Encroachment of shrubs into subalpine grasslands in the Pyrenees changes the plant-soil stoichiometry spectrum

Aims: shrub encroachment has been reported over a large proportion of the subalpine grasslands across Europe and is expected to have an important impact on the biogeochemical cycle of these ecosystems. We investigated the stoichiometric changes in the plant-soil system along the succession (e.g. increase in encroachment from unencroached grassland to mature shrubland) at two contrasting sites in the Pyrenees. - Methods: we analyzed the chemical composition (C, N,¹⁵N, P, K, Ca, Mg and Fe) in the soil and in the aboveground plant compartments (leaves, leaf-litter and stems) of the main herbaceous species and shrubs at three contrasting stages of the succession: unencroached grassland, young shrubland and mature shrubland. - Results: the plant-soil stoichiometry spectrum differed between the successional stages. Shrub encroachment generally increased the concentration of C and Ca and the C:N ratio and often reduced to concentrations of N, P and K in the leaves and leaf-litter, while several soil nutrient concentrations (N, P, K Ca and Mg) decreased. The stocks of C, N, P, Ca, and Mg in the total aboveground biomass increased with encroachment. - Conclusions: shrub encroachment favored the dominance of long-lived species with low concentrations of N and P in the plant-soil compartments, high C:nutrient ratios in the aboveground biomass and increase the uptake of N through ericoid or ectomycorrhizal fungi. We highlight the role of shrubs in the sequestration of C and nutrients through the allocation to the aboveground biomass. The changes in plant-soil elemental composition and stocks suggest a slowdown of the biogeochemical cycles in the subalpine mountain areas where shrub encroachment occurred

Plant community composition affects the species biogeochemical niche

Nutrients are essential for plant development, and their availability and stoichiometric ratios can influence the composition of plant communities. We investigated the possibility of the reverse influence: whether the conditions of contrasting species coexistence determine foliar element concentrations and plant stoichiometry, that is, species biogeochemical niche (BN). The experiment was conducted at the Ecological-Botanical Garden of the University of Bayreuth, Germany. We analyzed foliar element concentrations of two dwarf shrubs (Calluna vulgaris and Vaccinium myrtillus) and two grasses (Holcus lanatus and Arrhenatherum elatius) growing in different community compositions (monocultures and various mixed stands). Foliar nutrient concentrations and stoichiometry (taken as a proxy of species BN) were species specific; each species showed its own BN in all communities. Furthermore, V. myrtillus and H. lanatus species shifted their BN in response to changes in their community, accomplishing the "biogeochemical niche displacement" hypothesis. We conclude that plants can readjust their foliar element concentration if they grow in communities with contrasting plant composition, suggesting a differential use of element resources when the patterns of species coexistence change. These results also support the complementary niche hypothesis

GEOAI FOR MARINE ECOSYSTEM MONITORING: A COMPLETE WORKFLOW TO GENERATE MAPS FROM AI MODEL PREDICTIONS

Mapping and monitoring marine ecosystems imply several challenges for data collection and processing: water depth, restricted access to locations, instrumentation costs or weather constraints for sampling, among others. Nowadays, Artificial Intelligence (AI) and Geographic Information System (GIS) open source software can be combined in new kinds of workflows, to annotate and predict objects directly on georeferenced raster data (e.g. orthomosaics). Here, we describe and share the code of a generic method to train a deep learning model with spatial annotations and use it to directly generate model predictions as spatial features. This workflow has been tested and validated in three use cases related to marine ecosystem monitoring at different geographic scales: (i) segmentation of corals on orthomosaics made of underwater images to automate coral reef habitats mapping, (ii) detection and classification of fishing vessels on remote sensing satellite imagery to estimate a proxy of fishing effort (iii) segmentation of marine species and habitats on underwater images with a simple geolocation. Models have been successfully trained and the models predictions are displayed with maps in the three use cases

