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Browning events in Arctic ecosystems: Diverse causes with common consequences [1]

['Gareth K. Phoenix', 'Plants Photosynthesis', 'Soil', 'School Of Biosciences', 'University Of Sheffield', 'Sheffield', 'United Kingdom', 'Jarle W. Bjerke', 'Norwegian Institute For Nature Research', 'Fram - High North Research Centre For Climate']

Date: 2025-02 

Abstract Arctic ecosystems are experiencing extreme climatic, biotic and physical disturbance events that can cause substantial loss of plant biomass and productivity, sometimes at scales of >1000 km2. Collectively known as browning events, these are key contributors to the spatial and temporal complexity of Arctic greening and vegetation dynamics. If we are to properly understand the future of Arctic terrestrial ecosystems, their productivity, and their feedbacks to climate, understanding browning events is essential. Here we bring together understanding of browning events in Arctic ecosystems to compare their impacts and rates of recovery, and likely future changes in frequency and distribution. We also seek commonalities in impacts across these contrasting event types. We find that while browning events can cause high levels of plant damage (up to 100% mortality), ecosystems have substantial capacity for recovery, with biomass largely re-established within five years for many events. We also find that despite the substantial loss of leaf area of dominant species, compensatory mechanisms such as increased productivity of undamaged subordinate species lessen the impacts on carbon sequestration. These commonalities hold true for most climatic and biotic events, but less so for physical events such as fire and abrupt permafrost thaw, due to the greater removal of vegetation. Counterintuitively, some events also provide conditions for greater productivity (greening) in the longer-term, particularly where the disturbance exposes ground for plant colonisation. Finally, we find that projected changes in the causes of browning events currently suggest many types of events will become more frequent, with events of tundra fire and abrupt permafrost thaw expected to be the greatest contributors to future browning due to their severe impacts and occurrence in many Arctic regions. Overall, browning events will have increasingly important consequences for ecosystem structure and function, and for feedback to climate.

Citation: Phoenix GK, Bjerke JW, Björk RG, Blok D, Bryn A, Callaghan TV, et al. (2025) Browning events in Arctic ecosystems: Diverse causes with common consequences. PLOS Clim 4(1): e0000570. https://doi.org/10.1371/journal.pclm.0000570 Editor: Jamie Males, PLOS Climate, UNITED KINGDOM OF GREAT BRITAIN AND NORTHERN IRELAND Published: January 27, 2025 Copyright: © 2025 Phoenix et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The Event Drivers of Arctic Browning workshop was part-funded by P3-Plant Production and Protection, University of Sheffield. AMC was supported by NERC grant NE/M016323/1. JWB & HT were supported by the Research Council of Norway (grant 287402) and FRAM – High North Research Centre for Climate and the Environment (grant 369910). RGB was supported by the Swedish Research Council (grant 2018-04202 and 2023-04048). HEE was supported by the NASA Award 80NSSC18K0446 (Biodiversity and Ecological Forecasting). ACM was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 869471. TVC wishes to acknowledge the EU for funding (INTERACT grant agreement 871120). MML was supported by the US National Science Foundation (grant PLR-1417745). Competing interests: The authors have declared that no competing interests exist.

