United Airlines and Continental Airlines want you to save some trees and get in the habit of using mobile boarding passes instead of the printed-paper variety.

And, for Earth Day on Friday, April 22, the merged airlines are putting their money where their apps are — up to $50,000 for charity, that is.

United Continental Holdings, the holding company for the two airlines, pledges to donate $1 to Conservation International’s Protect an Acre project each time passengers use a mobile boarding pass on Earth Day.

When checking in for flights online, passengers can opt to get their boarding passes on their mobile devices and the airlines will email them to you.

With the mobile boarding passes, you just saunter up to the gate, scan your mobile pass and away you go.

And, Continental Airlines especially wants to push consumer use of its mobile apps for Android and iPhone and thus will kick in $2 each time a passenger uses a mobile boarding pass via their apps.

The total donation is capped at $50,000.

As we wrote March 30 here on the CheapOair travel blog, lots of hotels and other travel companies are getting in gear for Earth Day.

The Protect an Acre program preserves swaths of tropical forests, which help blunt carbon emissions.

United Continental Holdings says more than 10 million of their customers to date have used mobile boarding passes on their cell phones or PDAs at 89 domestic and international airports since the program was launched in 2007.

The airlines calculate that they have thus saved 1,200 trees in the process. Mobile boarding passes will undoubtedly increasingly become the norm over the next few years.