If every rock, river, and flame is an elemental, are there also wood elementals?
No. That would be silly. Plants are alive in the same way that other mortal creatures are alive. They are fragile. You can't kill a stone elemental and get dead stone (you just get smaller elementals), but you can kill a tree and get dead wood. Plants live in the same conceptual world as people, while elementals are live inscrutable, alien, and separate lives. Elementals play by different rules.
Trees
Some ideas nicked from here.
They think at a rate proportional to their growth speed, but it's always too slow. Fast-growing trees like poplar can actually react to mammal actions, but for most trees, people are like gnats. Fire is an explosion. A forest fire is like a nuclear blast. Chopping down a tree happens faster than they can react. It's like getting hit by lightning. Actually getting hit by lightning usually happens when a lightning elemental falls in love with a tree... or an ancient forest fire.
Lightning-stuck trees that survive become wizards. It's how you get magical wood for wands and staves.
Trees aren't malevolent by nature. They have to be taught. They have to be shown that axes are dangerous, that fire is deadly, that the strange gnat-things that buzz through the forest are evil. Only then will they act, and it's often too late. They can't see, but they can sense heat. Their sense of smell is very different and limited to other trees and plants. They can sense vibration and they can hear reasonably well, though only by their leaves and roots. Most trees are like blind and isolated snails, and are about as intelligent.
Talking to a young tree is like talking to a pigeon. You can do it, but you're not going to learn much. The older a tree gets, the smarter it gets, but unless there's a reason to shape its intelligence it will become fiercely stunted and introverted. They develop personalities after 50 years and complex inner thoughts after 100. They communicate via pheromones for formal discussions and, if they're smart, via vibrations for emergencies.
Any tree can get up and walk, but it's painful and uses up years of energy reserves. These can be replaced by magic, so if you see a walking tree, chances are a wizard did it.
Trees can be taught languages. Elves love doing this, which is why most trees speak elvish. They don't grow mouths and faces and they definitely don't talk, but if you've got a telepathy-like spell, or you're willing to be trained, you can commune with a tree. If the tree doesn't know a language this can be frustrating and deeply confusing. Druids burn talking trees.
Elves gain extra magic from trees because the High Elves made an ancient pact with primordial trees. No one is entirely sure what the bargain involved. A tree that is used to boost spells has a 1-in-6 chance of becoming a spellcaster itself (and learning its first spell in 1d100 years).
1d6 Ancient Scheming Trees
1. This tree has spoken to both druids and elves and secretly despises them both. If there is an elf in the party, it will drop a branch on it (2d6 bludgeoning damage, Save vs Dex for half) and laugh for weeks. If there are druids nearby, the tree can smell them, and will try and send the party to kill them (if the party is strong) or send the party away (if weak).
2. This tree is religious. It wants to be made into a temple or a holy icon. If its body serves the Authority, its soul will surely enter heaven. If you help, you might be visited by an angelic tree later in life.
3. This tree was struck by lightning and survived. It wisely hides the lightning scar behind new growth and, if questioned, pretends to be a helpful and slightly eccentric ancient tree. Three centuries of growth, also carefully hidden, have made this tree suspicious and hungry for power. It can cast (with 4 MD) the spells haste (trees), become delicious, lightning bolt, control weather, and fear. It has slowly and patiently trained the entire forest around it to think, speak, and obey. It is biding its time for now, but it may accept apprentices and allies, particularly if they are also spellcasters. A tree hiring a person is like a person hiring a mosquito to give someone else malaria.
4. This tree is dying, and knows it. The soil around it is foul or turning to sand. It wants the party to carry its seeds or seedlings far away and plant them somewhere. It's desperate. It's not entirely sure what to offer the party in return, but it does remember the site of a nearby battle or duel. Maybe there's something buried in the ground.
5. This tree found a buried creature that didn't want to stay dead. It has decide this is a matter for people to deal with. When the party approaches, it will release the creature from its roots. Shade from the tree conveniently shields the creature from direct sunlight. 1. vampire 2. wight 3. zombie 4. undead horse 5. skeleton 6. some ancient evil.
6. This tree is a murder-bomb planted by the druids and nurtured since infancy. If the party approaches, it will spend all its energy reserves killing them. Roots (1d6+2 damage per round, Save once to escape, otherwise they've got you), branches (2d6 bludgeoning, Save vs Dex for half), pheromones (call druids and other animals), and leaves (visibility reduced to 10'). The tree will die of exhaustion after 1d10+1 rounds.
