The Outer Hebrides Islands of Scotland are at the same latitude as northern Saskatchewan. However, palm trees grow naturally in the Hebrides while the dominant vegetation in this part of Saskatchewan is boreal forest (taiga). What could account for this difference in vegetation?

The water amount makes the difference

Explanation:

The vegetation is the result of different nature aspects. In this case there is an important feature of each local that determine the large difference on vegetation. The area of Saskatchewan is located within Canadian territory and Outer Hebrides are Islands bathed by the Atlantic ocean. Both territories suffer influence from the Arctic but the area on Scotland territory has the great amount of water at its favor. Considering the low specific heat capacity of water (which is extremely high) the coldness of the Arctic influence will be less severe on the scottish islands than at the Canadian territory.