What is the extent of natural forests in Šumava? Is the death of spruces on a larger area natural? What happens next and what processes shape the new forest? We have heard many answers. Senior politicians, mayors, officials, foresters, scientists and NGO representatives were interviewed. So far, however, few have asked the forest itself and the individual trees. At the same time, the quality management of the Šumava National Park and thus of all mountain national parks in Central Europe should be based on the natural development processes of the forest.
Scientists from the Faculty of Forestry and Wood Sciences at the CULS therefore tried to interview the Šumava forests and use the annual rings to reconstruct the dynamics of the mountain forest.
In natural forests, trees end their lives in different ways: due to storms, fires, various types of insects or fungal pathogens. Events in which more or less trees die in a forest are called disturbance. Disturbances are the main driving force of forest dynamics and determine what the forest will look like, such as how big and how old the trees will grow in it, what tree species will occur or to what extent the growth will be structurally heterogeneous. Disruption on the one hand has a destructive effect, but on the other hand also constructive, as it creates space for the renewal of young trees and for the life of a large number of other organisms. Therefore, it is essential to understand what disturbances occur naturally in the forest.
Systematic forest management began relatively late in the central part of Šumava, in the second half of the 19th century. Since then, there have been forest management plans for a large part of the forest, which summarize the age and species composition of the forest. From these maps it can be deduced that part of the current stands were created before the systematic management began. Even so (probably partly under the influence of the work of the writer Karel Klostermann), the Šumava forests are often referred to in general as non-native planted spruce monocultures dating from the end of the 19th century. As a result, these forests are said to be unable to develop independently as native forests, as they have lost their natural resistance to disturbance and are rapidly decaying. To clarify these discrepancies, the researchers focused their study on the history of stands older than 150 years in the area of ??mountain spruces (> 1150 m above sea level), where natural vegetation is dominated by Norway spruce. They wanted to find out what disturbances had occurred in these forests in the past and whether the trees died naturally due to wind or bark beetle or were logged by humans. They wondered how typical it is that trees in mountain spruces die in large numbers and on large areas, or when and how often this happens. At the same time, they wanted to clarify whether trees are currently dying in greater or lesser numbers than in the past.
So how exactly was the death of trees centuries ago revealed, when no chronicler wrote it? The trees themselves, which have their past hidden in annual rings (annual increments), helped to uncover this mystery. The annual ring analysis was performed on 26 areas located throughout Šumava. All the studied stands disintegrated during Hurricane Kyrill in January 2007 or later. The death of mature trees has a clear effect in the forest, the surviving trees suddenly begin to grow more intensely as their competition has been eliminated, and small seedlings begin to grow rapidly from the beginning. Intense growth will, of course, be recorded in the width of their annual rings. The historical disturbances were therefore placed at a time when there were enough new trees growing intensively at the beginning of their lives or enough older trees, which suddenly and permanently increased their growth. The identified periods were very often related to archival materials on the occurrence of storms and gradations of the bark beetle, which were available from the Šumava region. For this reason, wind and bark beetle are considered to be the main factors behind the death of spruces in the past. In addition, wind and bark beetle disturbances often work together, and it is likely that after most storms, at least some of the trees were also attacked by the bark beetle.
"Our findings thus significantly change the view of the Šumava mountain spruces and refute the traditional belief in non-native planted spruce monocultures dating from the end of the 19th century and the significant influence of man who caused the forest's susceptibility to extent deaths. At least a third of the landscape at locations above 1150 m above sea level dates from the period before systematic forest management and has never been affected by extensive logging or tree planting, as these stands were formed naturally after storms and gradations of the bark beetle. It is therefore highly probable that their properties (resistance to disturbance, age structure, etc.) correspond to the original forests, i.e. spontaneously developing forests without direct human influence. The marginal influence of man in the form of grazing livestock, hunting, or logging individual trees or dead wood could not affect the natural ability of spruce stands to develop and regenerate spontaneously, "explains the first author of the publication Vojtěch Čada.
More details on the history of Šumava for the curious reader
Large disturbances, in which more than 50% of the trees died, were found in the history of all but one of the studied stands. The stands, which have now died due to storms and bark beetle, were formed after similar major disturbances 133–263 years ago. In natural mountain spruces, trees usually die in larger quantities, which differs from forests in lower elevations. On the contrary, they resemble forests that are exposed to frequent hurricanes, for example in the north-eastern USA. Evidence of the widespread decay of these natural forests in the past and their subsequent restoration to the form we knew from Šumava, perhaps can serve to dispel concerns about the further development of dead vegetation.
