The European Forest Condition Monitor: Using Remotely Sensed Forest Greenness to Identify Hot Spots of Forest Decline

Abstract

Forest decline, in course of climate change, has become a frequently observed phenomenon. Much of the observed decline has been associated with an increasing frequency of climate change induced hotter droughts while decline induced by flooding, late-frost, and storms also play an important role. As a consequence, tree mortality rates have increased across the globe. Despite numerous studies that have assessed forest decline and predisposing factors for tree mortality, we still lack an in-depth understanding of (I) underlying eco-physiological mechanisms, (II) the influence of varying environmental conditions related to soil, competition, and micro-climate, and (III) species-specific strategies to cope with prolonged environmental stress. To deepen our knowledge within this context, studying tree performance within larger networks seems a promising research avenue. Ideally such networks are already established during the actual period of environmental stress. One approach for identifying stressed forests suitable for such monitoring networks is to assess measures related to tree vitality in near real-time across large regions by means of satellite-borne remote sensing. Within this context, we introduce the European Forest Condition monitor (EFCM)—a remote-sensing based, freely available, interactive web information tool. The EFCM depicts forest greenness (as approximated using NDVI from MODIS at a spatial resolution of roughly 5.3 hectares) for the pixel-specific growing season across Europe and consequently allows for guiding research within the context of concurrent forest performance. To allow for inter-temporal comparability and account for pixel-specific features, all observations are set in relation to normalized difference vegetation index (NDVI) records over the monitoring period beginning in 2001. The EFCM provides both a quantile-based and a proportion-based product, thereby allowing for both relative and absolute comparison of forest greenness over the observational record. Based on six specific examples related to spring phenology, drought, late-frost, tree die-back on water-logged soils, an ice storm, and windthrow we exemplify how the EFCM may help identifying hotspots of extraordinary forest greenness. We discuss advantages and limitations when monitoring forest condition at large scales on the basis of moderate resolution remote sensing products to guide users toward an appropriate interpretation.

A First Assessment of Canopy Cover Loss in Germany’s Forests after the 2018–2020 Drought Years

“…For a complete picture, it would be useful to combine canopy cover loss records with complementary products. These could either be anomalies of spectral indices related to forest vitality [80] or climatological and other environmental data [2,81].…”

“…While our product shows less canopy cover loss in deciduous forests, it records much larger canopy cover loss in coniferous forests compared to the maps of the global forest watch. Other European or nationwide products in Germany do not explicitly determine canopy cover loss (e.g., [80]) and are therefore difficult to compare with our map. While national forest disturbance mapping systems were recently proposed for other Central European countries (e.g., Austria [19]), a national system does not yet exist for Germany, even though remote sensing is considered an important asset to complement ground surveys and the technology is being established at the federal state level [82][83][84].…”

Analysis of multi-seasonal meteorological storylines leading to reduced forest greenness in Europe in 2000–2020

“…This study identified specific aspects of the meteorological storyline (the three-year evolution of seasonal temperature, T 2m ′ 90d , and precipitation anomalies, P ′ 90d ), which is characteristic for events of low summer forest greenness at the 50 km scale in Europe in 2000-2020. Forest greenness as measured by the N DV I is used to detect reduced forest performance -an early warning mechanism of forest decline (Buras et al, 2021). In temperate forests, low N DV I events are preluded by extraordinarily persistent warm periods, and an unusual accumulation of dry periods over two years.…”

“…Although some forest ecosystems are adapted to low summer water availability, e.g., in the summer-dry Mediterranean or in dry inner-Alpine regions, extended and more frequent hot droughts will strongly affect their dynamics including growth and survival (Rigling et al, 2013;Tague et al, 2019;Ogaya et al, 2020). In this context, forest greenness is an effective measure related to forest performance to monitor the recent forest decline in Europe (Orth et al, 2016;Buras et al, 2020Buras et al, , 2021.…”

Does fragmentation contribute to the forest crisis in Germany?

“…The information obtained in this study provides input to both forest management and landscape planning striving for ecosystem resilience and an ecosystem-based adaptation to climate change. Existing monitoring apps such as the European Forest Condition Monitor (Buras et al, 2021) or the Waldmonitor (Welle et al, 2022) focus on vegetation vitality. Extending this monitoring with analyses of forest fragmentation can contribute to the understanding of forest vulnerability.…”