To be blunt, I don't like these guys much because they every racial ability seems geared to druids, and some of these are over the top. But I think some work could make them better.
Merlion said:
Abilities: They receive a +2 racial bonus to Wisdom, and a -2 Racial penalty to Dexterity.
-2 Dex for +2 Wis is a fair trade, especially for druids. But it really helps clerics and low-dex fighters because they don't care about dex, they are going to be in full-plate anyway. And you add in natural armor later to make it even better for these guys. I would make these tree-people smaller, but still medium-sized, and give them a -2 STR instead of DEX.
Medium Size. As Medium size creatures Sylvani have no special bonuses or penalties due to size.
I suppose if you go with the -2 STR idea, you could make them actually small creatures. You'd have to tack on some more features though.
Good, but I could see them being 20 feet if you want to add another feature or keep one I want to cut. I wouldn't give them the dwarf heavy armor thing though. They are already geared toward heavy armor with the -2 DEX thing. Actually I'd only go to 20ft move if you go for -2 STR.
Low Light Vision: A Sylvani can see twice as far as a human in starlight, moonlight, or other conditions of poor lighting.
I've noticed this in all your races, haven't I? Low-light is very useful. I wouldn't be giving it out so easily. Yes the forest lurkers can justify it, but is it necessary for balance?
Barkskin: A Sylvani has a +2 Natural Armor bonus to armor class.
Totally out there. +2! No standard race has +2 to AC across the board. Halflings get effectively that via to different mechanics, but giving out +2 is too much. Even +1 offsets much of the DEX penalty for some people, and if you descide to go heavy armor (and who wouldn't) this natural armor is free. +1 Natural armor at most.
Speak with Plants: a Sylvani may communicate with plants, as though with a Speak with Plants spell, at all times.
I don't like this anymore than other people. Giving out 4th level spells at will isn't something for a +O LA race. I'd say they can communicate with plants for one round per level. Maybe add this full ability in at the end of a racial paragon class. Or how about Plant Empathy that works like Wild Empathy but with plants instead.
Plant Affinity: a Sylvani has a special connection to plant life. Due to this, whenever a Sylvani casts a spell that affects plants in some way, his caster level is considered to be 1 higher than its actual value.
This is okay. I wish there were a [Plant] descriptor so this could be clearer. So if a Sylvani wizard fireballs a person next to a tree, does the fireball deal extra damage to the tree, to the person, or both?
You might need to cut this for mechanical reasons. Instead how about plants suffer -1 on saves versus spells cast by Sylvani?
Skills: Sylvani receive a +2 racial bonus to Knowledge (Nature), Sense Motive, and Survival checks.
I like the Knowledge (nature) because it is INT based and stacks with the druidic ability making Druid Sylvani very knowledgable. But Sense Motive? First it is WIS based, so Sylvani really get +3 because of extra wisdom. Second, I don't see it, not at all. Survival makes more sense, but it is another WIS skill that is going to be in class. They don't need +3 on Survival. Are Sylvani supposed to be the best trackers in the land.
You seem to like giving out skill bonuses, so how about keeping the +2 to Knowledge Nature, and giving them some kind of forest sense like the dwarf's stonesence - The Sylvani can use search to detect traps in the forest as a rogue.
Additionally, Sylvani receive a +4 racial bonus to Hide checks in forested areas.
I don't mind this. +4 is a lot, but less than a halfling gets. If you make them small, the bonus should only be +2. Maybe add to move silently too if you drop the WIS skills from above.
Maybe add +4 to disguise skills when trying to look like normal foliage, or maybe not?
Of course, but you might consider going with a class that doesn't get so much for the bonus wisdom: ranger. They are woodsy would work just as well, but would give the humans and half-elves something reason to go druid.
In summary, my version would be
+2 WIS, -2 STR
Medium Size
Speak with plants for up to one round per level per day
Sylvani get +1 on DCs of their spells against plants
+1 Natural Armor
+2 Knowledge (Nature)
+2 hide and move silently in a forest
Forest sense - can search for forest traps as a rogue
I think with the natural armor this would be a very popular race.