Earth Observation Based Monitoring of Forests in Germany: a Review
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remote sensing Review Earth Observation Based Monitoring of Forests in Germany: A Review Stefanie Holzwarth 1,*, Frank Thonfeld 1,2 , Sahra Abdullahi 1, Sarah Asam 1 , Emmanuel Da Ponte Canova 1 , Ursula Gessner 1, Juliane Huth 1, Tanja Kraus 1 , Benjamin Leutner 1 and Claudia Kuenzer 1,2 1 German Remote Sensing Data Center (DFD), German Aerospace Center (DLR), 82234 Wessling, Germany; [email protected] (F.T.); [email protected] (S.A.); [email protected] (S.A.); [email protected] (E.D.P.C.); [email protected] (U.G.); [email protected] (J.H.); [email protected] (T.K.); [email protected] (B.L.); [email protected] (C.K.) 2 Institute of Geography and Geology, University of Wuerzburg, 97074 Wuerzburg, Germany * Correspondence: [email protected] Received: 25 September 2020; Accepted: 28 October 2020; Published: 31 October 2020 Abstract: Forests in Germany cover around 11.4 million hectares and, thus, a share of 32% of Germany’s surface area. Therefore, forests shape the character of the country’s cultural landscape. Germany’s forests fulfil a variety of functions for nature and society, and also play an important role in the context of climate levelling. Climate change, manifested via rising temperatures and current weather extremes, has a negative impact on the health and development of forests. Within the last five years, severe storms, extreme drought, and heat waves, and the subsequent mass reproduction of bark beetles have all seriously affected Germany’s forests. Facing the current dramatic extent of forest damage and the emerging long-term consequences, the effort to preserve forests in Germany, along with their diversity and productivity, is an indispensable task for the government. Several German ministries have and plan to initiate measures supporting forest health. Quantitative data is one means for sound decision-making to ensure the monitoring of the forest and to improve the monitoring of forest damage. In addition to existing forest monitoring systems, such as the federal forest inventory, the national crown condition survey, and the national forest soil inventory, systematic surveys of forest condition and vulnerability at the national scale can be expanded with the help of a satellite-based earth observation. In this review, we analysed and categorized all research studies published in the last 20 years that focus on the remote sensing of forests in Germany. For this study, 166 citation indexed research publications have been thoroughly analysed with respect to publication frequency, location of studies undertaken, spatial and temporal scale, coverage of the studies, satellite sensors employed, thematic foci of the studies, and overall outcomes, allowing us to identify major research and geoinformation product gaps. Keywords: remote sensing; earth observation; forest; forest monitoring; forest disturbances; Germany; review 1. Introduction 1.1. Forests in Germany: Relevance and Current Challenges Forests all over the world provide extremely valuable ecosystem services and contribute immensely to human well-being. Services can be grouped into provisioning services, supporting services, regulating services, and cultural services (Figure1)[ 1–4]. Forests provide raw materials, such as wood or plant fiber (construction wood, furniture wood, paper, coal, etc.), direct and indirect food Remote Sens. 2020, 12, 3570; doi:10.3390/rs12213570 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 3570 2 of 43 products (herbs, fruits, nuts, honey, mushrooms, game, insects etc.), and chemical substances and Remote Sens. 2020, 12, x FOR PEER REVIEW 2 of 44 medicinal products (turpentine, oils, resinate, etc.), as well as oftentimes granting access to pure water sources.and Forestsmedicinal support products habitats (turpentin fore, floraoils, resinate, and fauna, etc.), as are well home as oftentimes to a large grant wealthing access of biodiversity, to pure and contributewater sources. to soil Forests formation support andhabitats nutrient for flora cycling. and fauna, Furthermore, are home to a forests large wealth support of biodiversity, the protection of landand against contribute erosion, to soil such formation as coastal and nutrient erosion cycling. along Furthermore, shorelines, orforests slope sup erosionport the inprotection mountainous of regions.land Self-regulating against erosion, services such as include coastal watererosion filtration along shorelines, and air filtration,or slope erosion water retentionin mountainous as well as flood andregions. drought Self-regulating control,climate services changeinclude levellingwater filtration via the and fixation air filtration, of carbon water in retention plants and as well soils