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Studying the effect of surface ozone (O3) and water stress on tree growth is important for planning sustainable forest management and forest ecology. In the present study, a 22-year long time series (1998–2019) on basal area increment (BAI) and fructification severity of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) H.Karst.) at five forest sites in Western Germany (Rhineland Palatinate) was investigated to evaluate how it correlates with drought and stomatal O3 fluxes (PODY) with an hourly threshold of uptake (Y) to represent the detoxification capacity of trees (POD1, with Y = 1 nmol O3 m−2 s−1). Between 1998 and 2019, POD1 declined over time by on average 0.31 mmol m−2 year−1. The BAI showed no significant trend at all sites, except in Leisel where a slight decline was observed over time (−0.37 cm2 per year, p < 0.05). A random forest analysis showed that the soil water content and daytime O3 mean concentration were the best predictors of BAI at all sites. The highest mean score of fructification was observed during the dry years, while low level or no fructification was observed in most humid years. Combined effects of drought and O3 pollution mostly influence tree growth decline for European beech and Norway spruce. [ABSTRACT FROM AUTHOR]

• After the 2018–2020 drought, crown condition differed strongly within beech stands. • Growth dynamics and tree genetics of non–/vital individuals were compared. • Long-term growth decline was widespread in both groups. • Climate sensitivity and individual heterozygosity did not consistently differ. • Crown condition after drought is a poorer indicator of tree vitality than ring width. Global warming poses a major threat to forest ecosystems around the world. In Central Europe, vitality losses and tree mortality have already been observed in various regions, especially after extreme drought episodes such as the 2018–2020 drought. European beech (Fagus sylvatica L.), which is the dominating deciduous tree species in large parts of temperate Europe, also suffered from the recent drought. In Germany, for example, losses in crown condition were observed. Within individual stands, however, the crown condition of beech differed strongly from non–/weakly damaged trees (hereafter referred to as vital trees) to severely damaged/dead trees (non-vital trees). Tree characteristics and micro-site conditions were apparently similar. Hence, we checked whether differences in growth behavior exist and (or) developed over time and assessed if observed differences between the non–/vital individuals might be genetically driven. We found that the climate sensitivity as well as the drought resistance of tree growth did not consistently differ between non–/vital trees. In both groups, a growing importance of water availability for tree growth was apparent. Further, long-term growth decline was widespread in both vital and non-vital trees, suggesting severe stress and future risks of tree mortality irrespective of crown condition. Only at one out of nine sites, we found significant differences in individual heterozygosity, signaling potential differences in the adaptation of trees to environmental stress. As a consequence, our study highlights that crown condition after an extreme drought is a poorer indicator of tree vitality than ring width in beech stands in Germany. [ABSTRACT FROM AUTHOR]

Future increases in temperature and changes in precipitation patterns may negatively affect the growth performance of economically important tree species such as Norway spruce, which in the past have often been established and managed in monocultures. Structural diversity has been advocated as a silvicultural approach to increase resistance and resilience of forests to climate change extremes. Whether it promotes growth stability during and following drought years has not yet been analyzed. We investigated stem growth reactions to the extreme drought of 2003 in 23 uneven-structured, mixed Norway spruce and Silver fir stands in southwestern Germany. Using linear mixed-effects models we analyzed the resistance and resilience of basal area increment in relation to species identity, drought intensity, tree size, competition, density and diversity. Structural diversity, measured as variation in tree diameter at breast height, had no influence on increment stability during the extreme summer drought of 2003. Likewise, the effect of species diversity was weak and inconclusive. However, a higher presence of Silver fir in the mixture appeared to reduce increment stability in 2003 for both fir and spruce. Reducing competition through thinning counteracted this effect and promoted increment stability. Our findings indicate that the species identity of competitors in mixtures is a better predictor of stem growth reactions to drought than diversity. They support the conclusion that diversity does not generally increase stability to drought stress. Silver fir consistently showed a substantially higher increment resistance and resilience than spruce. Its resistance increased with diminishing drought intensity, yet spruce reacted uniformly, manifesting a low resistance across the whole drought intensity gradient. Spruce did not regain pre-drought growth levels within the first three years after drought, whereas fir did. We conclude that fir may be able to act as a silvicultural alternative to spruce under changing climatic conditions, given appropriate sites and thinning regimes. Considering the expected increase in drought intensity and frequency in the 21 st century, understanding species interactions at the local scale emerges as an essential prerequisite for developing resilient forest stands. [ABSTRACT FROM AUTHOR]

