Rapid beech decline under recurrent drought stress: Individual neighborhood structure and soil properties matter

During the summer of 2022, an acute drought once more afflicted central and southern Europe. This marked the third episode (after 2015 and 2018) of severe aridity in large parts of Germany within the last decade, leading to increased soil water depletion. Consequently, from July 2022 onward, European beech trees (Fagus sylvatica L.) exhibited early withering and pronounced premature defoliation. Nevertheless, crown defoliation exhibited substantial variation among trees within the same forest stands, prompting questions regarding the causal factors. In our study, we scrutinized twelve mature drought-impacted, beech-dominated forest stands in northern Bavaria, arranged along a gradient of different nutrient regime levels (base-rich, intermediate, base-poor), with co-occurring vital (& LE;40% crown defoliation) and declining (& GE;60% crown defoliation) trees. Within each stand, we selected an equal number of vital and declining trees, culminating in a total of 332 target trees. Dendrochronological patterns were analyzed to identify a potential timing of growth separation between vitality classes. Moreover, we used a Bayesian modelling framework to discern whether disparities in tree vitality hinged on competition, structure, small-scale differences in plant-available water capacity, and spatial clustering of declining competitors. We further explored the factors influencing the magnitude of growth decline post-2018 and how these were modulated by the site's nutrient regime. Our study unveiled that (i) low competition with increased size diversity bolstered tree vitality; (ii) declining trees were spatially aggregated; (iii) vital and declining trees exhibited strikingly similar growth trajectories in the past, which underwent a drastic shift following 2018, indicating a potential for a rapid vitality decline under recurrent severe drought stress; (iv) plant-available water capacity emerged as a crucial determinant of vitality and growth subsequent to 2018; (v) growth decline was most pronounced at base-poor and intermediate sites. Our findings underscore the importance of accommodating small-scale differences in soil and stand characteristics and advocate for silvicultural guidance towards reduced stand densities in combination with a more heterogenous structure to mitigate beech dieback in drought-prone forest stands.