as from the airflood and theand contributiondrought control, to pollinationclimate change and levelling the dispersion via the fixation of seeds, of carbon among in others. plants and Cultural soils from services includethe recreation air and the (e.g., contribution walking, hiking,to pollination cycling, and riding, the dispersion cross country of seeds, skiing, among hunting, others. etc.), Cultural aesthetics, services include recreation (e.g., walking, hiking, cycling, riding, cross country skiing, hunting, etc.), environmental education, and spiritual services [5,6]. aesthetics, environmental education, and spiritual services [5,6]. FigureFigure 1. Ecosystem 1. Ecosystem services services of forests,of forests, subdivided subdivided intointo provisioningprovisioning ser services,vices, supporting supporting services, services, regulatingregulating services, services, and and cultural cultural services. services. In Germany,In Germany, around around 11.4 11.4 million million hectares, hectares, a a share share of of 32%32% of the the country’s country’s surface surface area, area, are areforest forest covered.covered. The previouslyThe previously mentioned mentioned services services demonstrate the the enormous enormous indirect indirect value value forests forests play in play in ourour daily daily life. life. However, However, forests forests also also represent represent anan important economic economic factor: factor: forest forestss in Germany in Germany provide income for around two million forest owners, and 125,000 companies in the forestry and provide income for around two million forest owners, and 125,000 companies in the forestry and timber timber sector employ 1.1 million people—mainly in rural areas. In 2014, the sector generated a sectorturnover employ of 1.1 178 million billion people—mainlyeuros and 55 billion in euros rural in areas. gross Invalue 2014, added the sector[7]. generated a turnover of 178 billionAccording euros and to 55 the billion remote euros sensing in gross-based value Global added Forest [ 7Watch,]. Germany lost 754,000 hectares of Accordingtree cover from to the 2001 remote to 2019 sensing-based equivalent to Globala 6.0% Forestdecrease Watch, since 2000 Germany [8]. This lost loss 754,000 occurs hectares due to of tree coversettlement from and 2001 infrastructure to 2019 equivalent expansion to (e.g., a 6.0% urbanization, decrease road since construction), 2000 [8]. This resource loss exploration occurs due to settlement(e.g., andopencast infrastructure mining), and expansion agricultural (e.g., expansion urbanization, as well road as natural construction), hazards (storms, resource droughts, exploration (e.g., opencastpests, fires, mining), avalanches, and agriculturaletc.) [9]. In recent expansion decades, as well forests as naturalin Germany hazards have (storms, been facing droughts, a large pests, fires, avalanches,number of challenges, etc.) [9]. leading In recent to decades,increased forestsattention in in Germany public media. have been facing a large number of Severe summer droughts in 2003, 2018, and 2019 have led to stress, a much lower resilience and challenges, leading to increased attention in public media. death of many trees. In stands of lower overall tree health, bark beetle infestations spread to a much Severe summer droughts in 2003, 2018, and 2019 have led to stress, a much lower resilience and larger extent than in past decades. For 2017 and 2018, over 80% of the forested area in Germany deathshowed of many an trees. increased In stands crown of transparency lower overall over tree all health, species bark [10] beetle. All these infestations forest disturbances spread to aare much largerexpected extent than to increase in past decades.and accelerate For 2017in the and coming 2018, years over and 80% dec ofades the forested[11]. area in Germany showed an increased crown transparency over all species [10]. All these forest disturbances are expected to increase and accelerate in the coming years and decades [11]. Remote Sens. 2020, 12, 3570 3 of 43 Due to the alarming damage to over 32 million m3 of timber during the 2018 drought, and approximately 105 million m3 of wood damaged during the 2019 drought, national ministries and agencies in Germany have called for an action plan to develop counter measures and mitigation plans. Measures to be decided upon will be enacted at a federal level by the Federal Ministry of Food and Agriculture (BMEL), the Federal Ministry of the Environment, Nature Conservation and Nuclear Safety (BMU), and at a federal level state run and private organizations and agencies. These are supported by national and federal research institutions, non-governmental organizations (NGOs), as well as forestry-related chairs at universities in the individual federal states (see