Many people try to use less paper, in order to save trees. After all, who doesn’t like trees? And paper is made from wood, which means cutting them down. It seems like the more trees we use, the fewer we have left.
If trees were a non-renewable resource, like oil or helium, that reasoning would be totally valid. However, forests are one of our most renewable resources, since trees grow back. The more paper (and other wood products) we use, the more trees will get planted to satisfy demand, and the more total forest area we’ll have. With trees, the more we use, the more we’ll have, just as buying lots of beef means more cows.
Consider a company that owns a big stretch of undeveloped land. If people use lots of paper, then the demand for paper drives up the price of wood, making it valuable to grow trees on the land. If demand for wood is low, on the other hand, growing a forest won’t be profitable, and the company will do something else with the land, like turning it into parking lots and strip malls. Or, more commonly, turning it into farmland for growing yet more corn, which has problems of its own.
In the United States, we now have more forests than we did in 1900, despite producing about two hundred million tons of wood annually for the last four decades. The idea of old forests being “used up” en masse by logging and not replaced is simply untrue; according to one estimate, only 7% of US forest area is more than a century old, yet total forest area continues to increase. (Untouched, “old growth” forest is important environmentally, so it would be a bad idea to use it all for wood production. But about a third of all forested land is already in public parks, where it can’t be used in any case.)
Note, however, that this logic doesn’t hold in many developing countries, where it’s common to raze forests for wood and use the land for cattle-grazing or other agriculture. (The US did the same thing in the 19th century, when our economy was similarly undeveloped.) So, to protect trees, buy American wood products.
The Rationalist Conspiracy 
The problem is that the trees planted for produce stuff are not exactly the trees disappeared. Yes, there are more trees, but no, there will be less different kind of trees.
Not quite. The thing is, there’s a big difference between a tree farm and a functioning ecosystem. The people who want to save the trees are generally thinking of the old growth, which in many places (Brazil, British Columbia), is still being harvested.