Shifts in the elemental composition of plants during a very severe drought

Diverse plant functions (e.g., growth, storage, defense and anti-stress mechanisms) use elements disproportionally. We hypothesized that plants growing under different abiotic and biotic conditions would shift their elemental compositions in response to a very severe drought. We tested this hypothesis by investigating the changes in foliar stoichiometry and species composition from a very severe drought. We also tested the effects of previous droughts (acclimation) on this response. Different species growing in the same community responded more similarly to a very severe drought than did individual species growing in different communities. The stoichiometric shifts were thus more community-dependent than species-dependent. The results also suggested that plants grown in monoculture were less stoichiometrically plastic during the drought than plants grown in a more diverse community. Previous exposure to long-term drought treatments in the same communities did not significantly affect the stoichiometric shifts during the new drought. Differential use of resources may have been responsible for these responses. Monocultured plants, which used the same resources in similar proportions, had more difficulty avoiding direct competition when the resources became scarcer. Moreover, each species tested had a particular elemental composition in all communities and climatic treatments. The differences in foliar elemental compositions were largest between plant functional groups (shrubs and grasses) and smallest among species within the same functional group. Global principal components analyses (PCAs) identified a general tendency for all species, independently of the community in which they grew, toward lower concentrations of K, N, P, Mg and S, and to higher concentrations of C and Fe as the drought advanced. This study has demonstrated the utility of analyses of differences and shifts in plant elemental composition for understanding the processes underlying the responses of plants to changes in biotic and abiotic environmental conditions

Different "metabolomic niches" of the highly diverse tree species of the French Guiana rainforests

Tropical rainforests harbor a particularly high plant diversity. We hypothesize that potential causes underlying this high diversity should be linked to distinct overall functionality (defense and growth allocation, anti-stress mechanisms, reproduction) among the different sympatric taxa. In this study we tested the hypothesis of the existence of a metabolomic niche related to a species-specific differential use and allocation of metabolites. We tested this hypothesis by comparing leaf metabolomic profiles of 54 species in two rainforests of French Guiana. Species identity explained most of the variation in the metabolome, with a species-specific metabolomic profile across dry and wet seasons. In addition to this "homeostatic" species-specific metabolomic profile significantly linked to phylogenetic distances, also part of the variance (flexibility) of the metabolomic profile was explained by season within a single species. Our results support the hypothesis of the high diversity in tropical forest being related to a species-specific metabolomic niche and highlight ecometabolomics as a tool to identify this species functional diversity related and consistent with the ecological niche theory

31P-NMR metabolomics revealed species-specific use of phosphorous in trees of a French Guiana rainforest

Productivity of tropical lowland moist forests is often limited by availability and functional allocation of phosphorus (P) that drives competition among tree species and becomes a key factor in determining forestall community diversity. We used non-target³¹P-NMR metabolic profiling to study the foliar P-metabolism of trees of a French Guiana rainforest. The objective was to test the hypotheses that P-use is species-specific, and that species diversity relates to species P-use and concentrations of P-containing compounds, including inorganic phosphates, orthophosphate monoesters and diesters, phosphonates and organic polyphosphates. We found that tree species explained the 59% of variance in ³¹P-NMR metabolite profiling of leaves. A principal component analysis showed that tree species were separated along PC 1 and PC 2 of detected P-containing compounds, which represented a continuum going from high concentrations of metabolites related to non-active P and P-storage, low total P concentrations and high N:P ratios, to high concentrations of P-containing metabolites related to energy and anabolic metabolism, high total P concentrations and low N:P ratios. These results highlight the species-specific use of P and the existence of species-specific P-use niches that are driven by the distinct species-specific position in a continuum in the P-allocation from P-storage compounds to P-containing molecules related to energy and anabolic metabolism

Impacts of global change on Mediterranean forests and their services

The increase in aridity, mainly by decreases in precipitation but also by higher temperatures, is likely the main threat to the diversity and survival of Mediterranean forests. Changes in land use, including the abandonment of extensive crop activities, mainly in mountains and remote areas, and the increases in human settlements and demand for more resources with the resulting fragmentation of the landscape, hinder the establishment of appropriate management tools to protect Mediterranean forests and their provision of services and biodiversity. Experiments and observations indicate that if changes in climate, land use and other components of global change, such as pollution and overexploitation of resources, continue, the resilience of many forests will likely be exceeded, altering their structure and function and changing, mostly decreasing, their capacity to continue to provide their current services. A consistent assessment of the impacts of the changes, however,remains elusive due to the difficulty of obtaining simultaneous and complete data for all scales of the impacts in the same forests, areas and regions. We review the impacts of climate change and other components of global change and their interactions on the terrestrial forests of Mediterranean regions, with special attention to their impacts on ecosystem services. Management tools for counteracting the negative effects of global change on Mediterranean ecosystem- services are finally discussed

