Believe it or not, you don't have to go too sci-fi to have plants exerting a massive influence on their environment. Some plants can be very aggressive, and most plants can be very passive aggressive.
Weapons at a plant's disposal (in the real world).
- Sap and essential oil
Plants are the very basis of every food chain in the web, period. That puts them in a great position to influence even top predators. Animals eat leaves, shoots, nectar, fruits and seeds of plants, and the plants control when to produce each of these commodities. If you have a massive forest of even barely intelligent trees, you can control the migration of all animals within it by making leaves fall in one section, pollen release in another and fruit form in a third.
Many animals as part of their nature husband the plants that feed them. Squirrels bury nuts/seeds, apes and monkeys throw fruit (even when they are hungry).
Plants release oils when they are competing with other plants that inhibit their growth or even kill them. They can also just grow taller and block out their sunlight.
Plants can grow at phenomenal rates. Some edible stalks and fruit can grow by 8-12 inches per day. That's almost 30 c.
There are plants that release a pheromone that mimics those released by animals so that for instance, wasps will be attracted when the trees are being attacked by things that wasps eat or breed in.
I can't find it documented anywhere, but I have seen strawberry plants in planter boxes send out offshoots ONLY in the direction of another planter full of dirt.
To attract various types of insects, a tree might produce more nectar, more pollen, more fruit, or change colors in a given wavelength of light. For instance, a flower full of nectar looks just like an empty flower to us humans, but bees can see into the UV spectrum just far enough that there is a dramatic color change.
Mimosas and venus fly-traps will close up when touched, many plants will exude noxious oils, or fling seeds on contact, others will reach out vines and grasp firmly onto obsticles, and many plants supplement their nutrition by killing animals and using their decaying bodies as fertilizer.
Plants, even as tiny as grass can break slabs of concrete and reduce man made structures to base components in mere decades.
If the entire system could be likened to a single creature, the loss of a 'cell' is worth the survival of the whole. Trees could die off for many reasons. They could dry up to increase the risk of fire in a given area, drop branches or heavy seed pods to actively attack surface dwelling pariahs, but at the same time, dead trees offer homes and even food to other creatures they may want to promote. Your sentient trees could have gardens of fungi, just by growing and dying in the correct timeframes. They can purposely irritate these same fungi to cause them to release spores by dropping leaves or growing shoots through them.
Plants can hibernate like nobody's business, surviving intense cold and long periods of drought. Viable seeds have been planted after 30,000 years.
Don't forget that they also control oxygen production. Their respiration is a chemical bi-product of photosynthesis, so they could potentially halt it entirely and make things very difficult for larger creatures like us.
None of what I've added here inhibits any of your ideas on fast thinkers versus slow ones, or how intelligent they have to be. The main thing is they just need to be varied, with control over most if not all things on the above list. They can act on their own, or influence creatures that live within or nearby them to act for them. Heck, they may just be in charge of this world already...