Yellow birch (Betula alleghaniensis) and white cedar (Thuja occidentalis) are important components of mature forests in northeast Minnesota but recent evidence raises concerns about their continued regeneration. Saplings of both species are capable of reaching the canopy through large tree-fall gaps, but first these individuals must establish themselves, typically under a closed canopy. Our study examined the relative importance of substrate characteristics and plot level variables on seedling density. Our site in Crosby Manitou State Park is an old growth cedar-birch co-dominated stand that has seen minimal human disturbance. We quantified seedling densities in 20 100m² plots. For all seedlings, we recorded substrate and plot variables (e.g. canopy cover and composition) and then used generalized linear mixed effects models to determine which factors best explained variation in seedling densities.

Results/Conclusions

The overall density of birch between one year old and 50cm tall was 1950/ha (±366 SE). Almost all were found on coarse woody debris (CWD) that covered 3% to 7.5% of the forest floor. Elsewhere these seedlings can establish well on mineral soil but here the soil was covered by thick layers of litter and duff. The models showed that birch preferred CWD pieces that were, in order of decreasing importance: heavily decayed, mostly covered by moss, white cedar wood, or a stump. These substrate factors explained densities twice as well as variation in plot level factors. The density for cedar seedlings was very low, 155/ha (±48/ha SE). All were on CWD, and were more frequent on those with high moss cover, but none were taller than 10cm. We conclude that substrate traits strongly influence regeneration of both species at this site. In managed forests of this type, guidelines call for leaving a few low value trees, frequently Acer saccharum, as nurse logs. This work suggests that leaving low quantities of CWD of less suitable species like Acer, which also spend less time in suitable high decay stages, may not be sufficient to support historical levels of regeneration.