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Question: What are the drivers of structural changes and mortality in oak— beech forests over 50 yrs of natural succession? Location: Twelve unmanaged forest sites, comprising a large environmental gradient in the Swiss lowlands. Method: By using repeated inventory data from more than 17 600 individually tagged trees, the dynamics of oak—beech stands over the past 50 yrs were analysed. Generalized linear mixed-effects models were fitted to quantify annual mortality rates of oak and beech based on DBH, stand basal area, precipitation and slope. Results: Stand basal area increased, whereas tree density decreased over time. At most sites, the relative importance of oak decreased gradually compared to beech. Mortality increased over time for both oak and beech, but the increase was stronger for oak. Oak and beech mortality decreased with increasing DBH and tended to increase with precipitation. Additionally, oak mortality increased with stand basal area, whereas no such trend was found for beech. Conclusion: Our study indicates that mortality in Central European oak—beech forests is driven by a combination of stand structures (i.e. tree size and stand basal area) and climate. However, the influence of climate on oak mortality is comparably low. Increasing oak mortality with stand basal area is a plausible consequence of its lower relative competitiveness and higher demand for light. Thus, in forests developing towards higher stand basal area, the ecologically important oak is increasingly outcompeted by beech, unless competition is reduced through management or disturbances.

It is unclear how global warming will affect the El Niño/Southern Oscillation (ENSO), in part because the instrumental record is too short to understand how ENSO has changed in the past. Now a 700-year-long tree-ring record indicates that ENSO-related climate variability may increase in New Zealand with continued warming. It is not known how global warming will affect the El Niño/Southern Oscillation (ENSO). The instrumental record is too short to discern centennial-scale trends and modelling results are inconclusive 1 , 2 , 3 , 4 , 5 . Proxy reconstructions indicate that ENSO activity was relatively high during the late twentieth century 6 , 7 , 8 , 9 , 10 , but whether this was unusual in the millennial context remains uncertain. Here we present insights into these issues derived from rings of the kauri tree ( Agathis australis ), a rare long-lived conifer endemic to the forests of northern New Zealand. Our results indicate that the twentieth century was the most ‘ENSO-active’ century of the past 500 years, but may not be unique in the context of the past 700 years, and that ENSO activity comparable to or elevated above that experienced during the late twentieth century is plausible under warmer-than-present conditions. We also find evidence that there may have been significant changes in the ENSO teleconnection to the New Zealand region during the fourteenth and fifteenth centuries, and of multi-decadal fluctuations in ENSO-related activity building up to the present day. Although these two features may delay the expression of increased ENSO activity in the New Zealand region, our results indicate that New Zealand climate is likely to be more dominated by ENSO-related inter-annual variability as the world continues to warm.

Future forest composition is uncertain in many areas due to climate change. The spread of non-native species adds to these uncertainties, particularly in forests recently colonised by novel tree species. To anticipate future forest composition, and thus the provision of ecosystem services, a thorough understanding of the factors influencing the establishment of non-native tree species is essential. We studied the presence and abundance of regeneration of Ailanthusaltissima (Mill.) Swingle in 89 plots on a regular grid in three sites in southern Switzerland to determine the main drivers of its establishment. All sites are located in abandoned, i.e. currently unmanaged stands of Castaneasativa Mill. Propagule pressure is caused by single mature A.altissima that colonised the area ca 40 years ago. We found high rock cover, low litter cover and high light availability to be the most important predictors for the presence of A.altissima regeneration, whereas its abundance was positively influenced by high light availability, low litter cover and high browsing on regeneration of competing species. However, the presence models performed much better than the abundance models. Interestingly, the most important factors favouring the establishment of A.altissima in recently undisturbed sites were found to be similar in a nearby site after a severe forest fire, which suggests a similar establishment strategy after a disturbance as in recently undisturbed forests. Based on our results we expect a further expansion of the species in lowland forests currently dominated by C.sativa , likely controlled primarily by light availability.

