Measurement of water content in forest habitats is considered essential in ecological research on forests, climate change, or forest management. In the traditional forest habitat classification, two systems of habitat conditions analysis are found: single factor and multifactor methods. Both are laborious and therefore costly. Remote sensing methods provide a low-cost alternative. The aim of the presented study was to find the relationship between the spectral indices obtained from satellite images and the forest habitats moisture indices used traditionally in the Polish forest habitats classification. The scientific hypothesis of the research is as follows: it is possible to assess the variation in the humidity of forest habitats on the basis of spectral indices. Using advanced geographic information system (GIS) technology, 923 research plots were tested, where habitat studies performed with the traditional methods were compared with the analysis of 191 spectral indices calculated for Sentinel-2 image data. The normalized difference vegetation index (NDVI) has proved to be the most useful to the assessing of moisture of forest habitats. The ranking of the most correlated indices was calculated as E_intg—the product of the absolute value of the slope and the mean square error complement, and for the top five indices was as follows: NDVI = 0.248619, EXG = 0.242112, OSAVI = 0.239412, DSWI-4 = 0.238784, and RDVI = 0.236995. The results also highlight the impact of water reservoirs on the humidity and trophicity of forest habitats, showing a decrease in the fertility of habitats with an increase in distance from the water reservoir. The results of the study can be used to preparing maps of the diversity of forest types, especially in hard-to-reach places, as well as to assess changes in the moisture status of habitats, which may be useful, for example, in the assessment of the fire risk of forest habitats. We have proved that NDVI can be used in applications for which it was not originally designed.

Common ash (Fraxinus excelsior L.) forms stands of significant ecological and economic importance. However, a progressing phenomenon known as ash dieback has been recorded in Europe since 1992. The cause is primarily attributed to the fungus Hymenoscyphus fraxineus, but the initial factor allowing for the pathogenic role of fungi may be the weakening of the stands related to changing habitat conditions. Standard (ground-based) analysis of habitat conditions is tedious and costly, but remote sensing methods allow for the use of new tools. Among these are spectral indices, including the normalized difference vegetation index (NDVI). The aim of the study was to demonstrate the usefulness of this index in assessing habitat conditions of ash stands in Poland, a country located at the center of the Fraxinus excelsior range. By testing all ash stands in Poland where the share of ash in the stand was at least 80%, habitat conditions diversity covering 19 forest habitat types and 15 soil types was demonstrated. The conducted statistical analyses indicate that the tested habitat parameters affect NDVI in a complex and non-linear manner. The analysis showed that stand age has a minimal impact on NDVI. This means that other factors are responsible for the rapid drops in NDVI values. Differences in NDVI values can be considered the resultant of various habitat factors, illustrating better or worse conditions for a given stand type. Thus, the NDVI index can serve as a decision-support tool for evaluating the extent to which a given forest habitat type meets the ecological requirements of common ash.