Forest managers require the ability to predict the outcome of their management activities, among them, regeneration cuts designed to secure natural regeneration. Predicting these regeneration outcomes in forests that have many species that form a variety of associations across multiple environmental gradients is particularly challenging. Following a regeneration harvest, a new cohort is comprised of one or more of the following sources of regeneration: (1) new seedlings that become established during or soon after harvest; (2) advance reproduction, (i.e. seedlings and saplings that exist in mature stands at the time of harvest); (3) stump sprouts from harvested trees; and (4) root suckers (from those species capable of root suckering).

REGEN3 operates on a time step that simulates the interval following a treatment that reduces overall basal area below 50 ft2/acre (e.g., regeneration cut, intense thinning) until the canopy closure – defined here as the stage in stand development where dominant and codominant crowns can first be readily distinguished in the canopy. In REGEN3, canopy closure is assumed to occur by 10 years post-treatment, recognizing that time to canopy closure is variable and dependent on site quality. An initial period of new seedling and/or root sucker establishment is implied in the overall time step, with establishment taking place in the span of time immediately following harvesting. Although not explicitly specified in the model, the establishment interval can be assumed to be a single growing season. The main output produced by REGEN3 is a list of trees, by species, that survive and grow to attain dominant or co-dominant status at the time of canopy closure (assumed to be approximately 10 years post-harvest).

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