Environmental and Silvicultural Characteristics Influencing the Extent of Ash Dieback in Forest Stands

Extensive investigation of the impact of ash dieback in forest stands in the Czech Republic was conducted in 2013. Data on the defoliation of ash trees were collected from 1169 forest stands within the entire area of the Czech Republic. A set of 37 variables acquired from different databases (State Forests, GIS, Digital Terrain Model) describing silvicultural, environmental and landscape characteristics were used as explanatory variables. A generalized linear model (GLM) explained nearly 26% of the disease data variability. In the model, the extent of the disease was positively affected by the density of stocking, site class, vertical terrain heterogeneity, temperature and the presence and width of watercourse and negatively affected by mean tree height, the altitudinal zone of the forest, and the distance to the nearest ash stand. The model confirmed an important role of tree species composition of stands with ash. The disease extent was the highest in the presence of Quercus robur and the lowest in presence of Acer spp. and Abies spp. This finding is probably due to the different chemical composition of mixed litter and the leaching and translocation of nutrients from maple litter into ash petioles, which could accelerate decomposition, whereas fungistatic tannins and secondary metabolites from fir litter could inhibit microbial growth. The extent of the disease also significantly differed according to edaphic series of forests, and GLM models were successfully developed for them. These models differed from each other and explained 23-37% of disease variability; other factors influencing disease extent were also determined: distance to water, SD of slope, ash area, standing volume, aspect, TPI, landforms and the presence of other tree taxa such as Pinus spp., Quercus petraea, Fagus sylvatica and Betula pendula. The results indicated that the disease extent is substantially affected by environmental and stand characteristics and that the development of effective forest management strategies to address the epidemic in European forests (at least in central Europe) is possible.