Stephan Unger

Short-term dynamics of isotopic composition of leaf-respired CO2 upon darkening: measurements and implications.

Recent advances in understanding the metabolic origin and the temporal dynamics in delta(13)C of dark-respired CO(2) (delta(13)C(res)) have led to an increasing awareness of the importance of plant isotopic fractionation in respiratory processes. Pronounced dynamics in delta(13)C(res) have been observed in a number of species and three main hypotheses have been proposed: first, diurnal changes in delta(13)C of respiratory substrates; second, post-photosynthetic discrimination in respiratory pathways; and third, dynamic decarboxylation of enriched carbon pools during the post-illumination respiration period. Since different functional groups exhibit distinct diurnal patterns in delta(13)C(res) (ranging from 0 to 10 per thousand diurnal increase), we explored these hypotheses for different ecotypes and environmental (i.e. growth light) conditions. Mass balance calculations revealed that the effect of respiratory substrates on diurnal changes in delta(13)C(res) was negligible in all investigated species. Further, rapid post-illumination changes in delta(13)C(res) (30 min), which increased from 2.6 per thousand to 5 per thousand over the course of the day, were examined by positional (13)C-labelling to quantify changes in pyruvate dehydrogenase (PDH) and Krebs cycle (KC) activity. We investigated the origin of these dynamics with Rayleigh mass balance calculations based on theoretical assumptions on fractionation processes. Neither the estimated changes of PDH and KC, nor decarboxylation of a malate pool entirely explained the observed pattern in delta(13)C(res). However, a Rayleigh fractionation of (12)C-discriminating enzymes and/or a rapid decline in the decarboxylation rate of an enriched substrate pool may explain the post-illumination peak in delta(13)C(res). These results are highly relevant since delta(13)C(res) is used in large-scale carbon cycle studies.

Disentangling drought-induced variation in ecosystem and soil respiration using stable carbon isotopes

Combining C flux measurements with information on their isotopic composition can yield a process-based understanding of ecosystem C dynamics. We studied the variations in both respiratory fluxes and their stable C isotopic compositions (δ13C) for all major components (trees, understory, roots and soil microorganisms) in a Mediterranean oak savannah during a period with increasing drought. We found large drought-induced and diurnal dynamics in isotopic compositions of soil, root and foliage respiration (δ13Cres). Soil respiration was the largest contributor to ecosystem respiration (Reco), exhibiting a depleted isotopic signature and no marked variations with increasing drought, similar to ecosystem respired δ13CO2, providing evidence for a stable C-source and minor influence of recent photosynthate from plants. Short-term and diurnal variations in δ13Cres of foliage and roots (up to 8 and 4‰, respectively) were in agreement with: (1) recent hypotheses on post-photosynthetic fractionation processes, (2) substrate changes with decreasing assimilation rates in combination with increased respiratory demand, and (3) decreased phosphoenolpyruvate carboxylase activity in drying roots, while altered photosynthetic discrimination was not responsible for the observed changes in δ13Cres. We applied a flux-based and an isotopic flux-based mass balance, yielding good agreement at the soil scale, while the isotopic mass balance at the ecosystem scale was not conserved. This was mainly caused by uncertainties in Keeling plot intercepts at the ecosystem scale due to small CO2 gradients and large differences in δ13Cres of the different component fluxes. Overall, stable isotopes provided valuable new insights into the drought-related variations of ecosystem C dynamics, encouraging future studies but also highlighting the need of improved methodology to disentangle short-term dynamics of isotopic composition of Reco.

Allocation trade-off between root and mycorrhizal surface defines nitrogen and phosphorus relations in 13 grassland species

Background and aimsIn oligotrophic ecosystems efficient nutrient uptake mechanisms, like extensive root systems or the association with belowground symbionts (e.g. arbuscular mycorrhizal fungi, AMF), are crucial. Pursuing root- or AMF-dominated foraging may result in diverging success regarding nitrogen (N) and phosphorus (P) nutrition. In this study we identify species- and functional group-specific belowground allocation strategies and disentangle the role of root vs. hyphal allocation for N and P nutrition.MethodsAllocation patterns to both root and AM hyphal surface together with plant P- and N-relations were measured in non-mycorrhizal and mycorrhizal individuals of 13 common grassland species belonging to the functional groups of forbs, grasses, legumes and non-mycotrophic Brassicaceae.ResultsThe trade-off between predominant investments into either roots or hyphae showed high species- and functional group-specificity and clearly defined plant N:P relations, with root strategists gaining larger N- and lower P-benefits than mycorrhizal strategists. Further, P-delivery by AMF was accompanied by strong fungal N-competition.ConclusionsOur results demonstrate high relevance of the allocation trade-off between root and mycorrhizal surface for N- and P-nutrition in grassland species. Low soil N:P ratios may only allow for positive AMF effects in mycorrhizal strategists, whereas root strategists may experience negative effects, likely being linked to N-limitation in the AM-state.

