Completed Master Theses
The path and residence time of precipitation in a temperate forest investigated by an ecosystem-scale deuterium labelling experiment
By Till Buser
Supervisors: Prof. Ansgar Kahmen, Lars Dietrich
Despite the importance of water for terrestrial ecosystems, the flow-path that precipitated water takes through the soil profile and into trees is not well understood. In this study, I conducted a large-scale stable isotope labelling experiment in a mature temperate forest stand to trace the flow of water of a simulated precipitation event through a forest ecosystem and to assess the water uptake depths of four common temperate tree species. I irrigated the forest with water that had a d2H value of 2000 per mil. After the tracer addition, I collected soil cores and xylem samples for the next two months and extracted the water from these samples for the analysis of their d2H values. I did this to test how fast the precipitated d2H enriched water infiltrated into different soil depts. I also used a linear interpolation model and a Bayesian mixing model to assess from which soil depths the four different tree species (Picea abies, Fagus sylvatica, Pinus sylvestris, Quercus petraea) took up the d2h enriched water. My data show that the d2H enriched water infiltrated the soil up to a depth of -0.5 m within one day after the application to the forest ground. The water infiltration and uptake dynamics shown in this study point out the importance of preferential flow for the infiltration of water into the soil and suggest a mixing of infiltrating water with tightly bound soil water pools. A clear attenuation of the labelling pulse intensity was observed with depth. This is because the removal of the label from the soil profile by evapotranspiration and because of the mixing of the label along the soil column with residual water. Both statistical models revealed consistent and distinct differences in soil water uptake depths among the investigated tree species with P. abies relying on water from upper soil layers, F. sylvatica and P. sylvestris on intermediate and Q. petraea on deep soil layers. The labelling pulse was detected in the xylem water shortly after the labelling event, suggesting water uptake of all investigated species from upper soil layers in differing proportions. In summary, my study delivered important new information on the soil water uptake depths of four common European temperate forest tree species and challenges popular existing soil infiltration concepts, along with other recent studies.
First experimental evidence for increased temperature inducing carbon starvation in tropical cloud forest orchids of the genus Masdevallia
By Tiffany Fioroni
Supervisor: Dr. Günter Hoch
Climatic change like the increasing of emission rates of green-house gases (e.g. CO2, CH4 andN2O) and the predicted increase by up to 4.8 °C of the global average surface temperatures until the end of this century, will impact the whole world, but particularly ecosystem that are closely linked to specific climatic conditions like tropical montane cloud forest (TMCF) characterized by a permanent presence of clouds and the absence of climatic extremes throughout the year. With climate change, the altitude at which clouds will form in tropical mountains will increase of several hundred meters. The large number of endemic epiphytes, that promote biodiversity, productivity and resilience of the tropical forest, are especially threatened by these global and local changes in TMCFs, because their life is restricted to tree canopies.
In a phytotron experiment, 10 epiphytic orchids species of the genus Masdevallia, native to South-American cloud forest and originate from three different elevation groups (high: more than 1500 ma.s.l, low: under 1000 m a.s.l and overall: species that occur at high and low elevation) were submitted to three different temperature treatments: 1) control (23 °C/ 17 °C, day/night) simulating natural conditions, 2) slow temperature treatment (+ 0.75 °C every 14 days) and 3) fast temperature treatment (+ 1.5 °C every 14 days) to assess the impact of a rise in temperature on the plant's performance and their net carbon budget. Leaf gas exchange was determined every 14 days with a LICOR-6400xt portable photosynthesis system, while carbon reserves were determined form leaf samples collected throughout the experiment by analyzing non-structural carbohydrates (NSC). The results showed that, with an increase in temperature, the photosynthesis declined, while the day and the night respiration increased in both slow treatment and fast treatment, meaning that no adaptation of photosynthesis and respiration was observed when the temperature increase was applied at a slower rate. At 32 °C, the values of photosynthesis approached zero and the NSC pools of the orchids was depleted due to a drastically decline of the starch concentration, resulting in the death of all the investigated individuals. Species belonging to the lowland elevation were more temperature resistant than species belonging to the overall and high elevation. We can therefore conclude, that the death of all Masdevallia orchids was triggered by carbon starvation, when the rise in temperature exceeded the threshold of 32 °C. The dark reaction of thephotosynthesis was the most affected chemical reaction and probably the thermolability of the enzyme RuBisCO activase played an important role in the death of the orchids, but further research is needed to confirm the biochemical process limiting the photosynthesis at elevated temperature.