Question: How well can mortality probabilities of deciduous trees (Fagus sylvatica) and conifers (Abies alba) be predicted using permanent plot data that describe growth patterns, tree species, tree size and site conditions? Location: Fagus forests in the montane belt of the Jura folds (Switzerland). Method: Permanent plot data were used to develop and validate logistic regression models predicting survival probabilities of individual trees. Backward model selection led to a reduced model containing the growth-related variable ‘relative basal area increment’ (growth-dependent mortality) and variables not directly reflecting growth such as species, size and site (growth-independent mortality). Results: The growth-mortality relationship was the same for both species (growth-dependent mortality). However, species, site and tree size also influenced mortality probabilities (growth-independent mortality). The predicted survival probabilities of the final model were well calibrated, and the model showed an excellent discriminatory power (area under the receiver operating characteristic curve = 0.896). Conclusion: Mortality probabilities of Fagus sylvatica and Abies alba can be predicted with high discriminatory power using a well calibrated logistic regression model. Extending this case study to a larger number of tree species and sites could provide species- and site-specific tree mortality models that allow for more realistic projections of forest succession.

Key messageOur analysis of short-term growth responses to weather conditions provided evidence that the non-native tree species Ailanthus altissima may have a high growth potential in a warmer and drier environment, as predicted by global warming scenarios.Non-native tree species may alter ecosystem services in colonized forest communities. These novel forests represent a challenge for management, which is further accentuated by anthropogenic global change. Knowledge on the intra-annual growth performance and tree water relations of established and novel tree species is required to elucidate the drivers underlying forest dynamics. Here, we analysed intra-annual radial growth of the introduced tree species Ailanthus altissima (Mill.) Swingle and co-occurring Castanea sativa Mill. in novel forest communities in southern Switzerland. We used four years of continuously measured growth data in two sites to identify weather conditions in which the species achieved highest hourly growth and conditions in which largest proportions of seasonal growth occurred. While both species reached the largest proportion of seasonal growth and achieved the highest hourly growth under the most frequent weather conditions, A. altissima featured the highest growth rates in a wider range of weather conditions than C. sativa, particularly at high temperature and high vapour pressure deficit. We conclude that A. altissima has a greater potential than C. sativa to sustain growth in warm-dry conditions. Importantly, the advantages of A. altissima may be more accentuated in forest stands with low interspecific competition.

The tree of heaven ( Ailanthus altissima (Mill.) Swingle) is considered to be an early-successional, gap-obligate pioneer species with vigorous height growth, low shade tolerance, early fecundity and large seed production. It is a highly invasive species in many temperate and Mediterranean ecosystems outside its natural range, especially after disturbance. Due to its low shade tolerance, the potential of A. altissima to colonise undisturbed forests is thought to be low. In this study we analysed the potential of juvenile A. altissima to grow and survive in sweet chestnut ( Castanea sativa Mill.) forests in southern Switzerland. We used hemispherical photography to assess the light conditions of 204 individuals of A. altissima (31 % generative, 69 % vegetative) aged between 1 and 7 years (median: 3 years) in six sites. Generative (seed-borne) and vegetative (clonal ramet) offspring of A. altissima are able to grow in light conditions well below the requirements of shade-intolerant tree species such as European larch ( Larix decidua Mill.) and Scots pine ( Pinus sylvestris L.). The relatively low light conditions found to be sufficient for the growth and survival of generative regeneration of A. altissima suggest a higher shade tolerance for this species than previously stated, at least for early regeneration. Consequently, the colonisation frontier of A. altissima should be intensively monitored in both forest openings but also in closed canopy forests in the vicinity of seed-bearing A. altissima .

Question: How well can mortality probabilities of deciduous trees(Fagus sylvatica) and conifers (Abies alba) be predicted using permanent plot data that describe growth patterns, tree species, tree size and site conditions? Location: Fagus forests in the montane belt of the Jura folds (Switzerland). Method: Permanent plot data were used to develop and validate logistic regression models predicting survival probabilities of individual trees. Backward model selection led to a reduced model containing the growth‐related variable ‘relative basal area increment’ (growth‐dependent mortality) and variables not directly reflecting growth such as species, size and site (growth‐independent mortality). Results: The growth‐mortality relationship was the same for both species (growth‐dependent mortality). However, species, site and tree size also influenced mortality probabilities (growth‐independent mortality). The predicted survival probabilities of the final model were well calibrated, and the model showed an excellent discriminatory power (area under the receiver operating characteristic curve = 0.896). Conclusion: Mortality probabilities of Fagus sylvatica and Abies alba can be predicted with high discriminatory power using a well calibrated logistic regression model. Extending this case study to a larger number of tree species and sites could provide species‐ and site‐specific tree mortality models that allow for more realistic projections of forest succession.

