The final day of this year’s Long Play Festival appropriately began en plein air with a performance of John Luther Adams’ Crossing Open Ground – fitting for a composer whose music doesn’t simply reflect nature so much as exist within it. Led by percussionist and conductor Doug Perkins, the hour-long piece for forty musicians unfolded near the Prison Ship Martyrs Monument in the tranquil surroundings of Brooklyn’s Fort Greene Park. As with several other Adams works – including his Pulitzer Prize-winning Become Ocean – Crossing Open Ground emerged from the margins of silence, its opening sounds almost indistinguishable from the environment to which they belonged.