As the Aldeburgh Festival began to wind down, two exciting first performances recharged the batteries. And both premieres took place in the context of particularly enthralling programmes. The triumph of the week was the UK premiere of Harrison Birtwistle's new single-movement string quartet, The Tree of Strings. The title comes from a poem by the Raasay-born Gaelic poet Sorley MacLean. Birtwistle lived on Raasay in the late 1970s, but not until now has he distilled something of that experience into music. This is no romantic landscape portrait, though: Birtwistle has, in a compelling 30 minutes, tried to evoke the spirit of a music lost, to re-create imaginatively something of what could have survived in a hostile Presbyterian environment in which performances were forbidden.

The quartet begins with a juddering bare interval of sound, scaled through intense pianissimos, harmonics and tremblings on bridge and bow. It ends with a single player left on stage: a cello, whose thwack and growl of rage, grief, or perhaps of a dying pibroch's breath, haunts the work throughout. In between, the four instruments cross-etch fragments of spectral song and dance, brittle Scottish snaps, sighing dying falls, obsessive repetitions and ostinatos. There are pulsings and drones, a sense of a startling presence and a sudden vanishing. The Arditti Quartet were spellbinding.

The art of arrangement and reinvention had been celebrated the night before in a concert by the Northern Sinfonia, conducted by Thomas Zehetmair, in which five of Anton Webern's surprisingly emollient student orchestrations of Schubert songs provided quite some challenge for the baritone Florian Boesch's contrastingly tough and uncompromising way with Schubert.

This concert also provided the world premiere of an Aldeburgh commission: John Woolrich's single-movement Violin Concerto. Hot off the press, it fused the composer's own angular, flinty language with a virtuosity of orchestral colour, and an uneasy, unquiet lyricism, robustly realised in Carolin Widmann's playing.

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