The influence of flowering plants on the fecundity and survival of Cotesia rubecula and its host Pieris rapae
By Sabrina Leist
Supervisors: Prof. Dr. Ansgar Kahmen, Dr. Henryk Luka (FIBL), Shakira Fataar (FIBL)
Conservation biological control is a strategy that can be particularly effective in organic farming and is aimed at promoting natural enemies, for example by adding flowering plants to control pest species in agricultural systems. However,the selection of flowering plants for conservation biological control should becarried out carefully, as additional nectar sources might not only provide foodfor beneficial insects but promote the pest species as well, thus selectiveflowering plants are required. We conducted survival and fecundity experimentson the pest-antagonist complex Pieris rapae (pest butterfly) and Cotesia rubecula(beneficial wasp), to determine the selectiveness in promotion of the threeflowering plants Centaurea cyanus, Fagopyrum esculentum and Vicia sativa, whichare already used in a custom flowering strip mixture for cabbage cultivation.Additionally, we assessed if there are differences in promotion of the insectsbetween varieties of the same plant species, by comparing the wild and acultivated form of C. cyanus. Furthermore, we analysed the nectar compositionof the mentioned flowering plants, focusing on the three main sugars sucrose,fructose and glucose to provide further evidence to the selectiveness of theflowering plants. Survival of both insects was increased by three out of fourflowering plants (C. cyanus wildtype, its variety and F. esculentum). V. sativa didonly prolong the lifetime of the beneficial insect. However, it was offered withextrafloral nectar only, due to missing flowers. Therefore, any presumptions interms of selectivity of V. sativa should be done with care. Fecundity, on the otherhand, seems to be a function of the survival, as the daily realized fecundity wassimilar between the different flowering treatments of both insects. The floralnectar composition of F. esculentum was hexose-dominant, whereas theextrafloral nectar of V. sativa was sucrose-dominant. The nectar compositions ofboth cornflowers (C. cyanus wildtype and its variety) were surprisinglyinconsistent between the samples and, with one exception, making it impossibleto label the floral or extrafloral nectars as hexose or sucrose-dominant. Thesefindings underline the importance of the research in selective flowering plants tomaximise conservation biological control in agricultural systems.
Impact of tillage systems on the weed seedbank and weed dynamics in spelt.
Supervisors: Prof. Dr. Ansgar Kahmen, Dr. Paul Mäder (FIBL)
No-tillage (NT) and reduced tillage systems (RT) are well-known management tools for preventing soil erosion and conserving soil fertility. Introducing RT into organic farming is a challenging approach that is still in development. An important aspect is the increased weed infestation which is quite difficult to control when ploughing is omitted and herbicides are prohibited (Peigné, Ball, Roger-Estrade & David, 2007; Gruber & Claupein, 2009; Bio Suisse, 2017). The study was conducted in the long-term organic tillage trial in Frick, Switzerland (start autumn 2002, 47°30'N, 81°01'E, 350 m a.s.l), comparing reduced (RT) and conventional (CT) tillage system (Berner et al. 2008). The soil is a Vertic Cambisol (WRB classification). The soil cores were randomly sampled in 4 sub-plots out of the depths 0-7, 7-14 and 14-21 cm with with a 4-cm diameter soil auger in November 2015 after spelt was sown. The set-up of the weed seed trays was done according to the “seedling emergence method” described by Barberi and Lo Cascio (2001) during the period from December 2015 to August 2016 in a non-heated side opened greenhouse. Weeds were continuously determined, counted and removed. At five growth stages of spelt, a visual field assessment determined the weed density (seedlings/m²) and weed cover (%) linking the seedbank study with the actual weed infestation and crop performance in the vegetation period of spelt 2015/2016. The tillage showed a significant influence on weed abundance between the tillage systems in the seedbank study as well as in the visual field assessments. Weed density (seedlings/m²) in the seedbank was up to twice as high in RT as in the CT plots (RT: 39987 seedlings/m² vs. CT: 17 153 seedlings/m², p=0.029). The highest number of seedlings was found in the upper soil layer (0-7 cm) in RT and decreased significantly with increasing depth. In CT, the weeds were distributed more homogenously between the soil layers. In the field weed density (seedlings/m²), as well as the weed cover (%) were significantly higher in RT. The species richness and the Shannon Diversity Index were not significant for the tillage factors. For the spelt, we found significant differences in the spelt biomass (straw, spikes) between the tillage systems (RT: 10.5 t TS/ha-1, CT: 7.1 t TS/ha-1) but not for the yield (RT: 1.28 t TS/ha-1, CT: 1.04 t TS/ha-1). In summary, reduced tillage, although considered more sustainable for the soil in many studies, causes a significantly higher weed pressure with non-significant but absolute effects on yield.
