Drivers of basal area variation across primary late-successional Picea abies forests of the Carpathian Mountains

Disentangling the importance of developmental vs. environmental drivers of variation in forest biomass is key to predicting the future of forest carbon sequestration. At coarse scales, forest biomass is likely to vary along major climatic and physiographic gradients. Natural disturbance occurs along these broad biophysical gradients, and depending on their extent, severity and frequency, could either amplify or dampen spatial heterogeneity in forest biomass. Here we evaluate spatial variation in the basal area of late-successional Picea abies (L./Karst.) forests across the Carpathian Mountain Range of central Europe and compare the roles of coarse-scale biophysical gradients and natural disturbances in driving that variation across a hierarchy of scales (landscapes, stands, and plots). We inventoried forest composition and structure, and reconstructed disturbance histories using tree cores collected from 472 plots nested within 30 late-successional stands, spanning the Carpathian Mountains (approximately 4.5 degrees of latitude). We used linear mixed-effects models to compare the effect of disturbance regimes and site conditions on stand basal area at three hierarchical scales. We found that the basal area of late-successional Picea abies forests varied across a range of spatial scales, with climatic drivers being most important at coarse scales and natural disturbances acting as the primary driver of forest heterogeneity at fine scales. For instance, the stand-level basal area varied among landscapes, with the highest values (48-68 m(2) ha(-1)) in the warmer southern Carpathian Mountains, and lower values (37-52 m(2) ha(-1) on average) in cooler areas of the eastern and western Carpathians. Finer-scale variation was driven by local disturbances (mainly bark beetle and windstorms) and the legacies of disturbances that occurred more than a century ago. Our findings suggest that warming could increase the basal area of northern sites, but potential increasing disturbances could disrupt these environmental responses.