Ecosystem carbon dynamics differ between tundra shrub types in the western Canadian Arctic

Shrub expansion at high latitudes has been implicated in driving vegetation "greening" trends and may partially offset CO2 emissions from warming soils. However, we don't yet know how Arctic shrub expansion will impact ecosystem carbon (C) cycling and this limits our ability to forecast changes in net C storage and resulting climate feedbacks. Here we quantify the allocation of photosynthate between different ecosystem components for common deciduous Arctic shrubs, both of which are increasing in abundance in the study region; green alder (Alnus viridis (Chaix) DC.) and dwarf birch (Betula glandulosa Michx., B.). Using 13C isotopic labelling, we show that carbon use efficiency (net primary production: gross primary production) in peak growing season, is similar between the two shrubs (56 ± 12% for A. viridis, 59 ± 6% for B. glandulosa) and biomass production efficiency (plant growth per unit photosynthesis) is 56 ± 14% for A. viridis, versus 31 ± 2% for B. glandulosa. However, a significantly greater proportion of recent photosynthate is allocated to woody biomass in A. viridis vegetation (27 ± 5%), compared to vegetation dominated by B. glandulosa (4 ± 1%). Belowground C allocation also differs significantly; after 2.5 weeks we recovered 28 ± 6 % of recent photosynthate in root-free soil under B. glandulosa, but under A. viridis we were unable to detect recent photosynthate in the soil. We provide the first evidence that the impact of shrub expansion on Arctic C cycling will be species dependant. Where Betula dominates, ~1/3 of recently photosynthesised C will be rapidly allocated belowground to soil and microbial pools. Where Alnus dominates, more recently fixed C will be allocated to woody biomass. We conclude that models driven by remotely-sensed aboveground canopy characteristics alone (i.e. greenness) will be unable to accurately represent the impact of vegetation change on Arctic C storage.