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«Root Growth, Soil Nutrient Availability and Water Flow in Model Ecosystems of Young Oaks Subjected to Air-warming and Drought A dissertation ...»

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DISS. ETH No. 20577

Root Growth, Soil Nutrient Availability and Water Flow in Model

Ecosystems of Young Oaks Subjected to Air-warming and


A dissertation submitted to


for the degree of


presented by


dipl. Umwelt-Natw. ETH

born 2nd Februar 1980 citizen of Eschenbach/SG and Uznach/SG accepted on the recommendation of Prof. Dr. Rainer Schulin, ETH Zürich Dr. Madeleine Günthardt-Goerg, Eidg. Forschungsanstalt WSL Prof. Dr. Rainer Matyssek, TU München Zürich, 2012 Table of contents Table of contents



1. Introduction

1.1. Background

1.2. Objectives

2. Water regime and growth of young oak stands subjected to air-warming and drought on two different forest soils in a model ecosystem experiment


2.1. Introduction

2.2. Material and methods

2.2.1. Study site and experimental design

2.2.2. Measurements of soil water, leaf water potential and temperature regime........... 17  2.2.3. Biomass measurements

2.2.4. Statistical analysis

2.3. Results

2.3.1. Air and soil temperature

2.3.2. Evapotranspiration

2.3.3. Soil water regime and leaf water potential

2.3.4. Growth parameters

2.3.5. Water use efficiency

2.4. Discussion and conclusions


3. Root growth of different oak provenances in two soils under drought stress and air-warming conditions


3.1. Introduction

3.2. Material and methods

3.2.1. Study site & experimental design

3.2.2. Biomass sampling

ITable of contents

3.2.3. Statistical analysis

3.3. Results

3.3.1. Total and fine root mass

3.3.2. Effect of drought and soil properties on vertical root mass distribution............... 44 

3.4. Discussion

3.5. Conclusions


4. Root and shoot growth patterns of 1st-year Quercus robur seedlings as affected by drought conditions


4.1. Introduction

4.2. Material and methods

4.2.1. Experimental setup

4.2.2. Statistical analysis

4.3. Results

4.4. Discussion and conclusions


5. Nitrogen dynamics in oak model ecosystems subjected to air warming and drought on two different soils


5.1. Introduction

5.2. Material and methods

5.2.1. Study site and experimental design

5.2.2. 15N labelling and measurement of 15N, NH4+ and NO3- in soil, biomass and discharge water

5.2.3. Soil respiration

5.2.4. Statistical analysis

5.3. Results

5.3.1. Root and shoot biomass

5.3.2. Soil respiration

5.3.3. Nitrogen in soil and microbial biomass

5.3.4. Nitrogen losses with discharge

5.3.5. Total N concentration in fine roots, stem + twigs and leaves

5.3.6. 15N tracer fractions in drainage water, soil, and tree biomass

5.3.7. 15N recovery

5.4. Discussion

5.4.1. N status of the experimental trees

5.4.2. Air-warming effects


Table of contents

5.4.3. Drought effects

5.4.4. Nitrate vs. ammonium uptake

5.5. Conclusion


6. Synthesis

6.1. General conclusions

6.1.1. Drought effects

6.1.2. Air-warming effects

6.1.3. Interaction of soil types with climate treatments

6.2. Which oak species/provenance is the future forest tree?

6.3. Outlook



Other manuscripts of the “Querco” project


Curriculum vitae

–  –  –

IV Summary Global climate change is expected to increase annual temperatures and decrease summer precipitation in Central Europe. Being tolerant to heat and drought, oaks are considered promising trees to replace beech and spruce on forest sites that will become too warm and dry for these species. Oak provenances from dry sites are expected to be especially drought tolerant. However, little is known on how the oak species growing in Switzerland (Quercus robur, Q. petraea and Q. pubescens) will respond to the changing climate and how these responses depend on soil properties. Apart from direct drought and heat stress effects on biochemical and physiological processes, trees like other plants will also have to cope with indirect effects such as changes in soil nutrient availability. For example, nitrogen (N) availability depends on soil microbial processes, which are sensitive to soil temperature and moisture. We expected warming to increase and drought to decrease soil N availability and thus N supply to the trees.