Forest diversity-productivity relationships have been intensively investigated in recent decades. However, few studies have considered the interplay between species and structural diversity in driving productivity. We analyzed these factors using data from 52 permanent plots in southwestern Germany with more than 53,000 repeated tree measurements. We used basal area increment as a proxy for productivity and hypothesized that: (1) structural diversity would increase tree and stand productivity, (2) diversity-productivity relationships would be weaker for species diversity than for structural diversity, and (3) species diversity would also indirectly impact stand productivity via changes in size structure. We measured diversity using distance-independent indices. We fitted separate linear mixed-effects models for fir, spruce and beech at the tree level, whereas at the stand level we pooled all available data. We tested our third hypothesis using structural equation modeling. Structural and species diversity acted as direct and independent drivers of stand productivity, with structural diversity being a slightly better predictor. Structural diversity, but not species diversity, had a significant, albeit asymmetric, effect on tree productivity. The functioning of structurally diverse, mixed forests is influenced by both structural and species diversity. These sources of trait diversity contribute to increased vertical stratification and crown plasticity, which in turn diminish competitive interferences and lead to more densely packed canopies per unit area. Our research highlights the positive effects of species diversity and structural diversity on forest productivity and ecosystem dynamics. [ABSTRACT FROM AUTHOR]

Droughts and their negative effects on forest ecosystems are projected to increase under climate change for many regions. It has been suggested that intensive thinning could reduce drought impacts on established forests in the short-term. Most previous studies on the effect of thinning on drought impacts, however, have been confined to single forest sites. It is therefore still unclear how general and persisting the benefits of thinning are. This study assesses the potential of thinning to increase drought tolerance of the wide spread Scots pine ( Pinus sylvestris) in Central Europe. We hypothesized (1) that increasing thinning intensity benefits the maintenance of radial growth of crop trees during drought (resistance) and its recovery following drought, (2) that those benefits to growth decrease with time elapsed since the last thinning and with stand age, and (3) that they may depend on drought severity as well as water limitations in pre- and post-drought periods. To test these hypotheses, we assessed the effects of thinning regime, stand age, and drought severity on radial growth of 129 Scots pine trees during and after drought events in four long-term thinning experiments in Germany. We found that thinning improved the recovery of radial growth following drought and to a lesser extent the growth resistance during a drought event. Growth recovery following drought was highest after the first thinning intervention and in recently and heavily thinned stands. With time since the last thinning, however, this effect decreased and could even become negative when compared to unthinned stands. Further, thinning helped to avoid an age-related decline in growth resistance (and recovery) following drought. The recovery following drought, but not the resistance during drought, was related to water limitations in the drought period. This is the first study that analyzed drought-related radial growth in trees of one species across several stands of different age. The interaction between thinning intensity and time since the last thinning underline the importance to distinguish between short- and long-term effects of thinning. According to our analysis, only thinning regimes, with relatively heavy and frequent thinning interventions would increase drought tolerance in pine stands. [ABSTRACT FROM AUTHOR]

The carbon isotope composition (δ13C, ‰) and discrimination (Δ, ‰) of old grown North American Pinus ponderosa Dougl. Ex P. et C. Laws. and European Pinus sylvestris L. were determined using trees grown under almost identical growing conditions in a mixed stand in Bralitz, Northeast Germany. Single-tree δ13C analyses of tree-ring cellulose of both species were carried out at a yearly resolution for the period 1901-2001 and the results compared with growth (basal area increment). Annual mean δ13C values for P. ponderosa ranged from-21.6 ‰ to-25.2 ‰ and for P. sylvestris from-21.4 ‰ to-24.4 ‰. Accordingly, 13C discrimination (Δ) showed higher values for P. ponderosa throughout the investigation period. Five characteristic periods of Δ were identified for both the tree species, reflecting positive and negative influences of environmental factors. Good growing conditions such as after-thinning events had a positive effect on Δ, reflecting higher values, while poor conditions like aridity and air pollution had a negative influence, reflecting lower values. The dynamics of Δ were likewise reflected in the growth (basal area increment, BAI). Higher 13C discrimination values of P. ponderosa led to higher BAIs of P. ponderosa in comparison with P. sylvestris. Correlation function analyses confirmed that P. sylvestris was more dependent on precipitation than P. ponderosa, which showed a closer relationship with temperature. The results confirm that under predominantly dry growing conditions, P. ponderosa showed better growth performance than P. sylvestris, indicating better common intrinsic water-use efficiency and, therefore, higher rates of net photosynthesis at a given transpiration. In view of the prospect of climate change, the results are very significant for assessing both trees' physiological properties and, hence, their potential for coping with future growing conditions. [ABSTRACT FROM AUTHOR]