How cyclical and predictable are Central European temperate forest dynamics in terms of development phases

Questions Recently there have been vital discussions about the validity of the European patch-mosaic conceptual model of forest dynamics – the traditional concept of a shifting patch-mosaic of developmental stages and phases, also known as the forest cycle concept. Here we try to answer the fundamental questions of this debate: How much do the forest dynamics proceed along a predictable path (in a chronological sequence: growth—optimum—breakdown)? Or vice versa, are the patches rather a result of disturbances and/or other stochastic growth and mortality patterns? Location The study was carried out at 5 long-term research plots located in 4 different study sites of central European natural temperate forests. Methods The long-term evolution of forest developmental phases was analyzed by a GIS based, spatially explicit, fully reproducible method enabling proper verification of the functionality of the model forest cycle. We analyzed long-term transitions among forest developmental phases from the 1970's through the 1990's to 2000's. Observed phase-to-phase transitions were compared to a random transition model. We identified preferential pathways within the forest cycle model as well as the proportion of cyclic/ acyclic transitions. Results In total, across all sites and observation periods about 65% of all observed phase-to-phase transitions were realized through preferential pathways, about 28% of observed transitions went along pathways of random frequency and only about 7% of observed transitions were realized through uncommon developmental pathways. On the other hand, only less than 40% of all observed transitions might be classified as cyclic (following the model cycle), and thus more than 60% of the transitions were acyclic (moving across or backward in the model cycle). The overall pattern of all observed transitions resembled a complex web rather than a simple repeating cycle. Conclusions Although in all sites we documented signs of the cyclic and predictable development anticipated by the forest cycle concept, the predominance and stochastic nature of multiple acyclic developmental pathways gave rise to reasonable doubts on the legitimacy and usability of the concept for descriptions of forest dynamics. On the other hand, the verification of the concept may contribute significantly to our understanding of the complexity of forest dynamics. This article is protected by copyright. All rights reserved.