As a consequence of ongoing climate change, global forests have been facing repeated and prolonged droughts causing massive tree dieback (Hartmann et al. 2018, Schuldt et al. 2020). Under these circumstances, tree´s survival primarily depends on the extent to which tree function is impaired by drought (i.e. resistance, Lloret et al., 2011). To increase tree resistance and stand stability under these unfavorable conditions, mixtures of tree species have been promoted in Central Europe for many years (Pretzsch et al. 2010). However, in mature forest stands, the effect of long-term drought and species mixture are still poorly understood.

In Central Europe, Norway spruce (Picea abies [L.] KARST.) and European beech (Fagus sylvatica L.) are dominant tree species, accounting for 30% of the forest areas (Pretzsch et al. 2014). To elucidate the drought responses of these two tree species to a long-term and repeated drought in monoculture and mixed plantings, the “Kranzberg Roof Project” (KROOF, was initiated in 2013 and received funding by DFG as well as by the Bavarian State Ministry for Nutrition, Agriculture and Forestry and to the Bavarian State Ministry for Environment and Consumer Protection.

The phase I of this project has been focusing on the effect of a long-term drought and species mixture on mature beech and spruce trees. The experimental setup is comprised of 12 plots that contain clusters of 3–7 beech (c. 90 years old) and spruce (c. 70 years old) trees at the opposing sides of the plot (Figure 1a, for details see Grams et al. 2021). Six plots were equipped with roofs and assigned to throughfall exclusion plots (TE, Figure 1b). The other six plots were exposed to natural precipitation events as control plots (CO). The roofs were closed during the entire growing season (from April to November) for five years (2014-2018) and excluded ~70% of annual precipitation. The canopy crane located next to the plots enabled the measurements in sun-exposed crowns, such as leaf water potential and photosynthesis.

Figure 1: Map of Kranzberg (top, a), view of the roofs from the crane (bottom left, b) and overview of installed structures per plot (bottom right, c) (Taken from Grams et al. 2021).

Both tree species showed significant drought effects in the first drought years: e.g. reduced leaf/twig (Tomasella et al. 2018), stem (Pretzsch et al. 2020), root growth (Nickel et al. 2018, Zwetsloot & Bauerle, 2021), and C storage pools (Hesse et al. 2021). However, both tree species seem to have acclimated to the long water-limiting conditions: i.e. leaf-level and crown-level CO2 assimilation and water consumptions increased somewhat in the last drought years. Our current work is focusing on the mechanism behind this observation and their acclimation strategies.

KROOF Phase II (Recovery)

Since frequency of drought events is predicted to increase in the future (IPCC 2014), recovery is another important aspect of tree survival, which has attracted less attention compared to drought effect per se (Ruehr et al., 2019).

To elucidate if both species can recover their impaired function to the control level after a long-term drought, the KROOF project was shifted to the phase II with a watering experiment in the sixth drought year.

In early summer 2019, drought recovery was initiated through controlled watering of individual drought plots (for details see Grams et al. 2021). To accomplish simultaneous watering of an entire plot, a watering system composed of soaker hoses (CS Perlschlauch Premium, CS Bewässerungssysteme, Reichelsheim, Germany) and garden hoses was designed (Figure 2). All TE plots were watered with c. 90 mm over 36 h and the soil water content increased to the level of the CO plots within one week.

Figure 2: Soaker hose distribution, on one plot (left side up), schematic (right side), and detailed view of a dripping soaker hose (left side bottom) (taken from Grams et al. 2021).

This controlled watering system allowed detailed assessments of recovery processes. We have been mainly focusing on the following two aspects.

