After a weekend of modest, but wet snowfall, there were 4 trees down on our run yesterday. So, how do you get a double stroller that weighs in around 100 pounds over a tree? Well, it depends...
Level 1 crossings: Less than ~12"
Just pop that front wheel up and over like a curb. Depending on your wheel diameter, you may need to use the bottom of the stroller frame like a see saw to get the front wheel down on the other side.
Level 2: ~12-18"
Turn around and pull the back wheels over first. (And, yes, I do work out.)
Level 3: ~18-30"
Take the kid(s) out. Get them to the other side and then lift the empty stroller over.
Level 4: Over 30"
Find a way to go around it or change the plan to an "out and back" run.
There are other complicating factors such as branches or multiple trunks. You, of course, need to use your own good judgment depending on your strength, experience, and the weight of your children.
Finally, if you keep lots of random stuff in the under carriage basket as I do, you'll want to check that you haven't lost anything after the big tipping. Good luck.