Does warming affect growth rate and biomass production of shrubs in the High Arctic?

Few studies have assessed directly the impact of warming on plant growth and biomass production in the High Arctic. Here, we aimed to investigate the impact of 7 years of warming (open greenhouses) on the aboveground relative growth rate (RGR) of Cassiope tetragona and Salix arctica in North-Eastern Greenland. RGR was assessed for apical (leaves, stem, reproductive organs) and lateral meristems (secondary growth of stem and branches) and accompanied by measures of gross ecosystem production (GEP), branching and tissue carbon (C) concentration. Measurements were based on harvest and biometric methods (for RGR and branching) and gas exchange and chemical analysis (for GEP and C concentration). Warming nearly doubled the apical RGR of Cassiope, whereas it did not affect the apical RGR of Salix. Similarly, secondary growth increased for Cassiope but not for Salix. In particular, warming enhanced the secondary growth of old stem segments of Cassiope formed before the treatment began. The increase in Cassiope RGR was associated with an increase in gross photosynthetic uptake, branching and C concentration in old green tissues. Overall, the different growth measures consistently indicated that temperature limits the growth of Cassiope but not that of Salix in North-Eastern Greenland. Summer warming thus has the potential to stimulate biomass production in the High Arctic but major species-specific differences are expected.