Allah points out His great might and power in that He created theseven heavens with all their stars and planets, and the seven earths with everything in them of mountains, sands, oceans and wildernesses, and everything in between

He tells us to find the proof that He will re-create our bodies in His creation of these mighty things. This is like the Ayah:

لَخَلْقُ السَّمَـوَتِ وَالاٌّرْضِ أَكْـبَرُ مِنْ خَلْقِ النَّاسِ

In verse 80, it was said: جَعَلَ لَكُم مِّنَ الشَّجَرِ‌ الْأَخْضَرِ‌ نَارً‌ا (the One who created for you fire from the green tree). Two trees used to be well known in ` Arabia - Markh (Cynanchum Vinimale) and ` Afar. The Arabs were accustomed to cutting two twigs from these two trees as they did with siwak or miswak. These twigs would be solid, green, and full of fresh water inside. By rubbing one on the other, it worked like firestone. They could strike fire out of these. This is what has been alluded to in striking fire out of the green tree. (Qurtubi) And if we were to consider the ultimate fate of trees, every green tree that flourishes initially, then, later on, once dried, it becomes a source of fuel and fire. Seen from this angle, every tree can be meant here - as seems to be the sense in the following verse of the noble Qur'an: أَفَرَ‌أَيْتُمُ النَّارَ‌ الَّتِي تُورُ‌ونَ ﴿71﴾ أَأَنتُمْ أَنشَأْتُمْ شَجَرَ‌تَهَا أَمْ نَحْنُ الْمُنشِئُونَ (Now tell Me about the fire you kindle: Is it you who have originated its tree, are We the Orginiator? - 56:71-72).

But, in the present verse, since the quality of the 'shajar' or tree has also been mentioned as being 'akhdar' or green, therefore, here it seems that meant here are those particular trees that used to be a source of fire despite being green and moisturized.