Here are some silhouettes I did for a background for a new image. The trick with silhouettes is to treat them just as you would any other drawing--a generalized or abbreviated silhouette will be noticeably unconvincing. The trick with trees in particular is to avoid repetitive patterns. Even though the bows and limbs of certain varieties of trees have a unique growth pattern, each branch and leaf is unique. The more cookie cutter you draw them, the less real they'll seem, so it's important, even when drawing trees from your imagination, to be conscious of keeping the leaves and limbs diverse.