Mirjam Meischner

Summary: Temperate forests increasingly face extreme air temperature, but plant physiological responses, particularly alterations in carbon allocation or protection via volatile organic compound (VOC) emissions, remain poorly understood. We pulse-labelled well-watered saplings of Fagus sylvatica and Pseudotsuga menziesii in a controlled heat stress experiment with ¹³ CO ₂ to quantify heat-induced shifts in CO ₂ , VOC and C pool exchange, specifically analyzing compound-specific δ ¹³ C of terpenoids, water-soluble organic matter (WSOM) and dark respiration. Under heat stress, up to 50% of fresh assimilates were directed to maintenance respiration and 1-2% to VOC emissions, while net assimilation and water use efficiency decreased by 50-75% in both species. Heat directly affected metabolic processes and reduced turnover rates of fresh assimilates in F. sylvatica , but accelerated them in P. menziesii. Strong ¹³ C labelling of some compounds, particularly acyclic ones, suggested increased de novo synthesis of specific terpenoids for heat stress protection. By tracing the fate of recently assimilated ¹³ CO ₂ we demonstrate that heat stress reduces net carbon uptake and water use efficiency, disrupts turnover of C pools and increases carbon loss via respiration and de novo synthesis of specific VOCs, potentially diminishing net carbon uptake of forests under future heat extremes.

Summary: Globally, forest ecosystems face widespread mortality events. However, the independent impacts of distinct stressors, such as heat stress vs edaphic drought, remain poorly understood and physiological early warning indicators for tree mortality are urgently required. We exposed well-watered saplings of Fagus sylvatica , Pseudotsuga menziesii and Picea abies to summer heat waves and subsequent natural winter-desiccation. Physiological parameters (e.g. gas exchange, water uptake velocity via ² H labelling, and volatile organic compound emissions) were monitored throughout the growing season and survival was assessed regularly until subsequent spring to capture immediate and delayed mortality as a consequence of legacy effects. Heat exposure without soil water deficit, followed by winter desiccation, triggered species-specific mortality rates (51.8% F. sylvatica , 48.2% P. abies , 16.9% P. menziesii ), with P. abies exhibiting significantly faster mortality response than the other species. Reduced water uptake, lower stomatal conductance, impaired photosynthetic efficiency, and altered VOC emissions distinguished non-surviving from surviving saplings months before visible damage in all three species. Heat stress drives mortality independent of edaphic drought, with sub-lethal physiological indicators detectable up to 10 months before visual signs. These early warning indicators could enable damage detection before lethal thresholds are crossed, offering new strategies for mitigating climate change-driven forest decline.

Summary: Forest soils are an important source of volatile organic compounds (VOCs), yet little is known about how different tree species influence soil VOC emissions and the role of rhizosphere processes in mediating VOC release form roots. We analysed soil VOC emissions from the soil surface and bulk soil as well as from roots with intact rhizosphere and washed roots of Picea abies and Fagus sylvatica . Tree saplings were grown on natural forest soil, and VOC emissions and gas exchange of soils and roots were measured under controlled conditions using online gas analysers integrated into an automated system. To assess the contribution of rhizosphere soil and microbial communities to root VOC emissions, roots were analysed (a) without washing, preserving the rhizosphere, (b) water-washed, and (c) ethanol-washed (70 vol%) to minimize microbial contributions. Species-specific VOC emission patterns were observed in both soils and roots. P. abies showed higher total emission rates and a more diverse, terpenoid-rich VOC profile dominated by α-pinene, β-pinene, β-myrcene, and α-phellandrene than F. sylvatica . Notably, these differences were evident not only at the soil surface but also in root and litter free bulk soil. Root washing further revealed that the rhizosphere is a hotspot of terpenoid production in P. abies , with significantly higher monoterpenoid emissions from unwashed roots than from water or ethanol-washed roots. This study demonstrates how tree species shape net soil VOC emissions, potentially leading to cascading effects on atmospheric VOC concentrations, and highlights the importance of the rhizosphere in regulating belowground VOC production.

