Ericaceous dwarf shrubs contribute a significant but drought-sensitive fraction of soil respiration in a boreal pine forest

Boreal forests often have a dense understory of ericaceous dwarf shrubs
with ecological adaptations that contrast those of the canopy-forming
trees. It is therefore important to quantify contributions by understory
shrubs to ecosystem processes and disentangle shrub- and tree-driven
responses, and their interactions, to climatic factors. We quantified soil
respiration driven by the pine canopy and the ericaceous shrub understory
over three years, using a factorial pine root-exclusion and shrub-removal
experiment in a mature Pinus sylvestris forest. Soil temperature and
moisture-related responses of respiration attributed to autotrophic
(shrubs, pine roots) and heterotrophs were compared. Additionally, we
assessed effects of interactions between these functional groups on soil
nitrogen availability and respiration. Understory shrubs accounted for 22
± 10% of total autotrophic respiration, reflecting the ericaceous
proportion of fine root production in the ecosystem. Heterotrophic
respiration constituted about half of total soil respiration. Shrub-driven
respiration was more susceptible to drought than heterotrophic- and
pine-driven autotrophic respiration. While the respiration attributed to
canopy and understory remained additive, indicating no competitive
release, the plant guilds competed for inorganic N. Ericaceous understory
shrubs accounted for a small, yet significant, share of total growing
season soil respiration. Overlooking understory respiration may lead to
erroneous partitioning and modelling of soil respiration mediated by
functional guilds with contrasting responses to soil temperature and
moisture. A higher share of activity of both heterotrophs and pine roots,
under drought conditions, could have important implications for soil
organic matter accumulation and decomposition as the climate changes.