Bushes, Shrubs, and Grass
The lesser creatures of an ecosystem dominated by trees. Grass is as smart as krill. Shrubs and bushes are as smart as sheep, though much less mobile. They just don't have the energy reserves to uproot and run away.
In films, even the worst and most evil forest has nice clear paths around the base of trees. That's not always how it works in real life, most of the time. Woody bushes are everywhere. In some places, you can move through a forest about as fast as you can swim. The bushes will poke holes in your clothes and tear your hair and get down your boots and generally make life miserable.
If you're lucky, this is just a natural consequence of leaving the road. If you're unlucky, druids have been here and taught the bushes the smell of metal and worked leather. The bushes slow you down while the druids hunt you, or while a tree gets ready to drop a branch on your head. A well-trained bush will deliberately puncture your waterskins. Many of them have thorns and irritating poisons.
Everyone thinks ferns are useless.
1d6 Unusual Forest Plants
1. Water Daisies. Like quicksand, but flowers. Both druids and elves love them for the same reason.
2. Goatberry Bush. Grey, compact, and highly mobile. Stores all its energy in oils and waxes. Burns really well, if you can catch it. At the first sign of danger, the goatberry bush runs away on root-legs and plants itself somewhere else. You can tame it with offerings of dung. A handful of berries has a 1-in-6 chance of granting you a haste-like effect for 1 round, but no matter what, they will give you terrible indigestion. In the high mountains there are entire herds of goatberry bushes, and in some isolated regions, goatberry farms. Goatberry farmers are quick-witted but proverbially short-tempered. They covet bismuth.
3. Spelleater Ivy. Very rare. Any spells cast within 20' are absorbed by the ivy. It immediately grows a new 10'x10' section per magic dice of spell (or spell level) absorbed. The leaves are faintly octarine. Contact has a 1-in-6 chance of inducing temporary Wizard Vision. Eating or smoking the leaves causes astonishing visions and requires a Save vs Insanity. If you dry spelleater ivy, you can make anti-magic armour. Wise wizard don't bother. The armour explodes after absorbing 1d4 spells, releasing all the spells and burning the wearer.
4. Bloodbracken. A predatory plant. The seeds grow underground, sending out parasitic roots and drawing in nutrients. A few thin tendrils with unremarkable green leaves reach the surface. The bloodbracken is waiting, growing. The seedpod, now the size of a melon, strains and creaks. If a creature of sufficient weight steps on the soil above it, it will burst. A cone of red branches sprays upwards like a land mine, impaling and shredding. One moment, nothing. The next, a 6' tall red twiggy bush growing through the body of your friend. The body rots and provides nutrients for the bloodbracken to grow and produce seeds. It's entirely safe in this form. Dig around the base and you'll find bones and possibly treasure.
5. Molegrass. To most people, a completely ordinary tuft of flax-like grass. Molegrass sends out hollow roots. The roots are poisonous to prevent mice and other creatures from living inside them. If threatened or damaged, molegrass retreats down into its root-tunnels and emerges a few moments later from a different spot. Walking through a field of molegrass is like scattering an army of tuft-headed fairies before you. Molegrass is sometimes called "conqueror grass" for this reason.
6. Hornet Coral. A woody fibrous bush that's struck up a symbiotic relationship with hornets. The plant provides stability and fibre for the wasps to build their nests. The wasps sting the everliving fuck out of anything that tries to eat the bush. Assuming a good food supply, hornet coral can stretch for miles. Creatures that can tolerate the hornet's stings, parasites, and food-slaves (hornets also farm), also live in the reef. I'm surprised these don't exist in real life, to be honest.
Bonus: Reannual Grapes. A persistent error in Creation, reannuals are plants you harvest the year before you plant them. This is tricky work. Only a few monastic sites cultivate reannual grapes. It's easier than harvesting other reannual plants, but if you ever fail to plant or harvest last year's crop, paradox angels show up and burn down your monastery. Similarly, reannual wine (or counterwine, as opposed to table wine) gives you a hangover the morning before you drink it. If a PC mysteriously has a hangover in an area where counterwine is sold, they need to drink some as soon possible or paradox angels might arrive and set everything on fire.
