Urban forests are particularly affected by the consequences of global warming. Researchers and students at the university are investigating how such green spaces can be made more resilient to heat and drought using the example of Eversten Holz in the "Klimaoasen Oldenburg" project.
The "horse trough" still presents a sad picture in December. Several shallow puddles are all that remain of the historic body of water on the south-eastern edge of Eversten Holz. The numerous ditches that run through the city forest also carry no water at the beginning of winter.
"Climate change is clearly noticeable in Eversten Wood," says Prof Dr Dirk Albach from the Institute of Biology and Environmental Sciences (IBU). In addition to drought and higher temperatures, groundwater lowering and storm damage are also affecting the plants, reports the botanist. The 22-hectare woodland, which is just one kilometre from Oldenburg's city centre, is typical of many urban green spaces: these areas often suffer particularly badly from the consequences of global warming - partly because cities are generally hotter than the surrounding countryside, but also because they are more affected by traffic, air pollution or construction work than more rural forests.
Albach is leading a project at the university that is taking a closer look at the environmental changes in Eversten Wood. The researchers want to draw conclusions from their observations as to how the forest can adapt to the new conditions. The research is part of the City of Oldenburg's "Klimaoasen Oldenburg" project, which is led by the State Museum "Natur und Mensch" and funded by the Federal Ministry of Housing, Urban Development and Building as part of the "Adaptation of urban spaces to climate change" funding line. The aim of the project, which was launched at the beginning of 2022, is to make Eversten Holz and Oldenburg Castle Gardens fit for climate change. A large part of the funding totalling five million euros is available for the concrete redesign of the two green spaces and dialogue with the public.
Students analyse soil, water, flora and fauna
During the three-year project, the university will be responsible for the accompanying research, which will only take place in Eversten Holz. The results of the various studies will be presented at a scientific conference at the end of the project. They will also be incorporated into a travelling exhibition that is being developed by the State Museum. Dirk Albach is pleased that students in the project can carry out biodiversity research on site that is of practical use. The first step was a comprehensive milieu study carried out by students on the Environmental Sciences degree programme last summer semester. In groups of four and five, the participants focussed on the topics of soil, water, fauna, flora and use of Eversten Holz. They mapped soil types, calculated the amount of carbon stored and determined the groundwater level. They recorded the number of plant species, counted breeding birds and analysed how the population uses the forest.
"The environmental study forms the basis for a better understanding of the Eversten Holz ecosystem and its development," explains Albach. One important result: the woodland is a hotspot of biodiversity in the urban area. The juxtaposition of different types of biotope such as lawns, shady woodland and ponds means that strictly protected species such as the alpine newt and the broad-winged bat feel at home here. The horse trough, although not in a good ecological condition due to pollution and low water levels, nevertheless proved to be a particularly valuable biotope: specialised ground beetles and amphibians live on the banks and in the reeds, bats hunt over the open water surface and water birds breed in the reeds.
Changes can also be identified - thanks to an environmental study carried out by another group of students back in 2010. The forest now has more open areas, probably because some trees have fallen victim to drought and storms. As a result, a stronger herb layer has developed on the ground, which was previously absent in most areas. The students discovered more invasive species, including the particularly aggressive Japanese knotweed.
The groundwater level is crucial
The team also gave a forecast for the future: in order for native trees such as oaks and beeches to be able to survive hot summers in the future, the development of the groundwater level is particularly important. The city's project proposal envisages redesigning the ditch system. It currently drains the water from the formerly wet woodland, but in future the precipitation should remain in Eversten Holz as far as possible. In addition, the horse trough, which has silted up due to excessive nutrient inputs, is to be dredged.
This measure provides an opportunity for a further study by the IBU: using modern genetic methods, the community of unicellular organisms will be analysed before and after the desilting of the horse trough. Another project will analyse how the water level of the watercourse has developed over the past decades using photos taken by the public. A "forest research booklet" for schoolchildren has already been produced as part of a final thesis; in addition, two student assistants have recently started a tree register of all 6,000 trees in Eversten Holz.
"We can now find out why certain plants grow in one place and not in another, which species are under threat and how their growth is changing due to climate change," explains Albach. These findings are important for determining which species can be planted in the project to replace dying plants such as spruce or birch. Ultimately, other forests in the city will also benefit from this - as will the Botanical Garden, of which Albach is Director. "The 'Climate Oases' project is just the starting point," emphasises the researcher. "After that, we must continue to work continuously to ensure the long-term survival of these urban green spaces."