Introduction In recent years, Arctic ecosystems have been increasingly exposed to a range of extreme disturbance events that can cause acute and extensive loss of plant biomass at tens-of-meters to landscape and greater scales [1–3]. These abrupt losses of biomass are termed “browning events”, and contribute to the broader phenomenon of reduced biomass and productivity known as “Arctic browning” [4,5] which is an important driver of tundra vegetation dynamics and of complexity within the more widespread Arctic “greening” trend [6–9]. Browning events in Arctic ecosystems are caused by numerous and diverse disturbances, including climatic (such as extreme winter warming, frost drought, and icing), biological (such as invertebrate and vertebrate herbivore outbreaks, and pathogens) and physical disturbances (such as fire, or abrupt thermal erosion from permafrost thaw). The resulting browning is important because of the large magnitude of damage and the consequences for ecosystem structure and function, including loss of biomass, changes in biodiversity and vegetation dynamics, and feedback to climate through altered carbon (C) and energy balance [2,10,11]. The consequences of some browning events for local people and their livelihoods can also be significant [12]. It is therefore of concern that most types of disturbances that cause browning are expected to become more frequent, largely due to the direct or indirect influences of climate change [3,13–15]. Because of the often-substantial impact and increasing importance of browning events, it is now timely to synthesise current understanding of these diverse phenomena [7,8]. Within the press-pulse framework [16], the acute events that cause the browning are classed as “pulse” disturbances. They are also often referred to as “extreme events”, especially when referring to climatic extremes [17]. These pulse disturbances contrast with “press” disturbances that occur gradually over a longer duration, so within this framework it is the gradual ‘press’ of climate change that make the pulse disturbances that cause browning to be more frequent and/or intense [16]. Ecologists recognise that pulse disturbances (extreme events) are of major importance across ecosystems globally, with substantial impacts that are often disproportionate to their short duration, including increased probability of pushing organisms past lethal thresholds [17,18]. Recent research recognises this importance across high latitude ecosystems including the Arctic and the Antarctic [7,19–21], the boreal biome and alpine ecosystems [7,22]. For ground-based ecosystem responses that are the focus of this review, we define the browning events that result from the pulse disturbances as “significant declines in biomass, productivity, or in situ indices of these (such as Normalized Difference Vegetation Index (NDVI)) that occur within a year”. Browning events also have a definition in remote sensing, being referred to as “spectral browning events that are short-term decreases in vegetation indices” [6]. We focus on the ground-based definition given our aim to review the ecosystem responses that occur. At the scale of the individual browning event, the capacity for significant disruption of productivity and vegetation dynamics has been documented in clear examples. For instance, in 2007 the Anaktuvuk River fire in Alaska burned more than 1000 km2 of tundra and the resulting release of C was of similar magnitude to the annual net C sink for the entire Arctic tundra biome [23]. A single Extreme Winter Warming event in Northern Scandinavia (2007) caused widespread plant mortality resulting in a 26% reduction in NDVI over more than 1400 km2 [24]. However, despite the capacity for substantial impacts, the spatial and temporally discrete nature of browning events means they are unlikely to be fully apparent in long-term trends of satellite data [8], and therefore connecting browning events on the ground to the temporal and spatial dynamics in pan-arctic greenness remains challenging [6]. Part of the challenge is that the level of understanding among different types of browning events varies greatly, and the magnitude, spatial distribution and scale of some event types is either poorly understood or not known, so their contribution to the larger scale patterns of Arctic browning and greenness dynamics remain unclear. This also causes problems for incorporating browning events into models and projections. None-the-less, should the influence of browning events over Arctic ecosystems continue to increase, adjustments will be needed to models that may otherwise overestimate the increase in Arctic biomass with climate change, and therefore the consequences for C sequestration and climate feedbacks [25,26]. So, while modelling and remote sensing approaches provide the necessary tools for upscaling impacts and making future predictions, on-the-ground ecological understanding (the focus of this review) is needed to characterise the mechanisms causing browning for inclusion in modelling efforts and to robustly capture their impacts across the Arctic. Given the increasing importance of, and research on, Arctic browning events, our review aims to (i) synthesise understanding of browning events and their underlying pulse disturbance drivers; (ii) identify impacts on biomass and C dynamics; (iii) compare commonalities and key contrasts between browning events to provide a broad-based understanding of browning on the ground; and (iv) highlight research gaps and knowledge needs, considering the varying levels of understanding of the different types of browning events. Our review synthesizes observational and experimental studies of browning events in Arctic and sub-Arctic tundra ecosystems. We do not cover high latitude forest ecosystems [7,27,28], but we do include tundra-type vegetation (i.e., functionally and compositionally ‘tundra’) when it is the dominant vegetation community within sparse sub-Arctic woodlands. Because of the large diversity of different Arctic browning events, we do not provide an exhaustive review of all known effects of each, but instead focus on their key features and attributes, and the responses most widely assessed (including plant mortality, productivity and C cycling) that then allow us to draw out commonalities and key contrasts.