Abiotic and biotic factors determining the nutrient stoichiometry of contrasting terrestrial ecosystems /

Todo en la tierra es química. Esta afirmación tiene profundas implicaciones para las interacciones ecológicas. Los organismos vivos promueven y controlan flujos de materia y energía entre la atmosfera, hidrosfera y litosfera; modificando la composición química de la tierra de muchas maneras diferentes. La estequiometría ecológica estudia el balance y el papel de múltiples elementos químicos en las interacciones ecológicas y nos ayuda a entender patrones y procesos en la naturaleza. Representa el enlace entre la biogeoquímica y el funcionamiento de los ecosistemas, permitiéndonos describir procesos a todos los niveles de organización biológica, desde estructuras sub-celulares a ecosistemas.En esta Tesis he usado la estequiometría ecológica para describir procesos a nivel de organismo y ecosistema en tres condiciones ambientales terrestres diferentes. La estequiometría de los autótrofos se establece cuando estos usan luz para fijar carbono (C) y simultáneamente asimilan nutrientes. Las plantas son capaces de almacenar nutrientes en la vacuola intracelular y en diferentes órganos, lo que hace que su estequiometría sea muy flexible (baja homeostasis) y se adapten a diferentes ambientes, incluyendo condiciones del suelo limitantes para el desarrollo de las plantas. También, la interacción planta suelo se puede explorar a través de la estequiometría foliar, ya que se ha demostrado en todos los ecosistemas terrestres que el N:P foliar esta correlacionado positivamente con el N:P del suelo, sugiriendo que es un buen indicador de la disponibilidad de nutrientes.Las adaptaciones de las plantas a condiciones limitantes de nutrientes en el suelo son comunes en todos los ecosistemas terrestres, como es la fijación de nitrógeno, la asociación con micorrizas, producción de fosfatasas o la reabsorción de nutrientes desde las hojas senescentes para el reciclado interno de nutrientes. La composición elemental de las especies es afectada por esas interacciones abióticas y bióticas, y el intercambio de elementos químicos entre las especies y el espacio abiótico determinaran la composición elemental de las diferentes partes del ecosistema.En el Capítulo 2 exploramos el efecto biótico de la composición de las comunidades sobre la composición química foliar de distintas especies vegetales, a través del nicho biogeoquímico de cada especie. Encontramos que cada especie presenta su propio nicho biogeoquímico y fueron capaces de reajustar su composición química foliar en respuesta a las diferentes condiciones bióticas. Concluimos que las plantas pueden reajustar su composición elemental foliar cuando crecen en comunidades con diferente composición de plantas, a través del desplazamiento del nicho biogeoquímico, sugiriendo un uso diferencial de los recursos cuando los patrones de coexistencia cambian. En el Capítulo 3 hemos explorado el cambio en la composición química del sistema planta-suelo debido a la expansión de arbustos en los pastizales subalpinos del Pirineo. Esta expansión representa la transición desde pastizales puros a matorrales. Los pastizales son un ecosistema dominado por especies de ciclo de vida corto, rápido intercambio de nutrientes entre los compartimientos planta-suelo, altas concentraciones de nitrógeno (N), fósforo (P) y potasio (K) en el sistema planta-suelo, con alta productividad pero capacidad limitada de acumulación de biomasa. En cambio, los matorrales se caracterizan por ser un ecosistema dominado por especies de ciclo de vida largo, con estrategias más conservativas, con un intercambio de nutrientes más lento (relación de C:nutrientes altos en la biomasa aérea y baja concentración de N y P en el sistema planta-suelo) y mayor almacenamiento de nutrientes en la biomasa aérea de las plantas. La matorralización incrementa la dependencia de la adquisición de nutrientes como el N a través de micorrizas desde los pastizales puros a matorrales. Todos los cambios en el almacenamiento y composición elemental del sistema planta-suelo a lo largo de la sucesión desde pastizales a matorrales sugiere una desaceleración del ciclo biogeoquímico en las áreas montañosas donde la expansión de arbustos está presente. En el Capítulo 4, describimos la distribución de C y los nutrientes más importantes para el desarrollo de las plantas (N, P, K) en el sistema planta-suelo de bosques tropicales maduros en suelos pobres de la Guyana Francesa. También estudiamos el proceso de reabsorción de nutrientes desde las hojas senescentes, un mecanismo de nutrición de las plantas para evitar la perdida de nutrientes poco estudiado en este ecosistema. Nuestros resultados muestran que el P es el elemento más escaso presente en hojas, hojarasca y suelo. Las eficiencias de reabsorción de K y P fueron más altas que la de N y la estacionalidad solo afecto la reabsorción de K. La reabsorción de P fue la única que mostró una correlación, aunque débil, con el P en el suelo (total y disponible). Las relaciones entre la reabsorción de nutrientes y los rasgos funcionales de las especies (tasa de crecimiento, densidad de madera, diámetro a la altura del pecho y el área foliar específica) fueron débiles y variaron dependiendo del nutriente, en tanto que la relación filogenética no explica la variabilidad en las eficiencias de reabsorción de nutrientes de las especies. Nuestros resultados sugieren que la alta reabsorción de K y P desde las hojas senescentes es una estrategia adaptativa de las especies que les permite lidiar con la escasez de estos nutrientes en el suelo. Asimismo, el nivel de inmovilización de los nutrientes en los compuestos foliares (N > P > K) parece determinar significativamente el proceso de reabsorción. Concluimos que la reabsorción de nutrientes desde las hojas senescentes es un proceso clave de las plantas para la conservación de nutrientes en los bosques tropicales de la Guyana Francesa, especialmente para K y P, elementos que presentan una disponibilidad baja en el suelo, y esta depende principalmente del material parental y del proceso de lixiviación.En resumen, en esta Tesis hemos demostrado como la composición elemental del sistema planta-suelo refleja procesos e interacciones ecológicas, como son las interacciones intra e inter específica entre