Growth and mortality of Norway spruce and European beech in monospecific and mixed-species stands under natural episodic and experimentally extended drought. Results of the KROOF throughfall exclusion experiment

Abstract

Key message Under severe drought, growth of Norway spruce suffered much more than European beech. Norway spruce benefited from growing in the environment of beech, and both species acclimated slightly to 5 years of experimentally extended drought. Abstract Recent studies show that the detrimental effects of drought on stand growth are mitigated when the stand contains mixed tree species. We analysed the growth responses of Norway spruce and European beech to episodic and experimentally extended drought in intra-and inter-specific mature stands. We used annual diameter growth records dating back to 1998 to determine the impact of the natural episodic drought in 2003 and 2015. To analyse extended drought, spruce and beech trees were exposed to extreme drought under automatic throughfall exclusion roofs from 2014 to 2018. The growth of spruce in an inter-specific environment with beech was 20-50% less affected by natural episodic drought compared with an intraspecific constellation. When beech grew in an inter-specific environment, it was by 23% more affected by drought compared to intra-specific conditions, but seemed to recover faster. The induced drought from 2014 to 2018 resulted in a strong growth reduction in the first year particularly for spruce, followed by a slight acclimation to the dry conditions. Beech acclimated and recovered faster than spruce across all growing conditions, while spruce only acclimatized faster in the environment of beech. Both species showed a higher mortality under induced drought compared with the controls; for spruce, the mortality rate was fivefold higher compared to the long-term mortality. The long-term moderate-growth stabilization and the growth increase after the 5-year exposure to drought suggest a gradual acclimation to drought by beech. The resistance and acclimation to drought of spruce when growing in mixture should be considered when designing resource efficient and productive mixed conifer-broadleaved stands for future climates.

European beech stem diameter grows better in mixed than in mono-specific stands at the edge of its distribution in mountain forests

“…This intersection between the courses of growth in mono-and mixed-species stands as a function of elevation was indicated by a significant (p < 0.05) interaction between the variables MixMono and elevation (see parameter a 12 , Table 4). The growth of beech in mountain forests accelerated in the last century, as in many lowland forests (Spiecker et al 2012;Pretzsch et al 2014aPretzsch et al , 2020a, due to climate change. In the lower mountain forests, the positive growth trends for beech were greater in monoculture than in mixture.…”

“…However, there are also critical voices doubting the dominance of beech without humans due to an enhanced growth of beech since 1950 that can be primarily attributed to human influences like nitrogen fertilization or management effects (Scharnweber et al 2011(Scharnweber et al , 2019. Its dominance was reduced in the past for economic reasons, particularly in Central Europe, but its high mechanical stability against windthrow and comparatively high resistance and resilience to drought are leading to a comeback in view of climate change (Pretzsch et al 2020a;Paul et al 2019;Hanewinkel et al 2011). Presently beech forests cover about 12 × 10 6 ha in Europe (Brus et al 2012), and the area is steeply increasing due to a transition to close-to-nature forestry.…”

“…The downregulation of photosynthesis after the extreme drought and heat wave, which hit Central Europe in the summer of 2003, reduced the productivity of beech forests (Bosela et al 2018;Leuzinger et al 2005;van der Werf et al 2007). A decline in beech productivity in response to drought stress was also recorded in Mediterranean Europe (Jump et al 2006;Piovesan et al 2008), though populationspecific vulnerability to drought might be mitigated to some extent by acclimation (Dittmar et al 2003;Pretzsch et al 2020a;Tegel et al 2014;Tognetti et al 2014). Similarly, beech provenances may exhibit distinct drought vulnerability and resistance (Bolte et al 2016;Cocozza et al 2016).…”

“…Beech may benefit from spatial niche complementarity and light interception when mixed with other species such as Scots pine (Gonzalez de Andres et al 2017), Norway spruce (Pretzsch et al 2014b), Douglas-fir (Thurm and Pretzsch 2016) and silver fir (Bosela et al 2015;Jourdan et al 2019). Goisser et al (2016) and Pretzsch et al (2020a) indicated a temporal complementarity, possibly due to more favourable water and light use, when growing in mixture with conifer species.…”

Drought Resistance of Norway Spruce (Picea abies [L.] Karst) and European Beech (Fagus sylvatica [L.]) in Mixed vs. Monospecific Stands and on Dry vs. Wet Sites. From Evidence at the Tree Level to Relevance at the Stand Level

“…Resistance, resilience and recovery [21], as well as drought responses, of mixed and pure Norway Spruce (Picea abies [L.] Karst) and European Beech (Fagus sylvatica [L.]) have also been documented in studies recently [22][23][24]. Moreover, Pretzsch et al 2020 [25] reported spruce to be faster acclimated and by 20%-50% less affected within its interspecific neighborhood with beech, while the opposite was true for the beech. Neuner et al 2015 [26] report a higher survival probability of spruce in mixed stands with beech as compared to pure stands under warmer climate and poor soil conditions.…”