Abstract
Improving our ability to detect changes in terrestrial and aquatic systems is a grand challenge in the environmental sciences. In a world experiencing increasingly rapid rates of climate change and ecosystem transformation, our ability to understand and predict how, when, where and why changes occur is essential for adapting and mitigating human behaviors. In this context, long-term field research infrastructures have a fundamentally important role to play. For northern boreal landscapes, the Krycklan Catchment Study (KCS) has supported monitoring and research aimed at revealing these changes since it was initiated in 1980. Early studies focused on forest regeneration and microclimatic conditions, nutrient balances and forest hydrology, which included monitoring climate variables, water balance components, and stream water chemistry. The research infrastructure has expanded over the years to encompass a 6790 ha catchment, which currently includes 10 gauged streams, ca. 1000 soil lysimeters, 150 groundwater wells, >500 permanent forest inventory plots, and a 150 meter tall tower (a combined ecosystem-atmosphere station; ICOS, Integrated Carbon Observation System) for measurements of atmospheric gas concentrations and biosphere-atmosphere exchanges of carbon, water, and energy. In addition to field infrastructures, the KCS has also been the focus of numerous high resolution multi-spectral LiDAR measurements. This large collection of equipment and data generation supports a range of disciplinary studies, but more importantly fosters multi-, trans-, and interdisciplinary research opportunities. The KCS attracts a broad collection of scientists, including biogeochemists, ecologists, foresters, geologists, hydrologists, limnologists, soil scientists and social scientists, and many others bringing their knowledge and experience to the site. The combination of long-term monitoring, shorter-term research projects, and large-scale experiments, including manipulations of climate and various forest management practices have contributed much to our understanding of the boreal landscapes functioning, while also supporting the development of models and guidelines for research, policy and management.

