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Drought stress experiments

Overview

Forest ecosystems foster the bulk of terrestrial biodiversity and are a major driving force of the cycling of life-supporting elements (e.g., carbon, nitrogen). However increasingly frequent occurrences of increasingly intense heat and drought events have led to widespread forest mortality around the globe. Anticipated future increases of drought events may further threaten the survival of forests and hence jeopardize the sustained flow of the life-supporting services they provide. Surprisingly, there are large gaps in the fundamental understanding of how drought kills trees. The group of Carbon Allocation has carried out a series of drought stress experiments explicitly designed to investigate mechanisms of drought-induced tree mortality.

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Experimental design

Field experiment

Nursery trees (~ 1.8 m tall from root collar) in pots (~ 25 L volume, vermiculate-sand mixture) were placed under a roof covered with translucent acrylic sheeting (light transmittance factor ~0.90, see Figure 1). Continuous TDR reflectometer readings were compared on an hourly basis to treatment-specific soil water thresholds and an automatic watering system provided threshold-depended volumes of water individually to sample trees. We applied three treatments: (i) control - soil water potential maintained in the range of the field capacity, (ii) intermediate treatment - soil water potential will vary between drought and field capacity during several cycles and (iii) severe treatment - no irrigation at all.

Figure 1 The ‘rain exclusion’ roof of the field experiment for intercepting precipitation (left). An example of a study tree (Austrian pine) showing soil and stem chamber, dendrometer and the dripping hose for automatic watering (middle). The sapflow sensors, covered with aluminium-plated cardboard to prevent irradiative heating, are only partially visible on the back side of the stem. Whole-tree chambers (right) provided estimates of tree-level net primary production.

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Greenhouse experiment

Groups of four small trees (~ 0.8 m tall from root collar) in pots (~ 2 L volume, vermiculate-sand mixture) were placed in glass chambers in three replicates per treatment (water & CO2, water & low CO2, drought & CO2, drought & low CO2) with a total of 12 chambers (Fig. 2, left panel). The pots and up to ~ 2 cm of the lower stems were covered by a separate chamber (see Fig. 2, right panel). Chambers were continuously flushed with an air stream of either ambient or CO2-depleted air.

Figure 2 Experimental design of the greenhouse experiment with three replicates of four treatments each giving a total of 12 chambers (left panel). Individual chambers consisted of a group of four trees each (right panel). Green tubes indicate air intake whereas red tubes are the air exhaust. The right panel indicates also the installation of the several sensors (soil water potential, air & soil temperature, PPFD, and dendrometer).

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Growth chamber experiment

Young Thuja occidentalis L. trees (~ 0.8 m tall from root collar) were planted in 2 L pots in a vermiculate-sand mixture and grown in growth chambers at three different temperature levels ( 15, 25, 35oC, Fig. 3). The progressive drought treatment started from field capacity and until trees died (in the 30oC treatment). Trees were subjected to a 12h/12h dark/light (PAR: 390±10 μmol m-2 s-1) cycle.

Fig. 3 An brief overview of the experimental setups. We used a whole-tree chamber design and trees were separated into above- and belowground compartments. Carbon and water fluxes from each compartment were measured with Li-Cor 6262, and air samples were also taken for δ13C analysis.

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