Okay - got a question to ask you about this situation.
You say "TreeLINE". Now to me that indicates a number of trees and probably some distance from the camera. Now, you need a pretty BIG HONKIN flash unit to reach out more than 20~30 feet (most on camera flashes are good for 15~20 feet). So one issue here is you need a good sized flash unit if these trees are out 30~40 feet or more from your camera.
As a couple others have mentioned here, having a tripod with a slow shutter speed also works to get the lower amient light, while hoping your flash will catch the closer trees.
And Ms.Begin's idea of multiple flashes is cool too - if you have a hand-held strobe you can move to different areas of the treeline and expose in sections. We used to call this technique "painting with flash" or "Painting with light". It works best when the ambient light is REALLY low (dusk, or early evening after sunset - also works great in an old graveyard close to midnight!)
Another PHOTOSHOP solution to this situation takes lots of work, but can be quite striking: take TWO exposures of the scene... one exposed for trees, one for background light. You HAVE to use a tripod for this! Then, in photoshop with clever masking you combine elements of the two exposures to give you what you need. These shots can look Mucho Cool.
On a side note - using 1600 ISO / ASA on a digital camera is really NOT recommended at any time unless you really like that splotchy look (which can, at times, be cool). If you use a lot of higher IOS settings (above 400) I recommend a noise reduction software like Noise Ninja or NeatNoise... I have shot at 1600+ with my 30D and got it cleaned up nicely using these softwares.
And now, its Saturday here in Canada so I must go clean off the beaver droppings from the outer layer of my igloo before it attracts polar bears drawn in by the pungent odours!
Good luck with your TreeLine!
P.S. if it appears the trees move closer to you every time you take your eyes off them, you might want to switch to something lighter than whiskey!