Influence of bryophytes and microrelief conditions on Picea abies seed regeneration patterns in boreal old-growth swamp forests

The occurrence of naturally regenerated Picea abies (L.) Karst. emergent seedlings (i.e., 1 year old and up to 1.3 m in height) was monitored in relation to the occurrence of microrelief conditions in 10 old-growth swamp forests in northern Sweden. Further, nine different microhabitat types were defined in relation to the microrelief and bryophyte species present, and for five of the forest sites, a 4-year sowing experiment was performed by placing P. abies seeds in each microhabitat type. Although logs, roots, and stumps only occupied 14% of the forest floor, 43% of P. abies regeneration occurred in these microrelief conditions. Hummocks occupied 35% of the forest floor and harboured 48% of the regeneration, while hollows and flat ground occupied 49% of the forest floor but included only 9% of the regeneration. Seedling emergence from sown seeds was highest in the Sphagnum spp. microhabitats and lowest in the Hylocomium splendens (Hedw.) and Pleurozium schreberi (Brid.) microhabitats. Seedling establishment was highest in the presence of Sphagnum spp. and Pleurozium growing on logs. Seedling mortality was mainly caused by smothering by bryophytes. Elevated microhabitats are thus crucial for P. abies regeneration and should consequently be considered in the management of boreal swamp forests.