Kroof - Kranzberg Forest Roof Project - Interactions between beech and spruce in relation to drought

Extreme drought and dwindling water resources pose new challenges not only to humans, but also to forests. As a result of ongoing climate change, forests worldwide are facing repeated and prolonged droughts leading to massive tree mortality (Hartmann et al. 2018, Schuldt et al. 2020). The current increasing frequency of extremely warm and dry weather episodes will be the norm in the future.
Under these circumstances, the survival of a tree depends primarily on the extent to which tree functions to drought i.e., its resistance, resilience, and recovery functions are compromised (Lloret et al., 2011). To increase tree resilience and stand stability under these adverse conditions, tree species mixes have been promoted in Central Europe for many years (Pretzsch et al. 2010). However, the effects of long-term drought, species-specific physiological differences, adaptation or recovery effects, and the role of species mixing in adult forest stands are still poorly understood.
In Central Europe, spruce (Picea abies [L.] KARST.) and copper beech (Fagus sylvatica L.) are the dominant economic tree species, accounting for 30% of forest areas (Pretzsch et al. 2014). To elucidate the response of these two tree species to long-term and repeated drought in monocultures and mixed plantations, the Kranzberg Forest Roof Project (Kroof) was initiated in 2013, funded by the German Research Foundation (DFG) as well as the Bavarian State Ministry of Food, Agriculture and Forestry and the Bavarian State Ministry of the Environment and Consumer Protection.

The Kroof project includes two experimental approaches:

the "Kroof-gradient", which is an experimental plot transect on Bavarian forest sites with different precipitation totals and

the "Kroof-experiment", which is an intensively researched forest site in the Kranzberg forest with experimentally induced drought.Kroof gradient: Using an experimental plot transect through Bavaria, the effects of drought stress on the annual growth of beech and spruce in different mixing ratios are investigated along an ecological gradient from wet to dry sites.

Kroof experiment: An approximately 0.5 hectare forest area stocked with adult beech and spruce in the Kranzberger Forest (south of Munich) was selected as the intensive research site for the canopy experiment on rain exclusion to produce a long-term and repeated drought (Kroof experiment - througfall exclusion).

Kranzberger Forst (longitude: 11° 39′ 42″ E, latitude: 48° 25′ 12″ N, elevation 490 m a.s.l.) is located in southern Germany, about 35 km northeast of Munich. The experimental area is about 0.5 ha and lies 490 m above sea level. The annual average precipitation is 750-800 mm per year, including 460-500 mm during the growing season (May-September). The annual average air temperature is 7.8 °C, with a seasonal average of 13.8 °C. At the site, pure and mixed stands of spruce and beech grow on Luvisol of loess over Tertiary sediments, which provide high nutrient and water supply (Göttlein et al. 2012; Pretzsch et al. 1998). From about 1 m soil depth, a relatively dense layer of sandy-silty loam almost completely prevents deeper root growth and thus water uptake (Häberle et al. 2012).
The forest test plot is part of the long-term Bavarian yield research network (Bavarian State Ministry of Food, Agriculture and Forestry), accordingly stand structure and yield data are recorded tree-individually and spatially explicit (since 1992 permanent observation plot). Since 1998, several large-scale projects have been carried out on the forest test site.
In addition, a crane (45 m high, crane jib 50 m) has been located on the experimental plot since 2001, which allows direct access to the tree crowns for sampling and physiological measurements. The Kranzberger Forst test site is integrated into a worldwide canopy crane research network (International Canopy Crane Network, ICCA, Basset & Wright, 2003).

Kroof 1



After a 5-year desiccation period from 2014 to 2018, the study plots were re-watered from mid-2019 to reveal possible recovery effects or irreversible damage in subsequent years 2019-2023 and to clarify their mechanisms (Kroof experiment - recovery).

Kroof 2



A next project phase is planned starting probably in 2024. In the Kroof 3 phase in the Kroof experiment in the Kranzberg Forest, the focus of interest is on the significance of long-term effects of previous desiccations, so-called "legacy effects" (Kroof experiment - legacy/extremes).

The natural drought in the summer of 2022 has already indicated first results in this regard. The slow regeneration of the foliage area of previously drought-stressed spruce (Kroof1: 2014-2018) was beneficial to the trees during the 2022 summer drought. The previously drought-stressed spruce trees still had reduced foliage area compared to the unstressed control trees after three years of regeneration. The resulting reduced water consumption led to significantly lower drought stress symptoms. Thus, interesting long-term effects in the aftermath of drought periods are already indicated here. Further detailed investigations, also on the so far strongly neglected soil microbiology, promise very interesting new insights into the drought resistance of beech, spruce and pine.

For this purpose, a new drought stress phase is planned with a complete desiccation of the already drought-stressed experimental plots as well as additional new drought stress plots.

Das Kranzberg Forest Roof Projekt (Kroof) -
Buche, Fichte und Kiefer am Rande der Existenz
(in German only)

Kroof research questions

  • How do trees, especially our commercial tree species beech and spruce, cope with prolonged drought events?
  • Which species-specific reaction patterns are emerging?
  • What is the influence of mixing and management form?
  • Which adaptation strategies to water-limiting conditions do the two tree species develop?
  • How do the two tree species respond to rewatering?
  • Are there differences in regeneration?
  • What is the role of species composition of ectomycorrhizae on root and tree growth?

Forest scientists and biologists from TUM and Helmholtz Zentrum München as well as other associated national and international research partners are jointly investigating these questions in the Kroof project.