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Despite covering only 3 % of the planet’s land surface, peatlands store 30 % of the planet’s terrestrial carbon. The net greenhouse gas (GHG) emissions from peatlands depend on many factors but primarily soil temperature, vegetation composition, water level and drainage, and land management. However, many peatland models rely on water levels to estimate CH4 exchange, neglecting to consider the role of CH4 transported to the atmosphere by vegetation. To assess the impact of vegetation on the GHG fluxes of peatlands, we have developed a new model, Peatland-VU-NUCOM (PVN). The PVN model is a site-specific peatland CH4 and CO2 emissions model, able to reproduce vegetation dynamics. To represent dynamic vegetation, we have introduced plant functional types and competition, adapted from the NUCOM-BOG model, into the framework of the Peatland-VU model, a peatland GHG emissions model. The new PVN model includes plant competition, CH4 diffusion, ebullition, root, shoot, litter, exudate production, belowground decomposition, and aboveground moss development under changing water levels and climatic conditions. Here, we present the PVN model structure and explore the model's sensitivity to environmental input data and the introduction of the new vegetation competition schemes. We evaluate the model against observed chamber data collected at two peatland sites in the Netherlands to show that the model is able to reproduce realistic plant biomass fractions and daily CH4 and CO2 fluxes. We find that daily air temperature, water level, harvest frequency and height, and vegetation composition drive CH4 and CO2 emissions. We find that this process-based model is suitable to be used to simulate peatland vegetation dynamics and CH4 and CO2 emissions.

Plant traits reflect growth strategies and trade-offs in response to environmental conditions. Because of climate warming, plant traits might change, altering ecosystem functions and vegetation–climate interactions. Despite important feedbacks of plant trait changes in tundra ecosystems with regional climate, with a key role for shrubs, information on responses of shrub functional traits is limited. Here, we investigate the effects of experimentally increased permafrost thaw depth and (possibly thaw-associated) soil nutrient availability on plant functional traits and strategies of Arctic shrubs in northeastern Siberia. We hypothesize that shrubs will generally shift their strategy from efficient conservation to faster acquisition of resources through adaptation of leaf and stem traits in a coordinated whole-plant fashion. To test this hypothesis, we ran a 4 year permafrost thaw and nutrient fertilization experiment with a fully factorial block design and six treatment combinations – permafrost thaw (control, unheated cable, heated cable) × fertilization (no nutrient addition, nutrient addition). We measured 10 leaf and stem traits related to growth, defence and the resource economics spectrum in four shrub species (Betula nana, Salix pulchra, Ledum palustre and Vaccinium vitis-idaea), which were sampled in the experimental plots. The plant trait data were statistically analysed using linear mixed-effect models and principal component analysis (PCA). The response to increased permafrost thaw was not significant for most shrub traits. However, all shrubs responded to the fertilization treatment, despite decreased thaw depth and soil temperature in fertilized plots. Shrubs tended to grow taller but did not increase their stem density or bark thickness. We found a similar coordinated trait response for all four species at leaf and plant level; i.e. they shifted from a conservative towards a more acquisitive resource economy strategy upon fertilization. In accordance, results point towards a lower investment into defence mechanisms, and hence increased shrub vulnerability to herbivory and climate extremes. Compared to biomass and height only, detailed data involving individual plant organ traits such as leaf area and nutrient contents or stem water content can contribute to a better mechanistic understanding of feedbacks between shrub growth strategies, permafrost thaw and carbon and energy fluxes. In combination with observational data, these experimental tundra trait data allow for a more realistic representation of tundra shrubs in dynamic vegetation models and robust prediction of ecosystem functions and related climate–vegetation–permafrost feedbacks.

It is unclear whether plant trait relationships found at the global scale extend to climatic extremes. Here the authors analyse six major aboveground traits to show that known plant trait relationships extend to the tundra biomes and exhibit the same two dimensions of variation detected at the global scale.

