pocketmine\level\generator\populator\Tree::setBaseAmount PHP Method

    public function setBaseAmount($amount)
    {
        $this->baseAmount = $amount;
    }

 public function __construct()
 {
     parent::__construct();
     $trees = new Tree(Sapling::SPRUCE);
     $trees->setBaseAmount(10);
     $this->addPopulator($trees);
     $tallGrass = new TallGrass();
     $tallGrass->setBaseAmount(1);
     $this->addPopulator($tallGrass);
     $this->setElevation(63, 81);
     $this->temperature = 0.05;
     $this->rainfall = 0.8;
 }