look in the biomes.xml filethere are probabilities for trees
just search tree and look at some of the values, raise the one you want to
for example, this is taken from A17 biomes but the procedure will be the same for A18
<!-- Decorations Main: Snow -->
<decorations>
<decoration type="block" blockname="treeWinterEverGreen" prob=".003" rotatemax="7"/>
<decoration type="block" blockname="treeWinterPine13m" prob=".002" rotatemax="7"/>
<decoration type="block" blockname="treeWinterPine19m" prob=".001" rotatemax="7"/>
<decoration type="block" blockname="treeWinterPine28m" prob=".001" rotatemax="7"/>
<decoration type="block" blockname="treeSnowyDeadBush" prob=".005" rotatemax="7"/>
</decorations>
you will have to generate a new map for these changes to apply