So, why are the plant zones changing?

• Donald Boesch

The interesting front-page story about the shifts in the Agriculture Department's plant hardiness zone maps since 1990 ["New plant map shifts area to warmer zone," Jan. 26] included this headline on the continuing page: "Plant map doesn't measure climate change." However, nothing in the article discusses the relationship of the zone changes to climate change, whether global, national or regional. The nearly uniformly northward shifting zones reflect increases in "average winter low temperatures