Too wet for oaks? Inter-tree competition and recent persistent wetness predispose oaks to rainfall-induced dieback in Atlantic rainy forest

Forest dieback is usually triggered by climatic extremes, even if tree decline can be caused by diverse biotic and abiotic stressors acting synergistically on tree vitality. Many case studies worldwide illustrate the global importance of drought-induced forest dieback under a context of climate warming. However, forest decline is also occurring in regions that are not water-limited, but where increasing rainfall and exceptionally rainy events are observed. Here we assessed the influence of inter-tree competition, regional water availability, and large-scale climate variation on the decline and death of pedunculate oak (Quercus robur) in an Atlantic rainy forest in NW Spain. All healthy, declining, and dead trees in four replicated forest stands were mapped, and inter-tree competition was individually quantified with a distance-dependent competition index. Long-term variation of annual radial growth was analyzed on a selection of individuals per stand, and its dependence on climate variation was examined by correlation analysis with monthly climatic records. Trees under intense competition showed higher mortality risk. Increasing rainfall and the large-scale climatic pattern El Niٌo-Southern Oscillation (ENSO) have influenced tree growth during recent decades, acting as long-term stressors. A detrimental effect of water surplus during both the year preceding growth and spring of the current year has been noticed since 1980. Extremely rainy conditions in 2001 resulted in strong short-term stress that killed trees suffering from intense competition and wetness-induced stress. Our findings support that water excess is a relevant triggering factor for dieback of dominant forest trees in rainy temperate deciduous forest. This pattern is possible in regions where increasing precipitation and more frequent and intense rainfall extremes, associated with global climate warming, are happening. Since climate warming may lead to higher total annual rainfall, and to an increase in frequency and intensity of extreme precipitation events, forest dieback episodes associated with wetter conditions may become more common in the future.