Tree Advance Documented

A study of 166 sites around the world shows that trees are advancing to higher latitudes and higher elevations at more than half the locales, retreating in only two study sites, and remaining stable at the rest. Examining records from 166 areas where temperature and treeline records have been kept since 1900, scientists from New Zealand discovered that trees have advanced at 89 locations and remained stable at 77. The key factor in colonization of new areas appeared to be whether winter temperatures had risen in the past century, as treeline advance was most pronounced at sites where winters were warmer. Winter temperatures rose at 77 sites by an average of about 2 degrees C (3.6 F) over the past century, and summer temperatures increased at 117 of the 166 sites, rising by an average of 1.4 degrees C (2.5 F) since 1900. The findings, published in the journal Ecology Letters, seemed to suggest that rising winter temperatures were the crucial factor, because even if seedlings did colonize new areas in the warmer months, colder winter temperatures would kill them if they advanced to higher latitudes and elevations that had not warmed.