Potential advantages of highly mycotrophic foraging for the establishment of early successional pioneer plants on sand

Adaptive traits ensuring efficient nutrient acquisition, such as extensive fine root systems, are crucial for establishment of pioneer plants on bare sand. Some successful pioneer species of temperate, European sand ecosystems are characterised as obligate mycorrhizals, thus likely substituting fine roots with arbuscular mycorrhizal fungi (AMF). However, it is not clear whether AM fungal-mediated acquisition of scarce and immobile nutrients such as phosphorus (P) is an advantageous strategy on bare sand over foraging via roots. We compared the foraging performance of three obligately mycorrhizal forbs and two facultatively mycorrhizal grasses, regarding the influence of AMF on their capacity to acquire P from bare sand. Comparison of mycorrhizal and non-mycorrhizal individuals revealed a markedly higher AM fungal-dependency for P acquisition and growth in the forbs than in the grasses. Periodical soil core sampling, allowing for assessment of root and hyphal growth rates, revealed hyphal growth to markedly enlarge the total absorptive surface area (SA) in the forbs, but not in the grasses. Correlations between SA growth and P depletion suggest an AM fungal-induced enhanced capacity for rapid soil P exploitation in the forbs. Our study showed that AM fungal-mediated foraging may be an advantageous strategy over root-mediated foraging in sand pioneer plants.

Temporal Dynamics in δ13C of Ecosystem Respiration in Response to Environmental Changes

Publisher Summary Global atmospheric carbon dioxide concentration, which is a primary greenhouse gas, has been rapidly increasing as a result of fossil fuel combustion, land use change, and biomass burning. The rising CO2 concentration has now been identified as a primary cause for climate change. Facing the current threats to the worlds climate, a better understanding of the terrestrial carbon cycle, that is, the processes of CO2 exchange between the bio sphere and the atmosphere is of utmost importance. Stable isotopes are sensible tracers for the human impact on the atmosphere, as the δ13CO2 released from fossil fuel sources is highly depleted in the heavy carbon isotope. Hence, the isotopic composition of carbon in the atmosphere and biosphere parallels current environmental changes. This chapter gives an overview of the dynamics of ecosystem productivity and δ13CR at annual, seasonal, and diurnal timescales for a case study of a Mediterranean evergreen oak woodland. These ecosystems are vulnerable to climate change and are endangered by desertification processes. Furthermore, they are particularly useful to evaluate the temporal dynamics in δ13CR (at timescales of hours to days) due to the pronounced seasonal environmental changes and its link to net ecosystem productivity. Particularly, it focuses on the impact of the vegetation on ecosystem respiration. This chapter presents a survey on the knowledge of the mechanisms of carbon isotope fractionation during dark respiration and discusses the implications of rapid dynamics in δ13CR at larger temporal and spatial scales.

Role of mycorrhization and nutrient availability in competitive interactions between the grassland species Plantago lanceolata and Hieracium pilosella

Arbuscular mycorrhizal fungi (AMF) may serve as an effective substitute for root surface. As mycorrhizal benefits are related to nutrient availability, the trade-off between carbon investments into AMF versus roots may drive competitive interactions. We studied competitive interactions between mycorrhizal and non-mycorrhizal individuals of Hieraciumpilosella L. and Plantagolanceolata L., species differing in both mycotrophic degree and carbon allocation to roots. Three fertilization treatments were used to simulate nutritional differences over the course of succession. Species-specific differences in mycotrophy were reflected in markedly larger root/shoot allocation in P. lanceolata and higher mycorrhizal growth dependency in H. pilosella. P. lanceolata dominated competition in all fertilizer treatments, enabled by its comparatively larger root biomass allocation. In contrast, under intermediate and high fertilization, H. pilosella exhibited large investments into clonal shoot growth rather than in roots. Unexpectedly, the competitive imbalance between both species was amplified by the presence of AMF. The poor competitive strength of H. pilosella indicates that AMF-dominated foraging can be less effective than root-dominated foraging in competitive interactions, particularly under high nutrient availabilities. However, the competitive imbalance was reduced in favor of H. pilosella under nutrient deficiency. Our results lend support to the idea of differing competitive success of mycorrhizal- versus root-based foraging strategy over a nutritional gradient, which may play a role in the natural distribution of species over the course of succession.

Consequences of Changing Precipitation Patterns for Ecosystem Functioning in Grasslands: A Review

Grassland ecosystems worldwide provide agricultural goods and important ecosystem services. Productivity and other ecosystem processes in grasslands are, in most cases, strongly linked to the ecosystems’ water status, a factor that is predicted to experience major alterations with global climate change. Future predictions include changes in the amount, distribution, frequency, and intensity of precipitation, which, particularly in grasslands, may have important consequences for ecosystem state and functioning. This review analyses the effects of experimental precipitation manipulation on plant productivity, species diversity, soil/ecosystem respiration, and soil nitrogen in grassland-type ecosystems over a wide range of climate types, synthesising the results from 72 studies.

