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Verification between Douglas-fir Regeneration and Frost in Central British Columbia

Author(s) or contact(s): Robert M. Sagar and Michaela J. Waterhouse
Source: Ministry of Forests, Lands, Natural Resource Operations and Rural Development
Subject: Spruce, Douglas-fir, Frost Damage, Microclimate, Shelterwood
Series: Technical Report
Other details: Published 2018
 

Abstract

In the Sub-Boreal Spruce dry warm (SBSdw) biogeoclimatic subzone, on
the Interior Plateau of British Columbia, frost is a limiting factor for the
establishment and growth of Douglas-fir (Pseudotsuga menziesii) in clearcuts.
A replicated research trial, using a uniform shelterwood silvicultural system,
involved harvesting in three passes over a 20-year period to test how residual
basal area retention affected establishment, growth, and condition of natural
regeneration. Starting with even-aged stands of mature Douglas-fir averaging
61 m2/ha, a preparatory cut in 1991 resulted in 40 m2/ha retention, which
was followed by a regeneration cut in 2001 that reduced the basal area to
20 m2/ha. Final harvesting took place in 2011, which reduced the basal area of
mature trees to 0 m2/ha in the previously uncut control and shelterwood
treatments. Microclimate monitoring stations were installed at three research
sites in 2012 and were monitored for 5 years to compare vertical air temperature
profiles in the former 60 m2/ha uncut controls (60) and in the former
20 m2/ha shelterwoods (20) at each site.

In the 20 treatment units, the percent cover and modal height of the tall
(> 2 m) shrub layer, primarily Douglas-fir and hybrid white spruce (Picea
glauca engelmanni) regeneration, was much greater (27−53%, 3.5−5.7 m)
than in the 60 treatment units (< 10%, 3.1−3.6 m) that had no previous harvesting
history.

Air temperature profile data at 15, 40, 75, and 150 cm above ground showed
that strong temperature inversions frequently occurred in the 60 treatment
units on clear, calm nights, while well-mixed, neutral (isothermal) profiles
were observed in the 20 treatment units (2012–2016). This led to more frequent
frosts, greater duration of subfreezing minutes, and lower extreme air
temperatures during the growing season at the lower measurement heights in
the 60 treatment units. We assessed frost damage on planted Douglas-fir
seedlings in two of the 60 treatment units. Nighttime minimum air temperatures
were as low as –8.1C on the night of June 4–5, 2014 for the 15-cm
measurement height in the 60 treatment units, which caused frost damage
on 64% of the seedlings. In contrast, tall regeneration in the 20 treatment
units showed no apparent frost damage.

Over the 25-year study, air temperature data were collected during three
time periods to compare various treatments (uncut, clearcut, and shelterwood).
When combined for one of the research sites (Beedy Creek), they
clearly showed the relationship between overstorey forest cover and frost
frequency. Frost occurred frequently (up to 12 occurrences during the
May 15−July 31 period) following harvesting of the 60 treatment units in 2011,
compared with less than one occurrence per season in the same location
prior to harvesting (1993−1995). The frequency of frost occurrences increased
from less than one per season (1993–95) to about seven per season (2012–16)
in the same 20 treatment unit after basal area was reduced to 0 m2/ha in 2011.
However, the moderate frost frequency that occurred in the shelterwood
20 treatment units following final harvesting in 2011 was of little consequence
to the tall, well-established Douglas-fir regeneration.

A shelterwood system that uses a regeneration cut (or in combination
with a preparatory cut) can lower the frequency and duration of frost events,
as well as moderate extreme temperatures during the establishment phase,
iv and after the final cut, can leave a well-developed regeneration layer that can
moderate and survive frost.

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Updated November 16, 2018