When planting a palm tree sapling, without using bone meal, it will turn into an orange or lime sapling after a while.

When planting a lime sapling, it occasionally turns into an orange sapling, that grows a lime tree. When picking it up, you recieve an orange sapling however.

When a lime tree is chopped, it drops lime saplings, but also birch saplings.

Happens in forged minecraft 7.10, tropicraft-6.0.5.

Update: I noticed that birch saplings drop from the non-fruity leaf blocks on the lime trees. The fruit-containing leaf block only drop lime saplings.

Also, the mahogany leaves drop spruce saplings.

Apparently, commenting this out:

public class BlockRainforestLeaves extends BlockLeaves {
...
    @Override
    //@SideOnly(Side.SERVER)
    public Item getItemDropped(int metadata, Random random, int j) {
        if (metadata < 2)
            return Item.getItemFromBlock(TCBlockRegistry.saplings);
        else
            return null;
    }
...
}

Stops the leaves from dropping foreign saplings. However, palm trees still turn into other saplings over time. It seems that minecraft can randomly set the 4th metadata bit on sapling blocks.