Abstract:
A full range of experiments was carried out to investigate the
effects of environmental factors on the terminal bud dormancy in two
high-elevation true fir species, Shasta red fir (Abies magnifica A.
Murr. var. shastensis Lemm.) and noble fir (A. procera Rehd.),
seedlings. Photoperiod, thermoperiod and moisture stress were of main
interest.
In container-grown seedlings, it was found that warm thermoperiod
with moderate moisture stress and cool thermoperiod with low moisture
stress induced dormancy in the initial stage of the experiment. Short
photoperiod with warm thermoperiod and long photoperiod with cool
thermoperiod hastened onset of dormancy in a later stage. Short
photoperiod and cool thermoperiod, which acted independently, could
also induce dormancy.
Both Shasta red fir and noble fir seedlings showed a typical
terminal bud dormancy. Six to eight weeks of chilling were required
to overcome the dormancy. Dormancy induction method influenced
chilling requirements. Seedlings induced into dormancy by long cool
days required less chilling than did the seedlings induced into
dormancy by short warm days. Long photoperiod during chilling could
conpensate for inadequate chilling.
The survival, phenology and growth of non-chilled true fir
seedlings were determined for two seasons in two irrigation regimes.
Regardless of species, survival was not different between any
treatments. Unchilled terminal buds stayed inactive during the first
season after outplanting. These terminal buds resumed growth after
being chilled naturally during the winter. Lateral branch development
was observed for both growing seasons. Growth was greater in the
moist irrigation regime than the dry irrigation regime.
Dormancy induction in natural low- and high-elevation
environments was investigated by using a switch-back experimental
design. It was determined that lower night temperature in the high-elevation
areas triggered growth cessation in the seedlings and thus
earlier dormancy. High moisture stress early in the season could also
prematurely cease seedling growth and impose stress-induced dormancy.
