Mixed vs. monospecific mountain forests in response to climate change: structural and growth perspectives of Norway spruce and European beech

Mixed forests play a key role in terms of stability, production potential and adaptation to climate change. Norway spruce [PA, Picea abies (L.) Karst] and European beech (FS, Fagus sylvatica L.) are among the most important tree species in Europe. The aim of the study was to determine the influence of the species composition of these two tree genera on the production, structure, diversity and growth of mixed Fageto-Piceetum acidophilum stands in the Krkonose Mountains, in the Czech Republic. The following 5 variants (ratios) of mixture were compared in 6 replications (30 research plots in total): PA 100%, PA 75:25 FS, PA 50:50 FS, PA 25:75 FS and FS 100%. Based on 178 tree core samples, the research also focused on the influence of climatic factors (temperature, precipitation) and air pollution (SO2, NOX, AOT40F) on the radial growth of these tree species of particular variants. Mixed forests showed a timber production higher by 7.7% (-10.8 to 31.5%) in comparison to spruce monocultures, and by 47.3% (21.9-79.7%) compared to beech monocultures. The largest production as well as the highest diameter increment were documented in PA 75:25 FS (656 m(3) ha(-1)). In addition, this variant had the lowest extreme decreases/fluctuations in radial growth in both tree species. Over the last 50 years, the increment in beech increased by 7.9% and by 2.5% in spruce. The cyclical behavior in the radial growth of both tree species occurred in the short-term solar cycles of 9-11 and long-term periods of 50-75 years, while the spruce showed higher cyclic intensity. The concentration of both SO2 and NOX had a significant negative effect on the radial growth of spruce. In both tree species, the negative effect of air pollution lessened with their decreasing share in the stand. Similarly, precipitation and temperature had a more significant effect on the growth of monospecific variants in both tree species, especially in beech. Temperatures, when compared to precipitation, had a greater effect on the radial growth of both tree genera, especially during the vegetation period. In terms of diversity, mixed stands achieved significantly higher structural (diameter, height, crown) differentiation and overall diversity compared to monospecific variants. In general, mixed stands can achieve higher production potential, diversity and especially resistance to climate extremes and air pollution in relation to climate change in the water-sufficient highland and mountain areas of the Czech Republic. Differences between mixed stands vs. monocultures, i.e. the effect of tree species mixing, depend on suitable ratios of tree species and their spatial pattern.