Southern Anthracnose Resistance in Red Clover
By Milena Scandella
Supervisors: Prof. Dr. Ansgar Kahmen, Prod. Dr. em. Thomas Boller, Dr. Roland Kölliker (Agroscope)
Colletotrichum trifolii is an aggressive fungal pathogen mainly found in warm and humidenvironments and can infect Astaraceae, Cucurbitaceae, Malvaceae and Fabaceae. On foragelegumes, it causes southern anthracnose characterised by brown lesions, chlorosis and wiltingof the entire plant. Red clover (Trifolium pratense L.) is an important forage legume greatlyvalued worldwide for its high protein and nutrient content and in terms of symbiotic nitrogenfixation. As red clover is highly susceptible to southern anthracnose, valuable amounts offorage yield are lost every year. Resistance to the disease is, therefore, important to ensurepreservation of the easy to produce, high-quality fodder and cheap nitrogen supplier. At thefederal research station in Switzerland, red clover populations have been tested for resistanceby artificial spray inoculations with C. trifolii isolates. In this thesis, six SRAP primer combinationswere used to analyse molecular diversity among these isolates, which were thenapplied onto four red clover populations segregating for anthracnose resistance. The responseof plants arising from a cross with a heterozygous resistant and susceptible parental plant, waspheno- and genotyped to obtain information about the involved resistance genes. To distinguishfungal races and investigate gene-for-gene interactions in future studies, a nondestructivein vitro assay was developed. The outcome of a principal component analysis ofthe SRAP data revealed four distinguishable C. trifolii isolates. When only one of theseisolates was sprayed onto the plants, a 1:1 segregation in disease resistance was visible. Anadditional infection of the surviving plants with an isolate mixture, however, lead to 1:3, 3:1 and 1:1 segregations. It is therefore assumed that two or three resistance loci are involved inthe tested red clover plants that mediate resistance to different races of C. trifolii. In general,degree of resistance to southern anthracnose depends on both the red clover cultivars and theisolates of C. trifolii. Pathogenicity of the isolates could be studied with the in vitro assay butas immersion inoculation did not show the same outcome as spray inoculation, results shouldbe treated with caution. Evaluation of genotypic data from the resistant and susceptible plantswill hopefully facilitate the localisation of resistance genes for easier marker assisted breedingand the development of resistant red clover cultivars.
Soil fertility and its monitoring – comparing field and laboratory methods in two organic reduced tillage trials
By Kai Reinacher
Supervisors: Prof. Dr. Ansgar KahmenDr. Andreas Fliessbach (FiBL)
Anthropogenic land use practices have led to massive soil degradation and forced farmers to abandon huge areas of agricultural land over the years. This calls for new strategies of soil management and accurate monitoring of soil quality reducing costly laboratory experiments. Subplots of two Swiss field trials investigating the effects of reduced tillage (RT) under organic management were examined at two depths (0-10 cm and 10- 20 cm) for changes in soil quality by standard laboratory experiments and field tests being considered quick and cheap simultaneously. Tillage effects were studied with reference to the same soil layer and for matters of different soil stratification patterns with respect to tillage. Distinct tillage effects referring to soil structure and especially biological community were observed revealing a strong stratification of biological parameters near the surface under RT. In the 0-10 cm soil layer in Frick, microbial biomass and basal respiration were enhanced under RT compared to conventional tillage (CT) whilst the 10-20 cm layer was not different. In Aesch, compaction of the 10-20 cm layer was detected by penetration resistance and stratification of root growth. The investigated field tests gave quick results and the assessed parameters can be intuitively understood making them suitable for soil users. However, there is still potential for improvements. Although correlations between field test parameters and soil microbial properties were found, estimating microbial features from field tests alone remains challenging. Therefore, the combined use of field tests and laboratory measurements is recommended. Field tests can help improving our understanding for soil quality and play a major role in developing new agricultural management strategies.
