The Quran

وَإِذ قُلنا لَكَ إِنَّ رَبَّكَ أَحاطَ بِالنّاسِ ۚ وَما جَعَلنَا الرُّؤيَا الَّتي أَرَيناكَ إِلّا فِتنَةً لِلنّاسِ وَالشَّجَرَةَ المَلعونَةَ فِي القُرآنِ ۚ وَنُخَوِّفُهُم فَما يَزيدُهُم إِلّا طُغيانًا كَبيرًا

Muhammad Asad

And lo! We said unto thee, [O Prophet:] "Behold, thy Sustainer encompasses all mankind [within His knowledge and might]: and so We have ordained that the vision which We have shown thee¹ - as also the tree [of hell,] cursed in this Qur'an - shall be but a trial for men.² Now [by Our mentioning hell] Weconvey a warning to them: but [if they are bent on denying the truth,] this [warning] only increases their gross, overweening arrogance."

The vision (ru'ya) mentioned here is the Prophet's experience of the Ascension, preceded by the Night Journey (see Appendix IV). Inasmuch as this experience was and is open to most conflicting interpretations and, hence, may give rise to doubts regarding its objective reality, it becomes - as stated in the sequence - "a trial for men": the weak of faith and the superficial are shaken in their belief in Muhammad's veracity and, thus, in his prophethood; whereas those who firmly believe in God see in it an extraordinary evidence of the spiritual grace which He bestows on His chosen ones, and are, therefore, strengthened in their faith in the message of the Qur'an.
As regards "the tree cursed in this Qur'an", there is no doubt that it is the "tree of deadly fruit" (shajarat az-zaqqum) spoken of in 37:62 ff. and 44:43 ff. as one of the manifestations of hell (see 37:62-63 and the corresponding notes 22 and 23, the latter of which explains why this "tree" has become "a trial for men"). In the above context it is described as "cursed" because it obviously symbolizes hell itself. The reason why only "hell" - and no other manifestation of the hereafter - is specifically alluded to here becomes evident in the subsequent statement that it is meant to convey a warning.