Séminaire Ecobio - Andreas Prinzing et Soumen Mallick (Ecobio) - Observatoire Des Sciences De L'Environnement De Rennes (OSERen)

Séminaire1: All organisms need phosphorous (P). Across tree species, foliar P content increases towards higher latitudes, but it remains unclear whether this is due to higher availability of P, or higher demand for P, and whether the pattern across species also exists within species. Trees interact through competition and through enemies and mutualists shared among phylogenetically proximate trees. Such local interactions might increase access to P but also loss of P. It remains unclear whether interactions with phylogenetically proximate tree neighbours facilitate or hinder the maintenance of high foliar P at high latitudes. We used pan-European forest-plot databases to study nutrient contents of six late-successional tree species, each being represented in hundreds of plots. We inferred P availability from continental soil maps and P demand from climatic conditions constraining vegetation period and from high P resorption efficiency. Within five of the six species we found that trees show increased foliar P contents towards higher latitude – but only when growing in a phylogenetically distant neighbourhood. This pattern always reflected constraining climatic conditions increasing P demand, and occasionally the availability of P in the soil. We suggest that in order to maintain high P contents, trees may associate with distant relatives at high latitudes and with close relatives at low latitudes.

Séminaire 2: Trees suffer leaf herbivory by insect enemies, primarily during a short period after budburst. In response to this enemy pressure, shifting phenology has been identified as a defence strategy. However, previous localised studies, limited by human constraints, could only explore variations among a small number of trees. Therefore, the ecological significance of phenological shifts at a larger scale and under extreme variation in herbivory remains unknown. Nevertheless, recent advancements in remote sensing allow investigations to extend to entire landscapes. Using satellite data, we investigated phenology and herbivory dynamics from 2017 to 2021 in mixed-oak forests spanning approximately 80 km2 in Franconia, Bavaria, Germany. Trees that experienced higher levels of herbivory in a given year showed delayed budburst in the following year, resulting in reduced herbivory pressure. These patterns remained consistent even under extreme herbivory events such as herbivore outbreak in 2019 and experimental exclusion of herbivores, and under extreme variation in herbivory resulting from contrasting the exceptionally high herbivory during the outbreak with extremely low levels due to experimental herbivore exclusion. Our results indicate that at a landscape scale, trees consistently use heightened herbivory as a cue to strategically delay phenology in the following year, and this adaptive strategy helps mitigate enemy pressure. We suggest that this dynamic defence might play a crucial role in maintaining population equilibrium between the trees and their insect enemies.