Increases in dissolved organic carbon (DOC) have occurred in many freshwaters across Europe and North America over the last decades. Several mechanisms have been proposed to explain these trends, but consensus regarding the relative importance of recovery from acid deposition, climate change, and land management remains elusive. To advance our understanding of browning mechanisms, we explored DOC trends across 13 nested boreal catchments, leveraging concurrent hydrological, chemical, and terrestrial ecosystem data to quantify the contributions of different drivers on observed trends. We first identified the environmental factors related to DOC concentrations, then attributed the individual trends of DOC to potential drivers across space and time. The results showed that all catchments exhibited increased DOC trends from 2003 to 2021, but the DOC response rates differed five-fold. No single mechanism can fully explain the ongoing browning, instead the interaction of sulfate deposition, climate-related factors and site properties jointly controlled the variation in DOC trends. Specifically, the long-term increases in DOC were primarily driven by recovery from sulfate deposition, followed by terrestrial productivity, temperature, and discharge. However, catchment size and landcover type regulated the response rate of DOC trends to these drivers, creating the spatial heterogeneity in browning among the sub-catchments under similar deposition and climate forcing. Interestingly, browning has weakened in the last decade as sulfate deposition has fully recovered and other current drivers are insufficient to sustain the long-term trends. Our results highlight that multifaceted, spatially structured, and nonstationary drivers must be accounted for to predict future browning.

Since this is a HP today contribution I understand that it should not have an 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.