Svetla Bratanova-Doncheva

Background Long-term ecosystem observation networks provide an essential basis for understanding ecosystem dynamics and environmental change. However, the increasing complexity of environmental and societal challenges requires research infrastructures that extend beyond long-term monitoring by integrating harmonised observations, FAIR data management, scientific services, interdisciplinary collaboration and stakeholder engagement. The development of the European eLTER Research Infrastructure (eLTER RI) has established a common conceptual and organisational framework for this transition while simultaneously evolving through the experience and contributions of national Long-Term Ecosystem Research (LTER) networks. Objective This paper analyses the transformation of LTER-Bulgaria from a national long-term ecosystem monitoring network into an emerging national research infrastructure aligned with the principles, standards and services of eLTER RI. Rather than presenting a chronological history, it examines the gradual co-evolution of the Bulgarian network and the developing European research infrastructure, highlighting how scientific vision, organisational development and international collaboration have jointly shaped this transformation over the past two decades. LTER-Bulgaria: from sites to scientific community Established in 2006, LTER-Bulgaria is a distributed national network comprising Long-Term Ecosystem Research (LTER) sites and Long-Term Socio-Ecological Research (LTSER) platforms representing the major ecosystem types of Bulgaria, including forest (Petrohan, Parangalitsa, Belasitza), freshwater (Mesta river), wetland (Srebarna), coastal zone (Sozopol), marine (Black sea) ecosystems. The network provides long-term observations of ecosystem structure, processes and dynamics across different biogeographical regions and supports multidisciplinary ecological research at local, regional and national scales. However, the significance of the network extends beyond its physical components. Sites, platforms, cross-site observation systems (mobile labs) and digital resources constitute its structural basis, whereas its functioning depends primarily on the scientific community that develops shared methodologies, harmonises observations, manages and shares data, conducts analyses and plugs gaps in the ecosystem coverage of the network, builds long-term collaborations and continuously expands research capacity. This interaction between structural development and scientific cooperation has been the principal driving force behind the evolution of LTER-Bulgaria. Transformation pathway The transformation of LTER-Bulgaria has been characterised by continuous adaptation rather than by a predefined institutional roadmap. From its inception, the long-term vision extended beyond ecosystem monitoring towards developing an infrastructure capable of supporting ecosystem research, knowledge generation and evidence-based environmental management. Participation in successive European projects and policy documents such as the Methodological framework for mapping and assessment of ecosystem condition and ecosystem services in Bulgaria (ref.) provided opportunities to introduce new scientific concepts, harmonised methodologies and collaborative practices, while progressively strengthening integration with the emerging eLTER RI and widening its recognition beyond the environmental monitoring community. An important milestone in this process was the inclusion of LTER-Bulgaria in the Bulgarian National Roadmap for Research Infrastructures, which created the institutional conditions necessary to strengthen governance, improve coordination and develop the functional components of the infrastructure. Rather than initiating the transformation, this support enabled the implementation of a scientific vision that had evolved progressively through long-term collaboration and accumulated experience. The transformation encompasses several interconnected dimensions, including the strengthening of the national scientific community, harmonisation of observation protocols, implementation of FAIR data principles, development of interoperable digital services, adoption of eLTER Standard Observations, integration of research data infrastructures and increasing participation in European collaborative initiatives. Consequently, LTER-Bulgaria evolved not only technically and organisationally but also conceptually—from a network focused primarily on long-term ecosystem monitoring towards an integrated research infrastructure supporting interdisciplinary research, open science and evidence-based decision making. "Research infrastructures are built with sites and platforms, but they become sustainable through scientific communities." Conclusions The experience of LTER-Bulgaria illustrates that the transition from a monitoring network to a research infrastructure is neither linear nor driven solely by technological development. Instead, it represents a long-term process of co-evolution between scientific communities, organisational capacities and European research initiatives. The transformation involved not only the development of structural components of a research infrastructure but also the strengthening of its functional dimension through governance, scientific cooperation, shared methodologies, data stewardship and capacity building. Perhaps the most significant outcome of this continuing transformation is the gradual development of a collaborative scientific community capable of sustaining long-term observations, adopting common standards and jointly developing research capacities within the evolving eLTER RI framework. Rather than representing the culmination of a twenty-year history, the current stage marks the beginning of a new phase in which LTER-Bulgaria continues its evolution as an active national component of the European eLTER Research Infrastructure, contributing to integrated ecosystem research and sustainable environmental management.

To study climate impacts, data integration from heterogeneous sources is imperative for long-term monitoring in data sparse areas such as the High Mountain Ecosystems in the Rila Mountain, Bulgaria—difficult to both access and observe remotely due to frequent clouds. This task is especially challenging because discerning trends in vegetation location, condition and functioning requires observing over decades. To integrate the existing sparse data, we apply the Whole System framework adapted nationally in the Bulgarian Methodological Framework for Mapping and Assessment of ecosystem services. As the framework mainly relies on field data, we complement it with remote sensing vegetation indices (NDVI, NDWI and NDGI) for 42 years, together with Copernicus High Resolution Layer products and climate change reanalysis data for 40 years. We confirmed that the Whole System framework is extensible and semantically, ontologically and methodologically well suited for heterogeneous data fusion, co-analysis, reanalysis and joint interpretation. We found trends in ecosystem extent and functioning, in particular species composition, in line with climate change trends since around 1990 and exclusively attributable to climate change since 2015. Furthermore, we specified a data crosswalk between habitats and ecosystems at Level 3 (ecosystem subtype), and define new candidate indicators suitable for remotely monitoring climate change’s effects on the ecosystems’ extent and condition, as candidates for inclusion in the methodological framework.

Identifying patterns in roots spatial distribution and dynamics, and quantifying the root stocks, annual production and turnover rates at species level is essential for understanding plant ecological responses to local environmental factors and climate change. We studied selected root traits in four different stands, two European beech (Fagus sylvatica L.) forests and two Douglas fir (Pseudotsuga menziezii Mirb. Franco) plantations. Root system vertical distribution and dynamics were studied using sequential coring method and characterised into three root diameter size classes (0–2, 2–5 and 5–10 mm) sampled at three different soil depths (0–15, 15–30, 30–45 cm). Root annual production and turnover rates were analysed and quantified using Decision Matrix and Maximum-Minimum estimation approaches. The overall root mass (−1 and 0.76 yr⁻¹ based on mean and maximum biomass data, respectively. They were similar to the averaged values suggested for Central and Northern European forests but higher compared to those reported from Southern Europe.

Effects of ozone on the sensitive tree species Fraxinus excelsior L. exposed to ambient air were investigated. The dynamics of photosynthesis, transpiration, stomatal conductance and the activity of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT) in three-year-old ash seedlings were studied during a four-month period (June–September). Seedlings were exposed to ambient ozone in an urban (the Central City Park of Sofia – Borisova Gradina) and a mountain (Plana Mountain) area in Bulgaria. The sites were located near climate monitoring stations, providing data on ozone concentrations and meteorological parameters. Ozone exposure at the mountain site (AOT40) was more than two times higher compared to the urban site. Significantly higher values of sun radiation, transpiration, stomatal conductance and enzyme activity at the mountain site were also observed. At the urban site higher values of temperature and air humidity were registered. Effects of the measured variables on ash seedlings were complex and interdependent. No direct effect of ozone concentration in ambient air on the leaf physiology and biochemistry could be proved. However, intensified SOD and CAT activity in the presence of elevated ozone suggested antioxidant reaction in response to ozone uptake.