Effects of groundwater extraction and extreme drought events on vitality and growth of pedunculate oak (Quercus robur L.) trees

Hochschulschrift

Abstract: Background: Groundwater is one of the most crucial and reliable sources of fresh water for human and natural systems. Groundwater-fed and floodplain forests are typically characterized by high productivity and are extremely valued for their biodiversity and unique ecosystem functions and services they provide. Yet land-use change, groundwater extraction and river regulation have already resulted in reduced groundwater availability for these forests. In addition, more frequent and intense dry periods in future as direct and indirect consequences of climate change might expose these forests to potentially dramatic and unprecedented decreases in water availability. While multiple studies exist on the effects of drought on water-limited forest ecosystems, the influence of water deficits on groundwater-fed forests has been studied to a lesser degree and only very few studies have assessed the combined effects of groundwater extraction and drought on groundwater-fed forests.
Aim and objectives: The main aim of this thesis was to investigate the combined effects of groundwater extraction and extreme drought events on growth and vitality of groundwater-fed oak (Quercus robur) forests. To achieve that, the following specific objectives were defined:
1.To analyze the occurrence and severity of past drought events in the study region.
2.To assess the effects of reduced groundwater availability on growth sensitivity of Q. robur trees to drought.
3.To investigate the influence of groundwater availability on growth and xylem anatomical responses of Q. robur trees to drought.
4.To discuss limitations related to the calculation of commonly used growth-based indicators of tree responses to drought and to develop a more standardized procedure for their application and interpretation.
Data and study design: Two datasets were created for the purposes of this thesis. The first dataset was used to address objective 1 and constitutes an assembly of several drought related variables spanning the last 218 years including meteorological data, stream flow records, tree-ring data and recorded drought impacts from southwestern Germany.
The second dataset to address objectives 2 & 3 includes tree-ring data from three regions in the Upper Rhine Valley of Germany. Radial growth was analyzed in Q. robur stands where (a) water extraction had dramatically lowered groundwater levels so that trees presumably lost direct access to it, or (b) trees have not been affected by water extraction. In addition, (c) nearby sessile oak (Q. petraea) stands at upland sites were analyzed to isolate the climate signal present in tree rings for each study region from a groundwater signal. Younger and older forest stands for each site type and region were selected. In each stand twenty trees from all vitality classes were inventoried and two increment cores per tree were extracted. Tree-ring width series from a total of 529 trees were measured to develop 27 stand-wise chronologies. A subset of 216 trees (eight per stand) was used for wood anatomy analyses.
Analytical approach: The different drought related variables in the first dataset were transformed into continuous time series of anomalies or drought occurrences to explore the different characteristics (severity and frequency) of past drought events in southwestern Germany. Additionally, three metrics were used to quantify relationships, similarities and uniqueness of drought in different indices.
Series of ring-width and wood anatomical variables from the second dataset were used to assess differences in stem growth and xylem hydraulic function of oak trees from sites with contrasting groundwater availability. Climate sensitivity of growth and wood anatomical properties was assessed by calculating correlation functions between ring width and earlywood vessel chronologies and climate variables. To assess temporal changes in sensitivity of radial growth in response to climatic variation, moving-window correlation functions were computed between the detrended stand-wise chronologies and climate variables (precipitation, temperature and three different scales of SPEI index). Three indices, the components of resilience (as proposed by Lloret et al. 2011 resistance, recovery and resilience) were used to assess tree responses to drought in terms of growth and xylem hydraulic function. These were compared among sites and tree vigor groups (healthy vs. declining trees).
Due to inconsistencies and limitations in the use and interpretation of the growth-based indicators of tree responses to drought, a literature review and re-analysis of published data of resistance and recovery was carried out to identify, demonstrate and discuss options to overcome these shortcomings and pitfalls.
Results and discussion: The inclusion of different drought indices and impact information in this study allowed a comprehensive assessment of past extreme drought events (objective 1). Twenty extreme droughts since 1801 were identified as common events for the state of Baden-Württemberg when applying the different meteorological and hydrological indicators. All these events were associated with societal impacts and the majority of them with extremely low stream flow and tree growth. Over the same period two major peaks of increased drought frequency and severity were identified (1860-1870 and 2003-2018). Although these two periods were similar to each other in terms of overall drought frequency, the recent period (starting in 2003) was characterized by a higher frequency of extreme droughts.
Radial growth of oak trees in the region was strongly correlated to water availability in summer and spring months (objective 3). Series of earlywood and latewood width as well as of anatomical features showed more temporally specific climate signals than total ring-width, at shorter time periods (1 - 3 months compared to 6 - 9 months for total ring-width), demonstrating the direct influence of climate variability and especially of soil moisture on the different stages of wood formation. Sensitivity of oak growth to summer drought increased substantially following the onset of groundwater extraction (88–49 years ago) (objective 2). Compared to sites without extraction, trees at sites with groundwater extraction showed lower growth and hydraulic function both during periods of ample and of extremely reduced soil water availability. The highest proportion of trees with clear signs of vitality decline was observed at sites with groundwater extraction. These trees were not able to recover growth and hydraulic function following extreme drought events.
Shortcomings and pitfalls in the use of growth-based indicators of tree responses to drought (objective 4) were identified in terms of drought identification, the type of growth data to be used for index calculations and with regard to defining and calculating reference growth levels. As a consequence, no common framework to apply and interpret these indicators exists so far. Potential solutions for these limitations were proposed as well as a new analytical framework, the ‘line of full resilience’, which integrates several drought indicators and could be used for comparative drought-tolerance assessments of tree species.
Conclusions: Analyzing the frequency and severity of drought events over the last 218 years in the region revealed that the last two decades were characterized by the highest frequency of extreme drought events. These most likely resulted from increasing precipitation deficits in summer along with rising temperatures. Under such conditions, access to groundwater was beneficial for trees, as it appeared to mitigate the negative effects of increasing summer water-deficits on tree growth and hydraulic function. Increasing drought frequency and intensity combined with increasing needs for groundwater extraction in future could be detrimental for the vitality of groundwater-fed forest ecosystems. The evidence presented in this study suggests that the combination of long-term (groundwater extraction) and abrupt (extreme drought events) water deficits result in tree vitality losses and probably higher mortality risks in the future for trees at sites with reduced groundwater availability. Therefore, the location of groundwater extraction sites as well as the rates and timing of water extraction should be carefully considered to ensure that tree vitality and ecosystem health are not impaired

Object Details

• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Universität Freiburg, Dissertation, 2020

Classification and Topics

• Keyword: Groundwater ; Trees ; Vitality ; English oak ; Trockenheit ; Grundwasserabsenkung ; Eiche ; Dendrochronologie

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2021
• Creator: Skiadaresis, Georgios
• Contributor: Bauhus, Jürgen ; Stahl, Kerstin ; Albert-Ludwigs-Universität Freiburg. Fakultät für Umwelt und Natürliche Ressourcen

Further information

• DOI: 10.6094/UNIFR/194402
• URN: urn:nbn:de:bsz:25-freidok-1944022
• Rights: Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Last update: 25.03.2025, 1:42 PM CET