Abstract:
Douglas-fir (Psudotsuga menziesii (Mirb.) Franco) seedlings were outplanted
on eight dates (every three weeks from mid-August 2005 through mid-January 2006).
On each plant date, seedlings from a conventional dormancy (CONV) induction
treatment, including moisture and nutrient stress, and a shortened daylength (SD)
treatment were outplanted on three western Oregon sites ranging from the moist
coastal region (365 m above sea level, 315-355 cm annual precipitation) to the drier
valley fringe (275 m above sea level, 100-150 cm annual precipitation). SD treated
seedlings initiated primordial development earlier than CONV treated seedlings and
tended to have a lower shoot:root ratio due to larger stem diameter and root biomass.
On all three sites, seedlings planted on the first three plant dates (August-September)
had more root growth shortly after planting and greater seasonal height and stem
diameter growth after the first season than those planted on the later 5 dates (October-
January). On two of the three sites, seedlings from the CONV dormancy induction
treatment grew more in height and stem diameter than SD treated seedlings regardless
of planting date. On the wet coastal site seedlings planted on the earliest date (August)grew on average 20 cm more than seedlings planted on the latest date (January) and
averaged more than 2 mm in stem diameter growth when measured at the end of the
first growing season. Also on this site seedlings treated with CONV averaged almost 4
cm more in height growth and more than 1 mm in stem diameter growth than
seedlings treated with SD.
Soil moisture and temperature were monitored throughout the study period.
Soil moisture was above 20% (volumetric water content, soil core method) on all sites
at the time of planting and was not deemed a limiting factor when modeled with
seedling field performance. However, soil temperature at the time of planting was
found to have a significant influence, which is attributed to its effect on early root
egress. Seedlings planted into warm soils (>15ºC, first three plant dates) averaged
more height growth and stem diameter growth compared with seedlings planted in
cooler soils (<15ºC). With the exception of increased mortality for seedlings planted
on the first plant date on the driest site, survival was unaffected by plant date or
dormancy induction treatment due to adequate soil moisture throughout the planting
dates. Although these data are limited to one planting season, fall planting could be a
viable alternative to winter planting in PNW coastal areas where soil moisture is
adequate.
