Can anybody give me some advice on how to mod descr_ground_type_db.txt?

I am especially keen to know if by doing so it is possible to alter the battlemap composition in regard to trees, grass, and so on.

For example, is it possible to reduce the percentage of woods by altering some entries in this column:

Code:
climate temperate_grassland_fertile
{
	modifies default

	colour
	{
		237 20 91
	}

    types
    {
		dirt		DIRT		20.0	1000.0	0.0		40.0	0
		grass		GRASS_SHORT	20.0	1000.0	0.0		40.0	2
		grass1		GRASS_LONG	100.0   900.0   0.0     10.0	4	50.0
		forest		FOREST		50.0	900.0	0.0		10.0	4	150.0
		rock		ROCK		MIN		MAX		0.0		90.0	0
		sand		SAND		MIN     20.0f	0.0		30.0	1
		shrub_base	SCRUB		50.0	900.0	0.0		15.0	3	150.0
		snow		SNOW		1000.0	MAX		0.0		60.0	1
If someone could elucidate the significance of the numbers and entries for me, I'd be grateful.