void setup() {
  size(600, 600);
  background(0);
  stroke(255);
  drawTree(300, 550, 9);
}

void drawTree(float x, float y, int depth) {
  float forkAngle = radians(20);
  float baseLen = 10.0;
  if (depth > 0) {
    pushMatrix();
    translate(x, y - baseLen * depth);
    line(0, baseLen * depth, 0, 0);
    rotate(forkAngle);
    drawTree(0, 0, depth - 1);
    rotate(2 * -forkAngle);
    drawTree(0, 0, depth - 1);
    popMatrix();
  }
}