In a 3-year factorial model ecosystem experiment, we investigated the responses of young mixed oak stands with Quercus robur, Q. petraea and Q. pubescens, growing on either acidic or calcareous soil, to air-warming (+1 to 2 °C) and drought treatments (-43% to -60% precipitation), corresponding to the IPCC climate change Scenario A2. The air-warming and drought treatments were applied separately as well as in combination on both soils.

The mild air-warming treatment had no effect on soil water availability, evapotranspiration or stand biomass, neither above nor below ground. Water limitation, which was severe during drought periods as shown in low leaf water potentials, significantly reduced the stand foliage, stem + twigs and root biomass. However, no tree died during the

–  –  –

experimental period. The drought treatment led to higher root:shoot mass ratios and only little reduction in fine root biomass compared to total root mass. Allocating relatively more biomass into structures for water and nutrient uptake than for above ground assimilation can be seen as a response to balance impeded soil water extraction with reduced transpirational water loss. Moreover, evapotranspiration increased within a few days after re-watering to predrought levels, indicating a fast recovery potential of the oaks photosynthetic activity.

Decreased evapotranspiration from the drought-exposed stands led to significantly higher air and soil temperatures, which were attributed to reduced transpirational cooling. As a major finding of our results, we point out that additional air warming did not enhance the drought effects on evapotranspiration and biomass, although more negative leaf water potentials were observed.

In an additional neutron radiography experiment, it has been shown for the first time that root-shoot growth patterns of oak seedlings are not altered by either water availability or soil properties. Independent from soil type and drought treatment, peaks and dips of shoot and root growth rates occurred at the same time. We conclude therefore that during the first year after germination, environmental conditions were overruled by endogenous growth patterns.

Moreover, it has been shown for the first time that neutron radiography is a well-suited, nondestructive method to track root growth patterns of young oak trees.

Stands not subjected to the drought treatment produced significantly less biomass on the calcareous than on the acidic soil. This was probably due to phosphor (P) or manganese (Mn) limitation. In the acidic soil, root mass was maximal in the nutrient-rich topsoil, whereas in the calcareous soil maximum root mass occurred between 0.25 and 0.5 m depth. There was no difference in biomass and water regime between the two soils under drought conditions. As another highlight of our results, we therefore emphasise that nutrient availability was governed by water availability under these conditions.

–  –  –

While total amounts, vertical distribution and availability of soil N showed substantial differences between the two soils, neither warming nor drought had a significant effect on N availability in either of them, in contrast to our expectations. Air-warming also had no effect on N accumulation in the biomass of the trees, whereas the drought treatment increased or tended to increase N accumulation, indicating that growth was not limited by N. 15N-labelling revealed that this increase was associated with a higher fraction of tracer-N accumulation deriving from 15N applied in the form of nitrate to the soil surface, while the fraction of 15N accumulated in the plants decreased when it had been applied as ammonium. As a further highlight of our study, we therefore conclude that climate change is not expected to have major effects on the N dynamics of young oak stands in central Europe.

The provenances of Quercus robur showed more vigorous growth in all treatments than those of the other two species, irrespective of soil type, air-warming and drought treatments.

However, drought-tolerance in relation to root:shoot mass ratio, leaf stress injuries, photosynthesis and wood anatomy was higher in Q. petraea and Q. pubescens than in Q. robur. Therefore, Q. robur appeared to be the most promising species when drought periods are only short, whereas long and severe drought periods would favour Q. petraea and Q. pubescens. Within each species, provenances originating from drier sites produced more root mass than those from more humid sites. However, given that the provenances from drier sites reacted more sensitive to water limiting conditions than those from more humid sites, it remains questionable whether these provenances are better suited for a future climate. Also other aspects such as frost tolerance and wood quality must be considered in selecting provenances for future forestry. In any case, we demonstrated for the first time under controlled experimental conditions that independent from soil properties young saplings of all three central European oak species can successfully cope with severe drought periods and

–  –  –

increased air temperatures, and thus are suited from this point of view for future forestry in central Europe under conditions of climate change.

