How to decrease troops consumption of lumbers?

The simple answer is to reduce the number of cavalry troops, but you don't want to do this. Its better to have more troops.

You can reduce the consumption with stationing the right hero. The best hero for this is Sir Roguelin (orange hero). His skills include reducing consumption and improved training for cavalry. There is also alliance research that can help a little

However this will wont help a lot in the long term when you will have a lot of troops. In general just don't worry about the consumption. Earn as many rewards as you can from events that will give you lumber in boxes. Keep these in storage until you need them.

Use up any spade lumber you have in the warehouses on upgrades or charcoal production and you will not run out.