You could try dodging the tree. I just happened to have my Elements 4 book next to my head.
1-Go to the Layers palette & select "new layer" from clicking on the (more) button
2-In the "new layer" dialog box, change the mode to Overlay, and then click on the check box next to "fill with overlay-neutral color (50% gray)". This'll create a new layer over the picture of Neutral gray color, but because of the mode selected, it won't actually turn your picture grayish.
3-Select the brush tool and set up the options for it so it's medium sized & soft-edged. Make the Opacity 30%.
4-In the colors box in the bottom of the toolkit, make sure the foreground color is white. Select the gray layer and start painting white over where ya want things to lighten up.
I hope I explained that right. Here's what I got after fooling with it fer a bit. I didn't dodge everything to make it more obvious.