Influence of forest structure on habitat use by American marten in an industrial forest

American marten ( Martes americana) are associated with late-successional, coniferous forests throughout much of their range. In the northeastern US, however, marten also occur in forests dominated by mid-successional, mixed coniferous–deciduous and deciduous stands. Structural attributes other than forest-stand age and dominant overstory type may be important determinants of habitat quality for marten. To identify structural conditions associated with patterns of habitat use by marten, we compared characteristics of coarse woody debris (CWD), understory vegetation, and overstory vegetation between 16 ha areas receiving different intensities of use by 24 (13 male, 11 female) resident, nonjuvenile (≥1 year) marten in an extensively clearcut industrial forest in Maine. Overstory features related to stand maturity were most useful for discriminating between areas receiving high use, low use, and no use by marten. Used areas had taller trees, higher live-tree basal areas, and greater snag volumes than unused areas, which were regenerated by clearcutting 13–21 years prior to our study. Unused areas apparently lacked vertical structure required by marten. Among areas receiving use, higher use intensity was associated with greater basal area of deciduous trees. Understory vegetation and woody debris was abundant in all use-intensity categories, although areas used most intensively had larger downed logs. We conclude that where horizontal structure is not limiting, forest stands regenerated by clearcutting may provide suitable habitat for resident, nonjuvenile marten when the following thresholds are reached: live-tree basal area ≥18 m 2/ha, mean tree height ≥9 m for trees with dbh ≥7.6 cm, and snag volume ≥10 m 3/ha. Suitability of harvested stands may be enhanced if large woody debris is retained, and mean maximum diameter of downed logs exceeds 22 cm. Methods of harvesting that provide for retention and recruitment of woody debris and vertical structure characteristic of mature forests may be consistent with maintenance of marten populations in managed forests. Our proposed thresholds can be used for developing stand-scale harvesting strategies in forests that support marten, although landscape-scale habitat requirements of marten should also receive management consideration.