• Energy Research

Global climate change threatens ecosystem functioning worldwide. Forest ecosystems are particularly important for carbon sequestration, thereby buffering climate change and providing socio-economic services. However, recurrent stresses, such as heat waves, droughts and floods can affect forests with potential cascading effects on their carbon sink capacity, drought resilience and sustainability. Knowledge about the stress impact on the multitude of processes driving soil-plant-atmosphere interactions within these complex forest systems is widely lacking and uncertainty about future changes extremely high. Thus, forecasting forest response to climate change will require a dramatically improved process understanding of carbon and water cycling across various temporal (minutes to seasons) and spatial (leaf to ecosystem) scales covering atmosphere, biosphere, pedosphere and hydrosphere components. Many relevant processes controlling carbon and water exchange occur at small scales (e.g. rhizosphere, single leaf) with a high spatial and temporal variability, which is poorly constrained. However, interactions and feedback loops can be key players that amplify or dampen a system’s response to stress. Moreover, spatial and temporal scaling rules for these non-linear processes in structurally and functionally diverse ecosystems are unknown. Legacy effects, for example, altered response after previous stress and retarded recovery of forests after climate extremes, are not captured in state-of-the-art models. Currently, we are lacking the appropriate and interconnected measurement, data assimilation and modelling tools allowing for a comprehensive, real-time quantification of key processes at high spatio-temporal coverage in heterogeneous environments. Moreover, since climate impacts are highly unpredictable with respect to timing and location, future research will require novel mobile, easily deployable and cost-efficient approaches. ECOSENSE, therefore, assembles expertise from environmental and engineering sciences, both being excellently paired at the University of Freiburg. Our interdisciplinary research project will investigate all relevant scales in a next-generation ecosystem research assessment (ECOSENSE). Our vision is to detect and forecast critical changes in ecosystem functioning, based on the understanding of hierarchical process interaction. In the first phase, ECOSENSE will explore these process interactions by investigating pools and fluxes of water and carbon, i.e. CO2 exchange, isotope discrimination and volatile organic compounds (VOC), as well as stress indicators by remotely and in situ sensed chlorophyll fluorescence. To address these research tasks, ECOSENSE will develop, implement and test a distributed, autonomous, intelligent sensor network, based on novel microsensors tailored to the specific needs in remote and harsh forest environments. They will measure the spatio-temporal dynamics of ecosystem pools and fluxes in a naturally complex structured forest system with minimal physiological impact. Measured data will be transferred in real-time into a sophisticated database, which will be explored for process analysis, conducted by Artificial Intelligence and close to real-time process-based ecosystem models for now- and forecasting applications. Thereby, ECOSENSE will: i) break new ground for integrative ecosystem research by identifying hierarchies and interactions of abiotic and physiological processes of forest carbon and water exchange, ii) provide a profound understanding of complex ecosystem responses to environmental stressors and iii) enable the prediction of process-based alterations in ecosystem functioning and sustainability. Our novel ECOSENSE toolkit, tested and validated in controlled climate extreme experiments and our ECOSENSE Forest, will open new horizons for rapid assessment in vast and remote ecosystems. Thereby, ECOSENSE will allow for a unique avenue of data acquisition and, consequently, for unprecedented scale-crossing ecosystem understanding and modelling.

Mediterranean annual grasslands are species-diverse ecosystems of high economic and ecological value. CO2 and water fluxes in these grasslands are triggered by the first rains in autumn, after a long hot and dry summer. Climate change scenarios project altered rainfall patterns, such as prolonged dry season into the autumn, while simultaneously nitrogen (N) deposition is increasing globally. However, how these global change drivers will interact to affect Mediterranean grassland CO2, water fluxes and productivity is still unclear. In a greenhouse experiment, we subjected the seedbank of an annual Mediterranean grassland to a factorial treatment, by prolonging the dry season by 0 days (i.e. no autumn drought), 50 days and 100 days and crossing these drought treatments with two levels of N deposition: no N and N addition. A delayed onset of the rain season, i.e., a prolonged dry season, induced lower CO2 and water fluxes throughout the growing season and a lower aboveground biomass by the end of the study period. However, N addition attenuated the effects on NEE, Reco and GPP, but did not affect aboveground biomass or functional group composition. A prolonged dry season also lowered the productivity of forbs, the dominant functional group in our grassland. Our results anticipate important effects of interacting global change drivers on Mediterranean grassland functioning.

