Canopy gap characteristics and their implications for management in the temperate rainforests of southeast Alaska

Abstract

In the coastal temperate rainforests of southeast Alaska, much progress has been made in describing landscape-level natural disturbances and formulating management systems that emulate those disturbances. Little is known, however, concerning canopy gaps, the dominant form of natural disturbance in the region. During June–August, 1991–1993, we characterized canopy gap patterns and dynamics at three sites in the western hemlock/blueberry/shield fern plant association in the northern portion of the Tongass National Forest. Forest area in canopy gaps ranged from 5.8 to 12.6% and averaged 8.7%. The proportion of forest area in expanded gaps ranged from 18.1 to 43.9% and averaged 27.4%. Gap and gapmaker (tree whose death or crown displacement results in the creation or expansion of a canopy gap) characteristics were generally similar among sites. The majority of canopy gaps were <50 m 2 in area, had a D/ H ratio <0.50, were created from the death of one or two gapmakers, and had experienced gap expansion. The majority of expanded gap areas were <200 m 2. Gapmakers were usually snapped, had recently died (<20 years ago), and tended to be larger in diameter than surrounding overstory trees. Species composition of gapmakers was similar to surrounding overstory trees. A substantial amount of gap infilling takes place between 20 and 80 years following tree death, but gaps, or portions of gaps, can persist for >80 years. Forest turnover time was estimated to range from 230 to 920 years, and average 575 years. Canopy residence time was estimated to range between 210 and 840 years, and averaged 525 years. To emulate canopy gap dynamics in the plant association studied, forest managers should: (1) maintain a small proportion of a stand in openings within an otherwise undisturbed canopy; (2) use a combination of single tree selection and small group selection systems; (3) re-enter stands every 20–80 years; (4) select larger than average diameter crop trees in proportion to the species composition of the stand; (5) minimize soil disturbance and (6) select crop trees during re-entry so that the creation of new gaps and the expansion of old gaps is accomplished in approximately equal proportions.