Tundra is primarily a habitat for shrub growth, not trees, but growth of prostrate forms of trees has been reported occasionally from the subarctic tundra region. In the light of on-going climate change, climate sensitivity studies of these unique trees are essential to predict vegetation dynamics and potential northward expansion of boreal forest tree species into tundra. Here we studied one of the northernmost Larix Mill. trees and Betula nana L. shrubs (72 degrees N) from the Siberian tundra for the common period 1980-2017. We took advantage of the discovery of a single cohort of prostrate Larix trees within a tundra ecosystem, i.e., ca. 60 km northwards from the northern treeline, and compared climate-growth relationships of the two species. Both woody plants were sensitive to the July temperature, however this relationship was stable across the entire study period (1980-2017) only for Betula nana chronology. Additionally, radial growth of Larix trees became negatively correlated to temperatures during the previous summer. In recent period moisture sensitivity between Larix trees and Betula nana shrubs was contrasting, with generally wetter soil conditions favoring Larix trees growth and dryer conditions promoting Betula nana growth. Our study indicates that Larix trees radial growth in recent years is more sensitive to moisture than to summer air temperatures, whereas temperature sensitivity of Betula nana shrub is stable over time. We provide first detailed insight into the annual resolution on Larix tree growth sensitivity to climate in the heart of the tundra. The potentially higher Betula nana shrub resistance to warmer and drier climate versus Larix trees on a tundra revealed in our study needs to be further examined across habitats of various soil, moisture and permafrost status.

Broad-scale changes in arctic-alpine vegetation and their global effects have long been recognized and labeled one of the clearest examples of the terrestrial impacts of climate change. Arctic-alpine dwarf shrubs are a key factor in those processes, responding to accelerated warming in complex and still poorly understood ways. Here, we look closely into such responses of deciduous and evergreen species, and for the first time, we make use of high-precision dendrometers to monitor the radial growth of dwarf shrubs at unprecedented temporal resolution, bridging the gap between classical dendroecology and the underlying growth physiology of a species. Using statistical methods on a five-year dataset, including a relative importance analysis based on partial least squares regression, linear mixed modeling, and correlation analysis, we identified distinct growth mechanisms for both evergreen (Empetrum nigrum ssp. hermaphroditum) and deciduous (Betula nana) species. We found those mechanisms in accordance with the species respective physiological requirements and the exclusive micro-environmental conditions, suggesting high phenotypical plasticity in both focal species. Additionally, growth in both species was negatively affected by unusually warm conditions during summer and both responded to low winter temperatures with radial stem shrinking, which we interpreted as an active mechanism of frost protection related to changes in water availability. However, our analysis revealed contrasting and inter-annually nuanced response patterns. While B. nana benefited from winter warming and a prolonged growing season, E. hermaphroditum showed high negative sensitivity to spring cold spells after an earlier growth start, relying on additional photosynthetic opportunities during snow-free winter periods. Thus, we conclude that climate-growth responses of dwarf shrubs in arctic-alpine environments are highly seasonal and heterogenic, and that deciduous species are overall likely to show a positive growth response to predicted future climate change, possibly dominating over evergreen competitors at the same sites, contributing to the ongoing greening trend.

Climate change is expected to cause extensive vegetation changes in the Arctic: deciduous shrubs are already expanding, in response to climate warming. The results from transect studies suggest that increasing shrub cover will impact significantly on the surface energy balance. However, little is known about the direct effects of shrub cover on permafrost thaw during summer. We experimentally quantified the influence of Betula nana cover on permafrost thaw in a moist tundra site in northeast Siberia with continuous permafrost. We measured the thaw depth of the soil, also called the active layer thickness (ALT), ground heat flux and net radiation in 10 m diameter plots with natural B. nana cover (control plots) and in plots in which B. nana was removed (removal plots). Removal of B. nana increased ALT by 9% on average late in the growing season, compared with control plots. Differences in ALT correlated well with differences in ground heat flux between the control plots and B. nana removal plots. In the undisturbed control plots, we found an inverse correlation between B. nana cover and late growing season ALT. These results suggest that the expected expansion of deciduous shrubs in the Arctic region, triggered by climate warming, may reduce summer permafrost thaw. Increased shrub growth may thus partially offset further permafrost degradation by future temperature increases. Permafrost models need to include a dynamic vegetation component to accurately predict future permafrost thaw.