SNOWPACK AND THE DISTRIBUTION OF A MAJOR VEGETATION ECOTONE IN THE SIERRA-NEVADA OF CALIFORNIA

The upper montane red fir (Abies magnifica) forest of northern California experiences the highest snow-packs of any vegetation type in California. We found correspondence between the lower limit of this forest - the ecotone where red fir yields dominance to white fir (Abies concolor) - and changes in snowpack. Analysis of atmospheric soundings revealed that the mean freezing level during storms from December to March (the period of maximum precipitation) centres on the ecotone. There is a strong discontinuity with elevation in the form of winter precipitation (snow versus rain) that geographically coincides with the red fir-white fir ecotone. The proportion of red fir at fifty-one stations in northern California was more strongly correlated with long-term mean 1 April snow water equivalence (SWE) than with elevation. Four elevational transects at different latitudes through the white firred fir ecotone also showed positive, significant correlations between red fir importance and both snow depth and SWE.