The expansion of trees and shrubs is a natural process leading to the development of the forest. However, this is not always a desirable process, as it can threaten valuable and protected open ecosystems, which were formed as a result of agricultural use in areas from which the forest was removed long ago.

The article describes an effective method monitoring expansion of trees and shrubs using airborne data fusion developed within the HabitARS project. The project was conducted in years 2016-2019 and was co-financed by the Polish National Centre for Research and Development (NCBR) and MGGP Aero.

The earlier the better

Today the process of expansion of trees and shrubs is indicated as one of the main threats to biodiversity in Poland, including Natura 2000 for non-forest habitats. The abandonment of grassland as a result of socio-economic changes is a phenomenon observed throughout Central and Eastern Europe. This is particularly true for land with low productivity and high fragmentation. Conservation of a significant part of our native biodiversity depends today on active protection of non-forest ecosystems, carried out by nature conservation services.

Protecting open ecosystems by removing trees and shrubs and restoring regular use requires a detailed strategy. The first step should be to recognise the scale and dynamics of the phenomenon. Identification of this threat should be carried out at the earliest possible stage of expansion, i.e. when individual specimens are low and have a low density. Then, the active protection activities bring the best results. Today the remote sensing provides excellent tools for monitoring the threat of expansion of trees and shrubs.

How to balance the need for large range and high spatial resolution?

The average area of a national park in Poland is 136.7 km2 and the Natura 2000 special area of conservation is 46 km2. The strategy for protection of open ecosystems against the expansion of trees and shrubs requires acquisition of continuous spatial information for the whole protected area. The use of unmanned aerial vehicles for this purpose is inefficient due to highly insufficient platform capacity. On the other hand the satellite data does not currently provide sufficient spectral and spatial resolution to identify trees and shrubs at an early stage of expansion. The appropriate spectral and spatial resolution of the data can be however obtained using an an airplane, which is additionally able to acquire all data during one air campaign for an area of several hundred square kilometres.

What kind of sensors?

When analyzing the succession process, we take into account parameters related to the area covered by expansion, the height and species of trees and shrubs. The analysis of such variables requires image data from VNIR and SWIR spectrum as well as laser scanning data. The platform developed by our team acquires data using HySpex cameras and a RIEGL LiDAR scanner in one flight, which significantly reduces data acquisition costs and has a positive impact on analysis results.

- The acquisition of consistent VNIR and SWIR image and laser scanning for the analysis of the succession process requires an aerial platform with the appropriate parameters - says Katarzyna Osińska-Skotak, Assoc. Prof. from the Warsaw University of Technology, Team Leader for the identification of the succession process within the HabitARS project.

The fourth dimension of assessing the expansion of trees and shrubs

Developed risk assessment method provides information not only based on the analysis of current state, but also takes into account the dynamics of the process by including archival airborne images in the analysis. Including parameters relating to time-variation in this assessment shows the true scale of the threat, showing whether the process is progressing quickly or slowly. The project tested the use of technology of dense matching of archival aerial images to identify the tree.

Advantages of remote sensing

The greatest advantage of the HabitARS method is that the information it gives is:

• spatial continuity for vast areas;
• detailed, because it of its very high resolution data;
• objective, as it is generated by 100% repeatable automatic processes.

Implementation

The process of expansion of trees and shrubs threatens the biodiversity of many protected areas in Poland and around Europe. We are currently undertaking remote-sensing projects in three Polish national parks, in which we identify the current and historical range of trees and shrubs using the experience gained from the HabitARS project.

Authors: Dominik Kopeć, Martyna Wietecha, Łukasz Sławik

The study was co-financed by the Polish National Centre for Research and Development (NCBR) and MGGP Aero under the programme “Natural Environment, Agriculture and Forestry” BIOSTRATEG II.: The innovative approach supporting monitoring of non-forest Natura 2000 habitats, using remote sensing methods (HabitARS), project number: DZP/BIOSTRATEG-II/390/2015. The Consortium Leader is MGGP Aero. The project partners include the University of Lodz, the University of Warsaw, Warsaw University of Life Sciences, the Institute of Technology and Life Sciences, the University of Silesia in Katowice, Warsaw University of Technology.

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