Otto Nordenskjöld
Otto Nordenskjöld theorized that winter conditions also play a role: His formula is "W" = 9 − 0.1 "C", where "W" is the average temperature in the warmest month and "C" the average of the coldest month, both in degrees Celsius (this would mean, for example, that if a particular location had an average temperature of in its coldest month, the warmest month would need to average or higher for trees to be able to survive there).