1.    Focus water balance
a. More anisohydric beech recovers faster after drought than more isohydric spruce
b. Mixed stands recover faster than monocultures
Measurement parameters:
Soil water content, Leaf water potential, Leaf osmotic potential, Leaf stomatal conductance, Sap flow

2.    Focus carbon balance: 13C labeling experiment for carbon transport and allocation of mature spruce trees after drought release

Aside from water, carbon (C) supply to each tree organ is essential for tree survival. After drought release, belowground C sinks typically increase their C demand to restore the drought-impaired root system. The recovery of water uptake and other tree function can be expected only if the increased C demand after drought release can be met by available C. For mature trees, however, this is still poorly understood especially after a long-term drought.

I.    Drought-reduced C transport speed recovers rapidly after drought release
II.    Newly assimilated C is preferentially allocated to belowground C sinks after drought release
Experimental design:
In parallel with the watering, we conducted a 13C labeling experiment on four CO and three TE spruce trees on neighboring plots (Figure 3a, for details see Hikino et al. accepted). The whole crowns of the spruce trees, i.e. four CO and three TE trees, were fumigated with 13C-depleted tank CO2 (δ13C of -44.3 ± 0.2‰) using micro-perforated PVC tubes hanging vertically from a carrier structure (Figure 3b). For the Hypothesis I (C transport), arrival time of the 13C-tracer was determined in stem and soil CO2 efflux, and tips of living fine roots to calculate: 1) Aboveground carbon transport rates from crown to trunk base (CTRabove in m h-1), 2) belowground carbon transport rates from trunk base to soil CO2 efflux (CTRbelow in m h-1), and 3) the incorporation time of 13C label in fine root tips (Figure 4). For Hypothesis II, we investigated whole-tree C demand and allocation of newly assimilated C. Here, the fate of 13C-tracer was tracked in various above- and belowground C sinks (Figure 5).

Figure 3: (a) Overview of the two 13C-labeled plots (CO = control, TE = throughfall exclusion), giving positions of trees (red triangles = labeled spruce trees, green open circles = beech), sampling positions of canopy air (blue circles), stem CO2 efflux (x), and soil CO2 efflux (yellow circles). (b) Picture of the structure for the 13C labeling with PVC tubes hanging vertically through the spruce crowns. (taken from Hikino et al. accepted)
Figure 4: Overview of the carbon transport paths assessed in this study. (1) Aboveground carbon transport rates (CTRabove, in m h-1) from crown to trunk base (assessed as stem CO2 efflux), (2) Belowground carbon transport rates (CTRbelow, in m h-1) from trunk base to soil CO2 efflux, and (3) Incorporation time (in h) of current photoassimilates from trunk base to fine root tips. (taken from Hikino et al. accepted)
Figure 5: Overview of the C sinks assessed in this study to track the fate of newly assimilated C. (taken from Hikino & Danzberger et al. in prep.)

Measurement parameters:
Leaf CO2 assimilation rates, Whole-tree C demand and allocation of newly assimilated C in CO2 efflux and growth of branches, stems, coarse and fine roots, ectomycorrhizae, root exudates, and soil

Grams, T. E. E., Hesse, B. D., Gebhardt, T., Weikl, F., Rötzer, T., Kovacs, B., Hikino, K., Hafner, B. D., Brunn, M., Bauerle, T. L., Häberle, K.‑H., Pretzsch, H., & Pritsch, K. (2021). The Kroof experiment: realization and efficacy of a recurrent drought experiment plus recovery in a beech/spruce forest. Ecosphere, 12(3).

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Hesse, B. D., Hartmann, H., Rötzer, T., Landhäusser, S. M., Goisser, M., Weikl, F., Pritsch, K., & Grams, T. E. (2021). Mature beech and spruce trees under drought – Higher C investment in reproduction at the expense of whole-tree NSC stores. Environmental and Experimental Botany, 191, 104615.

Hikino, K., Danzberger, J., Riedel, V. P., Rehschuh, R., Ruehr, N. K., Hesse, B. D., Lehmann, M. M., Buegger, F., Weikl, F., Pritsch, K., & Grams, T. E. E. (accepted). High resilience of carbon transport in long-term drought stressed mature Norway spruce trees within two weeks after drought release.
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