How to save the object which has ArrayList of other objects using DataOutputStream?

Question

How to save the object which has ArrayList of other objects using DataOutputStream and then load them from the file? I'm using serialization, but ArrayList's objects don't want to load.

Here I save:

 void saveChunk(Chunk chunk) {
    String currentChunkName = SAVES_FOLDER + "\\" + name + "\\" + "chunk" + chunk.x0 + "_" + chunk.y0 + END_CHUNK_NAME;
    File f = new File(currentChunkName);
    if(!f.exists()) {
        try {
            f.createNewFile();
        } catch(IOException e) {
            e.printStackTrace();
            System.err.println("Can't create data for chunk " + currentChunkName);
        }
    }
    try {
        File chunkData = new File(currentChunkName);
        DataOutputStream dout = new DataOutputStream(new FileOutputStream(chunkData));
        try {
            ObjectOutputStream out = new ObjectOutputStream(dout);
            out.writeObject(chunk);
            for(Tree tree : chunk.trees) {
                out.writeObject(tree);
            }
        } catch(IOException e) {
            e.printStackTrace();
            System.err.println("Can't create ObjectOutputStream " + currentChunkName);
        }
    } catch (FileNotFoundException e) {
        e.printStackTrace();
        System.out.println("Can't save chunk " + chunk.x0 + "_" + chunk.y0);
    }
}

And here I load:

    Chunk loadChunk(int x0, int y0) {
    String currentChunkName = SAVES_FOLDER + "\\" + name + "\\" + "chunk" + x0 + "_" + y0 + END_CHUNK_NAME;
    try {
        File chunkData = new File(currentChunkName);
        if(!chunkData.exists()) {
            generateChunk(x0, y0);
        }
        DataInputStream din = new DataInputStream(new FileInputStream(chunkData));
        try {
            ObjectInputStream oin = new ObjectInputStream(din);
            try {
                Chunk chunk = (Chunk) oin.readObject();
                return chunk;
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    } catch (FileNotFoundException e) {
        e.printStackTrace();
        System.err.println("Can't find chunk " + x0 + "_" + y0);
    }
    return null;
}

There you can see the Chunk class with List (ArrayList) of trees:

public class Chunk implements Serializable {

public static final byte CHUNK_SIZE = 12;
private int[] floors = new int[CHUNK_SIZE * CHUNK_SIZE];
public List<Tree> trees = new ArrayList<>();

int x0, y0, x1, y1;

public Chunk(int x0, int y0) {
    this.x0 = x0;
    this.y0 = y0;
    x1 = x0 + CHUNK_SIZE;
    y1 = y0 + CHUNK_SIZE;
}

public void addNewTree(int x, int y) {
    Tree tree = new Tree(x, y);
    addTree(tree);
}

public void addTree(Tree tree) {
    trees.add(tree);
}........ect

Classes "Chunk" and "Tree" both implements interface Serializable. But when I'm trying to launch, that doesn't work: I have just a gray screen. But if I remove Serializable implementation in class "Tree", only chunks without trees are working, but the other cant load and I have black areas and this exceptions:

java.io.InvalidClassException: org.ixnomad.game.level.nature.trees.Tree; class invalid for deserialization
at java.io.ObjectStreamClass$ExceptionInfo.newInvalidClassException(ObjectStreamClass.java:150)
at java.io.ObjectStreamClass.checkDeserialize(ObjectStreamClass.java:790)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1782)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1353)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:373)
at java.util.ArrayList.readObject(ArrayList.java:791)
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:1058)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1909)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1808)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1353)
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:2018)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1942)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1808)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1353)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:373)
at org.ixnomad.game.level.Level.loadChunk(Level.java:44)
at org.ixnomad.game.level.InfiniteLevelRenderer.checkVisibleChunks(InfiniteLevelRenderer.java:70)
at org.ixnomad.game.level.InfiniteLevelRenderer.renderLevel(InfiniteLevelRenderer.java:55)
at org.ixnomad.MainComponent$SoftwareComponent.render(MainComponent.java:156)
at org.ixnomad.MainComponent$RunComponent.run(MainComponent.java:140)
at java.lang.Thread.run(Thread.java:745)