Thinning Young Stands for Diversity: Regeneration, Stand Structure and Microsite

Samuel S. Chan, USDA Forest Service, PNW Research Station, Corvallis, OR and Elizabeth C. Cole, Oregon State University, Dept. of Forest Science, Corvallis, OR

Density management through the thinning of young stands can potentially accelerate the growth of the overstory and increase the diversity and complexity of the understory. Franklin et. al 2001, cited the importance of the connectivity between the overstory and understory canopies as a key functional component of late-seral forests. We hypothesize that the regeneration of trees in the understory of thinned or partially harvested stands will contribute to the diversity, structure and functions of stands in Douglas-fir/western hemlock dominated stands in the Pacific Northwest. Thinning to create favorable conditions for overstory and understory development will obviously result in some short to intermediate changes to the environment. Depending on the degree of thinning and site factors, thinned stands are typically warmer (2-5 ^oC) during the warmest days of the year and warmest times of the day and have lower relative humidity (~10%) than closed stands.

A study on site productivity class I, 30-40 year-old stands in the Oregon Coast Range indicate that overstory canopies grow and fill in rapidly beginning about the third year after thinning. By the 8^th year, overstory canopies in stands thinned to ~100 TPA (QMD ~ 12”) that favor bole wood production, have closed to conditions approximating those of unthinned stands. Light levels in stands thinned to very wide spacing (30 TPA) have been reduced by nearly 50% from the initial thinned conditions. Vertical and horizontal overstory canopy growth accounts for much of the reduction in light availability. Heavily thinned stands showed lengthening live crown to tree height ratios, whereas the live crown ratio has decreased over 8 years in the unthinned and 100 TPA commercial thinnings. These expanding live overstory crowns are important structural components of late-seral trees. The ability of the overstory and understory canopies to co-mingle will depend upon: 1) the maintenance of adequate light and spacing, 2) the extent of artificial and natural regeneration, and 3) minimizing damage to regeneration from animals.

We find that artificial tree regeneration substantially accelerates the development of a diverse understory that includes trees as a major component. On study sites situated on site productivity class I plots in the Oregon Coast Range, we found higher growth and survival of underplanted tree regeneration in overstory densities lower than those commonly used to meet traditional goals for commercial wood production (less than 80-100 TPA or basal area of 80-100 ft2/a). Survival and growth of understory trees decreases with increasing overstory density. On our site class I study sites, species such as western hemlock, western red cedar and Sitka spruce have higher survival rates than Douglas-fir. Western hemlock typically had the highest survival and growth. We have found that Douglas-fir will survive successfully under heavily thinned stands, though growth will be less than those grown in the open. Little understory was present in these site class I stands prior to thinning. Another study conducted on drier site class potential II and III stands, grand fir and western redcedar had the highest survival and growth (note, on this site, western red cedar had little browse damage). Douglas-fir and western hemlock had poor survival. Seedling vigor, animal damage, competition from shrubs, ferns, herbs and overstory density all contributed to mortality. Where damage from browsing rubbing and clipping animals is not severe, the success of tree regeneration in the understory may be attributed to a lower degree of competition from shrubs and herbs. Maintaining light availability through the overstory canopy is critical for successful regeneration.

Keywords: understory, canopy development, light, microclimate, animal damage.