Volatile organic compounds (VOCs) facilitate above-ground plant communication, but below-ground signaling remains less understood. This study explored root–root interactions between Picea abies and Fagus sylvatica saplings in monospecific (Fagus–Fagus) and heterospecific (Picea–Fagus) pairs (n=6), excluding shoot-level VOC communication. Sender plants were treated with jasmonic acid (JA) to simulate herbivory and labeled with 13CO2 and 15NH4NO3 to trace nutrient transfer in a split-root design. VOC emissions and gas exchange were measured over 10 days using a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) and 13CO2 spectroscopy, and 13C and 15N were analyzed in roots and shoots via an elemental analyser coupled to an isotope ratio mass spectrometer (EA-IRMS). Our findings reveal, that (i) JA treatment induced strong de novo terpenoid emissions from P. abies and enhanced emissions of oxygenated VOCs and benzenoids from F. sylvatica, (ii) F. sylvatica receiver plants responded similarly to JA-treated neighbors, indicating below-ground signaling, and (iii) responses of receiver plants were more pronounced in the heterospecific treatment. Furthermore, formic acid emissions from soils increased following JA treatment, suggesting altered soil microbial activity. Isotopic analysis revealed carbon exudation into the rhizosphere and nitrogen transfer to receiver plants. These results suggest that below-ground signaling enables early up-regulation of herbivore-induced defenses in neighboring plants, and that the response intensity is modulated by species identity.

Compound droughts, i.e. the co-occurrences of heat and drought, represent a serious challenge for temperate forest trees leading to significant losses in forest biomass. We studied the physiological response of Norway spruce ( Picea abies ) saplings to heat and drought individually, and in combination. Continuous measurements of leaf gas exchange and VOC emission allowed us to identify fast-response reactions, while discrete VOC and root exudate samplings added qualitative information on compositional changes. Additionally, we used ¹³ CO ₂ and ² H ₂ O label pulses to investigate C-allocation and root water uptake in response to stress. Heat as well as drought reduced assimilation rates in the saplings, whereas transpiration, leaf VOC emission and root exudation rates increased in response to heat. Drought alone increased VOC emission but decreased exudation rates. Combined heat and drought triggered an amplified response in both processes despite negative net CO ₂ assimilation rates. Label incorporation showed compromised water uptake capacity of drought-stressed plants and illustrated de novo C-allocation to VOC emission and root exudates. The results point at the high susceptibility of Norway spruce saplings to drought and heat. Combined stress resulted in synergistic responses in VOC emissions and root exudates, showing the detrimental effect of compound droughts on Norway spruce. Highlight: In this study, we found synergistic effects of heat and drought on carbon losses from leaf VOC emission and root exudates despite negative assimilation rates in Norway spruce saplings.

The main sources of volatile organic compounds (VOCs) emitted from forest soils are roots and soil microorganisms. However, quantifying the relative contributions of these sources to net soil VOC emissions and the proportion of root VOCs degraded by soil microorganisms is challenging.In order to partition soil VOC emissions into root and microbial VOC emissions, we conducted a controlled mesocosm experiment using Picea abies and Fagus sylvatica saplings, as well as soil (Cambisol) collected from a temperate forest dominated by these two species. VOC emissions from the soil surface (combined root and soil emissions), as well as from excavated roots and root-free bulk soil (sieved to 2 mm), were analyzed using PTR-TOF-MS and GC-MS. To evaluate the role of microorganisms as a source or sink of VOCs in the rhizosphere, microbial colonization of roots was modified by applying three washing treatments: (1) no washing (intact rhizosphere, high colonization), (2) water washing (partial removal of root microbiome, intermediate colonization), and (3) washing with 70% (v/v) ethanol (disinfection, reduced colonization), with 6 replicates per species and treatment.This study aims to improve our understanding of the soil VOC fluxes in forest ecosystems by quantifying the contributions of roots and microorganisms to net soil VOC emissions, as well as the uptake of root VOCs by soil microorganisms.

