Spatial patterns with memory: tree regeneration after stand-replacing disturbance in Picea abies mountain forests

QuestionsWhat 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? LocationMountain Picea abies forests in the Sumava 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. MethodsWe 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 Matern point processes to the observed data. ResultsRecruits formed tight clusters (2-9m), whereas trees were distributed randomly or weakly clustered at short distances (1-2m). Both pre-disturbance and post-disturbance recruits were highly clustered (cluster radii<2m) 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 Matern process. ConclusionsTree 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.