Norway spruce (Picea abies) is the most widely cultivated and economically important tree species in southern Germany. However, due to the foreseen increase of drought occurrences to which spruce has been demonstrated to be particularly susceptible, spruce’s forests are expected to experience loss of stability and vitality. To maintain sustainable forest productivity and create resilient ecosystems, alternative conifer species are sought. In this study, we assessed the performance of presumably more drought tolerant species, such as Douglas fir (Pseudotsuga menziesii) and Silver fir (Abies alba).
The radial growth of these three species and their responses to climate variability was analysed within mixed stands in the Black Forest. Tree cores were collected from 18 sites, located within three altitudinal transects ranging from ca. 400 to 1.200 m a.s.l.. We used a dendroecological approach to assess the resistance, resilience, and recovery of these three conifers to the exceptional drought of 2003. Inventory data of neighbourhood density and composition were used to measure the effects of different mixtures.
Results/Conclusions
On average, when considering the mean radial growth of the last 30 years, Douglas fir is the fastest growing species, radial growth in Silver fir appears to be continuously increasing, whereas spruce shows a decreasing radial growth trend. Norway spruce was the least resistant and resilient to the summer drought in 2003. Silver fir showed the highest overall resistance to drought, similarly to Douglas fir. Tree ring growth was not significantly affected by the neighbourhood composition in „normal years”. However, Silver fir showed higher resilience to drought when growing in mixed-species neighbourhoods, while Douglas fir was more drought-stressed when growing in a mixture. The drought response of spruce was dependent on the density and species composition of the neighbourhood, showing both positive and negative mixing effects.
Our study indicates that Silver fir and Douglas fir are generally more resistant and resilient to drought extremes and are suitable alternatives to Norway spruce. Mixed stands containing these tree species could improve adaptation to drought because the risks of negative effects of extreme events are spread across species, and the performance of individual species is improved. A change in species composition is to be considered as a valuable approach to create resilient ecosystems across many Central European mountain forests capable of facing future climate change challenges.