Our knowledge of stem secondary growth of arctic shrubs (a key component of tundra net primary production, NPP) is very limited. Here, we investigated the impact of the physical elements of the environment on shrub secondary growth by comparing annual growth rates of model species from similar habitats at contrasting altitude, microtopography, latitude, geographical location, and soil type, in both the sub- and High Arctic. We found that secondary growth has a modest sensitivity to the environment but with large differences among species. For example, the evergreen Cassiope tetragona is affected by altitude, microtopography, and latitude, whereas the evergreen Empetrum hermaphroditum has rather constant secondary growth in all environments. Deciduous species seem to be most affected by microtopography. Furthermore, the impact of the environment on secondary growth differed from the impact on primary growth (stem apical growth, stem length, and apical growth of stem plus leaves), in some cases even with opposite responses. Thus caution should be taken when estimating the impact of the environment on shrub growth from apical growth only. Integration of our data set with the (very limited) previously published information on secondary growth provides an overview of its contribution to NPP and annual growth rates for 9 arctic species at 18 sites in Sweden, Greenland, Svalbard, Alaska, and the Alps.
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