Northeastern Minnesota’s logging history has altered the forests enough to cause concern about the reproduction of Thuja occidentalis and Betula alleghaniensis. I studied a rare old growth example of an already rare mesic Thuja-Betula forest and asked how well those species were regenerating in that mostly unaltered ecosystem. In managed forests, a lack of suitable substrate is thought to be limiting their seedling establishment so I asked which substrates the seedlings preferred in the old forest. To answer both questions, I measured the seedling densities of all canopy tree species across height classes and substrate types within twenty 100m2 plots. T. occidentalis, B. alleghaniensis, and the uncommon Picea glauca were found exclusively on coarse woody debris (CWD) rather than leaf litter. I used a generalized linear mixed effects model to rank the influence of each type of CWD on seedling density while comparing their influence to that of site level variables. T. occidentalis regeneration is failing, with no seedlings >10cm tall, due to either herbivory or a lack of light. Its seedlings did show significant preferences for CWD that was, in decreasing order of importance: moss covered, highly decayed, conifer wood. Site level variables did not influence T. occidentalis seedling density. B. alleghaniensis may be replacing itself in the canopy but only just, and is also limited by the availability of canopy gaps. Its seedlings preferred CWD that was, in decreasing order: moss covered, highly decayed, moderately decayed, Thuja wood, a stump (rather than a log). For B. alleghaniensis seedlings, CWD traits were almost twice as influential as site level variables. High decay class CWD was the most important factor for seedlings of the uncommon canopy species P. glauca, Abies balsamea, and Acer saccharum. Although CWD covered 5% of the ground in this old forest, it is much less abundant in rotation harvested forests. Adequate quantity and quality of CWD is a vital factor for natural regeneration of T. occidentalis, B. alleghaniensis, and P. glauca in Thuja-Betula forests.
Rossiter, Stephen C., "Seedlings’ Substrate Preferences in a Minnesota Old Growth Thuja-Betula Forest" (2009). Environmental Studies Honors Papers. 2.
The views expressed in this paper are solely those of the author.