To have a positive tone, it has to invoke a positive or "warm" feeling when you read it. The sentence has no positive or negative feel to it, it's just a statement.
The sentence isn't referring to anything that is put in place of something else — i.e. someone talking about a bird going silent but is actually referring to a mood or feeling — it's just a plain statement without talking about anything besides what is in the statement
The population of Marsh grasses might increase if the population of Herons decrease because the herons use the marsh to live in and potentially eat. I hope this helps
