# Detection of Forest Windstorm Damages with Multitemporal SAR Data--A Case Study: Finland
## 1 Introduction  
### Background and Objectives of the Study  
Windstorm damages have become more common in the past decades [1, 2]. Windstorms cause noticeable large area forest damages in Europe, including Scandinavia and Finland. For example, in southern Sweden, approximately 4.5 million cubic meters of timber was damaged in 1999 in a single storm [3], and in 2005 and 2007 approximately 70 and 12 million cubic meters of timber fell down in similar disastrous events, respectively [4]. The forest area reported to have been cut due to damages was over 30,000 ha on more than 20,000 forest stands in Northern Finland in 2014, and more than 6000 ha in Eastern Finland in July 2020. A rapid localization of the forest damages and removal of the fallen trees is the key for not only assessing the losses, but also avoiding further damage, caused, e.g., by insects. Severe storms require earlier sanitary cuttings (compared to original forest plans) to preventsuch insect outbreaks. These ad-hoc cuttings naturally increase harvesting and removal costs, cause losses in revenue and lower the future cutting possibilities [5]. The volume of damaged trees in windstorm has exceeded the volume of the normal annual cut in some countries in Europe, e.g., Germany, Poland and Sweden [1; 6]. Timely detection and mapping of a damaged forest allows additionally to optimize efforts in clearing potentially blocked roads and damaged power-lines in rural areas. A common method to localize the damage areas for operational forest regime purposes and obtain a rough overview of the damages has been monitoring with airplane using either visual assessment or optical sensors, e.g., video camera or airborne laser scanner (ALS). Most large-area studies with space-borne data have been conducted using optical satellite instruments [7]. A recent study by [PERSON] et al. [8] presents a summary of several demonstrated approaches in mapping windthrown forest areas. Our further in-depth analysis with SAR data is given in Section 1.2. A key prerequisite for successful operational forest management after a storm is a rapid, near-real-time localization of the damages. Damages are often large in area wherefore methods using space-borne data are appealing and cost-efficient alternatives. A central requirement is the timely availability of the remotely sensed data. SAR data are the only possibility for rapid monitoring due to their independence of light conditions and cloud cover. SAR backscatter depends on the forest structure and biomass, the environment and weather conditions such as moisture and temperature and sensor properties. From the current operative SAR satellites, EU Copernicus program's two C-band Sentinel-1 satellites probably have the best potential for rapid monitoring, primarily due to a frequent data acquisition and a free of charge data policy [9]. The only drawback of C-band data in forest application is the low penetration to forest volume due to short wavelength, which could restrict detectability of minor damages. The ALOS PALSAR-2 with L-band SAR, with deeper penetration depth than C-band and fully polarimetric capability, would likely better suit forest applications [10; 11], but the operational use is restricted by data availability [10; 11]. ESA's coming forest specific P-band BIOMASS mission may provide information for monitoring aboveground biomass and its change over large areas, but will not be operated over Europe [12]. The data availability and price also restrict the usability of high resolution X-band SAR data that could enable spatial texture analysis of SAR backscatter for forest disturbance [10] (a further detailed analysis of possible scenarios is given in Section 1.2) In the future, new satellites and constellations, such as NISAR [13] and ICEYE [14; 15] as well as planned DLR TanDEM-L [16] and ESA ROSE-L [17] may improve the situation significantly. Thus, Sentinel-1 presently and in the near future seems to be the most suitable tool for forest damage assessment in Europe at operational level.