Summary: Volatile organic compounds (VOCs) facilitate aboveground plant communication, but belowground signaling remains less understood. This study explored root-root interactions between Picea abies and Fagus sylvatica saplings in monospecific ( Fagus-Fagus ) and heterospecific ( Picea-Fagus ) pairs (n=6), excluding shoot-level VOC communication. Sender plants were treated with jasmonic acid to simulate herbivory and labeled with ¹³ CO ₂ and ¹⁵ NH ₄ NO ₃ to trace nutrient transfer in a split-root design. VOC emissions and gas exchange were measured over ten days using PTR-TOF-MS and ¹³ CO ₂ -spectroscopy and ¹³ C and ¹⁵ N were analyzed in roots and shoots via EA-IRMS. Our findings reveal, that (i) JA treatment induced strong de novo terpenoid emissions from P. abies and enhanced emissions of oxygenated VOCs and benzenoids from F. sylvatica , (ii) F. sylvatica receiver plants responded similarly to JA-treated neighbors, indicating belowground signaling, and (iii) responses of receiver plants were more pronounced in the heterospecific treatment. Furthermore, formic acid emissions from soils increased following JA treatment, suggesting altered soil microbial activity. Isotopic analysis revealed C exudation into the rhizosphere and N transfer to receiver plants. These results suggest that belowground signaling enables early priming of herbivore-induced defenses in neighboring plants, and that the response intensity is modulated by species identity.

The systemically induced production of volatile organic compounds (VOCs) in undamaged tissues of plants under herbivore attack is still not fully understood, particularly with respect to below‐ and aboveground signaling. Here, we test the hypotheses that treatment of trees with jasmonic acid (JA) to simulate local herbivory (i) systemically induces VOC emissions in leaves and roots by signal propagation via the vascular bundle system and (ii) that bidirectional signaling occurs between below‐ and aboveground organs. We applied JA to roots and branches of Fagus sylvatica and Picea abies in a controlled experiment and shielded untreated tissues from volatile cues. VOC emissions and gas exchange were measured continuously over 6–8 days and complemented by quantification of tissue terpenoid storage pools. In contrast to the strong increase in terpenoid emissions from directly treated leaves and needles, which were mainly composed of sesquiterpenes, no systemically induced terpenoid emissions were found. Direct JA treatment of shoots reduced net photosynthesis and stomatal conductance in P. abies by ~50%, while the gas exchange of F. sylvatica remained unaffected. In the root system of P. abies , terpenoid contents increased both locally and systemically in response to belowground JA treatment. Overall, our results challenge the concept of systemically induced terpenoid emissions through vascular JA signaling, which is commonly induced in trees in response to insect herbivory. Instead, our data point toward a possible role of volatile cues in intra‐plant signaling.

Many belowground processes, such as soil respiration and soil–atmosphere VOC (volatile organic compounds) exchange, are closely linked to soil microbiological processes. However, little is known about how changes in plant species cover, i.e. after plant invasion, alter these soil processes. In particular, the response of soil VOC emissions to plant invasion is not well understood. We analysed soil VOC emissions and soil respiration of a Mediterranean cork oak (Quercus suber) ecosystem, comparing soil VOC emissions from a non‐invaded Q. suber woodland to one invaded by the shrub Cistus ladanifer. Soil VOC emissions were determined under controlled conditions using online proton‐transfer time‐of‐flight mass spectrometry. Net soil VOC emissions were measured by exposing soils with or without litter to different temperature and soil moisture conditions. Soil VOC emissions were sensitive to C. ladanifer invasion. Highest net emission rates were determined for oxygenated VOC (acetaldehyde, acetone, methanol, acetic acid), and high temperatures enhanced total VOC emissions. Invasion affected the relative contribution of various VOC. Methanol and acetaldehyde were emitted exclusively from litter and were associated with the non‐invaded sites. In contrast, acetone emissions increased in response to shrub presence. Interestingly, low soil moisture enhanced the effect of shrub invasion on VOC emissions. Our results indicate that shrub invasion substantially influences important belowground processes in cork oak ecosystems, in particular soil VOC emissions. High soil moisture is suggested to diminish the invasion effect through a moisture‐induced increase in microbial decomposition rates of soil VOC.