Conditions Promoting Mycorrhizal Parasitism Are of Minor Importance for Competitive Interactions in Two Differentially Mycotrophic Species

Interactions of plants with arbuscular mycorrhizal fungi (AMF) may range along a broad continuum from strong mutualism to parasitism, with mycorrhizal benefits received by the plant being determined by climatic and edaphic conditions affecting the balance between carbon costs vs. nutritional benefits. Thus, environmental conditions promoting either parasitism or mutualism can influence the mycorrhizal growth dependency (MGD) of a plant and in consequence may play an important role in plant-plant interactions. In a multifactorial field experiment we aimed at disentangling the effects of environmental and edaphic conditions, namely the availability of light, phosphorus and nitrogen, and the implications for competitive interactions between Hieracium pilosella and Corynephorus canescens for the outcome of the AMF symbiosis. Both species were planted in single, intraspecific and interspecific combinations using a target-neighbor approach with six treatments distributed along a gradient simulating conditions for the interaction between plants and AMF ranking from mutualistic to parasitic. Across all treatments we found mycorrhizal association of H. pilosella being consistently mutualistic, while pronounced parasitism was observed in C. canescens, indicating that environmental and edaphic conditions did not markedly affect the cost:benefit ratio of the mycorrhizal symbiosis in both species. Competitive interactions between both species were strongly affected by AMF, with the impact of AMF on competition being modulated by colonization. Biomass in both species was lowest when grown in interspecific competition, with colonization being increased in the less mycotrophic C. canescens, while decreased in the obligate mycotrophic H. pilosella. Although parasitism-promoting conditions negatively affected MGD in C. canescens, these effects were small as compared to growth decreases related to increased colonization levels in this species. Thus, the lack of plant control over mycorrhizal colonization was identified as a possible key factor for the outcome of competition, while environmental and edaphic conditions affecting the mutualism-parasitism continuum appeared to be of minor importance.

Species-specific adaptations explain resilience of herbaceous understorey to increased precipitation variability in a Mediterranean oak woodland.

Abstract To date, the implications of the predicted greater intra‐annual variability and extremes in precipitation on ecosystem functioning have received little attention. This study presents results on leaf‐level physiological responses of five species covering the functional groups grasses, forbs, and legumes in the understorey of a Mediterranean oak woodland, with increasing precipitation variability, without altering total annual precipitation inputs. Although extending the dry period between precipitation events from 3 to 6 weeks led to increased soil moisture deficit, overall treatment effects on photosynthetic performance were not observed in the studied species. This resilience to prolonged water stress was explained by different physiological and morphological strategies to withstand periods below the wilting point, that is, isohydric behavior in Agrostis, Rumex, and Tuberaria, leaf succulence in Rumex, and taproots in Tolpis. In addition, quick recovery upon irrigation events and species‐specific adaptations of water‐use efficiency with longer dry periods and larger precipitation events contributed to the observed resilience in productivity of the annual plant community. Although none of the species exhibited a change in cover with increasing precipitation variability, leaf physiology of the legume Ornithopus exhibited signs of sensitivity to moisture deficit, which may have implications for the agricultural practice of seeding legume‐rich mixtures in Mediterranean grassland‐type systems. This highlights the need for long‐term precipitation manipulation experiments to capture possible directional changes in species composition and seed bank development, which can subsequently affect ecosystem state and functioning.

Overwhelming effects of autumn-time drought during seedling establishment impair recovery potential in sown and semi-natural pastures in Portugal

Climate change-induced alterations to precipitation may affect the regeneration dynamics of plant species, with the occurrence of drought influencing germination and seedling establishment. In Mediterranean grasslands, typically dominated by C3 annual species, germination occurs rapidly once sufficient rain falls in autumn. However, a single large precipitation event may be followed by a dry spell, with possible consequences for vegetation composition and productivity. We investigated the effects of autumn-time drought on germination and seedling establishment, and the subsequent recovery potential, in semi-natural and sown pastures. Although the majority of grasslands in Portugal are semi-natural, sowing legume-rich seed mixtures is an increasingly common agricultural practice in Portugal. Our results show an overwhelming effect of autumn-time drought on seedling establishment, with the semi-natural pasture being more compromised than the sown pasture. However, after the recovery period, the semi-natural pasture exhibited tendentially higher aboveground biomass recovery than the sown pasture. The differential species sensitivity to autumn-time drought and subsequent implications for recovery potential may be important for the agricultural practice of sown pastures, justifying additional research for drought-tolerant cultivar improvement to maintain productivity with climate change.