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.