Possible Clues:

• Upper limit of a forest
• Tall plants will not grow beyond it
• Woods can't develop past this point involving staggering sensation
• Above which mountainside is quite bare
• The upper limit for growth of a certain kind
• Where the timber ends
• No planes over this border?
• Point about film part neither section of Hollywood can get over
• Altitude above which woods don't grow
• Square piece of film equipment in Spain is the limit of Hazel's achievement