Swapping 2 different tree models

If you have multiple components accessing something like a universal/global time, I think it would be a good Idea to have a SeasonChanger Script. In this script you use Time.deltaTime to calculate how many days have elapsed. Your calculations would come out to things like int DayNumber, int SeasonNumber, etc.

Then on your tree prefab, you can reference your SeasonChanger script and change the model based on that. I find that it is easier to have two gameObject children on that tree prefab then disable one and enable the other to change the tree models. Good luck!

Ok here is a little Script if you use TerrainComponent.

Just be sure that the Lists of Seasonal Trees contain the Same Amount of Trees

EDIT adjusted the Code to four Seasons + Explanation.

---

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using System;
 
 [ExecuteInEditMode]
 public class SeasonalScript : MonoBehaviour
 {
     // Start is called before the first frame update
     public int DaysGone = 45;
     int SeasonalCounter = 0;
     public float Speed = 1;
     public float timer = 0;
     public bool LoadTrees = false;
     bool isOverDaysGone; 
 
     public GameObject[] SeasonalTrees_1 = new GameObject[0];
     public GameObject[] SeasonalTrees_2 = new GameObject[0];
     public GameObject[] SeasonalTrees_3 = new GameObject[0];
     public GameObject[] SeasonalTrees_4 = new GameObject[0];
 
     List<Action> SeasonVoids = new List<Action>();
 
     List<TreePrototype> TreeProtottypes = new List<TreePrototype>();
     
     TerrainData _ted;
     Terrain _ter;
 
     void OnEnable(){
         SeasonVoids.Clear();
         SeasonVoids.Add(SeasonOne);
         SeasonVoids.Add(SeasonTwo);
         SeasonVoids.Add(SeasonThree);
         SeasonVoids.Add(SeasonFour);
 
         _ter = GetComponent<Terrain>();
         _ted = _ter.terrainData;
         timer = 0;
         isOverDaysGone = false;
     }
     // Update is called once per frame
     void Update()
     {   
         if(LoadTrees){
             SeasonalCounter = 0;
             SeasonOne();
             LoadTrees = false;
         }
 
         if(UnityEditor.EditorApplication.isPlaying){
             timer += Time.deltaTime * Speed;
             if( timer> DaysGone){
                 
                 isOverDaysGone = true;
                 timer = 0;
             }
             if(isOverDaysGone){
                 SeasonVoids[SeasonalCounter]();
                 SeasonalCounter++;
                 if(SeasonalCounter == 4){
                     SeasonalCounter = 0;
                 }
                 isOverDaysGone = false;
             }
         }
     }
 
     public void SeasonOne(){
         TreeProtottypes.Clear();
         for(int i = 0; i< SeasonalTrees_1.Length; i++){
             TreePrototype TP = new TreePrototype();
             TP.prefab = SeasonalTrees_1[i];
             TreeProtottypes.Add(TP);
         }
         _ted.treePrototypes = TreeProtottypes.ToArray();
         _ted.RefreshPrototypes();
         _ter.Flush();
     }
     public void SeasonTwo(){
         TreeProtottypes.Clear();
         for(int i = 0; i< SeasonalTrees_2.Length; i++){
             TreePrototype TP = new TreePrototype();
             TP.prefab = SeasonalTrees_2[i];
             TreeProtottypes.Add(TP);
         }
         _ted.treePrototypes = TreeProtottypes.ToArray();
         _ted.RefreshPrototypes();
         _ter.Flush();
     }
     public void SeasonThree(){
         TreeProtottypes.Clear();
         for(int i = 0; i< SeasonalTrees_3.Length; i++){
             TreePrototype TP = new TreePrototype();
             TP.prefab = SeasonalTrees_3[i];
             TreeProtottypes.Add(TP);
         }
         _ted.treePrototypes = TreeProtottypes.ToArray();
         _ted.RefreshPrototypes();
         _ter.Flush();
     }
 
     public void SeasonFour(){
         TreeProtottypes.Clear();
         for(int i = 0; i< SeasonalTrees_4.Length; i++){
             TreePrototype TP = new TreePrototype();
             TP.prefab = SeasonalTrees_4[i];
             TreeProtottypes.Add(TP);
         }
         _ted.treePrototypes = TreeProtottypes.ToArray();
         _ted.RefreshPrototypes();
         _ter.Flush();
     }
 
 
 }
 

Ok, what i've done: I added to the SeasonVoids(an Action List) all Season Voids (SeasonOne, SeasonTwo, SeasonThree, SeasonFour).

Now if the Timer is above the given Value(DaysGone), it triggers the boolean isOverDaysGone. That will execute SeasonOne, we have now a Counter which Seasons will be next called SeasonalCounter.

After a Seasons is Executed, Seasonal Counter will be Added +1 (Operator ++). If The SeasonCounter reaches the fourth Season(SeasonalCounter == 3(Counts from 0 upwards(0,1,2,3))) then the Counter resets to 0 Again.

If you have any more Questions how it's done, feel free to Ask.

Dan