问一个JS继承的问题

Question

function Tree () {}

Tree.prototype.leaf = 0

Tree.prototype.addLeaf = function () {this.leaf++}

let tree1 = new Tree()

let tree2 = new Tree()

tree1.addLeaf()

tree1.addLeaf()

console.log(tree2.leaf) 

这里我把箭头函数去掉下
为什么输出是0.......求解惑