plantas (Capítulo 2), procesos fisiológicos poco estudiados en las plantas como la reabsorción de nutrientes (Capítulo 4) y la importancia de los estudios de estequiometría para describir cambios a nivel de ecosistema y predecir escenarios futuros (Capítulo 3). Estos estudios aportan nuevos conocimientos en el campo de la estequiometría ecológica y resaltan la importancia de este enfoque en los estudios ecológicos.Everything on Earth is based on chemistry. This statement has profound implications for ecological interactions. Living organisms generate and control fluxes of energy and matter among the atmosphere, lithosphere and the hydrosphere, shaping the chemistry of the Earth in many different ways. Ecological stoichiometry aims to explore the balance and role of multiple chemical elements in ecological interactions and help us to understand patterns and processes in nature. It represents the link between the biogeochemistry and the ecosystems' function and allows to describe processes across different levels of biological organization, from cellular structures to ecosystems. In this Thesis I use ecological stoichiometry to describe processes at organism and ecosystem levels in three contrasting terrestrial environment conditions. Autotrophs' stoichiometry is established when these organism use light to fix carbon (C) and simultaneously assimilate nutrients. Plants are able to store nutrients in the cells' vacuole and in different organs, which make them highly flexible (less homeostatic) in terms of their elemental composition. This feature explains the high adaptability of plants to different environments, including soil nutrient limitation conditions. Furthermore, plant-soil interaction could be explored through the foliar stoichiometry, because it has been shown that the foliar N:P is positive correlated with the N:P of soil in all terrestrial ecosystem, suggesting that foliar stoichiometry is a good indicator of the resource availability. Plant adaptations to soil nutrient limiting conditions are quite common in all terrestrial ecosystems, such as nitrogen fixation, mycorrhiza association, production of phosphatases and nutrient resorption before leave abscission. The species' chemical composition is affected by all these abiotic and biotic interactions, and these exchange of chemical elements between the species and the abiotic part of the system determine the elemental composition of different components of the ecosystems. In Chapter 2, we explore the biotic effect of the community composition on the species foliar stoichiometry, taken as a proxy of the species' biogeochemical niche. We found that each species has its own biogeochemical niche and is able to readjust its chemical composition in response to different biotic conditions. We conclude that plants can readjust their foliar element composition when they grow in communities with contrasting plant composition through the biogeochemical niche displacement, suggesting a differential use of the resources when the patterns of species coexistence change. In Chapter 3 we explore the plant-soil stoichiometry changes due the shrub expansion into the subalpine grassland in the Pyrenees. Shrub expansion had a clear impact on the plant-soil stoichiometry spectrum. This expansion represents the transition from pure grassland to shrubland. The grassland is an ecosystem dominated by short-lived species, fast nutrient turnover between the plant-soil compartments, high nitrogen (N), phosphorus (P) and potassium (K) concentrations in the plant-soil system, high productivity but low biomass stocks. The shrubland is an ecosystem characterized by long-lived species with more conservative strategy, slow nutrient turnover (low N and P concentrations in the plant-soil compartments, high C:nutrient ratios in the aboveground biomass) and high stocks of C and nutrients in the plant aboveground biomass. Shrub encroachment increase the acquisition of N through mycorrhizal associations. The changes in storage and elemental composition of the plant soil system along the succession from grassland to shrubland suggests that there is a slowdown of the biogeochemical cycle in the subalpine mountain areas where shrub encroachment occurred. In the Chapter 4, we describe the distribution of C and the most important nutrients for the plant development (N, P, K) in the plant and soil compartments in old-growth tropical forests growing in nutrient-poor soil in French Guiana. We also studied the nutrient resorption from senescent leaves, a poorly explored mechanism that plants use to avoid losing nutrients in this ecosystem. Our results showed that P was the scarcest nutrient in the leaf, leaf-litter and soil. Resorption efficiencies were higher for K and P than for N, and only K resorption efficiency was affected by seasonality. P resorption showed a negative and weak correlation with P in soil (total and available). Relationships between nutrient resorption and species functional characteristics (growth rate, wood density, diameter at breast height and specific leaf area) were weak and varied among the nutrients, and phylogenetic relatedness did not account for the variability in resorption efficiencies. Our results suggest that high K and P resorption from senescent leaves is an adaptive strategy allowing species to cope with soil nutrient scarcity. Furthermore, the level of nutrient immobilization in foliar compounds (N > P > K) seem to significantly determine the resorption process. We conclude that nutrient resorption from senescent leaves is a key process for plants to conserve nutrients in tropical forests of French Guiana, especially for K and P, where soil availabilities are low and depend mainly on soil parent material and leaching process. To sum up, in this Thesis we have demostrated how the elemental composition of the plant-soil system reflects ecological interactions and processes, such as intra and inter specific plant interactions (Chapter 2), poorly explored physiological processes such as nutrient resorption (Chapter 4) and the importance of stoichiometry studies for describing changes at ecosystem level and predicting future scenarios (Chapter 3). These studies add new knowledge to the ecological stoichiometry field and highlights the importance of this approach in the ecological studies

