FACTORS LIMITING THE ELEVATION OF TREELINES
· Winter temperatures at treeline level are distinctively different, comparing the two study areas (82 is colder). Summer temperatures are almost exactly the same at treeline level at both study sites (0.1 degree variation).
Average temperature at treeline level depending on season and study area:
|
|
study area 82: Rocky Mtns |
study area 92: Coast Mtns |
|
Mean January Temperature (deg C) |
-14.9 |
-8.7 |
|
Mean July Temperature (deg C) |
+8.0 |
+7.9 |
· This confirms what has been stated in research literature about treelines: The most crucial factor in determining treeline elevation is the mean temperature of the warmest month of the year (July in the northern hemisphere). This is not 10 (according to literature), but 8 degrees Celsius for study areas 82 and 92. As the employed temperature surface model is very simplistic, we cannot make conclusions about the actual temperature that determines treeline elevation. However, the close matching of results for summer temperatures is meaningful, as the same model has been used for both study sites.
· Snowfall in study area 92 (174 mm monthly precipitation for Dec.-Feb.) is more than 50% higher than in 82 (110 mm). According to literature, thick snow cover, and associated long duration of snow cover, are important limiting factors for treeline elevation. However, this does not alter the temperature requirement for tree growth (see above): if snow cover had a strong direct influence on treeline elevation, a higher July temperature could be expected at study area 92 (warmth trades off with strong snow cover). This is not the case. It has been stated in bioclimatic literature that snow cover rather has a direct influence on the atmospheric temperature in the area, i.e. snow cover decreases atmospheric temperature. However, from my analysis, no conclusions can be drawn about how much winter precipitation influences treeline elevation.
· Treeline elevation and minimum altitude of glacial extent closely correlate in the two study areas. The difference between the two study sites is approximately 425 m (82: 2400 m; 92: 1975 m), compared to a difference in treeline elevation of ca. 350 m. Taking aspect into account, it makes sense to compare only the difference in elevation between treelines at north-facing slopes, as this is where glaciers have furthest low-altitudinal extent due to a lack of solar radiation. This difference is 384 m (82: 2084 m; 92: 1700 m), which closely resembles the glacial elevation difference of 425 m. As a result, we can say that climatic factors in general are the crucial limiting factors for the elevation of treelines. Non-climatic factors such as soil type or tree species only play a very minor role in this context.
