Yosemite Vegetation Shifts from the 1930s to 1997

Overlay of 1930s vegetation survey map with 1997 vegetation cover map created from satellite images and ground truth observations. One can see the large expansion westward (uphill) of the montane forest (dominated by red fir, Abies magnifica) at the expense of the subalpine forest (dominated by lodgepole pine, Pinus contorta). We are searching for the cause of this shift. Climate or human impacts?

For the Yosemite project, climate data were obtained from Chris Daly's PRISM group at Oregon State University for an ~ 800m spatial grain size (30 arc second). For the 20th century as a whole, there is no meaningful linear trend in the average temperature of the coldest month (a.k.a. lowest mean monthly temperature). Instead, there is a downward trend in the first half of the century (slope = -0.02°C yr-1, r2= 0.62) followed by an upward trend in the second half (slope = +0.03°C yr-1, r2 = 0.89). The annual average monthly minimum temperature also shows an upward trend in the late 20th century, but it is a different pattern than the lowest mean monthly temperature in the first part of the century as there are only a few years of colder weather in 1911-16. The linear fit to 20th century monthly minimum temperatures is poor (r2 = 0.13).

The annual average of monthly maximum temperature shows no linear trend.There is no linear trend in growing degree days, but the two periods 1905-35 and 1967-97 have lower values relative to the rest of the century. Neither precipitation nor vapor pressure deficit shows any trend during the 20th century.

Note that he absence of alpine vegetation in the Wieslander VTM map does not mean that there was no alpine vegetation at that time, but is instead a result of the translation of the original Wieslander VTM vegetation classes into WHR ( California Wildlife Habitat Relationship) classes.