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Shrubs are expanding across a warming Arctic, evident from range expansion and increases in biomass, stature and cover. This influences numerous aspects of Arctic ecosystems. While shrub growth is generally positively associated with summer temperature, tundra ecosystems are characterised by abiotic gradients on small spatial scales (metres), and the Arctic climate and its year‐to‐year variability are changing rapidly. Hence, it is often unclear to what extent climate‐growth associations are scalable to future climate scenarios and across environmental gradients within ecosystems. Here, we investigate the stability of climate–growth associations of Arctic dwarf shrubs across small‐scale (metre to kilometre) topographic gradients and decadal timescales.We constructed ring width series (1974–2018) for a common Arctic dwarf shrub (Betula nana) for three representative types of subsites in the Siberian lowland tundra: higher elevation, lower elevation and thermokarst‐affected (thaw ponds) terrain. We quantified decadal variability in climate–growth associations across subsites using partial least squares regression and a moving window approach.We found consistently positive association of shrub radial growth with summer temperature, but substantial spatial and temporal variability in precipitation response. Association of shrub growth with summer rainfall increased in recent decades. Shrubs on elevated sites showed particularly strong response to rainfall following drier periods, and a negative association with recent snowfall extremes. Shrubs sampled from thaw ponds showed strong positive association with rainfall, followed by high shrub mortality after an extremely wet summer. This likely resulted from waterlogging due to thermokarst.Synthesis. Our findings imply that the response of shrub growth to changes in Arctic precipitation regimes is regulated by (i) macro‐ (kilometre‐scale) and micro‐topographical (metre‐scale) gradients, (ii) colimitation between temperature and moisture and (iii) potentially nonlinear responses to precipitation extremes. This suggests that the scalability of precipitation‐growth relationships for Arctic shrubs across dynamic tundra landscapes and future climate scenarios is limited. We recommend that future climate–growth studies on Arctic tundra shrubs simulate future precipitation changes across spatial gradients and include detailed microsite and shrub physiological monitoring. [ABSTRACT FROM AUTHOR]

Contrary to the general "greening of the Arctic", the Siberian Indigirka Lowlands show strong "browning" (a decrease in the Normalized Difference Vegetation Index or "NDVI") in various recent satellite records. Since greening and browning are generally indicative of increases and losses in photosynthetically active biomass, this browning trend may have implications for the carbon balance and vegetation of this Arctic tundra region. To explore potential mechanisms responsible for this trend break from general Arctic greening, we studied timeseries of Landsat summer maximum NDVI, weather data, and high‐resolution maps of vegetation compositional change, topography, geomorphology and hydrology. We find that a significant proportion of browning (lower summer NDVI) is explained by moisture dynamics, with high snow depths and resulting floods as well as summer drought coinciding with low NDVI. Relations between seasonal weather variables and NDVI are spatially heterogeneous, with floodplains, drained thaw lake basins and Yedoma ridges showing different patterns of association with weather variables. Low summer NDVI after high snowfall was particularly evident in floodplains, likely explained by early summer floods. Local small‐scale vegetation changes explained only small amounts of variance in browning rates in Landsat NDVI. Local expansion of Sphagnum vegetation in particular may have contributed to recent browning of our study site, but higher resolution NDVI timeseries are necessary to accurately constrain the role of small‐scale vegetation shifts. Overall, associations identified in this study suggest that future increases in Arctic precipitation variability and extremes may limit tundra greening, but to different extents even across comparatively small topographical contrasts. Plain Language Summary: Across Arctic regions, satellite images show that tundra ecosystems have been "greening". This suggests that plant growth, and thereby uptake of carbon from the atmosphere, is increasing in the Arctic. The Indigirka Lowlands in Siberia provide a stark contrast; in this lowland tundra region, satellite images show "browning", which suggests a decline in plant growth. We explored why this might be the case, in order to better assess whether browning should be expected across the wider Arctic in a changing climate. We compare local browning rates, calculated as changes in spectral indices from Landsat satellite images, to annual weather data, detailed maps of local vegetation changes and maps of terrain properties such as elevation and hydrology. We find that browning is associated with high snow depths and resulting floods, but also with dry summer conditions. Browning was not strongly related to local, small‐scale changes in vegetation (the most common of which was expansion of peat moss). We also find that the degree of association with potential drivers of browning (e.g., flooding, dry summers) differs across landforms. Floodplains show particularly strong browning following high snowfall, likely explained by early summer floods. This suggests that expected increases in year‐to‐year variability and extremes in Arctic precipitation may limit future greening. Key Points: Our study site in the Indigirka Lowlands shows strong (−0.0039 NDVI units yr−1) and ubiquitous (76% of area) recent browning in Landsat NDVIHigh snowfall and low summer rainfall were associated with lower summer NDVI, with differential impacts across topographical gradientsLocal small‐scale shifts in vegetation types and thermokarst activity show weak association with Landsat NDVI trends [ABSTRACT FROM AUTHOR]