Use the Knife.

While you feel sure that you could hit the vines binding your crewmate without fail, you would prefer not to take the chance. You sling the rifle over your shoulder, and withdraw your knife from it's sheath. Rising to you feet, you charge down the hill to where she is imprisoned, and begin to whittle away at the vines ensnaring her. The tree reacts almost as if defending itself. It tries to wrap around your arms and legs and trap you as well. But your motions are too quick, and you are able to both keep yourself free from the tree as well as eventually free Serena from her bonds. She collapses on your shoulder, exhausted from her travails. You drag her away from the evil tree as it's vines wave about in what you would swear was impotent fury.

You finally find a clearing out of range of any of the dangerous trees, and gently lower the weak woman to the ground. Taking out your water canteen, you slowly pour some of the liquid over her lips and into her mouth. Eventually, her eyes flicker open and go wide...the horror of what she endured patently obvious there. She grasps your arms fearful for a moment before burying her head in your shoulder and beginning to sob uncontrollably. You gently stroke her hair for a few moments, trying to calm herself and get herself under control.

Eventually, you realize you need to get moving. Gently, pulling the distraught girl away from you, you look down at her and whisper. "Ensign....I need you get a hold of yourself. We still aren't safe here. And there could be others that need our help. Ok?" You lift her head up, and force her to look into your eyes. She sniffles for just a moment, before straightening up. She a look for resolve tighten her face as she takes a deep breath and nods. You help her up to her feet, still weak but a bit more strength in her motions. "Thank you, Major," she says. "For saving me."

You nod to her. And turn back to the direction where you saw the smoke. "There was a crash over that way. We are going to investigate. After me," you say, your rifle coming out and ready once more. You set off in motion, trusting in the Ensign's training to get her in motion...and sure enough you hear her moving quickly behind you. Together you begin your march toward the smoke...