An RGB file starts with three channels: one each for red, green, and blue. Photoshop also displays a composite, one for all of the channels combined, although this composite isn't actually a channel itself. You can view and edit each channel individually, giving you total control over your image.
Now the trees are extracted from the background.
You use any tool, run any adjustment or effect on a channel.
Click ok.
Using the brush tool set to overlay paint over the trees to strengthen the contrast, notice that the white values are unaffected by the paint strokes.
Open the channels palette and pick the channel with the most contrast.
Duplicate the Channel by dragging down to the new channel icon.
Use levels to boost the contrast on the duplicated channel.
Click on the load selection icon or command click on the Green Channel. The image should now be selected. Choose select inverse. Return back to the layers palette. Now the dark pixels are selected.
Click on the layer mask icon at the bottom of the layers palette.
