How can we know that trees don't feel any pain when we cut off their branches?
They do feel pain due to morphic resonance.
Morphic resonance has been scientifically proved btw and is a fact, unlike Darwinism which is based on a claim that if you open a jar of peanut butter then worms should come out.
pain is primarily a signal of distress or harm in a sufficiently advanced organism. plants do feel pain, as well as "listen" to sounds and feel through touch. don't be a simpleton, there is more to this than your presumptions.
We use models to make predictions about the world, both intentionally and intuitively.
Sapience and sentience are models which we create intuitively through introspection and theory of mind, for the purpose of making predictions about behavior.
We apply the models of sentience to all animals and sapience to some because it helps us make predictions about their complex behavior. We do this intuitively as a way to understand and control our world, like a evolutionary adaptation.
Trees do not have the complex behavior that results from a connectivist computational architecture like a nervous system. Instead their "behavior" can currently be accurately predicted with chemical and physical models. So the efficient but inaccurate models of sapience and sentience need never be applied to trees. Note that before accurate chemical or physical models existed, people were far more inclined to attribute souls to some plants. Therefore, barring the influence of spirituality, there is no impetus for us to assume that trees possess sapience or sentience. Without this impetus almost no one will share the intuition that trees feel pain, so the question of whether or not they do feel pain will be forever pushed to the fringes of philosophical thought.
>Trees do not have the complex behavior that results from a connectivist computational architecture like a nervous system.
Plants excrete scents when being eaten by caterpillars that cause a particular type of fly to come and eat the caterpillar but not the plant.
Vegtables have been said to scream "When a leaf or stem is sliced, the plant signals pain by releasing the gas ethylene over its entire surface." This is indicative that the plant is indeed a biological connection center and when one part of the plant is harmed, just like humans, the rest of the plant must react. The lack of a nervous system is not proof that a plant cannot feel pain.