Quite often, minor disturbances also occurred, which caused the death of individual trees or smaller groups. In the dynamics of the forest, the rule is that with increasing force of disturbance, the interval between individual events growths. This rule also applies to Šumava spruces. While disturbances, in which at least 10% of the trees died in the area, occurred on average once every 41 years, disturbances in which at least 50% of the trees died in the area occurred on average once every 174 years. At the same time, one event exerted a different force on different surfaces. Such a wind event probably created several completely uprooted areas unevenly distributed throughout the Šumava and at the same time uprooted in individual areas, for example, only individual trees or smaller groups.
The character of the forest is therefore very variable over time. The number of dead trees has varied in different periods over the last 550 years (Fig. 1). Most trees died around 1820 (more than a third of the population) and later today. The current wave of death was preceded by about a century, when there was no major disruption. Therefore, these events have disappeared from people's memory and, like the floods before 1997, they no longer perceived them as part of their lives. Over the centuries, without much disturbance, most of the forest has aged and reached an age when it was prone to wind and bark beetle. Both factors affect mainly older trees, while young spruces are resistant. The current wave of dying was, therefore, due to the aforementioned age of the forest, slightly more extensive than what we have observed in the past. In addition, the current climatic conditions have also probably supported the decay of the forest, because, for example, higher temperatures increase the condition of the bark beetle.
Figure 1: Reconstruction of mountain spruce disturbance in Šumava according to annual rings. The height of the column roughly corresponds to the share of adult trees that died in our area in the given period. The red line shows the distribution of tree age, i.e. for each decade it is the share of newly grown trees (from the total number of trees). The largest wave of death and at the same time the largest growth of new trees occurred around 1820. (Axis x = decades, axis y = share)
On the ridges of the mountains and in their immediate vicinity, spruces died more often. In more protected positions on steeper slopes farther from the ridge, disturbances occurred less frequently, and the trees lived to an older age. However, the main factor that affects whether a tree will be uprooted by the wind or attacked by a bark beetle is the age of the tree. The stand itself therefore determines the most whether or not disturbance occurs and how many trees die. Thus, the dynamics of disturbance in the mountain forests of Central Europe is influenced retroactively by the stand structure and a possible increase in the frequency of storms or the activity of the bark beetle due to climate change will not lead to proportionally the same changes in the forest. A new vegetation is being created on the disturbed areas, which will be resistant to wind and bark beetle for several decades. We know from a historical analogy that a new mature stand will emerge within 30 years. If there is then an intense storm or gradation of the bark beetle, as was the case during the 18th and 19th centuries, only a small part of the trees will die. In order for a larger wave of death to occur again, the forest would have to develop for a longer period of time without much disturbance and age properly.
"Old spruce stands in higher altitudes are therefore very likely to be disturbed. Protected areas of mountain spruces must therefore be designed, with the proviso that trees may die in large areas. Protected areas should be as large and compact as possible, so that the potential for nature protection can be fully developed and at the same time the undesirable migration of the bark beetle into the surrounding commercial forests can be eliminated as effectively as possible, ”further recommends Vojtěch Čada.
"In addition, natural disturbances in the forest have a major impact on biodiversity, as they create specific habitats that exploit different species of organisms. For example, it has been found that far more species of insects, spiders, lichens, mosses and other groups live in dead stands than in dense living stands that resemble commercial forests. Our results, which show a very frequent occurrence of disturbances, confirm that disturbances have a dominant effect on the structure and biodiversity of the forest. Hurricanes and gradations of the bark beetle were so frequent that they did not allow most spruces to reach absolute age, which exceeds 500 years. Therefore, the trees did not die here with age, and the "primeval" structure was not created by succession, but by variously strong disturbances. This allows for the existence of many species of organisms that are absent in ordinary commercial forests,” the first author summarizes other aspects of natural forest development.
Let us therefore believe that the testimony of the forest will be taken into account in the human approach to nature and landscape protection, and we realize that the primary purpose of national parks is to protect unique nature and that dead trees belong to the forest as well as living trees. Experience from the non-intervention areas of the Šumava National Park and the neighbouring Bavarian Forest National Park shows that, despite initial concerns, nature surprises us with its strength and ingenuity and convinces us that it is working at its own pace.
Čada, V., Morrissey, R. C., Michalová, Z., Bače, R., Janda, P., and Svoboda, M. (2016). Frequent severe natural disturbances and non-equilibrium landscape dynamics shaped the mountain spruce forest in central Europe. For. Ecol. Manage. 363, 169–178.
Ing. Vojtěch Čada, PhD. (* 1985)
He graduated from the Faculty of the Environment of the Czech University of Life Sciences in Prague, majoring in Engineering Ecology. Since 2009 he has been working as a doctoral student at the Faculty of Forestry and Wood Sciences, CULS in Prague, Department of Forest Cultivation.
Prepared by: Jiří Lehejček, (Vojtěch Čada)