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• In general, the Sub­Boreal Pine­Spruce zone (SBPS) has cold, dry winters and cool,
dry summers. Substantial moisture deficits are normal during the middle and latter
parts of the growing season. The low precipitation, dry air, and clear skies in the Coast
Mountains rainshadow result in significant night‐time radiation cooling and low
overnight temperatures. Frost can occur at any time of the year, especially in low‐
lying areas. The SBPS zone is one of the least productive areas for tree growth in the
region, outside of the Bunch Grass and Arctic Tundra
zones which are generally
considered “non‐forested.”
• The Very Dry Cold Sub­Boreal Pine­Spruce subzone (SBPSxc) occurs in the southern
and western parts of the SBPS zone in the study area. This subzone is strongly
affected by the Coast Mountains rainshadow, and the SBPSxc has the lowest annual
precipitation of the SBPS subzones. Vegetation production and soil development are
severely limited by the cold, very dry climate.
• The Dry Cool Interior Douglas­fir subzone – Chilcotin variant (IDFdk4) is the
coldest biogeoclimatic unit of the IDF zone in British Columbia and is climatically
transitional from the generally warmer portions of the IDF zone to the cold, dry SBPS
zone.
Areas that are colder or drier than the pine and spruce forests of the Chilcotin Plateau are
generally not forested. Steen and Coupe (1997) comment that the total annual precipitation
near Tatla Lake at the western part of the Plateau is only 338 mm. For comparison, any
region that receives less than 250 mm of precipitation annually is generally defined as a
desert. Moist forested areas in the Cariboo region, such as the Interior Cedar‐Hemlock Zone
to the east, receive 700 to 800 mm of precipitation annually while some of the wetter areas
of the Coast Mountains to the west receive in excess of 2500 mm of precipitation per year
4.3. Historical Climate Trends
Definition of scientific baseline based on 1961­1990 Station Climatology Data 19
Long‐term climate trends show that considerable warming has taken place in the Cariboo‐
Chilcotin and surrounding areas over the last century. A recent report from the Pacific
Climate Impacts Consortium 20 shows that during this time period, the mean annual
temperature has increased about 1°Celsius in the region. Even though data show a clear
warming trend, there are large year‐to‐year variations in temperature, with ENSO climatic
cycles having had a strong impact on temperature. Historical changes in precipitation are
less clear and consistent than for temperature. What is certain, however, is that historical
changes in temperature have already had real implications for important hydrological
variables, including snow accumulation and timing of snow melt. In addition, divergences in
temperature and precipitation from average conditions due to natural cycles and climate
change have affected ecosystems and resource management over the past century in this
region.
The following provides a baseline for climate information by using data for the 1961‐1990
period. Note that the 1961‐1990 climate period is used as a baseline against which the
climate change projections shown
in section 4.4 are compared. Tables 2 and 3 show
baseline
temperature and precipitation data for the Tatlayoko Lake, which is the weather
19 The information in this section is extracted from Pacific Climate Impacts Consortium (PCIC) 2008.
20 Pacific Climate Impacts Consortium (PCIC), 2008, p.30.
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