Is climate the key factor limiting the natural regeneration of silver fir beyond the northeastern border of its distribution range?

Abstract

Silver fir (Abies alba Mill.) is one of the most important forest tree species in Europe potentially prone to climate changes. The northern limit of its distribution is in central Poland but fir stands were planted also in northern part of the country. It gives opportunity to investigate the potential for silver fir to expand into northern (cooler) Europe which could be an important issue in the face of changing climate. The aim of this research was to identify factors influencing the establishment and advancement of silver fir regeneration within and outside its natural range. Using data collected on sample plots in 48 stands, we examined influences of climatic and stand factors on establishment and development of silver fir regeneration. The Boruta algorithm was used to identify variables relevant for particular stages of fir regeneration (short and tall seedlings and saplings). The variable location (indicating that the analysed plot was located inside or outside the natural range of silver fir) was not the most important variable for any size classes of silver fir regeneration (Boruta algorithm). We found that the mean values of diameter of breast height (DBH) and height of the dominant firs were larger for northerly plantations than for stands within the natural range. The density of seedlings was similar within and outside the natural range but density of saplings was lower, especially tall saplings. Climatic variables (continentality index and summer heat: moisture ratio) were significant in the models predicting the probability of establishing tall seedlings and short saplings. Stand composition was an important factor for all the regeneration stages. For short seedlings, local (sample plot) stand composition was important. The establishment of short seedlings was negatively influenced by shade-tolerant species (sum of the heights of the shade-tolerant trees per plot). Species composition at the forest stand spatial scale was more important for tall seedlings and saplings. The proportion of Scots pine in forest stand volume positively influenced the establishment of tall seedlings. We found that with the growth of regeneration, the role of free space availability increased. The occurrence of tall seedlings and saplings was negatively influenced by increased space filling. Introducing silver fir outside of its natural range requires adjustment to management practices such as creation of vertically diversified mixed stands, decreasing stand stocking, regulation of stand species composition. A lack of experience with silver fir silviculture in northern Poland is one of important factors limiting the advancement of regeneration to the stage of high saplings outside the natural range.