Perspectives and future challenges While most browning events are either known or expected to be increasing in frequency due to ongoing climate change (Table 1), it remains unclear which events will be the most important for driving browning in the future. Here it is helpful to categorize events in terms of their size and spatial distribution. Some browning events are widespread and numerous throughout many regions of the Arctic; this especially applies to fire and abrupt permafrost thaw events [11,13]. While the spatial extent of individual permafrost thaw events may often be small when compared with other events considered here, their relatively frequent occurrence and wide distribution indicate that they will remain among the primary drivers of browning events at the pan-Arctic scale. Fires can affect a larger area with a single occurrence, and also occur over a large spatial domain. These factors, combined with their increasing frequency, suggest fires may also become the most important pan-Arctic driver of browning. Climatic and biotic events fit a category that can be described as often large in size (for an individual event), but either less common throughout the Arctic (biotic events) or common but currently restricted to some regions (as far as the evidence suggests, e.g., extreme winter warming). These events may be the main drivers of Arctic browning in particular regions but have no importance in others. Such regionally-limited events may spread as the Arctic continues to warm, but work is needed here to establish projections with confidence. At least for permafrost and fire, there are strong and coordinated efforts addressing uncertainties (e.g., [11,26,13,69]). In contrast, efforts to predict future changes in frequency and extent of climatic and biotic events are at an earlier stage, and currently there is a need for greater links between studies projecting these pulse disturbances and the resulting browning that can arise. Furthermore, pulse disturbances can interact (see [7]), for instance dead, dry, plant material from drought may promote fire, or fire may promote conditions for permafrost thaw. However, given most work focusses on one browning event type, more work is needed to understand the impacts of a future of interacting disturbances. Predicting the frequency and understanding the impacts of future browning events will be challenging [5,149]. These events are generally hard to study, occur abruptly and are difficult to predict; so being on site to study them in progress is difficult, especially in the vast expanse of the Arctic, where logistics pose an additional challenge to observations. Arctic research broadly should have much improved and genuine collaboration with Indigenous people and other Arctic residents from the onset, and the challenges posed in studying browning events will be better addressed with such an approach [152,153]. Furthermore, while remote sensing provides a powerful tool for studying browning events, there can be a disconnect between what is observed remotely and what is observed on the ground [6,9,154]. Additionally, the remotely-sensed signals of some browning events can be very similar (e.g., the damage resulting from frost drought, icing, and extreme winter warming), preventing attribution to a single causal mechanism, unless there are supporting ground observations or fine-scale meteorological data. To better address the challenges of understanding the future consequences of browning events in the Arctic, we propose the following priorities: (1). Characterise the baseline conditions - i.e., how frequent were browning events in the past, and what is their current frequency and extent geographically? This will ensure we can better understand how browning events have naturally influenced Arctic ecosystems, and therefore better understand the possible impacts of increases in frequency. Here historical remote sensing data can be used to link to known past browning events. When linked with climate data, greater understanding of the causes of browning can also be achieved [155]. People that live in areas of browning events are an important source of knowledge on historic frequency (see also point 6, collaboration with indigenous and local communities). (2). Quantify the links between pulse disturbances (extreme events) and press climate change, and therefore improve predictions of how browning event frequency or intensity will change. For some types of browning events, evidence and understanding is focussed in one or a few regions. There are large areas of the Arctic where it remains unclear which, if any, browning events occur on the ground (to “western” scientists at least, see point 6), and this restricts our ability to project event importance and spread in the future. Data to address this will need more sampling and observations in regions with low representation. Collection of this data can work best when done in standardized ways across locations (for example, the CAFF’s extreme event monitoring tool provides a helpful solution [156]). Better understanding of the climate/weather conditions that cause pulse browning events will facilitate modelling of future frequencies and intensities in a changing climate. This can be better informed also from greater understanding of the past base-line (point 1), and also with more experimental efforts, both outdoors (field experiments) and in climate-controlled chambers (see also point 3). (3). Examine how impacts of browning vary among vegetation types, plant functional types, populations, and life stages. The level of knowledge varies greatly among types of browning events. The issue of knowledge sometimes being limited to only some Arctic regions (point 2 above) also applies here, meaning that we do not know the consequences for all vegetation types in which browning events may occur. Collecting data from events when they occur provides an opportunity, because many events are at a scale that allows assessment of a number of different plant communities. Such work, should include more data on biodiversity-ecosystem structure responses because to date there has tended to be a greater focus on plant mortality and biomass loss, with less on compositional change. For data on responses at the level of species, functional types and life stages, growth chamber and experimental plot studies can be useful due to the large number of different species that can be included [157]. The experimental control possible in these more reductionist approaches can also provide data on what the thresholds are for different species to result in the mortality that creates the browning (especially for climatic events, e.g., how long do warm temperatures need to last to initiate premature bud burst in extreme winter warming events). (4). Test how browning affects the biogeochemical and biophysical feedbacks to climate. For many browning event types (especially climatic and biotic), biophysical feedbacks have been little studied, hindering our ability to incorporate these into Earth System Models. Understanding is better for fire and permafrost thaw, but all event types need more data. Addressing this data need could include more space-for-time studies, targeting specific neighbouring sites with and without recent browning events, and with data focussed on comparing specific biophysical feedbacks (e.g., changes in evapotranspiration, albedo, carbon loss). Data can be drawn from remote sensing and field-based instruments, depending on the needed resolution, accuracy and precision. For biogeochemical cycling, much more data is needed on below ground responses (especially for climatic and biotic events), given we should expect significant impacts on microbial communities and biogeochemical cycling resulting from the abrupt cut-off of C supply from plants, and impacts on ground temperature (loss of shading) and moisture (loss of plant transpiration). For many events, more data is also needed on the consequences for nutrient availability especially as this could impact recovery rates [158]. (5). Improve the inclusion of browning in models. This work should include bottom-up approaches, where environmental variables with quantifiable causal influence over the occurrence, rate and biogeochemical consequences are identified (e.g., [159]), with a focus on environmental variables that can be quantified across heterogeneous Arctic regions [149]. Data on bottom up controls include information on plant species identity both geographically (what species are where particular events occur, and their response and sensitivity in terms of thresholds that cause mortality), and topo-graphic controls that modify response. More data is needed on top-down climatic controls that create the conditions for the browning event (see points 2 and 3). (6). Collaboration and reciprocal knowledge sharing with Indigenous communities in browning research. Browning events can have direct consequences for Indigenous peoples of the Arctic through impacts on local infrastructure and natural resources. Acknowledging the collective benefit for including Indigenous perspectives, research should adopt recommendations for co-production with Indigenous peoples for improved recording, prediction and adaptation [153]. Such work should include Indigenous knowledge holders from the outset to inform research design, should promote Indigenous governance and self-determination in research, and leave a legacy of capacity building and training [160,161]. Use of standardised protocols for data collections (point 2) can apply here, but also Indigenous knowledge holders have a wealth of insight into occurrence of extreme events. Through reciprocal knowledge sharing, better understanding of which events are most impactful to livelihoods and resources should also be used to help set priorities for future research.

