Altered climate memory characterizes tree growth during forest dieback

Warming temperatures and droughts are driving widespread forest dieback and growth decline worldwide. In forests experiencing dieback, declining trees may exhibit altered climate memory of growth, indicative of physiological impairment. Thus, we evaluated climate-growth responses of trees in four drought-disturbed forests dominated either by gymnosperms (Abies alba, Pinus sylvestris) or angiosperms (Fagus sylvatica, Quercus humilis) in Northern Spain, where we compared responses of declining (heavily defoliated) and non-declining (slightly or not defoliated) trees. To disentangle the effects of forest dieback and past climate on tree growth, we applied the stochastic antecedent modeling (SAM) framework to annual tree-ring widths to quantify climatic memory. Declining trees had lower recent growth than non-declining conspecifics. All species responded positively to precipitation and temperature, independent of their vigor class, except for declining silver fir (A. alba) and European beech (F. sylvatica) individuals, which showed a negative effect of warmer temperatures on growth. Declining trees of these two species were also more sensitive to recent temperature and precipitation conditions, whilst climatic conditions further into the past were more important for non-declining trees. Silver fir and European beech from both vigor classes were also coupled to climate conditions during markedly different seasons, with dry summer conditions particularly affecting declining trees. Declining and non-declining Scots pine (P. sylvetris) and pubescent oak (Q. humilis) trees did not show different responses to past climate. While drought-triggered dieback differentially impacted silver fir and European beech individuals, Scots pine and pubescent oak trees suffered from a chronic process of loss in tree growth and vigor. Our results highlight the differences in climate sensitivity and climate memory of tree growth in forests experiencing ongoing dieback.