Fig 1.
Location of the study area and the experimental setup of the two planted dike sections.
(A) Overview of the study area at the southeastern North Sea coast, Germany, on the Butjadingen Peninsula, Wesermarsch (red star). (B) The summer dike in front of the main sea dike (shaded in green), on which the two dike sections were investigated (red square). (C) Simplified cross-section of the summer dike according to the experimental setup with a 20 cm thick layer of new bare clay soil and the two established plant communities ‘Mix-Herb’ (herb-dominated) and ’Mix-Grass’ (grass-dominated); not to scale. The maps (A-B) were created using Natural Earth (CC0, public domain) and DOM1 data republished from OpenGeoData NI under a CC BY license, with permission from Landesamt für Geoinformation und Landesvermessung Niedersachsen (LGLN), original copyright 2020.
Table 1.
Statistical results based on the Mann-Whitney U test for soil temperature and moisture.
Fig 2.
High temporal resolution soil temperature and moisture data reveal vegetation-depending differences.
Daily minimum and maximum values of soil temperature (A, orange and red bands) and soil moisture (B, cyan and blue bands) with darker, bold lines for the corresponding daily averages of both parameters during the measurement period from November 22, 2022 to November 22, 2023. The soil temperature and moisture values are given for a depth of 4 cm and for dike sections ‘Mix-Herb’ and ‘Mix-Grass’. Key events are the two drought periods (gray bands), vegetation mapping (olive green band) and mowing (light green band). Data gap between May 9 and May 25, 2023, on the ‘Mix-Herb’ dike section was due to temporary sensor breakdowns.
Fig 3.
Detecting meteorological drought periods based on SPI values and precipitation history.
Standard Precipitation Index (SPI) based on precipitation reference data from 1991 to 2020 from the weather station Burhave from November 22, 2022 to November 22, 2023 and corresponding daily precipitation. Periods, which by definition represent a drought, correspond to the negative SPI values (orange), whereby a distinction is made between moderate (−1.00 to −1.49 SPI), severe (−1.50 to −1.99 SPI) and extreme events (≤−2.00 SPI; [56]).
Fig 4.
Changes in soil physical conditions as a function of mowing.
Diurnal temperature variations (difference between daily maximum and minimum soil temperature) of ‘Mix-Herb’ (orange circles) and ‘Mix-Grass’ (red squares) recorded at a depth of 4 cm for the periods May 1, 2023 to August 13, 2023 (before mowing; A), and August 15, 2023 to November 22, 2023 (after mowing; B). Heatmap strip represents daily average differences in soil moisture between the ‘Mix-Herb’ and the ‘Mix-Grass’ vegetation areas. Grey shaded bands highlight both drought periods (negative SPI values), while olive green and light green bands mark the vegetation mapping and mowing event, respectively. Data gap between May 9 and May 25, 2023 on the ‘Mix-Herb’ dike section is due to a temporal sensor breakdown.
Fig 5.
Influence of vegetation composition on the rate of soil moisture loss during drought periods.
(A) Soil moisture during the first phase of the drought period from May 24, 2023 to June 6, 2023 with SPI values from −0.53 to −1.75 and linear regression with soil moisture change (Δy) over time (Δt) in days. (B) Soil moisture during the first phase of the drought period from September 04, 2023 to September 11, 2023 with SPI values from −0.19 to −1.79 and linear regression with soil moisture change (Δy) over time (Δt) in days.