Aj. Hansen et al., ALTERNATIVE SILVICULTURAL REGIMES IN THE PACIFIC-NORTHWEST - SIMULATIONS OF ECOLOGICAL AND ECONOMIC-EFFECTS, Ecological applications, 5(3), 1995, pp. 535-554
New silvicultural strategies to sustain both ecological and human comm
unities are being developed and implemented on federal forest lands in
the Pacific Northwest (PNW) United States. Two important stand-level
components of the new silviculture regimes are rotation age and retent
ion level of live trees in harvest units. Ecologists have suggested th
at canopy tree retention and longer rotations will create patterns of
stand structure in managed forest that are similar to those in natural
forests, and promote long-term ecological productivity and biodiversi
ty. Forest economists, however, are concerned that canopy tree retenti
on and long rotations may reduce wood production, although the high va
lue of large logs produced by these new silvicultural regimes may comp
ensate for reduced growth rates. We used the forest model ZELIG to per
form a factorial simulation experiment on long-term responses of ecolo
gical and economic variables to nine retention levels and four rotatio
n lengths. ZELIG output on forest structure and composition was input
to a forest economics model that calculated net value of wood products
in 1989 dollars. The simulated stand data were also linked with regre
ssion equations to predict the densities of 17 bird species as a funct
ion of tree size class distribution. Five replicates of each treatment
were run for the 240-yr simulation period. Results indicated that sta
nd structure under each of the canopy tree retention levels was more s
imilar to the pre-treatment natural forest than following clear-cuttin
g. Variation in tree size under intermediate levels of retention, howe
ver, did not reach the level of the natural forest during the simulati
on period. Tree species composition was strongly related to retention
level and rotation age. Shade-intolerant Douglas-fir (Pseudotsuga menz
iesii) lost dominance to shade-tolerant species under intermediate ret
ention levels and longer rotations. Wood production decreased signific
antly with increasing retention level and rotation age, with a notable
threshold between retention levels of 0 and 5 trees per hectare. Net
wood products value did not decrease as rapidly with retention level,
and did not differ much among rotation ages, because of the high value
of large logs. Bird species responded individualistically to retentio
n level and rotation age. Some had peak densities under short-rotation
clear-cutting, but most were associated with structurally complex, cl
osed-canopy forest. Consequently, bird species richness increased sign
ificantly with retention level and rotation age. Within the assumption
s and limitations of our models, this application provided knowledge o
n trends and thresholds that can help land managers to choose silvicul
tural regimes that best balance their management objectives. We conclu
ded that retention level and rotation age strongly influence ecologica
l and economic responses in PNW forests; efforts are needed to reduce
uncertainty about these effects.