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

Abstract

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 largescale 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 Niño-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.

Individual-based approach as a useful tool to disentangle the relative importance of tree age, size and inter-tree competition in dendroclimatic studies

“…When radial growth of sampled individuals was averaged into a mean stand tree-ring chronology, there is no relationship between the climatic response of mean stand chronology and stand tree density or basal area (results not shown). A previous study in this forest showed that oaks that died, as a consequence of the extremely rainy conditions in the year 2001, were smaller and had lived under higher competition levels than surviving oaks (Rozas & García-González 2012). This evidence supports the theory that competition can predispose trees to the damaging effects of a climatic stress (Linares et al 2010).…”

“…The dominant woodland is an Ibero-Atlantic acidophilous oak forest (G1. (Rozas & García-González 2012).…”

“…The monthly or seasonal climatic variables that significantly influenced radial growth were identified by means of the percentage of trees that showed significant correlations with climate in 1971-2000, the most recent 30-yrs period common to all individual treering chronologies prior to 2001, which was the wettest year on record for the site (Rozas & García-González 2012). Individual climatic responses were calculated as bootstrapped correlations of the monthly and seasonal climatic time series with individual tree-ring chronologies.…”

Oak decline in a southern Finnish forest as affected by a drought sequence

“…An enhancing effect of precipitation is widely observed in the Mediterranean environments (Santini et al, 1994;Tessier et al, 1994;Čufar et al, 2008) and the Atlantic Spain (Rozas, 2001(Rozas, , 2005, where high precipitation creates favourable and high temperature unfavourable growth conditions during the summer period. On the other hand, water excess can be limiting for growth and is triggering forest dieback in rainy temperate deciduous forest (Rozas and García González, 2012). Anyway, the importance of summer precipitation for oak tree-ring width is evident across Europe.…”

Dendroclimatic responses of four European broadleaved tree species near their south-western range edges

“…Temperate trees suffer from additional stress sources at the transitional areas between Atlantic and Mediterranean regions revealing a more complex picture (González-González et al, 2014). For example, high levels of precipitation combined with relatively high winter temperatures has been shown to be the main cause for growth reduction and dieback of Q. robur in the north-western Iberian Peninsula (Rozas & García-González, 2012). Analyses of F. sylvatica tree-ring chronologies from south-western Europe have shown that spring temperature, together with the amount of winter precipitation, coexist with water availability in summer as major drivers of radial growth (Lebourgeois et al, 2005;Piovesan et al, 2005).…”