ABSTRACTIncreasing durations and frequencies of droughts under climate change endanger the sustainable functioning of forests worldwide. The admixture of species with complementary resource use may increase the resilience of forests towards drought; however, little is known about modifications of species interactions (i.e. facilitation and competition) by increasing drought severity in mixed forests. In particular, knowledge on the regulation of central ecohydrological processes, such as tree water fluxes, is lacking. Therefore, we conducted a literature review to assess the impact of species interactions on tree resilience (resistance + recovery) under increasing drought severity. The classification of studies into three drought classes suggested that beneficial species interactions, i.e. through improved water relations, were prevalent under mild droughts. However, with increasing drought, negative effects, such as interspecific competition, occurred. These negative interactions were prominent under extreme droughts, where even trees with complementary resource‐use strategies competed for water resources. Fewer data are available on recovery of water fluxes. The limited evidence supported the patterns observed for drought resistance, with facilitation and complementarity of species in mixtures enhancing tree recovery after moderate droughts. However, after extreme droughts, competition effects and reduced recovery for some species were observed, which can strongly compromise tree resilience. While we acknowledge the importance of mixed forests for biodiversity, ecosystem services or pest resistance, we caution that beneficial species interactions may shift under extreme droughts. Thus, there is an urgent need to investigate species interaction effects on resilience in more depth to adapt forest trees to increasing drought stress.

There are many reasons that the isotopic signature of deposited N is unlikely to be causing the declines in plant δ15N. Although our analysis of foliar δ15N was limited to after 1980, declines in foliar, tree-ring and sediment δ15N pre-date the onset of widespread inorganic N fertilizer use, as well as any timing ofshifts to more reduced forms of N in deposition. Also, despite variation in the isotopic signatures of N deposition and its sources, there is no evidence currently that the signature of atmospheric N deposition has been declining overtime. A global, comprehensive dataset on the signature of N deposition does not exist and would be helpful to generate. Yet, even if the isotopic signature of N deposition has been declining, changes in N availability can have a stronger influence on plant δ15N than the signature of added N. Fil: Peri, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones y Transferencia de Santa Cruz. Universidad Tecnológica Nacional. Facultad Regional Santa Cruz. Centro de Investigaciones y Transferencia de Santa Cruz. Universidad Nacional de la Patagonia Austral. Centro de Investigaciones y Transferencia de Santa Cruz; Argentina Fil: Guerrieri, Rossella. Centre for Ecological Research and Forestry Applications; España Fil: Gundale, Michael J.. Swedish University of Agricultural Sciences; Suecia Fil: Hietz, Peter. University of Natural Resources and Life Sciences; Austria Fil: Werner, Christiane. Albert Ludwigs University of Freiburg; Alemania Fil: Fang, Yunting. Chinese Academy of Sciences; República de China Fil: Elmore, Andrew J.. University of Maryland; Estados Unidos Fil: Nelson, David M.. University of Maryland; Estados Unidos Fil: Templer, Pamela H.. Boston University; Estados Unidos

Mediterranean ecosystems are threatened by climate change and shrub encroachment. An increase in shrub cover can intensify the competition for water, aggravating the impacts of drought on ecosystem functioning. The effects of shrubs can be positive or negative, depending on the shrub species and density. We used a Mediterranean cork oak (Quercus suber) woodland to investigate the effects of increasing gum rockrose (Cistus ladanifer) cover on leaf carbon assimilation (Amax) and tree growth. The experiment consisted of a gradient of shrub cover, with four treatments: control, LD, MD, and HD, with 0%, 25%, 45%, and maximum shrub cover (>90%), respectively. Increasing shrub cover significantly decreased Amax in trees from HD (−15%) compared to control treatment, with intermediate effects on trees from LD (−5%) and MD (−12%). There was a large variability in tree growth, resulting in no significant effects of shrub cover, despite higher trunk diameter relative increments in LD (+40%), and lower in MD (−17%) and HD (−32%) compared to the control. The results indicate that a dense shrub cover (>90%) affected cork oak carbon assimilation and growth, while a low-to-medium shrub cover (≤45%) only induced mild intermediate effects. This information is important for the effective management of shrub density to improve the health and productivity of cork oak woodlands.

Mechanisms controlling the successful invasion of resource demanding species into low-resource environments are still poorly understood. Well-adapted native species are often considered superior competitors under stressful conditions. Here we investigate the competitive ability of the resource demanding alien Acacia longifolia, which invades nutrient-poor Mediterranean sand dunes such as in coastal areas of Portugal. We explore the hypothesis that drought may limit invasion in a factorial competition experiment of the alien invasive versus two native species of different functional groups (Halimium halimifolium, Pinus pinea), under well-watered and drought conditions. Changes in biomass, allocation pattern, and N-uptake-efficiency (via 15N-labeling) indicated a marked drought sensitivity of the invader. However, highly efficient drought adaptations of the native species did not provide a competitive advantage under water limiting conditions. The competitive strength of H. halimifolium towards the alien invader under well-watered conditions turned into a positive interaction between both species under drought. Further, low resource utilization by native species benefited A. longifolia by permitting continued high nitrogen uptake under drought. Hence, the N-fixing invader expresses low plasticity by continuous high resource utilization, even under low resource conditions. The introduction of novel traits into a community like N-fixation and high resource use may promote A. longifolia invasiveness through changes in the physical environment, i.e., the water and nutrient cycle of the invaded sand dune system, thereby potentially disrupting the co-evolved interactions within the native plant community.