How much of a firbolg's Speech of Beast and Leaf replaces Speak with Animals/Plants?

Question

I am simply having a hard time wrapping my head around the versatility of this feature, and the meaning of the last aspect: (emphasis my own)

You have the ability to communicate in a limited manner with beasts and plants. They can understand the meaning of your words, though you have no special ability to understand them in return. You have advantage on all Charisma checks you make to influence them.

Does this mean that you can attempt to persuade them to do simple favours in the same way you would do with an NPC? For plants, would that allow you to mimic the difficult terrain feature of Speak with Plants? Or is this feature meant for something else entirely?

Answer 1

This is entirely up to the DM

As a player, you neither to get to ask to make Charisma checks, nor do you get to decide what the outcome of these checks is. (Page 236-237, DMG):

Dice are neutral arbiters. They can determine the outcome of an action without assigning any motivation to the DM and without playing favorites. The extent to which you use them is entirely up to you. (...) When a player wants to do something, it's often appropriate to let the attempt succeed without a roll or a reference to the character's ability scores. For example, a character doesn't normally need to make a Dexterity check to walk across an empty room or a Charisma check to order a mug of ale.

If you are able to talk to animals and plants, and they are able to understand you, then yes, you can try to convince them to help you or do you a favor, if you ask them nicely and treat them well. But what they'll do without a check, and what they'll do with a successful check, and what they can or cannot do at all, is the DM's call.

Speak with Animals, and Speak with Plants are their own, separate spells and effects. As these are spells, they may be able to do things that your normal ability to speak with beasts or plants cannot do. You therefore cannot assume that you can transfer the spell results to this ability. However, as the behaviours and abilities of normal plants or animals have very little coverage overall in the rules, it might be a reference point for the DM to look at these spells and get some idea what you could achive from speaking with beasts or plants. Clearly, as in the case of Speech of Beast and Leaf they cannot answer, you cannot do anything related to questioning the beasts or plants of what is going on in the area.

If you can ask plants make terrain difficult or stop making it so, is also up to the DM. If I were DM, I'd probably look at the densitiy of plant growth, and if it is sufficient (brambles, hedges or such) I'd allow it.

To me, this whole quality has a fairy-tale like vibe. I'd let the animals or plants provide minor help (or minor hindrance to opponents) in line with their own self-interest, nature and need of survival. I'd not let them fight on the PC's behalf, unless they were some kind of long-time friends or companions of the PCs.