Terrain.treeCrossFadeLength  

var treeCrossFadeLength : float

Description

Total distance delta that trees will use to transition from billboard orientation to mesh orientation.

Decreasing this value makes the transition happen faster. Setting it to 0 will produce a visible pop when switching from mesh to billboard representation

function Start () {
    Terrain.activeTerrain.treeCrossFadeLength = 20;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Start() {
        Terrain.activeTerrain.treeCrossFadeLength = 20;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Start():
        Terrain.activeTerrain.treeCrossFadeLength = 20