What are the risks to the silviculture of oak and walnut tree species during climate change?
Global climate change presents a significant challenge for the forestry sector. This includes uneven distribution of precipitation, rising temperatures, more frequent extreme weather events, and a longer growing season, all of which are altering the ecological conditions for tree growth. The most threatened regions in Europe are the Mediterranean and the continental temperate zone, including the Czech Republic.
Photo: Oak in the landscape
In the context of climate change, experts also discuss the movement of tree species’ ranges, both across the continent and within a specific region in the vertical direction from lowlands to middle and higher (mountainous) locations. Forests growing at altitudes of around 400 to 500 meters are considered the most endangered. In addition to changes in the non-living environment, climate change activates a number of living factors (diseases, pests) that can affect the successful growth of forests in the newly formed environment.
Scientists from the Mendel University in Brno conducted a study to show how climate change impacts the emergence and activity of plant pests. They specifically looked at how these pests can affect the cultivation of oak (Quercus spp.) and walnut (Juglans spp.) trees, which are commonly grown in Central Europe and the Czech Republic for economic purposes.
Twelve tree species were chosen for the research. Two of these species are native to North America, nine originate from Europe, and one is a hybrid of a North American and European species. Scientists examined current silviculture practices for these species and suggested how they could be silvicultured in the face of climate change.
Photo: Young oak forest
Scientists have compiled a list of the 19 most significant disease-causing pests that could potentially greatly impact the successful economic use of certain tree species in the future. The list includes the known host range for each pathogen.
The scientists published their findings in the article titled “Fytosanitární rizika pěstování vybraných dřevin rodu Quercus a Juglans v době klimatické změny“ (Phytosanitary risks of the silviculture of oaks and wallnuts in the period of climatic change: review). The paper was published in the journal Reports of Forest Research (Zprávy lesnického výzkumu) 2/2024 and was created as part of the NAZV project number QK22020045 „Potenciál geograficky nepůvodních druhů dřevin v lesním hospodářství ČR“ (The potential of non-native tree species for forest management in the Czech Republic).
Both oaks and walnuts are primarily planted for their highly valuable timber. The main types of oak planted are the summer oak and the winter oak, which are suitable for specific habitats. In the Czech Republic forests, the North American black walnut is the only type of walnut tree used, especially in floodplain habitats.
The occurrence of insect pests is on the rise in oak forests in southern Moravia. For example, there has been an increase in the overbreeding and denudation of the gypsy moth and browntail moth in 2021 and 2022, as well as a chronic problem with cockchafers in Bzenec region. With ongoing climate change, we can anticipate more such events in the future.
Photo: Mature oak forest
There is a serious risk of significant pests and diseases affecting northern red oak being introduced into Europe. This could lead to widespread dieback of red oak stands in the US. In several western European countries, red oak is seen as an invasive species. As a result, deliberate planting of red oak should be significantly limited in the Czech Republic.
The silviculture of native oak species in Europe, such as pedunculate and sessile oak, is becoming riskier as their representation in forest stands increases due to the death of other species. Serious pests are already destroying oak forests in Great Britain.
The expected vertical shift of vegetation zones, or the silviculture of pedunculate oak and especially sessile oak in central locations, will require changes in silviculture practices.
According to the precautionary principle and the predicted extreme climate events (such as late frosts and heavy snow), it is important to focus on creating mixed stands and transitioning to support the stability and vitality of individual trees.
Photo: Walnut Tree ‘Broadview’ (Juglans regia) in the landscape
Because oak tree species require a lot of light, it’s challenging to silviculture forests where oak is the dominant species using traditional management methods that maintain a wide range of tree ages. However, we can use the concept of mosaic forests to address this challenge. When establishing oak stands in intermediate positions, we recommend using looser spacing, such as rows, individual trees, or groups, and possibly more intensive tending methods.
The Turkey oak (Quercus cerris) and Hungarian oak (Quercus frainetto) are considered promising tree species for the most drought-prone areas of the Czech Republic. Due to the limited number of potentially suitable tree species for these areas, it may be necessary to incorporate shrubs or non-native tree species (such as Turkish hazel, sweet chestnut, etc.) more extensively to create mixed stands.
Consider creating mixed stands and low to medium forest forms in habitats that are exposed to drought. This allows for the utilization of less common oak species such as pubescent oak, white oak, Oak of Virgil, and Dalechamp’s Oak. Another option is transitioning to cultivating loose stands and solitary trees, which could lead to forest management being replaced by agroforestry management.
Walnuts have a wide range of applications in agroforestry systems, a common practice in other countries due to the use of both their wood and fruits. With climate change advancing, there may be increased use of the Walnut Tree ‘Broadview’ and its hybrid with the black walnut, which can to some extent replace the black walnut. This is important because the black walnut appears to be at high risk from a phytosanitary point of view.
Photo: Group of Walnut Tree ‘Broadview’s
Based on the current research and the environmental needs of walnut trees, it is recommended to cultivate stands of these trees in mixtures with other species such as hornbeam, linden, or field maple, as well as shrubs and pioneer tree species. It is also advisable to provide intensive care for these stands due to the potential risks involved.
In conclusion, the scientists emphasized:
In terms of pathogen occurrence, cultivating black walnut and red oak can be considered risky. The Walnut Tree ‘Broadview’, along with the above-mentioned hybrid, seems to be a suitable replacement for the black walnut in certain types of habitats. Similarly, the traditional pedunculate oak tree can also be cultivated in floodplain habitats.
The optimal target state should consist of individually mixed stands with a relatively small number of essential trees. This state can be achieved by establishing mixed stands and providing intensive care.
Scientists suggest planting more trees like the Turkey oak (Quercus cerris) and Hungarian oak (Quercus frainetto), as well as other oak species that can tolerate drought. This is especially important in areas currently experiencing or expected to experience severe drought in the future. In these areas, it may be necessary to transition to low or medium-density forests, and in extreme cases, consider planting individual trees or switching from traditional forestry to agroforestry.
Paper Fytosanitární rizika pěstování vybraných dřevin rodu Quercus a Juglans v době klimatické změny (Phytosanitary risks of the silviculture of oaks and wallnuts in the period of climatic change: review) can be downloaded here.
Authors: Antonín Martiník, Jiří Rozsypálek, Petr Martínek, Anna Mária Mitrová, Roman Longauer; Mendelova univerzita v Brně (Mendel University in Brno), e-mail: martinik@mendelu.cz
Prepared according the original by Jan Řezáč, VÚLHM, v. v. i., e-mail: rezac@vulhm.cz