Encroachment of shrubs into subalpine grasslands in the Pyrenees changes the plant-soil stoichiometry spectrum

Aims: shrub encroachment has been reported over a large proportion of the subalpine grasslands across Europe and is expected to have an important impact on the biogeochemical cycle of these ecosystems. We investigated the stoichiometric changes in the plant-soil system along the succession (e.g. increase in encroachment from unencroached grassland to mature shrubland) at two contrasting sites in the Pyrenees. - Methods: we analyzed the chemical composition (C, N,¹⁵N, P, K, Ca, Mg and Fe) in the soil and in the aboveground plant compartments (leaves, leaf-litter and stems) of the main herbaceous species and shrubs at three contrasting stages of the succession: unencroached grassland, young shrubland and mature shrubland. - Results: the plant-soil stoichiometry spectrum differed between the successional stages. Shrub encroachment generally increased the concentration of C and Ca and the C:N ratio and often reduced to concentrations of N, P and K in the leaves and leaf-litter, while several soil nutrient concentrations (N, P, K Ca and Mg) decreased. The stocks of C, N, P, Ca, and Mg in the total aboveground biomass increased with encroachment. - Conclusions: shrub encroachment favored the dominance of long-lived species with low concentrations of N and P in the plant-soil compartments, high C:nutrient ratios in the aboveground biomass and increase the uptake of N through ericoid or ectomycorrhizal fungi. We highlight the role of shrubs in the sequestration of C and nutrients through the allocation to the aboveground biomass. The changes in plant-soil elemental composition and stocks suggest a slowdown of the biogeochemical cycles in the subalpine mountain areas where shrub encroachment occurred