Seasonal Effects of Drought on the Ecohydrological Performance of the two Common Swiss Grassland Species Dactylis glomerata L. & Lolium perenne L.
By Florian Cueni
Supvervisor: Prof. Dr. Ansgar Kahmen
It is predicted that due to climate change, drought events will occur more often and be more severe by the end of this century (Trenberth et al. 2003, IPCC 2007). Grasslands, which are very sensitive biomes, are likely to be strongly affected by those predicted changes in rainfall (Chou et al. 2008). Considering, that in Switzerland 70% of the agricultural land is used as grassland for fodder production (BfS 2014), it is very important to understand the effects of drought on this ecosystem. Many drought experiments like Kahmen et al. (2005), Knapp et al. (2008), Fay et al. (2000), Finger et al. (2012) or Bollinger et al. (1991) have found that drought stress reduced the biomass production of grasslands. Since grasslands show a seasonality in their biomass production, but most drought stress studies have been carried out during the summer month, this study investigated the influence of seasonal droughts on the water status of agriculturally important temperate grassland species, Dactylis glomerata L. and Lolium perenne L.. Like Fay et al. 2008 already showed, in this study the absence of precipitation reduced the soil water potential, which in turn lead to a reduced water availability for the plants. This consequently resulted in a reduced aboveground biomass production under drought. Moreover, the results show that 1) seasonal drought events have different effects on the aboveground biomass production of grasses 2) either drought period is increasing water stress in the plants in a similar amount and 3) the biomass reduction observed can to some extend be explained by the water stress the pants experience under drought.
Duckweed Spirodela polyrhiza (L. Schleiden) as an alternative protein source for rainbow trout Oncorhynchus mykiss (Walbaum)
By Svenja Förster
Supvervisors: Prof. Dr. Ansgar Kahmen; Dr. Timo Stadtlander, Research Institute of Organic Agriculture (FIBL)
Two in-vitro screening experiments with six different duckweed species and five growth media were conducted which revealed Spirodela polyrhiza grown on 1:10 diluted cattle slurry not only as the fastest growing species (96.2 g fresh weight m-2 day-1) but also as the species with the highest protein content (up to 30.6% of dry matter). However, in the subsequent upscaled field experiment on the same growth medium a lower growth rate of 11.6 g fresh weight m-2 day-1 (equivalent to 1.26 g dry matter m-2 day-1) as well as a lower protein content (18% of dry matter) were observed. Afterwards, the suitability of dried and ground duckweed Spirodela polyrhiza grown on 1:10 diluted cattle slurry as a dietary protein component for rainbow trout (Oncorhynchus mykiss) was evaluated. Three iso-nitrogenous (50% crude protein), iso-lipidic (15% crude fat) and iso-energetic (23.8 kJ g-1 gross energy and 18.4 kJ g-1 digestible energy) diets were fed to rainbow trout fingerlings for 28 days. One experimental diet contained 6.25% and the other 12.5% dried Spirodela polyrhiza while a diet without duckweed served as control. Growth performance and nutrient utilization of fish were evaluated by individual weight gain, specific growth rate, feed conversion ratio, protein efficiency ratio and individual final body weight. There was no significant difference (p < 0.05) between fish receiving control diet and fish fed the 6.25% diet regarding one of the above mentioned factors. Also, there was no significant difference between fish fed the two experimental diets. However, fish fed control diet showed a significantly higher specific growth rate (SGR = 5.80) and percentage weight gain (406%) in comparison to fish fed the diet containing 12.5% duckweed (5.41 and 355%, respectively). Among all treatments an average feed conversion ratio of 0.66 and a protein efficiency ratio of 3.1 were observed. The body compositions of the fish were analyzed revealing an average dry matter of 19.4% of fresh weight, a protein content of 63.4% of dry matter and a fat content of 18.7% of dry matter. No significant differences were detectable among the treatments regarding these factors. Despite that, fish fed the diet containing 12.5% Spirodela polyrhiza showed the highest (p < 0.05) ash content with 9.7% of dry matter in comparison to the other two treatments (9.4%). There were no pathological differences regarding the gut histology of fish among all treatments.
