Mapping tree species ecograms based on soil pH and soil water availability across Denmark

A prerequisite for successful afforestation and reforestation is understanding the quality of a site before establishing a specific tree species. Ecograms have been widely applied to determine the suitability of different sites for different species by a simple assessment of nutrient and water availability. Their graphical representation of suitability into classes allows ecograms to be easily understood. However, ecograms have generally been mapped for small areas only and their validity has rarely been documented. The aim of this study is to map ecograms for five tree species across Denmark and validate the maps using forest stand production data. For this purpose, we classified the landscape into six nutrient classes and nine water classes based on four variables to generate the ecogram maps. Based on these classes, the generated maps depict if a tree species is unsuitable, suitable or optimal for a specific site. The absolute average misclassification for nutrient and water supply was 1 and 2 classes, respectively. Stepwise linear regression was implemented to determine if the four variables used to create the ecograms were able to predict production as observed from forest experiment and management planning data obtained from across the country. All five species used the full model to explain variation in production. However, the average production values per ecogram growing condition were not significantly different for all species. The range of R-2 for the five species was 0.05-0.32, indicating that one ecogram template might not work for all species. The high-resolution national ecogram maps incorporate large-scale variables important for tree growth and will be beneficial when selecting new land for afforestation. The simplicity of ecograms allows for easy interpretation, meaning that foresters can quickly determine which regions of a landscape are suitable, saving time and resources.