Effects of soil moisture, needle age and leaf morphology on carbon and oxygen uptake, incorporation and allocation: a dual labeling approach with 13CO2 and H218O in foliage of a coniferous forest

Dew benefits on alpine grasslands are cancelled out by combined heatwave and drought stress

Increasing frequencies of heatwaves combined with simultaneous drought stress in Europe threaten the ecosystem water and carbon budgets of alpine grasslands. Dew as an additional water source can promote ecosystem carbon assimilation. It is known that grassland ecosystems keep high evapotranspiration as long as soil water is available. However, it is rarely being investigated whether dew can mitigate the impact of such extreme climatic events on grassland ecosystem carbon and water exchange. Here we use stable isotopes in meteoric waters and leaf sugars, eddy covariance fluxes for H₂O vapor and CO₂, in combination with meteorological and plant physiological measurements, to investigate the combined effect of dew and heat-drought stress on plant water status and net ecosystem production (NEP) in an alpine grassland (2000 m elevation) during the June 2019 European heatwave. Before the heatwave, enhanced NEP in the early morning hours can be attributed to leaf wetting by dew. However, dew benefits on NEP were cancelled out by the heatwave, due to the minor contribution of dew in leaf water. Heat-induced reduction in NEP was intensified by the combined effect of drought stress. The recovery of NEP after the peak of the heatwave could be linked to the refilling of plant tissues during nighttime. Among-genera differences of plant water status affected by dew and heat-drought stress can be attributed to differences in their foliar dew water uptake, and their reliance on soil moisture or the impact of the atmospheric evaporative demand. Our results indicate that dew influence on alpine grassland ecosystems varies according to the environmental stress and plant physiology.

Water status and macronutrient concentrations, but not carbon status, of Viscum album ssp. album are determined by its hosts: a study across nine mistletoe-host pairs in central Switzerland

Introduction

European mistletoe, Viscum album L., is a hemiparasite that can infect various tree species, yet our understanding of its physiological interactions with host species is limited.

Methods

Nine mistletoe-host pairs (i.e. V. album ssp. album growing on nine different broadleaf tree species) under different growth conditions in central Switzerland were selected to examine the carbon, water and nutrient relationships between mistletoe and its hosts. We measured leaf morphological traits, isotopic compositions (δ13C and δ15N), concentrations of non-structural carbohydrates (NSC) and specific compounds (i.e. mobile sugars and starch), and macronutrients (i.e. N, P, K, Ca, Mg, S) in leaf and xylem tissues of both mistletoe and its hosts.

Results and Discussion

There were only non-significant relationships between NSC concentrations in mistletoe and in its host species across the nine mistletoe-host pairs, suggesting the carbon condition of V. album ssp. album is determined by both the heterotrophic carbon transfer and self-photosynthetic capacity among different mistletoe-host pairs. However, mistletoe leaf morphological traits (single leaf area and mass, and leaf mass per unit leaf area) did not change across the nine mistletoe-host pairs, and mistletoe leaf δ13C, water content and macronutrient concentrations were linearly correlated with those in the host leaves. Macronutrients showed accumulations in mistletoe across the nine pairs. Further, tissue N concentrations were significantly higher in mistletoe grown on N-fixing hosts than on non-N-fixing hosts. Finally, leaf N:P in mistletoe was significantly correlated with the ratio in the host across the nine mistletoe-host pairs. Overall, our results indicate strong relationships between mistletoe and its hosts for water- and nutrient-related traits, but not for carbon-related traits, which demonstrates that V. album ssp. album can adjust its physiology to survive on different deciduous tree species hosts and under different site conditions.

High resilience of carbon transport in long-term drought-stressed mature Norway spruce trees within 2 weeks after drought release

Under ongoing global climate change, drought periods are predicted to increase in frequency and intensity in the future. Under these circumstances, it is crucial for tree's survival to recover their restricted functionalities quickly after drought release. To elucidate the recovery of carbon (C) transport rates in c. 70-year-old Norway spruce (Picea abies [L.] KARST.) after 5 years of recurrent summer droughts, we conducted a continuous whole-tree ¹³ C labeling experiment in parallel with watering. We determined the arrival time of current photoassimilates in major C sinks by tracing the ¹³ C label in stem and soil CO₂ efflux, and tips of living fine roots. In the first week after watering, aboveground C transport rates (CTR) from crown to trunk base were still 50% lower in previously drought-stressed trees (0.16 ± 0.01 m h^-1 ) compared to controls (0.30 ± 0.06 m h^-1 ). Conversely, CTR below ground, that is, from the trunk base to soil CO₂ efflux were already similar between treatments (c. 0.03 m h^-1 ). Two weeks after watering, aboveground C transport of previously drought-stressed trees recovered to the level of the controls. Furthermore, regrowth of water-absorbing fine roots upon watering was supported by faster incorporation of ¹³ C label in previously drought-stressed (within 12 ± 10 h upon arrival at trunk base) compared to control trees (73 ± 10 h). Thus, the whole-tree C transport system from the crown to soil CO₂ efflux fully recovered within 2 weeks after drought release, and hence showed high resilience to recurrent summer droughts in mature Norway spruce forests. This high resilience of the C transport system is an important prerequisite for the recovery of other tree functionalities and productivity.

Radial and axial water movement in adult trees recorded by stable isotope tracing

The capacity of trees to release water from storage compartments into the transpiration stream can mitigate damage to hydraulic functioning. However, the location of these 'transient' water sources and also the pathways of water movement other than vertical through tree stems still remain poorly understood. We conducted an experiment on two tree species in a common garden in eastern Australia that naturally grow in regions of high (Eucalyptus tereticornis, 'Red Gum') and low (Eucalyptus sideroxylon, 'Ironbark') annual precipitation rates. Deuterium-enriched water (1350% label strength) was directly introduced into the transpiration stream of three trees per species for four consecutive days. Subsequently, the trees were felled, woody tissue samples were collected from different heights and azimuthal positions of the stems, and stable isotope ratios were determined on the water extracted from all samples. The presence/absence of the tracer along the radial and vertical stem axes in combination with xylem hydraulic properties inferred from sapflow, leaf and stem water potentials, wood moisture contents and anatomical sapwood characteristics elucidated species-specific patterns of short-term stem water storage and movement. The distribution of water isotopes at natural abundance among woody tissues indicated systematic differences with highest values of sapwood water and lower values in inner bark and heartwood. Presence of tracer in water of the inner bark highlighted the importance of this tissue as capacitor. Although injected at the northern side of stems, tracer was also discovered at the southern side, providing empirical evidence for circumferential flow in sapwood, particularly of Ironbark. Greater vertical water transport in Red Gum compared with more radial and circumferential water transport in Ironbark were associated with species-specific sapwood anatomy. Our study highlights the value of combining information from stable isotope tracers and wood anatomy to investigate patterns of water transport and storage of tall trees in situ.