MYCORRHIZATION OF NARANJILLA (SOLANUM QUITOENSE) AS A STRATEGY FOR A MORE SUSTAINABLE CULTIVATION TECHNIQUE
By Michelle Gisler
Supervisors: Prof. Dr. Ansgar Kahmen, Prof. Dr. em. Thomas Boller, Dr. Paul Mäder (FIBL)
AMF (arbuscular mycorrhizal fungi) can act beneficially in various ways, for example byimproving plant nutrition and growth, water relations, soil structure stability and protectionof plants against biotic and abiotic stresses. These properties make them promisingcandidates in the naranjilla-problematics. The current cultivation practices of this tropicalfruit-shrub not only cause deforestation and large scale soil degradation but also makeplants highly susceptible to a range of pests and diseases. Moreover, due to the intensiveuse of chemical input, these practices present a major problem for the health of the farmers.In this study, we have assessed the impact of AMF inoculation on growth and nutritionparameters of naranjilla (inoculation experiment). As inocula we used three referenceAMF strains (Rhizoglomus irregularis, Claroideoglomus claroideum and Scutellospora pellucida)as well as soils coming from three differently managed naranjilla sites in Ecuador.With this setting we were able to compare the performance of exotic strains (referencestrains) versus local strains contained in the three soil inocula. We expected a superiorityof the local strains and for this reason we set the conditions for a planned further determinationand isolation of the most effective local AMF strains that bear the potential tobe used as inocula in the future. For this purpose, we set up trap cultures where the nativeAMF community has been assessed in a first instance for abundances of commonly foundAMF species, namely R. irregularis and C. claroideum. In the inoculation experiment it hasbeen shown that two of the three reference AMF strains and soil from a permaculture siteled to significantly better acquisition of phosphorus. This result suggests that the use ofAMF represents a valid approach towards a more sustainable cultivation practice of naranjilla.The outstanding results of inoculation with Ecuadorian soil from a site cultivatedafter permaculture principles indicates that this soil harbours beneficial AMF strains thatcould be identified by sequencing, propagated and used for further inoculation experiments.
Effects of Growth Limiting Cold Temperatures on Cell Wall Chemistry in European Tree Species
By Sue Rauss
Supvervisor: PD Dr. Günter Hoch, Prof. Dr. Ansgar Kahmen
The growth of trees is constrained at low temperatures but the underlying mechanisms are still unknown. Temperature thresholds around 5°C have been reported to be growth limiting across temperate tree species and in other functional types of higher plants. Further, roots of tree seedlings which were grown in soils at temperatures below 6°C, were previously reported to be pale and brittle, indicating changes in the amount and the chemical composition (especially reduced lignification) of cell walls at growth limiting low temperatures.
In this study, I investigated seedlings of 12 European tree species treated with low temperatures to quantify the effect of cold temperatures on the total cell wall fraction of plant tissue, as well as the relative proportion of different compounds of the cell wall (hemicellulose, cellulose and lignin) in root tips, coarse roots and stem wood. The cold temperature effect on roots was tested in two water bath experiments, where roots were treated with soil temperature gradients from room temperature at the top soil to ca. 3°C in lower soil depths. Cold temperature affected stem wood was analyzed from a previous phytotron experiment, in which tree seedlings were treated with two short and cool growing seasons, simulating climatic conditions at the alpine treeline.
Across all analyzed tissues and species, there was a decrease of cell wall mass per total tissue dry mass in seedlings grown at low soil temperatures, compared to the warm control treatments. On average this decline was 30% in root tips, 15% in coarse roots and ca. 10% in stem wood. A comparison of the major components of the cell walls, revealed higher fractions of hemicelluloses and lower fractions of cellulose at cold temperatures in all tissues. The lignin content of cell walls was also reduced in cold-treated roots, but not in stem wood of seedlings growing at low temperatures.
Overall, this study could show that growth limiting low temperatures led to a general reduction of cell wall production. This finding thus indicate that cell wall synthesis might be a critical process for cell growth at low temperatures. Considering the decreased proportion of cellulose and (for roots) lignin in cell walls of cold-treated seedlings, the synthesis of these two compounds appears to be a bottleneck for tree growth in cold climates.
