In article ,
Bogdan wrote:

I assume there must be something like a "dictionary" somewhere, where
I*can search for traits and find the matching species. I looked around
on the net but found only small boy-scout-type "know the trees in your
area"*info. But my trees are in France, and besides, I*have no idea if
those particular trees are "native" or not.

What you're looking for is a taxonomic key, a sort of flow chart or
decision tree that you follow to identify a plant, a process called
'keying it out'. Keys generally apply to plants of one geographical
area or related group. So if these are native trees, you can find them
in a key for trees of France or western Europe.

Unfortunately, plants found in non-wild areas are often not native.

One of the trees is some kind of chestnut according to the fruits;
it's probably quite old, the trunk is more than a meter in diameter.
It's weird in that the low part (up to knee-height)*is wider than the
rest of the trunk by almost a half, and very gnarly; I suppose it's
because of early pruning; the crown is more wide than tall. The other
trees in the area don't look like that, even if the fruit seem
identical at first look.

You're probably seeing an old graft union -- the top of the tree was
grafted onto a rootstock which grew faster than the top. This would
have taken place long ago, when the tree was only about as thick as
a pencil. The other trees may not have been grafted, or may have
been grafted onto a more compatible rootstock.

The tree is probably a chestnut (Castanea) or horse chestnut (Hippocastanea).

The other tree is strange (to me) because it has spiny leaves like
those of conifers, but it has small (0.5cm)*cylindrical red fruits
with a single seed inside.

Probably a yew (Taxus). The seeds are quite poisonous. IIRC, the
fruits -- seeds removed -- are edible, but please don't take my word
for it and try them!

Thanks for any pointers.

People generally learn the common names of common local trees when they
are children, so your friends may be able to give you a start. You can
also ask the grounds workers. There may be a local naturalists or
hiking club with knowledgable members. Ditto gardening or landscaping
people. You should be able to find info on trees of your geographical
area in the library or on the web. A government horticultural or
forestry site may help. Also, local nurseries may have sites with
pictures of plants they sell.

I hope this helps! Have fun learning the flora of a new area!