Nonvascular contribution to ecosystem NPP in a subarctic heath during early and late growing season

Bryophytes and lichens abound in many arctic ecosystems and can contribute substantially to the ecosystem net primary production (NPP). Because of their growth seasonality and their potential for growth out of the growing season peak, bryophyte and lichen contribution to NPP may be particularly significant when vascular plants are less active and ecosystems act as a source of carbon (C). To clarify these dynamics, nonvascular and vascular aboveground NPP was compared for a subarctic heath during two contrasting periods of the growing season, viz. early-mid summer and late summer-early autumn. Nonvascular NPP was determined by assessing shoot biomass increment of three moss species (Hylocomium splendens, Pleurozium schreberi and Dicranum elongatum) and by scaling to ecosystem level using average standing crop. For D. elongatum, these estimates were compared with production estimates obtained from measurements of shoot length increase. Vascular NPP was determined by harvesting shrub and herb apical growth and considering production due to stem secondary growth of shrubs. Hylocomium splendens and Pleurozium schreberi showed highest biomass growth in late summer, whereas for D. elongatum this occurred in early summer. Maximum relative growth rates were ca. 0.003–0.007 g g−1 d−1. For D. elongatum, production estimates from length growth differed from estimations from biomass growth, likely because of an uncoupling between length growth and biomass shoot growth. Nonvascular NPP was 0.37 and 0.46 g dry weight m−2 d−1, in early and late summer, respectively, whereas in the same periods vascular NPP was 3.6 and 1.1 g dry weight m−2 d−1. The contribution of nonvascular NPP to total aboveground NPP was therefore minor in early summer but substantial in late summer, when 25% of the C accumulated by the vegetation was incorporated into nonvascular plant tissue. The expected global change-induced reduction of nonvascular plant biomass in subarctic heath is likely therefore to enhance C release during the late part of the growing season.

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