Location and Design


AFERP’s long-term, expanding-gap experiment is located at the Penobscot Experiment Forest (PEF) in Bradley, Maine (Figure 1).

Figure 1. PEF location in east-central Maine


Forest description

Under a recent ecological land classification, the location of the PEF is within the Central Maine Coastal and Interior Section of the Laurentian Mixed Forest Province. It is dominated by mixed northern conifers, including eastern hemlock (Tsuga canadensis); spruce, mostly red (Picea rubens) with some white (P. glauca); balsam fir (Abies balsamea); northern white cedar (Thuja occidentalis); eastern white pine (Pinus strobus); and, infrequently, tamarack (Larix laricina) or red pine (P. resinosa). The most common hardwoods are red maple (Acer rubrum); paper birch (Betula papyrifera); gray birch (B. populifolia) and aspen, both quaking (Populus tremuloides) and bigtooth (P. grandidentata). Sugar maple (A. saccharum), yellow birch (B. alleghaniensis), American beech (Fagus grandifolia), white ash (Fraxinus americana), black cherry (Prunus serotina), northern red oak (Quercus rubra), and American basswood (Tilia americana) are scattered throughout the Forest. Vegetation types are typically more diverse than the industrial spruce-fir forest farther north.


Experimental design

Control and two treatment levels of expanding gap harvesting are replicated 3 times in a randomized block design (See table below and Figure 2 & 3).

Figure 2.  Canopy profiles for a typical 20-10 treatment after the first (1995), third (2015), and fifth/final (2035) entries.

Figure 3. Simulated expansions for research areas 1 & 2 (20-10 and 10-20, respectively).  The double solid line in 20-10 figure corresponds to the transect representing canopy profiles in Figure 2.



Both harvesting treatments permanently retain 10 % basal area or 3.75 m2/ha across a dispersed network of reserve trees.  The objectives for retaining trees in perpetuity include, structural complexity enhancement, maintaining seed sources for high-value and rare tree species, providing material for dead wood recruitment, and improving wildlife habitat, to name a few.  In particular, large trees were favored during selection with the intention of providing structure for cavity-dwelling wildlife and large-diameter downed dead wood.

Gap Types

Two scenarios exist during gap creation and expansion depending on the status of tree regeneration (see figure below):

Scenario 1: If an area is considered “regenerated” with a sufficient number of large (at or above breast height) and well-distributed seedlings, the area is categorized as a Type A gap/expansion and the overstory will be treated with a one-stage removal cut reducing basal area to the long-term target of 10%.

Scenario 2: If an area is not considered “regenerated”, the area is categorized as a Type B gap/expansion and the overstory will be treated in two-stages.  First, an initial reduction to 30 % basal area followed by a final reduction to the long-term target of 10% basal area at the end of the regeneration period (see Table 1 for details on treatment-specific regeneration period).