31Since several studies have been reporting an increase in the decline of forests, a major issue in ecology 32 is to better understand and predict tree mortality. The interactions between the different factors and 33 the physiological processes giving rise to tree mortality, as well as the individual between-tree 34 variability to mortality risk, still need to be identified and assessed. 35This study is based on a survey of 4323 European beeches (Fagus sylvatica L.) since 2002 in a rear-36 edge population within a natural reserve. We combined two types of approaches: (1) statistical 37 models were used to quantify the effects of competition, tree growth, size and decline on mortality 38 and (2) an ecophysiological process-based model (PBM) was used to separate out the different 39 mechanisms giving rise to temporal and inter-individual variations in mortality by simulating carbon 40 reserves, hydraulic conductance and late frosts in response to climate. 41The mortality rate at population level was associated to the combination of conductance loss, carbon 42 reserve depletion and occurence of late frosts simulated with the PBM. In the statistical models, the 43 individual probability of mortality decreased with increasing mean growth, and increased with 44 increasing crown defoliation, earliness of budburst, fungi presence and increasing competition. The 45 interaction between tree size and defoliation was significant, indicating a stronger increase in 46 mortality associated to defoliation in smaller than larger trees. Finally, the PBM predicted a higher 47 conductance loss together with a higher level of carbon reserves for trees with earlier budburst, while 48 the ability to defoliate the crown was found to limit the impact of hydraulic stress at the expense of 49 the accumulation of carbon reserves. 50We discuss the convergences and divergences obtained between statistical and process-based 51 approaches and we highlight the importance of combining them to identify the different processes 52 underlying mortality, and the factors modulating individual vulnerability to mortality. 53 54 55 56
Abiotic and biotic stresses related to climate change have been associated to increased crown defoliation, decreased growth and a higher risk of mortality in many forest tree species, but the impact of stresses on tree reproduction and forest regeneration remains understudied. At dry, warm margin of species distributions, flowering, pollination and seed maturation processes are expected to be affected by drought, late frost and other stresses, eventually resulting in reproduction failure. Moreover, inter-individual variations in reproductive performances versus other performances (growth, survival) could have important consequences on population’s dynamics.We investigated the relationships between individual crown defoliation, growth and reproduction in a drought-prone population of European beech, Fagus sylvatica. We used a spatially explicit mating model and marker-based parentage analyses to estimate effective female and male fecundities of 432 reproductive trees, which were also monitored for basal area increment and crown defoliation over nine years.Female and male fecundities markedly varied among individuals, more than did growth. Both female fecundity and growth decreased with increasing crown defoliation and competition and increased with size. Male fecundity only responded to competition, and decreased with increasing competition. Moreover, the negative effect of defoliation on female fecundity was size-dependent, with a slower decline in female fecundity with increasing defoliation for the large individuals. Finally, a trade-off between growth and female fecundity was observed in response to defoliation: some large trees maintained significant female fecundity at the expense of reduced growth in response to defoliation, while some other defoliated trees rather maintained high growth at the expense of reduced female fecundity.Synthesis. Our results suggest that while decreasing their growth, some large defoliated trees still contribute to reproduction through seed production and pollination. This non-coordinated decline of growth and fecundity at individual-level in response to stress may compromise the evolution of stress-resistance traits at population level, and increase forest tree vulnerability.
Background and Aims Abiotic and biotic stresses related to climate change have been associated with increased crown defoliation, decreased growth and a higher risk of mortality in many forest tree species, but the impact of stresses on tree reproduction and forest regeneration remains understudied. At dry warm margin of species distributions, flowering, pollination and seed maturation are expected to be affected by drought, late frost and other stresses, eventually resulting in reproduction failure. Moreover, inter-individual variation in reproductive performance versus other performance (growth, survival) could have important consequences for population dynamics. This study investigated the relationships among individual crown defoliation, growth and reproduction in a drought-prone population of European beech, Fagus sylvatica Methods We used a spatially explicit mating model and marker-based parentage analyses to estimate effective female and male fecundities of 432 reproductive trees, which were also monitored for basal area increment and crown defoliation over nine years. Key Results Female and male fecundities markedly varied between individuals, more than did growth. Both female fecundity and growth decreased with increasing crown defoliation and competition, and increased with size. Moreover, the negative effect of defoliation on female fecundity was size-dependent, with a slower decline in female fecundity with increasing defoliation for the large individuals. Finally, a trade-off between growth and female fecundity was observed in response to defoliation: some large trees maintained significant female fecundity at the expense of reduced growth in response to defoliation, while some other defoliated trees maintained high growth at the expense of reduced female fecundity. Conclusions Our results suggest that while decreasing their growth, some large defoliated trees still contribute to reproduction through seed production and pollination. This non-coordinated decline of growth and fecundity at individual-level in response to stress may compromise the evolution of stress-resistance traits at population level, and increase forest tree vulnerability.
Comparing statistical and mechanistic models to identify the drivers of mortality within a rear-edge beech population, PeerCommunity Journal, 1: e55.
Wild bees are declining, mainly due to the expansion of urban habitats that have led to land-use changes. Effects of urbanization on wild bee communities are still unclear, as shown by contrasting reports on their species and functional diversities in urban habitats. To address this current controversy, we built a large dataset, merging 16 surveys carried out in 3 countries of Western Europe during the past decades, and tested whether urbanization influences local wild bee taxonomic and functional community composition. These surveys encompassed a range of urbanization levels, that were quantified using two complementary metrics: the proportion of impervious surfaces and the human population density. Urban expansion, when measured as a proportion of impervious surfaces, but not as human population density, was significantly and negatively correlated with wild bee community species richness. Taxonomic dissimilarity of the bee community was independent of both urbanization metrics. However, occurrence rates of functional traits revealed significant differences between lightly and highly urbanized communities, for both urbanization metrics. With higher human population density, probabilities of occurrence of above-ground nesters, generalist and small species increased. With higher soil sealing, probabilities of occurrence of above-ground nesters, generalists and social bees increased as well. Overall, these results, based on a large European dataset, suggest that urbanization can have negative impacts on wild bee diversity. They further identify some traits favored in urban environments, showing that several wild bee species can thrive in cities.
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