Mortality, dynamics and structure of mountain forests in Central Europe

Ing. Michal Synek

Doctoral thesis

Abstract:

Natural tree mortality, driven by disturbances, their legacies and climate, represents a key process shaping forest dynamics and influencing forest structure. Shifting tree mortality rates are a global-change syndrome influencing forests across the world. However, determining whether environmental changes are altering mortality rates is complicated by the fact that tree mortality inherently changes through time as forests develop following natural disturbances. Exploring the mortality patterns in primary forests in the context of their disturbance regimes and stand structure provides information on natural dynamics of the forests under ongoing climate change. Different mortality agents leave various biological legacies that influence future stand structure and sensitivity to disturbances. The composition of mortality agents across different tree species and sizes is therefore crucial for stand development. The thesis aims to 1) evaluate the mortality rates and composition of mortality agents in the context of stand structure in primary mountain Norway spruce forests (Chapter 5.1), 2) explore the relationship between disturbance history and current mortality (Chapter 5.2), 3) evaluate the overall and species-specific mortality rates and mortality agents’ composition in primary European beech-dominated mixed stands (Chapter 5.3), and 4) analyze the main drivers of basal area variability in primary forests (Chapter 5.4). An extensive database of permanent study plots from primary forests in the Carpathian Mountains was used for these studies. Structural, dendrochronological, and re-census-based mortality data from a total of almost 700 sample plots were used to achieve the goals. The main findings of this thesis are: 1) The estimated overall annual mortality rate in primary Norway spruce stands was within the previously published range of background mortality, however, substantial variability was detected on both stand and plot levels. The overall distribution of mortality was balanced up to 50 cm DBH, however, the distributions for individual mortality agents were different, because some mortality agents are prevalent in certain size classes. Bark beetle (Ips typographus L.) was the most frequent mortality agent even in stands with low overall mortality. The results suggest the role of bark beetle as a mortality agent does not equate to severe mortality at large scales (Chapter 5.1). 2) Recent mortality was highest in the historically least disturbed plots. The strongest relationships were observed for the 120-year period immediately preceding the inventory-based observation of current mortality at the stand level. The patterns varied among subregions. The stronger results on stand level than plot level, coupled with the absence of significant results for some subregions, indicate an important role of scale when assessing the relationship of past disturbances and current tree mortality. The results suggest not only intensity, but also variability through time, are parameters of disturbance regimes which shape current forests. The past disturbance period with the strongest relationship to mortality rate in the target decade is shifting (Chapter 5.2). 3) The estimated overall annual mortality rate in mixed stands was well within the previously reported range of background mortality. Stand- and plot-level mortality rates varied, although, not as much as has been reported from other forest types in the Carpathians. Most recently dead trees were found in the smallest size classes, pronouncing the effects of competition. However, individual mortality agents have different distributions across the size classes. Climatic extremes as a major mortality agent were reported for only a limited number of trees, despite the fact that most study stands are experiencing drought conditions. Competition and abiotically caused damage are prevalent mortality agents overall, however, insect infestation is the most frequent mortality agent in

Abstract:

Norway spruce. The significant differences in mortality rates among regions and species and species-specific distributions of mortality agents emphasize the importance of structural and compositional heterogeneity of forest stands (Chapter 5.3). 4) We found that the basal area of Norway spruce forests varies on multiple spatial scales. Climatic drivers are most important at coarse scales while natural disturbances represent the primary driver of forest heterogeneity at fine scales. Finer-scale variability is driven by local disturbances (mostly bark beetle and windstorms) and the legacies of disturbances that occurred more than a century ago. Climate change can increase the basal area of northern sites, however, a potential increase in disturbance frequency and severity can disrupt environmental responses (Chapter 5.4). Additionally, based on our results and their context within published literature, the thesis provides general recommendations and guidelines for forest management and conservation.

Keywords

Disturbance legacies, Forest dynamics, Forest structure, Annual mortality rate, Mortality gents, Natural disturbances, Primary forest

Language used: English

Date on which the thesis was submitted / produced: 2. 1. 2022

Thesis defence

• Supervisor: prof. Ing. Miroslav Svoboda, Ph.D.

Czech University of Life Sciences Prague

Faculty of Forestry and Wood Sciences

Doctoral programme / field:
Forestry Engineering / Silviculture