But, small pine trees emit unpleasant, if not toxic, resins shortly after being placed underwater and rot away comparatively rapidly. They re hardly worth the effort. Hardwood brush and Mexican cedar trees last a lot longer. That's why vertical tree trunks without limbs attract few bass, but stumps with exposed roots and laydowns are good attractors. It's the spaces, not the surfaces that provide cover.
Small spaces offer cover to small fish, and large spaces similarly attract larger fish.
Bass enter cover to rest in a semi-sleep and hide from giant predators like yellow catfish while they rest and digest. Holes with closed backs offer more security than open-ended tubes that a catfish might enter from the rear. The downside of such artificial structures is that bass inside them tend to be inactive and uninterested in striking either live or artificial baits. These homebody bass are vulnerable and catchable only as they exit to make a feeding run and during the few moments when they have just returned, and have yet to enter their shut-down, digestive, inactive and uncatchable state. Actively feeding bass come and go from brush piles, looking at each blob of cover to see if any careless preyfish will blunder out of the cover and into attack range.