Effect of dew and fog on Swiss grasslands using stable isotopes of water

Non-rainfall water (NRW), mainly dew and fog, have been identified as important water sources for many ecosystems. NRW, as an additional water source besides rainfall, is expected to become more important for regions suffering from longer and more frequent dry spells, as they are predicted with climate change. This also affects the midlatitudes, where regionally more frequent droughts and dry spells are expected during the warm season. Stable isotopes of water are an efficient and naturally available tracer to investigate hydrological processes. Differences in saturation vapor pressure and diffusivity of different water molecules inducing so-called isotopic fractionation during phase changes can be linked to the key processes in the hydrological cycle. Therefore, stable isotopes can be used for analyzing soil–plant–atmosphere interactions associated with NRW input related to dew and fog. My doctoral thesis aimed to investigate the effect of dew and fog on Swiss grasslands through event-based field campaigns at three temperate grassland sites (CH-CHA, CH-RHB, CH-FRU) and one alpine grassland site (CH-AWS) in 2018–2021. I explored the role of dew and fog in local water cycling, and the effect of dew and fog on plant water status and net ecosystem exchange (NEE). This was done by measuring the isotopic composition (δ18O and δ2H) in all relevant compartments of the hydrological cycle: (1) atmospheric water vapor, (2) NRW droplets on foliage, (3) leaf water, (4) root xylem water, and (5) soil water, combined with (6) eddy-covariance (EC) flux measurements of water vapor, (7) meteorological and (8) physiological measurements. The primary aim of this thesis is to emphasize the ecological importance of dew and fog for temperate and alpine grasslands during summer dry spells. The functions of NRW inputs have been rarely investigated in the past, because rainfall gain tends to exceed annual evapotranspiration loss in these regions during normal years. However, the fact that NRW inputs may play a key role at the event timescale in overcoming a dry spell without irreversible eco-hydrological damage has recently attracted more attention towards this important moisture source for plants in temperate regions. After a general introduction in Chapter 1, I illustrate in Chapter 2, the role of dew and fog in local water cycling through analyzing the isotopic composition of atmospheric water vapor, and NRW droplets on foliage, complemented by EC and meteorological variables during three dew and fog events in summer 2018 at the CH-CHA site located in the bottom of a broad valley. The environmental conditions relevant for the formation of dew and fog are discussed. Deuterium excess (d = δ2H – 8δ18O) derived from δ18O and δ2H was employed to analyze the main drivers of diel water vapor isotope variability. Furthermore, a simple two-end-member mixing model based on an isotope mass balance was employed to partition the input pathways of NRW droplets on foliage into condensation from ambient water vapor and soil water distillation. The results in Chapter 2 showed that dew droplets on foliage were mixed with 9–42 % of condensate from soil vapor diffusion, and 52–91 % of condensate from atmospheric water vapor. The water vapor d was found to be strongly linked with local surface relative humidity, highlighting the dominant role of local moisture as a source for ambient water vapor in the synoptic context of the studied dry spells with very limited impact from large-scale advection. Chapter 2 underlines the importance of NRW inputs to temperate grasslands during dry spells and reveals the complexity of the local water cycle in such conditions. In Chapter 3, I focused on the complexity of water vapor isotopes in the near-surface atmosphere as affected by the temporal evolution of radiation fog and cloud coverage at the CH-CHA site, Water vapor isotopes were combined with EC and meteorological measurements, as well as COSMO-1 model (Consortium for Small-scale Modeling) output. Five events were selected: (1) event 1 with shallow radiation fog occurring around sunrise; (2) event 2 with the transition of fog from shallow radiation fog to deep radiation fog; (3) event 3 with the transition of fog from shallow radiation fog, and further to deep radiation fog, which persisted after sunrise with a low-level cloud cover aloft; (4) event 4 with midnight fog dissipation due to low-level clouds aloft; and (5) event 5 with low to mid-level frontal cloud coverage but without fog in the context of a cold frontal passage. The results in Chapter 3 showed that shallow radiation fog and deep radiation fog are associated with different characteristic signatures in the variability of the water vapor isotope signals depending on the relative strength of mixing, condensation, and deposition processes. Furthermore, Chapter 3 highlights the importance of cloud coverage on the temporal evolution of the isotope signals of fog events. The results reveal the highly variable isotopic composition in the near-surface atmosphere affected by land–atmosphere interactions relating to radiation fog evolution and cloud coverage. In Chapter 4, I investigated the effect of dew and fog on plants of a temperate grassland at the CH-CHA site through analyzing the isotopic composition of atmospheric water vapor, NRW droplets on foliage, leaf water, root xylem water, and soil water, complemented by the changes of leaf water potential (LWP) and leaf relative water content (RWC), EC and meteorological measurements. The controls of radiative cooling and leaf wetting on nighttime stomatal opening were investigated. The physical prerequisite of dew and fog formation, with cooler leaf surface allowing for warmer atmospheric water vapor condensing on foliage, controlled the directions of water exchange from the atmosphere to the leaf. Leaf conductance computed from an isotopic mass balance model indicates nighttime stomatal opening, and minor influence of root water flux on leaf conductance during dew and fog formation. The results in Chapter 4 underline the ecological relevance of radiative cooling and leaf wetting for land–atmosphere interactions in natural temperate grasslands which is hardly considered in laboratory experiments, thereby complementing previous chamber experiment studies. In Chapter 5, I concentrated on the effect of dew water on plant water status and gross primary production (GPP) of ecosystem, and in an alpine grassland site CH-AWS during the June 2019 heatwave. GPP partitioned from net ecosystem CO2 flux was analyzed during no-rain period of the heatwave from 25 to 30 June 2019. To allow for natural drought treatment, at the end of the heatwave periods (28–30 June), the isotopic composition of atmospheric water vapor, NRW droplets on foliage, root xylem water, soil water, leaf water and leaf sugar, as well as leaf water potential (LWP) was measured. A chamber-tracer experiment in a custom-made canopy chamber was carried out on 28–29 June by amending isotopically depleted water on plant surfaces for leaf water and sugar isotopic analysis. The results indicate the suppression of CO2 uptake by heat-drought stress, and the promotion of dew water on CO2 uptake in the early morning hours. The effect of dew water on LWP varied by species and is related to species-specific dependences on soil water. In the chamber-tracer experiment, the leaf water isotopes were found to be depleted by the tracer, which could not be successfully transferred to leaf sugar, suggesting that the effect of dew water on GPP was not directly via influencing photosynthesis, but via indirectly benefiting CO2 uptake of plants suffering from drought and heat stress by suppressing atmospheric evaporative demand. Chapter 5 underlines the effect of dew on GPP of alpine grassland during the June 2019 heatwave. As synthesized in the conclusions in Chapter 6, this doctoral thesis used stable isotopes of water combined with site-based EC and meteorological measurements, regional-scale COSMO-1 model output at km resolution, and event-based physiological measurements to investigate the role of dew and fog in grassland–atmosphere water and carbon exchange. The results of this doctoral thesis contribute to advance our understanding on the complex ecologically relevant processes associated with the occurrences of dew and fog during droughts