| Summary: | The influence of four commonly used site preparation methods (disc
trenching, pile-and-burn, broadcast burn, no treatment) on soil microclimate, soil
nutrients, bulk density, and early survival and growth of hybrid interior spruce
(Picea glauca [Moench] Voss x engelmannii Parry ex. Engelm.) and lodgepole pine
(Pinus contorta Dougl. ex. Loud.) seedlings were examined in plots established in the
Engelmann Spruce - Sub alpine Fir (ESSF) zone in the northwest interior of British
Columbia.
Soil moisture was monitored in 13 potentially plantable microsite types at
weekly intervals over the first two growing seasons after site preparation. Although
growing season soil moisture in 1994 was significantly effected by the microsite
types under study, soil moisture was not believed limiting to seedling growth in any
microsite studied and soils did not reach slightly to moderately dry conditions (> 2
bars tension) until late July, and only in three microsites, disc trench berm, disc
trench hinge, and pile-and-burn-scalp (mineral soil exposed).
Soil and air temperature were strongly affected by the four site preparation
treatments and nine microsites studied, particularly in early spring periods between
mid-May and mid-June. Soils (10-cm depth) warmed sooner, and for longer
duration, in treatments which promoted soil drainage. Treatments which removed
or incorporated insulating forest floor layers resulted in greater cumulative hours of
soil temperature greater than 8°C in early spring periods, giving a distinct growth
advantage over planting sites having intact forest floors. Untreated soils (slash left)
were consistently colder for longer periods during the growing season. Broadcast
burning decreased the frequency of frost events 20 cm above the ground surface
during the 1994 growing season by 43 and 121%, relative to pile-and-burn and no
treatment plots, respectively. Site preparation had strong effects on coarse fragment free bulk density of
near surface (2-7 cm) and rooting zone (0-20 cm) layers one season after treatment.
The coarse fragment free bulk density of the disc trench and hinge microsites 0-20
cm layer were 50 and 24% less, respectively, as compared to pre-treatment density.
The disc trench berm and hinge microsites had significantly (p<0.05) smaller 0-20
bulk density one season after treatment relative to other microsites studied. Both
the disc trench trench-bottom and pile-and-burn under-excavator-track microsites
had greatest near surface (2-7 cm) soil density.
Several soil nutrients in decayed wood, forest floor and 0-20 cm mineral
layers were significantly affected by site preparation treatments. With the
exception of total S, exchangeable K, mineralizable N in most treatments and total
N in the pile-and-burn burned-pile microsite, soil nutrients and pH in decayed wood
materials increased from pre-treatment to one season post-treatment. Nutrients of
forest floor materials revealed greater microsite effects than decayed wood materials
and may reflect greater biological activity in this uppermost layer. Nine and 12
months after pile and broadcast burning, respectively, there were significant
increases in forest floor pH and exchangable and total Ca and Mg relative to
untreated soils. Site preparation treatments which promote mixing (i.e. at the berrh
and hinge) resulted in significant increases in many soil nutrients in the 0-20 cm
layer, most notably total C and N . Greatest one-season losses of total N , S, and C
from all three soil layers studied occurred in the pile-and-burn burned-pile microsite.
Superior second-year root collar diameter and height increment growth of
both planted species occurred on the broadcast burn and disc trench hinge
microsites. Poorest growth of both species occurred on the pile-and-burn and
untreated plots and was believed at least partially associated with higher soil
moisture content and slow soil warming in early spring periods in this moist, cold
biogeoclimatic subzone (ESSFmc). === Land and Food Systems, Faculty of === Graduate
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