Canopy feedbacks and microtopography regulate conifer seedling distribution in two Minnesota conifer-deciduous forests

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Abstract
Successful regeneration of some species may be linked to microsite availability, which in turn may be influenced by dominant canopy species. We investigated the relationship between canopy feedbacks and microsite heterogeneity as they affect conifer regeneration in two Minnesotan mixed species forests: Abies balsamea (L.) Miller-Populus tremuloides Michx and Thuja occidentalis L.-Betula papyrifera Marshall. Objectives were to examine (i) microtopographic differences resulting from canopy feedbacks and (ii) conifer regeneration at three stages (seed dispersal, germination and establishment). We quantified seed rain and surveyed microtopography and regeneration of T. occidentalis and A. balsamea. Microtopographic features included pits, mounds, stumps, logs, and level ground. Differences in microtopography beneath deciduous and conifer canopy types were generally minor. Conifer seeds fell in larger numbers under conifer canopy types than under adjacent hardwood canopy types in both forest types, but substantial seed rain occurred up to 20 m into hardwood canopy patches. In the Thuja-Betula stands, both germinants and seedlings of T. occidentalis occurred almost entirely under the Thuja canopy type, and there predominantly on decayed wood (rotting stumps and logs), which occupied only a small fraction of the forest floor. In the Abies-Populus stands, A. balsamea germinants and seedlings were relatively common beneath both canopy types and on all microtopographic features. The distribution of T. occidentalis was more restricted than that of A. balsamea, indicating that A. balsamea may be more of a generalist.