ALBERT

Description: Questions What are the spatial patterns of adult trees and recruits in natural Picea abies forests? How are these patterns related to each other? Does the relationship differ before and after stand-replacing disturbance? What are the ecological processes behind these spatial patterns? Location Mountain Picea abies forests in the Šumava Mts. (Bohemian Forest), Czech Republic, affected by a high-severity outbreak of bark beetle ( Ips typographus ) that caused large-scale dieback of the forest canopy. Methods We measured the spatial coordinates and heights of all recruits and the coordinates and DBH of all adult trees in nine plots across a wide range of recruit densities. We distinguished pre- and post-disturbance recruits, trees killed by the disturbance and trees already dead before it. To analyse spatial relationship among these groups, we used univariate and bivariate pair-correlation functions. To provide further insight into the mechanisms behind the observed patterns, we fitted Thomas and Matérn point processes to the observed data. Results Recruits formed tight clusters (2–9 m), whereas trees were distributed randomly or weakly clustered at short distances (1–2 m). Both pre-disturbance and post-disturbance recruits were highly clustered (cluster radii 〈 2 m) around trees before and after the stand–replacing disturbance. This fine-scale pattern was likely driven by a combination of: (1) seed accumulation in tree wells during winter; (2) nurse effects of tree trunks extending the vegetation period, suppressing competitive vegetation and enhancing nutrient supply from decomposed litter; and (3) suitable seedbeds on some decaying wood. The Thomas point process fitted the observed pattern of decreasing recruit density with increasing distance from mature trees better than the Matérn process. Conclusions Tree spatial pattern in mountain P. abies forests showed high resilience to stand-replacing disturbance. After a self-thinning of recruits tightly clustered around parental trees, their spatial pattern will mirror the pattern of trees that formed the stand before the disturbance. This ‘memory’ of tree spatial patterns is an important biological legacy and should be viewed as a fundamental property of natural P. abies forests. What are the spatial patterns of trees and recruits after the stand-replacing disturbance in mountain Norway spruce ( Picea abies ) forests? We found that spruce recruits are tightly clustered, both around disturbance-killed trees and snags that were already dead before the disturbance. Therefore, after self-thinning, the forest will have the same spatial pattern of trees as before the disturbance.

Description: The severity and spatial extent of bark-beetle outbreaks substantially increased in recent decades worldwide. The ongoing controversy about natural forest recovery after these outbreaks highlights the need for individual-based long-term studies, which disentangle processes driving forest regeneration. However, such studies have been lacking. To fill this gap, we followed the fates of 2,552 individual seedlings for 12 years after a large-scale bark-beetle outbreak that caused complete canopy dieback in mountain Norway spruce ( Picea abies ) forests in SE Germany. Here we explore the contribution of advance, disturbance-related and post-disturbance regeneration to forest recovery. Most seedlings originated directly within the three-year dieback of canopy trees induced by bark-beetle outbreak. After complete canopy dieback, the establishment of new seedlings was minimal. Surprisingly, advance regeneration formed only a minor part of all regeneration. However, because it had the highest survival rate, its importance increased over time. The most important factor influencing the survival of seedlings after disturbance was their height. Survival was further modified by microsite: seedlings established on dead wood survived best, whereas almost all seedlings surrounded by graminoids died. For 5 cm tall seedlings, annual mortality ranged from 20% to 50% according to the rooting microsite. However, for seedlings taller than 50 cm, annual mortality was below 5% at all microsites. While microsite modified seedling mortality, it did not affect seedling height growth. A model of regeneration dynamics based on short-term observations accurately predicts regeneration height growth, but substantially underestimates mortality rate - thus predicting more surviving seedlings than were observed. We found that Picea abies forests were able to regenerate naturally even after severe bark-beetle outbreaks owing to advance and particularly disturbance-related regeneration. This, together with microsite-specific mortality, yields structurally and spatially diverse forests. Our study thus highlights the so far unrecognized importance of disturbance-related regeneration for stand recovery after bark-beetle outbreaks. This article is protected by copyright. All rights reserved.

Description: Atmospheric circulation types, blockings, and cyclones are central features of the extratropical flow and key to understanding the climate system. This study intercompares the representation of these features in 10 reanalyses and in an ensemble of 30 climate model simulations between 1980 and 2005. Both modern, full-input reanalyses and century-long, surface-input reanalyses are examined. Modern full-input reanalyses agree well on key statistics of blockings, cyclones, and circulation types. However, the intensity and depth of cyclones vary among them. Reanalyses with higher horizontal resolution show higher cyclone center densities and more intense cyclones. For blockings, no strict relationship is found between frequency or intensity and horizontal resolution. Full-input reanalyses contain more intense blocking, compared to surface-input reanalyses. Circulation-type classifications over central Europe show that both versions of the Twentieth Century Reanalysis dataset contain more easterlies and fewer westerlies than any other reanalysis, owing to their high pressure bias over northeast Europe. The temporal correlation of annual circulation types over central Europe and blocking frequencies over the North Atlantic–European domain between reanalyses is high (around 0.8). The ensemble simulations capture the main characteristics of midlatitudinal atmospheric circulation. Circulation types of westerlies to northerlies over central Europe are overrepresented. There are too few blockings in the higher latitudes and an excess of cyclones in the midlatitudes. Other characteristics, such as blocking amplitude and cyclone intensity, are realistically represented, making the ensemble simulations a rich dataset to assess changes in climate variability.