KROOF 2 Experiment - Re-watering and 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 2 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 minimize differences between drought-stressed plots (TE) and control plots (CO), care was taken to bring the TE and CO plots to the same soil water content during rewatering. 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 1).

Controlled irrigation of drought-stressed (TE) plots was initiated in early summer 2019 (see Grams et al. 2021 for details). 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.

In the rewatering year and subsequent years 2019 -2023, research has been and will be conducted to determine if and to what extent both species, depending on the mix ratio, can restore their impaired function to control levels following an extended drought.

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

1.    Focus water balance
Hypothesis:
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
Background:
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.

Hypotheses:
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 2a, 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 2b). 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 3). 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 4).

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