1.0 Introduction

Water balance, carbon dynamics, and the ecological integrity of northern regions are expected to change in response to global warming. This, in combination with a growing human population, increased environmental pressure, and more intensive resource extraction will amplify the stressors imposed on terrestrial and freshwater resources in the north. Such changes may be gradual or abrupt, but in any case, the outcome is likely to be unexpected, owing to nonlinearities in catchment-scale storage and release of water, carbon, nutrients, and a wide range of additional processes. Already, a cascade of changes to northern landscapes have been observed in response to shorter and milder winters (Spence et al. 2015) and enhanced summer warming (Isles et al. 2016). Understanding and predicting how this trajectory toward a warmer climate will reshape the physical, chemical, and ecological properties of the natural environment at northern latitudes will be a critical challenge for the scientific community in the decades to come.
High quality empirical data of adequate spatial and temporal resolution are central for deciphering patterns, dynamics, and trends in environmental variables. Catchment hydrology, biogeochemical processes, and landscape carbon balance are inherently complex and often scale dependent, making reliable predictions of future conditions difficult (Laudon and Sponseller, 2018). Sound predictions are even more challenging in regions where appropriate empirical data to develop, test, and validate models are sparse. Despite the undeniable value of field observations and experiments, a transition from field-based empirical studies to model-only approaches has been an accelerating trend in environmental science (Burt and McDonnell, 2015). This trend is especially noticeable at northern latitudes, where the number of long-term research sites have declined rapidly during the last decades, leaving only a few locations with enough field data generated to answer the most pertinent questions about the future of our water resources (Laudon et al. 2017).
Most expected changes in response to anthropogenic forcing will be superimposed on natural variability that can mask responses and make changes difficult to detect and mechanistically explain. Unravelling the mechanisms responsible for various degrees of environmental perturbations in northern ecosystems therefore requires more than just basic monitoring information, standard models, and insights from other regions. The boreal region is dominated by nutrient limited forests and peatlands representing large carbon stores (Loisel et al. 2014) that contain at least a third of the Earth’s soil carbon pool (Bradshaw et al. 2015). Despite their global importance and vulnerability to ongoing climate change, boreal catchments have been subject to comparatively little experimental, integrative, and process-oriented research in the past. This knowledge-gap is of concern, yet at the same time represent a tremendeous opportunity for cutting edge research on global warming feedbacks to both the atmosphere and water resources.
Most stream hydrological and biogeochemical research is based on individual, well-studied catchments, or alternatively on data from regional monitoring datasets. While a major advantage of small research catchments is the large amount of ancillary data that can provide mechanistic insights, a disadvantage is that the results are often based on limited replication and provide poor geographic representation. Conversely, a limitation of environmental monitoring is that data collection is often not designed to answer process-oriented questions, which can make it difficult to infer causal relationships. To overcome the constraints of both approaches, one way forward is to combine the strengths of these two approaches into a framework that promotes basic research in long-term monitored catchments (Tetzlaff et al. 2018), especially when they include several catchments of different scales and land-use (Laudon and Sponseller, 2018). The boreal landscape heterogeneity provides a unique template in this context because of the large spatial variability in the coverage of forest and peatlands that regulate much of the spatial and temporal complexity of soils, hydrology and biogeochemistry (Laudon et al. 2011; Fork et al. 2020). The study of stream networks also makes it possible to ascertain the influence of more seldomly studied headwaters to downstream ecosystems (Bishop et al., 2008)
Understanding the role of thresholds, tipping points, and other forms of nonlinearity is crucial for assessing the consequences of future environmental change. Detecting these responses requires field studies that go well beyond standard, disciplinary approaches. Instead, what is needed for solving future challenges is research infrastructures that combine field measurements from a multitude of disciplines in the same catchment. This type of effort goes well beyond what one research project or research group can achieve, and requires a well-coordinated infrastructure that can support and combine the collection of critical long-term monitoring data, provide large sets of ancillary empirical information, and host complementary long-term/large-scale experiments that are crucial to achieve mechanistic understanding of ecosystem responses to environmental change.
While long-term, process-based research at the landscape scale is clearly needed to address the influence of environmental perturbation on terrestrial and aquatic resources, few research sites exist that capture the spatial and temporal dimensions required. One existing example is the Krycklan Catchment Study (KCS), located in northern Sweden, which has provided a unique opportunity for integrated, process-based research in the boreal region for decades (Laudon et al. 2013). The over-arching objectives of KCS are to (1) provide a state-of-the-art infrastructure for experimental and hypothesis driven research, (2) maintain a collection of high quality, long-term climatic, biogeochemical, hydrological, and other environmental data, and (3) support the development of models and guidelines for research, policy and management. An important step in this direction is to make the field infrastructure even more visible for potential users at the same time as we make more of the field data accessible.

2.0 Site description

The Krycklan Catchment Study (KCS,www.slu.se/Krycklan) is located in the heart of the boreal landscape (64˚, 14’ N, 19˚ 46’ E), approximately 50 km northwest of the city of Umeå in northern Sweden (Laudon et al. 2013). The KCS includes 6790 ha land that comprises a mosaic of instrumented and well-studied forests, wetlands, and lakes, all drained and connected by a network of streams and rivers (Fig. 1). Over the past 35 years, the existing field research infrastructure has generated data resulting in approximately 1000 peer-reviewed publications, and over 110 PhD theses in diverse areas such as hydrology, biogeochemistry, carbon dynamics, climate change, geography, soil science, ecology, forestry, land-use history, and political science.
Forestry and peatland research began in area in the 1910s, with the first field station being built in 1923 at the Kulbäcksliden research park (Grip, 2015). Forest regeneration was a primary motivation for establishing the nearby Svartberget research station in the center of the KCS in 1979. The specific questions at that time were primarily related to microclimatic conditions after forest harvesting, nutrient limitation to tree growth, and forest hydrology. Ten years later, the role of acid deposition and natural acidity became the major water related research question at the site (Bishop et al., 1990). The combined focus on climate, forestry, and hydrology resulted in a wide range of high quality field measurements that are now of particular importance for documenting responses to current environmental changes.