Drought and masting affect growth allocation along the stem axis in beech, oak, fir and spruce in temperate forests

In temperate forests, tree growth and vitality are significantly influenced by drought, often reflected by low stem diameter growth. Additionally, masting, the cyclic production of seeds, can also affect growth, particularly through its interactions with environmental factors like drought. Despite extensive research on the individual impacts of drought and masting, their combined effects on growth allocation remain underexplored, especially for a broader variety of species. This study aims to bridge this gap by investigating how drought and/or masting influence growth patterns in the coniferous species Abies alba and Picea abies, as well as the deciduous broadleaved species Fagus sylvatica and Quercus petraea. Utilising tree-ring data collected over several decades, we analysed how basal area increment (BAI) at 1.3 m stem height as well as along the stem axis responds to various scenarios of drought and masting. Our results showed that drought consistently reduced BAI in all species. F. sylvatica and P. abies experienced significantly lower BAI during mast years that coincided with drought, suggesting a potential trade-off between vegetative and reproductive growth under resource limitation. In contrast, Q. petraea exhibited higher BAI during mast years, indicating a distinct carbon allocation strategy. Furthermore, our study revealed that differences in BAI between mast and non-mast years were most prominent in the upper stem in the deciduous species, particularly during drought periods. These findings highlight the complex interactions between environmental stress, stem diameter growth and seed production, emphasising the need for further research into whole-tree carbon allocation processes. Drought and masting jointly affect basal area increment along the stem axis with species-specific patterns. Overall, beech and spruce are severely affected, while oak and particularly fir show less impact.