Conclusions The pulse disturbances that cause Arctic browning events are numerous and diverse, which presents challenges in understanding current and future browning and its consequences. First, we identified a suite of pulse disturbances that cause browning events that are most frequently observed with the greatest impacts (fire, abrupt permafrost thaw, extreme winter climatic events, invertebrate herbivore outbreaks). Data, modelling, or extrapolations from climate change projections indicate many pulse disturbances that cause browning are either already increasing or are likely to increase in the future. Second, we found that Arctic ecosystems show capacity for relatively rapid recovery from many browning events especially where only certain plant functional types are impacted and live biomass is left for re-growth. Third, we identified that for many (though not all) events, impacts on C sequestration can be lessened by compensatory mechanisms. In the longer term, evidence suggests that browning events can lead to greater biomass production, and - ultimately - become drivers of Arctic greening. Fourth, we identified six research priorities based on perceived knowledge gaps. Given that many types of browning events are already increasing in frequency, greater understanding is needed if we are to adequately predict the future for Arctic terrestrial ecosystems, consequences for biodiversity, feedbacks to climate, and ultimately the consequences for Arctic societies that rely on these landscapes.

Acknowledgments This review arose from the “Event Drivers of Arctic Browning” workshop, held at the University of Sheffield, UK, We thank Holly Croft, Chuixiang Yi and Jill Edmondson for helpful comments on the manuscript.

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