Silver Fir Decline in Pure and Mixed Stands at Western Edge of Spread in Croatian Dinarides Depends on Some Stand Structure and Climate Factors

Abstract

Silver fir is one of the most threatened conifer species in Croatia, especially at the western edge of its spread in Croatian Dinarides, where the decline in fir trees has resulted in significant ecological and economic issues. The aim of this study was to determine, over an 18-year monitoring period, the relationships of silver fir crown defoliation with climatic factors and structural attributes. We further analyzed the tree retention time in a given defoliation class and transition dynamics between defoliation classes, as well as the survival/mortality of trees. Data on silver fir defoliation were analyzed in two different forest types: in pure silver fir and in mixed silver fir and common beech stands. The climatic factors, primarily vegetation period air temperature, potential evapotranspiration, and dry season water deficit, were correlated with crown defoliation. Regarding the structural attributes, in the mixed stand with predominantly smaller trees, crown defoliation increased with reduced diameter at breast height, crown diameter, social class, and crown illumination. In the pure fir stand, crown defoliation increased with reduced crown diameter, greater crown asymmetry, greater crown illumination, and on trees with a stork’s nest crown. The retention time in defoliation classes differed for research sites. Transition dynamics were different only for trees in the highest defoliation class (dead trees). At the end of the study period, silver fir mortality was higher in the pure fir stand. Increased silver fir defoliation and mortality can be expected in the future, particularly in overmature stands under prolonged drought stress. Permanent forest monitoring could ensure the high-quality data needed for adaptive management of fir stands that could positively influence the structure of these stands and, thus, improve their health status.

Which Are the Best Site and Stand Conditions for Silver Fir (Abies alba Mill.) Located in the Carpathian Mountains?

“…This fact confirms the silver fir's predilection for mixed stands, while pure and even aged silver fir stands (composition: 100%) are extremely rare [66]. Unlike pure stands, mixed stands offer a better resilience against harmful factors (biotic and abiotic) and, implicitly, a better phytosanitary stand state [47,75]. In addition, a silver fir participation percentage of up to 20% in the stand's composition contributes significantly to ensuring the sustainability of forest ecosystems [76].…”

“…Over this altitude the presence of silver fir in stand composition decreases sharply [17]. Furthermore, a high percentage of silver fir in the stand's composition will lead to the decline of this species especially in mountain areas where the effects of climate changes are more evident and where they diminish the vitality of this species [46,75].…”

Influence of Tree Attributes on Silver Fir (Abies alba Mill.) Transitioning to Higher Defoliation Classes Determined by Logistic Regression

“…Site B has reverse-J diameter distributions, while Site A has a depleted selection structure with a greater amount of wood per unit area and a higher proportion of trees with larger-diameters (Figure 3). In selection forests, these larger diameter trees often form the top layer [47], leading to a relatively uniform stand at Site A.…”

Nutrition of silver fir (Abies alba Mill.) and its comparison with Norway spruce (Picea abies L. H. Karst) from the same forest sites in the Czech Republic

“…According to palaeoecological studies, silver fir was a very common tree species within the European continent in the past (Wick, Möhl 2006;Tinner et al 2013;Birks, Tinner 2016). A decrease in its representation in forests can be explained and the most important explanations include: (i) land-use pressure, specifically excessive anthropogenic fire and site browsing disturbance; (ii) forest management using clearcuts and oriented to even-aged unmixed stands; and (iii) sensitivity to abiotic and biotic factors, particularly SO 2 pollution and climate change (Schütt et al 1984;Uhlířová, Kapitola 2004;Elling et al 2009;Hájek et al 2016;Mauri et al 2016;Ugarković et al 2021).…”