Sacred Trees

 Trees are everywhere in the real world. Yet very rarely do specific species get a mention in adventure modules. And its hardly a surprise. What difference does it make? Well, I suppose it might be 'realistic'/'authentic' for the Ranger to know that say, willow, or alder only grow on wet ground of near water. But how often is a ranger going to need to do that? It isn't part of the game we usually focus on.

Killing vampires, however...

Well, it doesn't have to be specifically vampires. They're an obvious example, because, well, wooden stakes...? But any kind of 'evil' or supernatural entity might be thwarted more easily if your players have access to these sacred trees. Or may be less inclined to pursue them into a grove. You don't want to be making them omnipresent, of course, and there may be additional rituals or ceremonies or simply acts of preparation that you want to make significant to the acquiring of these woods. But its always nice, I think, to know which kind of wood might be most helpful in the vanquishing of a specific kind of entity. As well as what sort of trees there actually are in the landscape. 

Historically, of course, this sort of knowledge was second nature. People's lives were intimately intertwined with the natural world. It was second nature for them to look around them for the tools the natural world could provide and, especially before the advent of monotheism, spirits, both benign and malevolent and anywhere in between, were thought to interact with the mortal or human world on a regular basis. Nature included natural focus points for this, places where our world bordered the 'otherworld'. And everybody knew this. So ANY player character should get at least a chance to know this stuff but you might want to give the ranger or the magic user an edge by at least making other classes make an attribute check for the knowledge.

Alder

The alder grows near water, literally with their roots in the stream if they can.  The wood is also resistant to splitting, so they were often used in the manufacture of cart wheels. Alder was associated with divination, so any divination spells might work better beneath an alder tree or with a wand of alder wood in hand, and this was especially true when the diagnosis of disease was involved. So a stream that grew thickly with alder might even have healing properties of its own (that's my own extrapolation there) and make it very much easier to Cure Disease etc. 

Cutting one down was once punishable.

Apple

As an emblem of fruitfulness, this wood was often used in association with fertility or youth and was associated with immortality. Druid wands are thought to have been crafted from this wood, or from yew. I think if I were staking a vampire, I'd probably favour this or Yew wood. 

Hazel

Sacred to poets and thus probably important to Bards. Also a taboo to burn. Strongly associated with magic and authority. Also, incidentally, one of the oldest kinds of tree in Europe and one of the earliest we have evidence for building with. Witches' wands and divining rods were made of this and eating the nuts brought wisdom, especially when they grew at the head of a sacred stream or river or at a sacred well. Salmon that eat them convey magical wisdom to those that catch and eat them. I'd make that a prerequisite for anyone wishing to start learning magic! It might provide a nice , 'incidental' adventure, where the party has to travel across wild country for a while and deal with a couple of wilderness encounters. 

Oak

of course. This is the one everybody knows is special. It was prominent in many cultures. it grew in sacred groves. According to Pliny The Elder, druids would climb one on the 6th day of the moon and cut boughs of mistletoe and sacrifice two white bulls for fertility. There was a goddess, Daron, associated with the tree, which is why 'derry' is such a popular feature in place names across Ireland. Maybe mistletoe increases the potency of all spells or all potions. But you have to placate that goddess 9if that's what she really is...) before you can safely take it. 

Yew

one of three evergreens indigenous to Britain, it isn't surprising that it has powerful folkloric association. It also has poisonous berries. It symbolizes death, but is that because for a long time now, the only place you can easily find it is in stately home gardens and churchyards 9they get left alone there)? I'd suggest that this association with death might be to do with the power of its wood to encourage Undead to accept their rightful place in the cosmos... 

I've only included five tree species here. there are more if you look for them, such as Rowan, but you really want to limit the number you include, in my view, so the players have a chance to really get familiar with them and be able to remember offhand, which ones to go looking for when something supernatural rears its head.