Rapid climate change induced carbon starvation in orchids from tropical cloud forests
By Helena Römer
Supervisor: Prof. Dr. Ansgar Kahmen, PD Dr. Günter Hoch
The world’s average temperature is predicted to increase in this century due to anthropogenic greenhouse gas emissions. Tropical forests are expected to respond most adversely to those changes in climate. In montane cloud forests, these impacts are even more severe due to resulting changes in the hydrological cycle. Formulated in a hypothesis, plant carbon reserves are predicted to eventually deplete during environmental stress as a result of insufficient carbon assimilation to meet carbon demand for tissue maintenance. This carbon starvation hypothesis has been debated in numerous studies over the past decades, yet empirical evidence for plants depleting their carbohydrate reservoirs in response to severe drought or temperature increases are scarce. To address this hypothesis, three different temperature treatments were applied to ten epiphytic cloud forest orchid species that are endemic to the highly diverse cool and moist tropical cloud forests of western South America. The response of CO2 exchange (photosynthesis, respiration during day and night) to these increasing temperatures was measured using an infrared gas analyzer and carbon reserves were determined by analyzing non-structural carbohydrates (NSC). While increasing temperatures are frequently associated with reduced water
availability, drought was deliberately avoided in this study, by regular watering of the plants and by keeping relative humidity at an average of 80%. Plants treated with a fast temperature ramp (+ 1.5 K per 10 days) died within 80 days after the start of the experiment, while plants within the slow temperature increase treatment survived for more than 130 days. In both treatments leaf respiration during day and night did not show significant changes. Leaf photosynthesis, however, started to decline significantly when daytime temperatures exceeded 29°C, resulting the NSC levels to decline significantly in both treatments. We were able to show that moderate temperature increases can lead to a lethal depletion of carbohydrate reservoirs as a result of rapidly declining photosynthetic activity and increasing demand of carbon for tissue maintenance.
Assessing minimum levels of C-reserves in carbon limited tree saplings
By Andrea Schwendener
Supervisor: Dr. Günter Hoch
Over the last years, numerous studies have used tissue concentrations of non-structural carbohydrates (NSC) to determine a tree’s net carbon (C) - balance as well as to predict the presence or absence of C-limitation for growth and survival under environmental stress like drought. However, an exact prediction of C-starvation by comparative analyses of NSC concentrations is only possible, if the minimum tissue NSC concentrations at lethal C-starvation are known. To address this question, I conducted an experiment in which two broad-leaved tree species (Acer pseudoplatanus and Quercus petraea) were exposed to C-limitation and finally lethal C-starvation, induced by complete darkening for varying time periods. To my knowledge, this study provides the first experimental data for minimum tissue NSC concentrations at lethal C-starvation. Darkening led to a fast and significant decrease in sugar concentrations and an even stronger and complete decline in the starch concentrations in all investigated tissues. Additionally, darkened saplings were able to survive for several weeks with virtually no NSC reserve pools, and NSC concentrations recovered fast in darkened saplings that were re-introduced to light. I conclude, that C-starvation in tree saplings can only be assumed when NSC reserves are close to zero in all tree organs, and broad-leaved tree saplings are able to survive C-limitation for several weeks, even after the depletion of all NSC pools. Finally, I could show that NSC tissue concentrations react very fast to C-source-sink imbalances, indicating a surprisingly short legacy of previous C-limiting conditions in the NSC pool of tree saplings.
Sarah Hasler: Impacts of spring-, summer- and fall drought on above- and belowground biomass of two common temperate grass species
Aim of the project
To study the impact of seasonal drought events on above- and belowground biomass in two cultivars of Lolium perenne L. and Dactylis glomerata L..
On a field site of the Agroscope a seasonal drought experiment was carried out. With rainout shelters drought was simulated for 10-11 weeks. In the middle of each treatment and at the end, the aboveground biomass was harvested. Additionally at the end of each seasonal drought event, soil samples with 14cm depth were collected. This was also done after 6-8 weeks of recovery. The roots were collected, dried, weighed and then grinded for stable carbon isotope analysis.
In the aboveground biomass a decreasing treatment effect in all seasons was observed. The strongest decrease of aboveground biomass was visible in summer. There is an overexpression of
aboveground biomass after the end of drought treatment. In comparison, there is no effect of seasonal drought visible in belowground biomass in all species. Moreover, there is no change
in the δ13C of the root biomass during the drought treatment.