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Vegetation composition shifts, and in particular, shrub expansion across the Arctic tundra aresome of the most important and widely observed responses of high-latitude ecosystems to rapidclimate warming. These changes in vegetation potentially alter ecosystem carbon balances byaffecting a complex set of soil–plant–atmosphere interactions. In this review, we synthesize theliterature on (a) observed shrub expansion, (b) key climatic and environmental controls andmechanisms that affect shrub expansion, (c) impacts of shrub expansion on ecosystem carbonbalance, and (d) research gaps and future directions to improve process representations in landmodels. A broad range of evidence, including in-situ observations, warming experiments, andremotely sensed vegetation indices have shown increases in growth and abundance of woodyplants, particularly tall deciduous shrubs, and advancing shrublines across the circumpolar Arctic.This recent shrub expansion is affected by several interacting factors including climate warming,accelerated nutrient cycling, changing disturbance regimes, and local variation in topography andhydrology. Under warmer conditions, tall deciduous shrubs can be more competitive than otherplant functional types in tundra ecosystems because of their taller maximum canopy heights andoften dense canopy structure. Competitive abilities of tall deciduous shrubs vs herbaceous plantsare also controlled by variation in traits that affect carbon and nutrient investments and retentionstrategies in leaves, stems, and roots. Overall, shrub expansion may affect tundra carbon balancesby enhancing ecosystem carbon uptake and altering ecosystem respiration, and through complexfeedback mechanisms that affect snowpack dynamics, permafrost degradation, surface energybalance, and litter inputs. Observed and projected tall deciduous shrub expansion and thesubsequent effects on surface energy and carbon balances may alter feedbacks to the climatesystem. Land models, including those integrated in Earth System Models, need to account fordifferences in plant traits that control competitive interactions to accurately predict decadal- tocentennial-scale tundra vegetation and carbon dynamics.

Biodiversity conservation is often considered to be an important co-benefit of REDD+ and other mechanisms aiming to increase carbon in biomass and soil to mitigate climate change. This reasoning is based on the assumption that the level of biodiversity and ecosystem carbon are positively correlated. Firstly, however, studies have shown both positive and negative relationships. Secondly, incentives for additional ecosystem carbon do not trigger random or all potential changes in land-use, but often concentrate on one or a few specific changes that could have an opposite effect than the general trend indicates. Therefore, it is important to study biodiversity impacts of plausible measures to increase carbon. We obtained land-use scenarios on pathways to increase carbon based on 97 face-to-face interviews of local land-use experts in twelve landscapes in seven countries and five continents. We then conducted another set of face-to-face interviews with biodiversity experts yielding 2963 estimations concerning the value of land-use classes for 264 taxa of fauna and flora in these landscapes. We found positive carbon to biodiversity relationships in ten of the twelve landscapes. The biodiversity impacts of measures to increase carbon were positive in eleven of the twelve landscapes. Our results indicate that a random land-use change that increases biodiversity is also likely to increase carbon and vice versa.

Ecosystem carbon (C) uptake in terrestrial ecosystems has increased over the past five decades, but with large interannual variability (IAV). However, we are not clear on the attribution and the processes that control the long-term trend and IAV of land C uptake. Using atmospheric inversion net ecosystem exchange (NEE) data, we quantified the trend and IAV of NEE across the globe, the Northern Hemisphere (NH), and the Southern Hemisphere (SH), and decomposed NEE into carbon uptake amplitude and duration during each year from 1979–2013. We found the NH rather than the SH determined the IAV, while both hemispheres contributed equivalently to the global NEE trend. Different ecosystems in the NH and SH had differential relative contributions to their trend and IAV. The long-term trends of increased C uptake across the globe and the SH were attributed to both extended duration and increasing amplitude of C uptake. The shortened duration of uptake in the NH partly offsets the effects of increased NEE amplitude, making the net C uptake trend the same as that of the SH. The change in NEE IAV was also linked to changes in the amplitude and duration of uptake, but they worked in different ways in the NH, SH and globe. The fundamental attributions of amplitude and duration of C uptake revealed in this study are helpful to better understand the mechanisms underlying the trend and IAV of land C uptake. Our findings also suggest the critical roles of grassland and croplands in the NH in contributing to the trend and IAV of land C uptake.