Stem Secondary Growth of Tundra Shrubs: Impact of Environmental Factors and Relationships with Apical Growth

Abstract

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.

Acknowledgments

This work was supported by Methusalem funding (OEC ECO, University of Antwerp), by the Danish Council for Independent Research | Natural Sciences, by the EU ATANS Grant (FP6 506004), and by a FWO Travel Grant to M. Campioli. Special thanks are due to the Abisko Scientific Research Station and the Zackenberg Research Station for the logistic support. We thank: Gus Shaver for support, Annemie Vermeulen for determination of stem secondary growth, Willem De Smet for sampling at Ny Ålesund, John King for the English edit, and the subject matter Editor and two Reviewers for their constructive comments on a previous version of the manuscript. Field research at Fontainemore fits within collaboration with Michele Freppaz in the framework of a project sponsored by the Municipality of Fontainemore and the Regione Autonoma Valle d'Aosta–Servizio Aree Protette.