I don't know the specifics for the plant/locations mentioned, but generally higher altitudes (and northern latitudes) have lower average temperatures, and therefor lower evaporation, allowing plants to thrive with lower precipitation levels. Much of Alberta's productive agricultural area receives rainfalls similar to drylands farther south, but with lower average temperatures- and also the timing of precip, heaviest rain from late may to mid july, plants can do well with lower overall precipitation.
edge of the boreal mixed woods zone, just east of the Rocky Mtn Foothills, z 2/3
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