adding the trunk
function tree() {
  var trunk=150;
  bk(trunk);
  fd(trunk); vee(100);
}
function test() {
  tree();
}

Looking good but we need to move it all down. This means we need to be able to move without drawing a line.

So, two new functions: pu() and pd() for pen up and pen down.

We'll also need a new global g_pen so fd() knows what to do.

function pu() {
  g_pen='up';
}
function pd() {
  g_pen='down';
}
function fd(d) {
  g_ctx.beginPath();
  g_ctx.moveTo(g_x,g_y);
  var rad=g_h*Math.PI/180.0;
  var dx=g_step*d*Math.sin(rad);
  var dy=-g_step*d*Math.cos(rad);
  g_x=g_x+dx; g_y=g_y+dy;
  if (g_pen=='down') {
    g_ctx.lineTo(g_x,g_y);
    g_ctx.stroke();
  } else {
    g_ctx.moveTo(g_x,g_y);
  }
}