Terrain.treeCrossFadeLength
var treeCrossFadeLength: float;
float treeCrossFadeLength;
treeCrossFadeLength as 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

public class Example(MonoBehaviour):

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