FRST 320
Extreme Low Temperatures
Page 1 of 7
FRST 320: ABIOTIC DISTURBANCES—(Earth) Wind and Fire
Fall 2014
EXTREME LOW TEMPERATURES AND COLD TOLERANCE
Notes by Wolf Read and Lori Daniels
Outline:
1. Climatology and meteorology of cold weather in BC
2. Nighttime cooling and frost
3. Cold tolerance and adaptations in plants
4. Freeze symptoms: Tissue, tree and stand levels
5. Contributing factors to cold events
6. Management responses for cold temperatures
1. CLIMATOLOGY AND METEOROLOGY OF COLD WEATHER IN BC
Mean Annual Temperature
Where do expect the coldest temperatures?
- high latitude, high elevation, north-facing slopes
When?
- winter, late at night under a clear sky
Why?
- continental climate, large diurnal extremes
- cold season, no energy input + radiative heat loss
Winter - extreme cold temperatures when large surface high-pressure systems typically develop over the
interior of BC
- interior temperatures ≤ -50ºC
- coastal temperatures ≥-20ºC.
Despite extreme cold snaps, winter conditions are not a problem for cool temperate tree species. Why?
- adaptation of cool-temperate vegetation to extreme winter cold = dormancy
- winter cold events are not typically a problem in forestry (except frost-cracking)
So, when are plants most susceptible to cold?
1. Spring strong Artic fronts
- coincide with trees dehardening and breaking from dormancy
- can cause damage due to freezing
2. Late spring and summer frost events
- regularly occur in the interior, problematic
- despite high daily maximum temperatures (mid-30ºC) during the growing season, many interior
locations have sub-freezing episodes in summer (e.g. 100 Mile House)
- interior air is drier than coastal air, but other factors (eg elevation) are relevant
FRST 320
Extreme Low Temperatures
Page 2 of 7
Frost-Free Days (FFD)
FFD≥150? Why?
- Coast = moderated by ocean
- Interior valleys = low elevation, orographic
effects
FFD<50? Why?
- Majority of BC = continental, dry air masses,
elevation effects
http://www.agf.gov.bc.ca/resmgmt/maps/frostfre.jpg
2. NIGHTTIME COOLING AND FROST
Radiative cooling occurs on calm, clear nights.
- heat stored in the ground and objects on the surface (like trees) during the day is radiated into the sky
- ground surface can cool to <0ºC.
Advection is horizontal movement of air
- occurs when air that has radiatively cooled at another location flows, or is blown (advected) over a
site
Latent heat of condensation
- complicating factor for understanding cooling is the latent heat release during condensation of water
vapour to liquid
- when air cools and reaches saturation, water vapor condenses as dew or fog
- latent heat release from condensation slows the rate of cooling and contributes to heat the air
- explains why the moist coastal atmosphere does not get as cold as drier air found in interior
higher-elevation valleys
Elevation
- at lower elevations air does not heat or cool as quickly as air at higher elevations due to differences in density
between the two air parcels (lower elevation air has higher density in general)
- at higher elevations air is more likely to be cooler, or have experienced cooler episodes during its
development, and therefore tends to be drier than air at lower elevations, a situation that allows for
faster temperature changes
Temperature Inversion
- condition of the atmosphere where the temperature increases with height
- develops in a stable atmosphere indicated by clear skies and no wind
- stable atmosphere stratifies at night with cold air pooling at the surface + warmer air above
- sub-zero temperatures can occur very near the ground (even in summer)
- temperatures above freezing only 1-2 m above the ground surface
- cold layer (even if shallow) can damage or kill actively-growing tissues of young trees
FRST 320
Extreme Low Temperatures
Page 3 of 7
3. COLD TOLERANCE AND ADAPTATIONS IN PLANTS
Winter
- dormancy during high stress (winter) = key adaptation to cold for plants in seasonally variable climates
Growing Season
- active growth is typically done during the warm season
- below-freezing temperatures during the active growth period can be harmful to plants
- early spring – arctic outbreaks can cause damage
e.g. yellow-cedar decline
• 250,000 ha in coastal BC and Alaska
• warmer temperatures = reduced snow pack
• tree roots are susceptible to freezing during late spring arctic outflow after snow has thawed
• Yellow-cedar trees are injured by extreme cold weather during generally warmer climate and die of
drought in a rainforest.
Cold Hardiness
Winter hardiness = lowest temperature tolerable in cool season
Summer hardiness = lowest temperature tolerable in warm season
Species- and tissue-specific cold hardiness
- Plants vary in cold-hardiness, but share higher cold sensitivity during the growing season
- Annual variation in cold tolerance for different conifer species
Table 1 from Bigras and Colombo 2001 (p. 59-60) – full table provided to respond to questions in class
- Douglas-fir = least cold-hardy of BC's trees during the growing season (Hardiness = “min” in Table 1)
- western redcedar shoots = similar, not cold hardy
- subalpine fir = freeze-tolerant during the growing season
- Cold tolerance varies among tissue types
Roots = least cold hardy, insulated by soil and winter snow
Shoots = most cold hardy - dormancy - increased solute concentrations and reduction of moisture in cell
Summary: most freezing damage occurs during the growing season, when seasonal temperatures tend to be
their warmest (late spring through summer) but diurnal variation can be great
FRST 320
Extreme Low Temperatures
Page 4 of 7
Source:
Bigras, FJ and SJ Colombo (eds). 2001. Conifer Cold Hardiness. Kluwer Academic Press, The Netherlands.
