je dirais Helvella fusca

It seems Helvella fusca. You should control the underside surface, it should be venose-reticulate. H. fusca is typical in spring under Populus. The ectal excipulum is made of a thin layer of clavate cells (almost like an hymenium of basidia). In H. crispa the ectal excipulum is more thick and complex, with tufts of fascicled hyphae projecting out from surface, thus the surface at bare eye is finely velvety (naked in H. fusca and H. lacunosa). Helvella crispa is an autumnal species.
From H. lacunosa, the difference is thin. Only the colour and habitat, may be. But it is important to note that the brown colors of H. fusca are due to an intracellular pigment. Helvella lacunosa has almost only wall pigments. It is true that the reticulated pattern of the underside of H. fusca is typical, but sometimes also H. lacunosa is veined (but in a less charachteristic way).
There are two monograph on Helvella: Dissing 1966 The genus Helvella in Europe, and Häffner 1987 Die Gattung Helvella. I don't know if there is still some copies to sell.
If you need more, I am here.

Dear Angelos, I think also in Helvella fusca

Mario, thank you so much for your detailed and helpful answer, as well as for your willingness to help further. JJ and Miquel Angel, thanks for your answers, too.

I've just taken a picture of the underside surface and I think it is definitely a H. fusca. By the way, it was found (along with several more) under poplars (Populus) a couple of days ago.

We gathered this species on last saturday, under Populus. I posted a new entry in the Ascofrance database but it's not yet published...

There is also a monographic revision of the Helvellaceae by Abbott & Currah in Mycotaxon 62: 1-125. The descriptions are shorter, and the species concept to my personal judgment is a little too large, but the work can be very useful. You can find it free at
http://194.203.77.76/librifungorum/SearchResult.asp?ItemID=280

With that kind of underside, there is no doubts left. Helvella fusca.

You're right Mario, the article of Abbott & Currah is very interesting but most of the European taxa are "undervalued" (this is the same for the genus Gyromitra).
I hope you'll publish the next monograph of Helvella :-)

Again, many thanks to all of you nice people!

Dear Nicolas, it is somewhat strange for me to read you in English...!
Ciao

All the messages were in English, so... mais si tu veux, je peux continuer en français ! :-)

J'ai suivi la discution à propos de Helvella fusca. J'ai également trouvé à la base d'un peuplier une quarantaine d' Helvella que j'avais classé comme H. crispa faute de mieux. Je ne possède aucune litterature dans laquelle H.fusca est décrite surtout la microscopie. La face intérieure veineuse était un aspect que je n'avais observé.

Helvella cf fusca

Jackyi, la photo n'est pas trancheante... Tous les 40 ascomes étaient comme ça...?

il a des photos plus parlantes, sinon, ce n'aurait pas été facile pour moi de les reconnaître