Gardens
Shrines to domesticated plants. A big wheat field is impressive, but a garden is a tiny segment of the wilderness made perfect and smooth. Dogs are tamed wolves. Gardens are tamed forests. We tell stories about how real life ought to be, and we build gardens to make nature the way it ought to be.
Needless to say, druids hate gardens. If they ever found out about greenhouses filled with rare jungle plants they'd probably die of rage.
Garden Wizards are powerful because they tame the forest and drive back starvation. They'll walk a belt of trees down the hill to stop a flood or protect a field from the wind. They can clear a new field in hours instead of years. Rural folk might applaud an Orthodox Wizard and his fancy tricks, but they'll line up to marry a Garden Wizard.
Dryads
Some people think dryads are "tree-lings", a race of people with plant-like features. They're wrong.
Ghosts are the souls of creatures that don't know they're dead, or are too insane to care. Dybbuks and vampires are created when a creature knows it is dead but has the strength to carry on, at least for a time. A fleeing soul can possess an animal but the arrangement is temporary and unpleasant for everyone involved. Ever wondered why cats are so weird?
But a fleeing soul can also possess a young plant. Trees are preferred because they are magical reservoirs with weak souls. As the tree grows, the new soul inside of it forms a burl, and if conditions are right, eventually springs free as a dryad. Their past life is mostly forgotten. All motivations and ambitions are cleansed. Having wood for brains does that to you.
Dryads are ludicrously magical, like unicorns or dragons. They have simulated biologies. They have a heart that pumps sap, leaf-lungs that breathe air, but they don't need them to live. They're vestigial, or comfortable, or eccentric. They don't start out gendered, but sometimes they remember bits of their past life and change their forms accordingly. Alternatively, they pick forms that appeal to credulous travelers. Dryads build strange little houses or clearings, filled with half-remembered glimpses of their past life.
It's difficult to fight a dryad. They can walk through plants. The forest will never harm them. They are also mostly harmless, so druids and elves and people leave them alone. If a dryad likes you, it will try and make you stay and live with it. Dryads are very forgetful. They might not realize you've died of dehydration. If a dryad hates you, it will retreat to its grove and unleash nearby trees and shrubs.
Dryad Stats
As a vampire, but replace "needs blood" with "needs sunlight" and "weak to sunlight/holiness/garlic/silver/whatever" with "weak to cold, darkness, and unkindness". I don't care if the statblock doesn't make sense. Also, dryads aren't undead.
Spells: charm, fear, sleep
Plant Walk: dryads move through trees like water. They can swim up a tree and emerge in the crown, or burrow into the roots. They can compress to fit in a 1' tree trunk. They love playing hide-and-go-seek.
Heal: the touch of a dryad restores any character to full HP, but does not heal any injuries. Plants flourish and grow.
Dryad Past Lives
1. Evil Sorcerer King
Not evil anymore, just kind of callous. Has a small army of 1" high disobedient plant servants. Lives in a tree turned into a tower. Is working on a fragmented and forgotten scheme, like a child building a moon rocket out of motorcycle parts.
2. Spurned Lover
Driven to suicide, in fine romantic tradition. Wants a loving companion who can never leave. Ever. Starts off as adorably awkward, ends up stabbing you with roots through the eyeballs.
3. Peasant Farmer
Has a tiny garden with rare and, in some cases, unique plants enhanced by magic. Will offer you a feast. Feast may have side-effects. If you've got a random potion table, apply it to the tomatoes.
4. Lost Child
Adorable, tiny, and capricious. Has a puppy named Dog. Actually a giant, extremely surly bear that will sit patiently while the dryad puts flowers in its hair. Will swat the heads off anyone who makes the dryad cry.
5. Highwayman
Sleek, elegant, and charming. A mature dryad, pretending it knows more than it does. Wants shiny things to bury in the ground. Thinks it's growing a golden forest. 1-in-6 chance that the next time the party passes by, the dryad returns their valuables plus a basket full of tiny identical copies, with stems attached.
6. Explorer.
A mobile grove. Might be a walking tree, might just be the dryad hopping along and spending the night building a nest. Wants to look at maps. The dryad knows a lot about the local region but 50% of the information is false or nonsense.
Mushrooms
Are not plants, and deserve their own post.