Future forest composition is uncertain in many areas due to climate change. The spread of non-native species adds to these uncertainties, particularly in forests recently colonised by novel tree species. To anticipate future forest composition, and thus the provision of ecosystem services, a thorough understanding of the factors influencing the establishment of non-native tree species is essential. We studied the presence and abundance of regeneration of Ailanthusaltissima (Mill.) Swingle in 89 plots on a regular grid in three sites in southern Switzerland to determine the main drivers of its establishment. All sites are located in abandoned, i.e. currently unmanaged stands of Castaneasativa Mill. Propagule pressure is caused by single mature A.altissima that colonised the area ca 40 years ago. We found high rock cover, low litter cover and high light availability to be the most important predictors for the presence of A.altissima regeneration, whereas its abundance was positively influenced by high light availability, low litter cover and high browsing on regeneration of competing species. However, the presence models performed much better than the abundance models. Interestingly, the most important factors favouring the establishment of A.altissima in recently undisturbed sites were found to be similar in a nearby site after a severe forest fire, which suggests a similar establishment strategy after a disturbance as in recently undisturbed forests. Based on our results we expect a further expansion of the species in lowland forests currently dominated by C.sativa, likely controlled primarily by light availability.

Forest succession depends strongly on the life history strategies of individual trees. An important strategic element is the ability to survive unfavourable environmental conditions that result in strongly reduced tree growth. In this study, we investigated whether the relationship between growth and mortality differs among tree species and site conditions. We analysed 10 329 trees of nine tree species (Picea abies, Taxus baccata, Fagus sylvatica, Tilia cordata, Carpinus betulus, Fraxinus excelsior, Quercus robur, Betula spp. and Alnus glutinosa) from unmanaged forests of Europe: the continental Białowieża forest (Poland) and several oceanically influenced Swiss forest reserves. For each species, we calculated a set of flexible logistic regression models with the explanatory variables growth (as measured by relative basal area increment), tree size and site. We selected the species-specific model with the highest goodness-of-fit and calculated its discriminatory power (area under the receiver operating characteristic curve, AUC) and calibration measures. Most models achieved at least a good discriminatory power (AUC>0.7) and the AUC ranged from 0.62 to 0.87; calibration curves did not indicate any overfitting. Almost all growth-mortality relationships differed among species and sites, i.e. there is no universal growth-mortality relationship. Some species such as F. excelsior showed reduced survival probabilities for both unfavourable and very good growth conditions. We conclude that the growth-mortality relationships presented here can contribute to the life-history classification of trees and that they should also help to improve projections of forest succession models.

1. Tree growth and mortality are key elements of forest dynamics, and thus are of great concern for forest managers. It is widely accepted that tree mortality can be predicted using tree growth data. Several approaches have been proposed for modelling the growth-mortality relationship, differing in terms of data sources and model flexibility. However, little is known about their ability to reliably reconstruct the shape of the real growth-mortality relationship due to a lack of long-term data. 2. We adopted a 'virtual ecology' approach to this problem, simulating forests with either of two a priori specified growth-mortality relationships. Different sampling regimes in these virtual forests resulted in virtual tree-ring data, forest inventory data, or a combination of both. We used eight existing or newly developed models of different structural flexibility to analyse the growth-mortality relationship. The accuracy of the different model outputs, i.e. the deviation from the a priori specified growth-mortality relationships, was quantified with the Kullback-Leibler distance. 3. For all data sources, reliable growth-mortality models could be identified. The highest accuracies were found for tree-ring based models, which require only a small sample size (60 dead trees). High model accuracies were also found for forest inventory based models, starting at sample sizes of 500 trees. 4. Flexible statistical approaches turned out to be superior to less flexible models only for large sample sizes (totally 2000 trees). The additional use of Bayesian statistics, specifically designed for small sample sizes, led to high model accuracies only when model flexibility was constrained. 5. Synthesis. Our study shows that simulated experiments are a powerful tool for selecting reliable approaches to analyse ecological processes such as tree mortality. Reliable models are fundamental for gaining novel ecological insights into the growth-mortality relationship of tree species. The use of more accurate growth-mortality relationships in forest succession models would allow for strongly improved projections of past and future forest dynamics. Our study provides the theoretical basis for a sound estimation of such growth-mortality models, and it also provides guidelines for efficient sampling schemes in real forests.