VIII Zusammenfassung Als Folge des globalen Klimawandels wird erwartet, dass bis Ende dieses Jahrhunderts in Mitteleuropa die Sommer wärmer und trockener sein werden als heute. Die in Mitteleuropa vorherrschenden Stiel-, Trauben-, und Flaumeichen (Quercus robur, Q. petraea und Q. pubescens) sind als trocken- und hitzetolerante Bäume bekannt. Damit dürften sie in Zukunft die heute vorherrschenden Buchen und Fichten in warmen und trockenen Tieflagen ersetzen. Eichenprovenienzen aus niederschlagsarmen Regionen dürften dabei besonders erfolgreich sein. Die Effekte von höherer Lufttemperatur und Trockenheit auf das Eichenwachstum sind bereits gut erforscht. Allerdings haben bisherige Studien diese Effekte getrennt voneinander untersucht, weshalb nur wenig über die Auswirkungen von Interaktionen dieser zwei Faktoren untereinander und mit Bodeneigenschaften bekannt ist.

Neben den direkten Auswirkungen von Trockenheit und Hitze auf Stoffwechselprozesse der Eichen müssen auch indirekte Effekte des Klimawandels auf Bäume berücksichtigt werden, zum Beispiel eine Veränderung der Stickstoffverfügbarkeit (N). Dabei wird erwartet, dass höhere Lufttemperaturen die Verfügbarkeit und damit das N-Angebot für Eichen erhöhen, während Trockenheit die Verfügbarkeit von N verringert.

–  –  –

Lufterwärmung (+1 bis 2 °C) und Trockenstress (-43% bis -60% Niederschlag) auf Eichenbestände mit jungen Stiel-, Trauben- und Flaumeichen auf einem sauren Boden und auf einem Kalkboden untersucht. Die Behandlungen Lufterwärmung und Trockenstress wurden entweder voneinander getrennt oder in Kombination angewendet und orientierten sich

–  –  –

dabei am IPCC Szenario A2, welches auf einem hohen Populationswachstum, einer langsamer Wirtschaftsentwicklung und einem geringen Technologiewandel basiert.

Die Lufterwärmung hatte keinen Effekt auf die Verfügbarkeit des Bodenwassers, die Evapotranspiration oder die Biomasse der Eichenbestände. Im Gegensatz dazu reduzierte

–  –  –

Trockenstressbehandlung erwies sich als ziemlich extrem, was gut sichtbar an den sehr tiefen Blattwasserpotentialen war. Trotz dieser starken Trockenstressbehandlung starb während des ganzen Experiments keine einzige Eiche. Darüber hinaus stieg die Evapotranspiration innerhalb weniger Tage nach Beendigung der Trockenperioden auf das Niveau der Kontrolle an, was auf ein hohes Regenerationspotential der Eichen hindeutet. Unter Trockenstress entwickelten die Eichen ein höheres Wurzel:Spross Verhältnis. Ausserdem waren die Effekte der Wasserlimitation auf die Feinwurzelbiomasse geringer als auf die Gesamtwurzelbiomasse.

Diese Anpassungen ermöglichten es den Eichen auch unter Trockenheit effizient Wasser und Nährstoffe aufzunehmen und gleichzeitig Wasserverluste durch Transpiration zu reduzieren.

Die Evapotranspiration von trockengestressten Eichenbeständen war zeitweise sehr gering, was durch eine geringe Transpirationskühlung zu höheren Luft- und Bodentemperaturen führte. Als wichtige Erkenntnis dieser Studie möchten wir hervorheben, dass die zusätzliche Lufterwärmung in der kombinierten Behandlung jedoch zu keiner Verstärkung der Trockenstressbehandlung auf die Evapotranspiration oder Biomasse der Eichen führte obwohl tiefere Blattwasserpotentiale gemessen wurden.

In einem zusätzlichen Neutronenradiographieexperiment wurde zum ersten Mal gezeigt, dass periodisches Wurzel- und Sprosswachstum von Eichenkeimlingen weder durch Wasserverfügbarkeit noch durch Bodeneigenschaften verändert wird. Unabhängig von Trockenstressbehandlung und Bodentyp traten die höchsten wie auch die tiefsten Wachstumsraten von Spross und Wurzeln immer zur gleichen Zeit auf. Wir folgern daher,

–  –  –

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