Making Sense of Forest Terrain

Edgar Ironpelt · May 25, 2019

Either the "Forest Terrain" rules in the 3.5 SRD don't make any sense, or I'm grossly misinterpreting them.

There's an old thread about how the percentages for "trees, massive trees, etc." are suppose to be guidelines for GM mapping rather than 'Roll for each 5 ft square to see what terrain it has.' That would make sense - except that the percentages, as near as I can tell, are off by an order of magnitude.

Googling gives me figures of 100 trees/acre or less as sparse forest, and 200 trees/acre or more as "overgrown" in the real world. But using the SRD figures, 50% trees per 5 ft. square - supposedly "sparse" works out to ~860 trees per acre, with dense forest being twice that.

As a quick fix, I'm thinking of taking the SRD percentages and applying them to "per 15x15 grid of 9 squares" rather than "per square." That gives ~100 trees/acre in sparse forest, ~150 trees/acre in medium forest, and ~200 trees/acre in dense forest.

What do other people do?

ccs · May 25, 2019

I make up whatever details I need.

Why in the world would I need rules to tell me how many trees there are in any given acre??

Edgar Ironpelt · May 25, 2019

ccs said:
I make up whatever details I need.

Why in the world would I need rules to tell me how many trees there are in any given acre??

Well, I want rules or guidelines that give me an approximately correct number of trees per acre because I want my forests to look like forests. More generally, I want to be approximately realistic, in places where realism isn't a buzzkill, for the sake of adding verisimilitude to an otherwise bald and unconvincing narrative.

And in this case, by making forests more realistic, I'm also making them less of a fun-killing hindrance for my players. Win-win!

ccs · May 25, 2019

Edgar Ironpelt said:
Well, I want rules or guidelines that give me an approximately correct number of trees per acre because I want my forests to look like forests. More generally, I want to be approximately realistic, in places where realism isn't a buzzkill, for the sake of adding verisimilitude to an otherwise bald and unconvincing narrative.

And in this case, by making forests more realistic, I'm also making them less of a fun-killing hindrance for my players. Win-win!

Hey, if you want to spend your prep time counting fictional trees within a hypothetical acre, knock yourself out....

Edgar Ironpelt · May 25, 2019

ccs said:
Hey, if you want to spend your prep time counting fictional trees within a hypothetical acre, knock yourself out....

And if you're able to just wing it without spending a lot of time on "Hey, why do my forests look so fugly and suck to run encounters in?" then more power to you.

jaelis · May 26, 2019

I think you all are missing the forest for the trees here.

Man in the Funny Hat · May 27, 2019

jaelis said:
I think you all are missing the forest for the trees here.

With 860 trees per acre I don't think you're missing ANY tree. Ever.

LordEntrails · May 27, 2019

Edgar Ironpelt said:
Either the "Forest Terrain" rules in the 3.5 SRD don't make any sense, or I'm grossly misinterpreting them.

There's an old thread about how the percentages for "trees, massive trees, etc." are suppose to be guidelines for GM mapping rather than 'Roll for each 5 ft square to see what terrain it has.' That would make sense - except that the percentages, as near as I can tell, are off by an order of magnitude.

Googling gives me figures of 100 trees/acre or less as sparse forest, and 200 trees/acre or more as "overgrown" in the real world. But using the SRD figures, 50% trees per 5 ft. square - supposedly "sparse" works out to ~860 trees per acre, with dense forest being twice that.

I don't see how you are converting "50% trees" to 860 trees per acre. Are you assuming 1/2 a tree per 5 foot square? Cuz that's not how trees work. What they are saying is that in that case trees cover approximately half of the area. Given that a single tree might have a diameter of 15 or 20 feet, that means it covers 6 to ~10 squares. So then you need to have that much open space.

Or, use the area of a circle, Pi*Rsquared. So each tree covers ~150-300 sqft (depending upon a radius of 7.5' to 10'). So 50% of an acre is 21,780 sqft. 21780sqft / 200sqft/tree = 108.9 trees per acre.

Funny how well that matches with Google 100 tree per acre huh?

Edgar Ironpelt · May 27, 2019

LordEntrails said:
I don't see how you are converting "50% trees" to 860 trees per acre. Are you assuming 1/2 a tree per 5 foot square? Cuz that's not how trees work. What they are saying is that in that case trees cover approximately half of the area. Given that a single tree might have a diameter of 15 or 20 feet, that means it covers 6 to ~10 squares. So then you need to have that much open space.

OK, that's one way to look at it. Except... this means that standing under a tree allows you to use the tree as cover, or to climb it, even if you are standing 10 or 15 feet away from the trunk, and even if the branches are high enough that you don't have to duck. Also, how then do you interpret massive trees in medium or dense forest?

If a "massive tree" square is occupied by the trunk of a tree thick enough to prevent entry, then "10% massive trees" in medium forest comes to 174 trees per acre and "20% massive trees" in dense forest comes to 348 trees/acre. Which is still an awfully lot, especially when we're talking about big trees with trunks thick enough to completely block a square. Alternatively, a massive tree has to completely block some of the squares under it with something other than the tree's trunk. How does that work?

ccs · May 27, 2019

Edgar Ironpelt said:
And if you're able to just wing it without spending a lot of time on "Hey, why do my forests look so fugly and suck to run encounters in?" then more power to you.

Well, my encounters "look" however I describe them.

Wich has nothing to do with wasting my prep time trying to figure out exactly how many trees are in any particular acre.
All I need to do is tell you that your in an area of sparse/light/moderate/heavy etc woods, describe it a bit, & apply whatever modifiers for cover, movement, perception, etc apply.
It's all pretty easy.