Il bosco a forma di Italia

Participio passato come aggettivo
Past participle as adjective

“…primo centenario dell’Unità d’Italia, avvenuta nel 1861”
(the first centenary of the Unification of Italy, which took place in 1861)

“Avvenuto”, in the sentence above, is a past participle of the verb “avvenire”, meaning “to happen”, “to take place.”
In this sentence, it is used as an adjective to describe the “unità d’Italia”, which occurred in 1861.

In Italian, past participles are often used as adjectives to modify nouns. They agree in gender and number with the noun they modify.

For example:

• Ho mangiato una pizza cotta al forno a legna. (I ate a pizza cooked in a wood-fired oven.)
  In this sentence, “cotta” is the past participle of the verb “cuocere” (to cook) used as an adjective to modify “pizza”.

• Ho visto un film girato in Spagna. (I watched a movie filmed in Spain.)
  In this sentence, “girato” is the past participle of the verb “girare” (to film) used as an adjective to modify “film.”