treeLength = irandom_range(3, 9);

create = (irandom(100) <= 40)

if create
{
      for(i = 0; i < treeLength; i += 1)
      {
          tree = instance_create(x, y - 32 * i, objBlockWood);
      }

      instance_create(tree.x, tree.y, objBlockLeaf);
      instance_create(tree.x, tree.y - 32, objBlockLeaf);
      instance_create(tree.x - 32, tree.y, objBlockLeaf);
      instance_create(tree.x + 32, tree.y, objBlockLeaf);

}

instance_destroy();