In the tree example it's not the tree that's collecting interest, its the 7 dollars today that I could simply put in a bank at 5% and which would become $1000 dollars in 100 years without having to DO any tree planting at all. This is really very basic NPV math, if you still don't understand then go educate yourself on this concept because it has a huge bearing on what activities the monetary system is encouraging and discouraging.
Ah, thank you, I was confused as to how you got from $900 profit to $93 loss. In that case though, why not just plant $93 worth of trees, and invest the $7 at 5%, and have $1930 at the end of 100 years, plus a plot of land that you can continue to plant trees on?
Assuming, of course, that you can find an investment that will pay out 5% for 100 years.