The 2018 European heatwave led to stem dehydration but not to consistent growth reductions in forests

Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes

Nitrogen deposition is the most important environmental driver of growth of pure, even-aged and managed European forests

Changing environmental conditions may substantially interact with site quality and foreststandcharacteristics,and impactforestgrowthandcarbonsequestration.Understandingtheimpactofthevariousdriversofforestgrowthis thereforecriticaltopredicthowforestecosystemscanrespondtoclimatechange.Weconductedacontinental-scale analysis of recent (1995–2010) forest volume increment data (ΔVol, m3 ha−1 yr−1), obtained from ca. 100,000 coniferous and broadleaved trees in 442 even-aged, single-species stands across 23 European countries. We used multivariatestatisticalapproaches,suchasmixedeffectsmodelsandstructuralequationmodellingtoinvestigatehow Europeanforestgrowthrespondtochangesin11predictors,includingstandcharacteristics,climateconditions,air andsitequality,aswellastheirinteractions.Wefoundthat,despitethelargeenvironmentalgradientsencompassed bytheforestsexamined,standdensityandagewerekeydriversofforestgrowth.Wefurtherdetectedapositive,in some cases non-linear effect of N deposition, most pronounced for beech forests, with a tipping point at ca. 30kgNha−1 yr−1.WiththeexceptionofaconsistenttemperaturesignalonNorwayspruce,climate-relatedpredictorsandground-levelozoneshowedmuchlessgeneralizedrelationshipswithΔVol.Ourresultsshowthat,together withthedrivingforcesexertedbystanddensityandage,Ndepositionisatleastasimportantasclimatetomodulate forestgrowthatcontinentalscaleinEurope,withapotentialnegativeeffectatsiteswithhighNdeposition