FRST 320
Extreme Low Temperatures
Page 5 of 7
4. FREEZE SYMPTOMS: TISSUE, TREE AND STAND LEVELS
Lethal freezing damage
- includes freezing of water, leading to ice crystal formation in the cell sap within living plant cells
- intracellular freezing ruptures the plasma membrane and can draw water out of the cell causing dehydration
Non-lethal freezing damage
- includes disrupting photosynthesis by affecting the light-sensitive enzymes in the photosynthetic pathway
- freezing damage to differentiating cambial cells (e.g. xylem mother cells) leads to thin-walled, abnormallyshaped trachieds
Tissue Level
- symptoms include ruptured cell walls in the cambium, phloem and leaves
- "frost rings" with irregular xylem cells and pitch
- secondary fungal attack may also be present
- consequences include reduced growth of trees, direct mortality of trees, secondary infection of trees, and the
desired species may be out-competed by more tolerant species
Tree Level
- symptoms include dead or wilted current-year shoots, missing needles on previous-year shoots, old dead
buds or partially developed shoots, multiple leader forks and a reduced growth rate.
Stand Level
- symptoms include: red or chlorotic seedlings, multiple forked leaders and reduced growth rates
- symptoms vary among species based on their tolerances
- presence of spirea and other frost-tolerant shrubs is also diagnostic
- frost pockets enhance landscape-level heterogeneity
5. CONTRIBUTING FACTORS TO COLD EVENTS
Atmospheric and site-related factors that affect frost occurrence:
Factor
Influence
Air temperature
Further heat loss required for freezing to occur
Air humidity
Latent heat content of the air
Absorption and emittance of longwave radiation
Windspeed
Mixing of warmer air toward the ground surface
Cloud cover
Emittance of longwave radiation from the atmosphere toward the
ground surface
Slope angle
Cold air drainage and accumulation
Slope position
Size of uphill cold air source
Aspect
Daily total amount of solar radiation, soil heat storage and time of day
when sites are coldest
Elevation
Air temperature, humidity, cloud cover, windspeed
Topography
Windspeed, cold air drainage and accumulation
Vegetation cover
Windspeed, radiative heat loss from the ground surface and soil profile
heat storage
Soil composition
Soil profile heat storage and transfer properties
FRST 320
Extreme Low Temperatures
Page 6 of 7
Location
- integrates slope, elevation, topography etc. and is critical
- valley bottoms that are sheltered from wind and/or places far to the north where the sun angle remains low
are strongly favored for extreme cold events during the warm season.
- topography influences cold-air drainage and ponding
- cold air = denser than surrounding warm air, migrates downslope + accumulates in hollows + valleys
- these locations are typically sheltered from mixing winds
- the atmosphere to stratifies and air near the surface to become quite cold
- in contrast, ridge-tops are exposed to mixing winds (prevents stratification and inversions) and the
cold air tends to drain away from the uplands
- higher elevations are warmer relative to the bottomlands where cold-air drainage occurs
- mid-slopes are less prone to hard freezes than the valley bottoms
- cold air moving downslope provides a gentle breeze that mixes the atmosphere
Canopy cover
- shelters the ground from clear skies and strongly influences surface temperatures
- canopies emit longwave energy
e.g. contrasting minimum ground-surface temperatures Lubrecht, Montana, on 29 Aug 1978:
-5.0ºC in a clearcut but 2.2ºC in the uncut control
6. MANAGEMENT RESPONSES FOR COLD TEMPERATURE
1) Identify frost prone sites
2) Assess severity of frost hazard at frost-prone sites
3) Determine whether you can modify cold air drainage patterns via
harvest design
4) Retain overstory trees to conserve heat
5) Design site preparation and vegetation management to conserve heat
in microsites
6) Use more frost-resistant species and provenances for reforestation
7) Ensure nursery stock is phenologically in-phase with the growing site
when planted
Canopy cover can be a useful management tool. Surface temperatures tend to stay significantly warmer in a
shelterwood, for example, providing protection for regeneration. The seedlings, however, will not
receive as much light as an open-grown tree because of the partial canopy cover.
Site preparation is an important tool for mitigating surface freezes. Exposed mineral soil tends to stay warmer
at night than an organic layer—even in trenches. Mounds tend to have drier soil than trenches or flat,
bare ground; these drier mounds will reradiate heat more quickly than the wetter soil.
Frost-tolerant trees are often best planted in the sites that are prone to freezing temperatures during the
growing season, with frost-intolerant species most wisely placed outside of frost pockets, say on mid
and upper slopes.
High Frost Tolerance (ideal for frost-prone sites): lodgepole pine, trembling aspen, black cottonwood,
white pine, black spruce, Englemann spruce, mountain hemlock
Medium Frost Tolerance: ponderosa pine, western larch, noble fir, subalpine fir, Sitka spruce, amabilis
fir, grand fir
Low Frost Tolerance (plant away from depressions): red alder, Douglas-fir, western redcedar, western
hemlock
FRST 320
Extreme Low Temperatures
Page 7 of 7
References
Spittlehouse, D. L., Stathers, R. J. 1990. Seedling microclimate. Land Management Report No. 65, BC Ministry of
Forests, Victoria, BC, 28 p.
Stathers, R. J. 1989. Summer frost in young forest plantations. F.R.D.A. Report No. 073, BC Ministry of Forests,
Victoria, BC, 24 p.