The subtitle of Peter Wohleben's The Hidden Life of Trees is What They Feel, How They Communicate. I don't think he really can know what they feel. Certainly they react to stimuli, and in ways a lot more complex than one might have thought. Still, if sending out a chemical "distress" signal when an insect starts chewing your leaves counts as the experience of pain– well, then I imagine every living thing shares in similar feelings. I'm not sure that would pass Peter Singer's test of sentience as the ability to experience suffering.
Which is not to say Wohleben is necessarily wrong about tree feelings. He has spent a lifetime with trees, and he shares some wisdom about them in his book. For me there are two key takeaways. First, trees are social organisms. Not just in the sense that they live in groups and offer obvious mutual support as windbreaks and soil stabilizers. They also communicate, through commingling of roots and chemical signals sent across dense networks of fungal filaments– the "wood-wide web." And they share resources, in what any economist would see as risk-pooling arrangements. Of course, they are also in competition– Wohleben is especially good in describing the struggle between species in the process of forest succession, where scrappy, fast-growing pioneer varieties pounce on openings created by fire, flood, or avalanche, but inevitably are overshadowed by their more patient rivals.
Second, trees experience the world in a completely different time scale, which comes from the fact of being rooted, and the extraordinary lifespans of many species. The realization of the transitory nature of a human life compared with trees is one of the things that makes it so humbling to stand in a grove of old-growth redwoods. Fantasy filmmakers like to scare you with the trees that "come to life": think Oz, Lord of the Rings, and Harry Potter. Real trees are actually quite active and mobile in their fashion– they just take their time about it. Mobility, in fact, occurs only on an intergenerational time scale.
The alien slowness of tree experience raises difficulties for thinking about tree sentience. I never really believed all that woo-woo business about plants having feelings, singing to your houseplants to make them happy, etc. As someone who loves forests and trees, I value trees deeply for what I understand them to be: beautiful, fascinating, and utilitarian organisms. In the past I would have welcomed Wohleben's approach only as a useful metaphor– a plea for treating trees with respect, but not to be taken literally.
Nowadays, I'm less confident in my skepticism. We take seriously that humans are animals, all the way deep down, that other animals can have cognition and consciousness, and that the bright lines dividing organisms one from another are a lot blurrier than we thought: 90% of our cells, after all, are "non-human." So who is to say that there is not some very slow-moving form of consciousness embodied in the complex network of roots, filaments, and soil microorganisms of the forest? Perhaps a consciousness that can suffer?
Wohleben is not a great writer, and his book begins to plod within the first hundred pages. But I'd recommend the first half, at least. I'm not yet sure how it will change the way I interact with trees, but I think it will.