cfgSurfaceCharacters defines what clutter types are placed on the terrain and in which density.

a3\map_data\config.cpp/cfgSurfaceCharacters:

class CfgSurfaceCharacters
{
	class StratisForestPineClutter
	{
		probability[] = { 0.05, 0.012, 0.01, 0.1, 0.05 };
		names[] = { "StrBigFallenBranches_pine", "StrBigFallenBranches_pine02", "StrBigFallenBranches_pine03", "StrGrassDryGroup", "StrGrassGreenGroup" };
	};
	class StratisDryGrassClutter
	{
		probability[] = { 0.21, 0.35, 0.21, 0.09, 0.06, 0.02 };
		names[] = { "StrGrassDryGroup", "StrGrassDryMediumgroup", "StrWeedBrownTallGroup", "StrWeedGreenTall", "StrThistleYellowShrub", "StrPlantMullein" };
	};
	class StratisGreenGrassClutter
	{
		probability[] = { 0.95, 0.02, 0.02, 0.01 };
		names[] = { "StrGrassGreenGroup", "StrThornGreenSmall", "StrThornGreenBig", "StrPlantGreenShrub" };
	};
	// etc ...
};

See also how_cfgsurfaces_cfgsurfacecharacters_and_clutter_work.