Over the past decades, vegetation and climate have changed significantly in the Arctic. Deciduous shrub cover is often assumed to expand in tundra landscapes, but more frequent abrupt permafrost thaw resulting in formation of thaw ponds could lead to vegetation shifts towards graminoid-dominated wetland. Which factors drive vegetation changes in the tundra ecosystem are still not sufficiently clear. In this study, the dynamic tundra vegetation model, NUCOM-tundra (NUtrient and COMpetition), was used to evaluate the consequences of climate change scenarios of warming and increasing precipitation for future tundra vegetation change. The model includes three plant functional types (moss, graminoids and shrubs), carbon and nitrogen cycling, water and permafrost dynamics and a simple thaw pond module. Climate scenario simulations were performed for 16 combinations of temperature and precipitation increases in five vegetation types representing a gradient from dry shrub-dominated to moist mixed and wet graminoid-dominated sites. Vegetation composition dynamics in currently mixed vegetation sites were dependent on both temperature and precipitation changes, with warming favouring shrub dominance and increased precipitation favouring graminoid abundance. Climate change simulations based on greenhouse gas emission scenarios in which temperature and precipitation increases were combined showed increases in biomass of both graminoids and shrubs, with graminoids increasing in abundance. The simulations suggest that shrub growth can be limited by very wet soil conditions and low nutrient supply, whereas graminoids have the advantage of being able to grow in a wide range of soil moisture conditions and have access to nutrients in deeper soil layers. Abrupt permafrost thaw initiating thaw pond formation led to complete domination of graminoids. However, due to increased drainage, shrubs could profit from such changes in adjacent areas. Both climate and thaw pond formation simulations suggest that a wetter tundra can be responsible for local shrub decline instead of shrub expansion.

Vegetation changes, such as shrub encroachment and wetland expansion, have been observed in many Arctic tundra regions. These changes feed back to permafrost and climate. Permafrost can be protected by soil shading through vegetation as it reduces the amount of solar energy available for thawing. Regional climate can be affected by a reduction in surface albedo as more energy is available for atmospheric and soil heating. Here, we compared the shortwave radiation budget of two common Arctic tundra vegetation types dominated by dwarf shrubs (Betula nana) and wet sedges (Eriophorum angustifolium) in North-East Siberia. We measured time series of the shortwave and longwave radiation budget above the canopy and transmitted radiation below the canopy. Additionally, we quantified soil temperature and heat flux as well as active layer thickness. The mean growing season albedo of dwarf shrubs was 0.15 ± 0.01, for sedges it was higher (0.17 ± 0.02). Dwarf shrub transmittance was 0.36 ± 0.07 on average, and sedge transmittance was 0.28 ± 0.08. The standing dead leaves contributed strongly to the soil shading of wet sedges. Despite a lower albedo and less soil shading, the soil below dwarf shrubs conducted less heat resulting in a 17 cm shallower active layer as compared to sedges. This result was supported by additional, spatially distributed measurements of both vegetation types. Clouds were a major influencing factor for albedo and transmittance, particularly in sedge vegetation. Cloud cover reduced the albedo by 0.01 in dwarf shrubs and by 0.03 in sedges, while transmittance was increased by 0.08 and 0.10 in dwarf shrubs and sedges, respectively. Our results suggest that the observed deeper active layer below wet sedges is not primarily a result of the summer canopy radiation budget. Soil properties, such as soil albedo, moisture, and thermal conductivity, may be more influential, at least in our comparison between dwarf shrub vegetation on relatively dry patches and sedge vegetation with higher soil moisture.

Deciduous shrubs are expected to rapidly expand in the Arctic during the coming decades due to climate warming. A transition towards more shrub-dominated tundra may have large implications for the regional surface energy balance, permafrost stability and carbon storage capacity, with consequences for the global climate system. However, little information is available on the natural long-term shrub growth response to climatic variability. Our aim was to determine the climate factor and time period that are most important to annual shrub growth in our research site in NE-Siberia. Therefore, we determined annual radial growth rates in Salix pulchra and Betula nana shrubs by measuring ring widths. We constructed shrub ring width chronologies and compared growth rates to regional climate and remotely sensed greenness data. Early summer temperature was the most important factor influencing ring width of S. pulchra (Pearson's r = 0.73, p < 0.001) and B. nana (Pearson's r = 0.46, p < 0.001). No effect of winter precipitation on shrub growth was observed. In contrast, summer precipitation of the previous year correlated positively with B. nana ring width (Pearson's r = 0.42, p < 0.01), suggesting that wet summers facilitate shrub growth in the following growing season. S. pulchra ring width correlated positively with peak summer NDVI, despite the small coverage of S. pulchra shrubs (< 5 % surface cover) in our research area. We provide the first climate-growth study on shrubs for Northeast Siberia, the largest tundra region in the world. We show that two deciduous shrub species with markedly different growth forms have a similar growth response to changes in climate. The obtained shrub growth response to climate variability in the past increases our understanding of the mechanisms underlying current shrub expansion, which is required to predict future climate-driven tundra vegetation shifts.