Web storage is simple. You're just storing pairs of keys and value. The keys are strings, the values are strings or numbers.
For example, to store the player score:
WebStorage: Set local key "score" to 500
Then when you want to get that value from web storage you do:
Set variable to WebStorage.LocalValue("score")
You can easily save an entire array or dictionary to webstorage by saving it as JSON.
WebStorage: Set local key "savedArray" to Array.AsJSON
Array: Load from JSON string WebStorage.LocalValue("savedArray")
For your tree I suppose you could do something like
System: Every x seconds
System: Compare two values floor(random(100)) < n
-> Tree: Subtract floor(random(a, b)) from var
(Every x seconds there is a n% chance to subtract a value from a to b-1)