Resilience and synergism in larch-beech-spruce mixed forests

The current environmental conditions in Central Europe negatively affect tree growth and forest vitality. Mixing tree species may stabilize or even increase stand productivity and mitigate the effects of drought caused by climate change. However, the effects of mixing European larch (Larix decidua Mill.) with other species have not been assessed by systematic empirical studies. Accordingly, we studied the growth response of larch trees growing in middle-aged mixed stands with European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) in the highlands of the Czech Republic - Drahanská vrchovina (around 500 m a.s.l.). Four research sites with similar environmental conditions were selected to observe various forest mixtures: larch-beech-spruce, larch-beech, larch-spruce, and pure larch. 10 wood cores were collected at breast height from each tree species at the site (a total of 80 samples). Cores were measured and cross-dated by the dendrochronological software Past4. The radial increment of target trees with respect to the competition of the 10 nearest neighbouring trees was evaluated. Positions were measured with a Filed-Map system (IFER, Czech Republic). The competition indexes were calculated as the ratio between the DBH of the target tree and of the competitor divided by the distance between them. The resistance (growth reduction during the extreme year), recovery (growth response after the extreme year), and resilience (capacity to reach pre-disturbance growth) of target trees were calculated.

The even-aged stands with a different proportion of larch, beech, and spruce exhibited a different growth response under the same climatic conditions. In 2018 (the year with the most stressful climatic conditions in the last decade), a decrease in the tree-radial increment of spruces and beeches was observed. This did not occur in larches. They had a similar (only marginally better) growth response in monoculture compared to mixed stands. Tree resistance to the environmental conditions in 2018 was lower than a value of 1 for all tree species. However, beeches growing in the triple mixed stand were the most resistant. Surprisingly, larches in a monoculture exhibited a higher resistance compared to mixed stands. The highest recovery after the extreme conditions of 2018 was observed in larches growing in all variations of mixed stands. On the other hand, larches in a monoculture demonstrated nearly their lowest growth in the period 2019 - 2021. Only larches growing with beeches had a wood increment as high as prior to 2018 (higher resilience of larches growing with beeches). The other tree species reached their previous (2015 – 2017) increment in all stands with larches, so we can assume a positive effect of larch presence (increases the resilience of other tree species). We determined that the larches are not significantly influenced by competition with their neighbours. Furthermore, larches in monoculture are more resistant and larches growing in mixtures are more resilient with superior recovery. 

Key words: competition, stem increment, recovery, resilience, resistance, tree-rings, wood cores