The plants growing on the forest floor are one of the most diverse and least understood components of forests worldwide. These plant communities are especially important because they assist in the cycling of nutrients and can influence what happens to the trees and the canopy above. Measuring the appearance of the forest floor (understory), or its reflectance, over time and different locations is important to accurately estimate the parameters of forest canopy, and therefore for ecosystem modelling. However, this reflectance data from different types of forests is currently missing.

Currently, carbon models often treat the forest floor vegetation as an unknown due to the difficulties in measuring it properly and consistently on a larger scale. Remote sensing could be the technology to provide consistent data in this area. Multi-angle satellite-based remote sensing can capture signals of different forest layers because the observed proportions vary with the viewing angle, making it possible to separate the signals from forest canopy and forest floor.

New approach and promising results

The aim of the study published by Pisek and colleagues was to combine past efforts of tracking forest floor reflectance and its dynamics with multi-angle satellite remote sensing. The approach, originally developed over the Canadian boreal forests within the research group of prof. Jing Ming Chen at the University of Toronto, was validated against in situ understory reflectance measurements over an extended set of ICOS forest ecosystem sites.

The main findings show that the novel modelling method can deliver good results, especially over different forest types with open canopies. The performance of the method was found limited over forests with closed canopies (high foliage cover), where the signal from understory gets hidden. This actually makes sense – the modelling algorithm relies on the condition that at least a small part of the forest floor is seen from different directions through the overstory canopy. If the tree cover is dense and the forest floor cannot be seen from above, the algorithm cannot provide the correct, independent estimation of the understory signal. “I would be actually really concerned if we were still retrieving ‘correct’ understory signal under these conditions of too dense tree cover. The fact that the algorithm indeed fails when it is supposed to fail, helps me to have more confidence in its performance,” Pisek says.

Using ICOS forest ecosystem sites to validate estimations from remote sensing satellites

To validate satellite measurements, Pisek and colleagues used the data from different sites of the ICOS research infrastructure. The mission of ICOS RI is to enable research to understand the greenhouse gas budgets and perturbations in Europe and adjacent regions. ICOS is based on the collection of high-quality observational data by measurement stations operated long-term (15+ years) as national networks in the RI member states.

Estonia currently has a pending membership status with ICOS. Through collaborations carried within the framework of this project, Pisek and colleagues demonstrated commitment and showcased an innovative way how Estonia-based research can make an impact on the ICOS infrastructure and efficiently use it already now. The researchers showed that ICOS forest ecosystem sites can serve as network for validation of pixel resolution Earth observation products.

“The ICOS RI provided an excellent tool for truly comprehensive and efficient validation exercise thanks to the wide variety of forests types represented in the network,” Jan Pisek says.

For the first time, the study also provides comprehensive spatial representativeness assessment of the included ICOS sites across different spatial scales. “We were happy to see that most of the included ICOS forest ecosystem sites are indeed representative and suitable for validation of Earth observation products across different scales,” adds Pisek. Identifying ways how to properly scale and compare the in situ ground-based measurements with satellite measurements is one of the objectives of the SENSECO project, in which Pisek participates.

Jan Pisek acknowledges support by the Eesti Teadusagentuur (grant nos. PUT1355 and Mobilitas Pluss MOBERC11).