Areas occupied by mountain hemlock generally have a cool to cold maritime climate that includes mild to cold winters, a short, warm to cool growing season and moderate to high precipitation (table 1). Annual and summer precipitation and the proportion of precipitation as snow show notable latitudinal climatic trends in the range of mountain hemlock (table 1). Latitudinal trends in mean temperatures are not evident.
Table 1- Climatological data from 14 weather stations within the range of mountain hemlock Temperature Precipitation Location and number of stations
Annual
January
July
Annual June to August Portion¹ as snowfall °C mm mm pct Alaska², 3 4 -7 13 1681 346 14 British Columbia, 4 3 -3 11 3021 361 29 Washington, 2 4 -3 12 2728 270 51 Northern Idaho, 1 3 -7 15 971 124 65 Oregon, 1 4 -4 13 1643 99 81 California, 3 3 -4 13 1048 51 88 °F in in pct Alaska² (80) 39 20 56 62 13 14 British Columbia (9,45) 38 26 52 104 14 29 Washington (20) 39 27 53 107 11 51 Northern Idaho (79) 37 19 59 38 5 65 Oregon (20) 39 25 56 65 4 81 California (64,77) 38 24 56 39 2 88 ¹Estimated snowfall by assuming 10 cm (4 in) of snow is equivalent to 1 cm (0.4 in) of rain, for all locations but those in British Columbia.
²Stations in Alaska are near sea level. Mountain hemlock grows at higher elevations where temperature and precipitation levels are likely to vary from those recorded. The high snowfall results in snowpacks with maximum depths that range from 245 cm (96 in) in Idaho to 380 cm (150 in) in British Columbia (9). A snowpack may cover the ground for long periods (7 to 10 months in southwestern British Columbia) (9). The relatively short growing season (frost-free period) ranges from 95 to 148 days in southwestern British Columbia (9,45) and from 49 to 63 days in the central Sierra Nevada (64).
Climatic extremes include a temperature range of -29° to 38° C (-20° to 100° F) (11), annual snowfall in excess of 2200 cm (866 in) (66), snowpack up to 750 cm (295 in) (20), and persistence of the snowpack until August or September (9,66). Because there are few weather stations in the range of mountain hemlock, reported extremes are probably often exceeded.
Mountain hemlock grows in an altitudinal band 300 to 1000 m (1,000 to 3,300 ft) wide that increases in altitude from north to south:
Altitude m ft Alaska (83) 0 to 1067 0 to 3,500 Northern British Columbia (46) 300 to 900 1,000 to 3,000 Southern British Columbia (46) 900 to 1800 3,000 to 5,900 Northern Washington (20) 1300 to 1700 4,200 to 5,600 Rocky Mountains (12,33,63) 1550 to 2100 5,100 to 6,900 Southern Oregon (42) 1600 to 2300 5,200 to 7,500 Northern Sierra Nevada (64) 2400 to 3050 7,900 to 10,000 Southern Sierra Nevada (62) 2750 to 3050 9,050 to 10,000 The presence of mountain hemlock in the Rocky Mountains is closely correlated with the eastward penetration of moist maritime air masses (33). On the east side of the Coast Mountains in British Columbia, mountain hemlock is limited to relatively moist sites where snow accumulates early in the fall (46). Krajina (46) proposes that mountain hemlock does not grow on sites with later, thinner snowpacks because it cannot tolerate the frozen soils there.
Throughout most of the range of mountain hemlock local climate differentiates two types of subalpine mountain hemlock forest. A parkland subzone of single trees and small tree clumps (average canopy cover less than 25 percent) extends from treeline or near treeline to the lower forest subzone of relatively continuous forest cover (canopy cover more than 25 percent) (2,9,21,82). Most climatic data (table 1) are from the forest subzone. Detailed microclimatic data (9) and data extrapolated upslope from the weather stations (45) in southwestern British Columbia indicate that the parkland subzone has shorter frost-free and snow-free periods and that maximum snowpack, range of summer temperatures, and actual evapotranspiration are greater than in the forest subzone. Mountain hemlock also grows above treeline in the alpine environment as prostrate krummholz (elfinwood) throughout most of its range (10,20,35).
If climate warms as hypothesized for western North America (24), existing mountain hemlock forests will probably increase in productivity; upper and lower boundaries of the mountain hemlock zone, within which new mountain hemlock forests become established after disturbance, will increase in elevation; and the zone will decrease in area. Near Mount Baker, Washington, ring width of mountain hemlock increases with increasing monthly temperatures in the preceding 12 months, decreasing winter precipitation (37), and decreasing spring snow depth, down to about 1 m (3.3 ft) (27). This implies productivity should increase with predicted temperature increases. Graumlich and others (28) estimated that productivity increased 60 percent in the last century in four high-elevation stands in Washington, three of which contained 48 to 96 percent mountain hemlock. They related this increase most strongly to the increase in growing-season temperature during this period (about 1.5° C or 2.7° F). Thus, further increases in temperature may cause further increases in productivity.
Based on the current elevational distribution of major forest zones in the Oregon Cascades and a mean temperature lapse rate of 4.4° C/100 m (2.4° F/1,000 ft), Franklin and others (24) have hypothesized the effects of two warmer climates. If mean annual temperature increases 2.5° C (4.5° F), the mountain hemlock zone in Oregon may be shifted upwards 570 m (1,900 ft) and decrease in area from 9 to 2 percent; an increase of 5.0° C (9° F) may move it upwards 1140 m (3,700 ft)-above all but the tallest peaks-so it is effectively eliminated.