Books' worth! Any airflow disturbance creates sounds of characteristic volume, frequency, decay, damping, attack, and oscillation. Trees' songs change with wind speed and direction. Higher branches have different shapes and textures of leaves, and encounter higher winds. Thin, threadlike or needle-like leaves or petioles shed vortices as the wind oscillates round them, creating the high-pitched, romantic whisper of conifers. To experiment with the process, whiffle a straightened wire hanger through the air, and compare the effect with the same wire bent into a smooth  scimitar-shape. Then reverse the scimitar, point leading. Its voice suddenly weakens, the vortices flying off alone, instead of trailing along the wire and amplifying new vortices.  Hold threads of various thicknesses under various tensions beside your ear as Aeolian speedometers; you may be surprised at the clarity of their sound even at walking pace.  

Flat leaves flap like flags, depending on thickness, firmness, edge outline, and surface texture. This is commonly the main component of the rustling sound. Pointed, narrow willow leaves shed wind energy with whisperings rather than flappings. Colliding leaves suffer damage, so they grow in patterns that avoid touch, but in high winds impact is inevitable, causing another kind of rustling. Smooth, large, simple leaves tend to give low notes except when flapping vigorously; trees with small or compound leaves, prominent veins, complex outlines, furry surfaces, and rough bark, seem quieter, but they produce more ultrasonics. Crisp autumn leaves act as rattles. Hollow leaves galled by aphids, and swollen Acacia thorns hollowed by ants, may actually whistle. Dense foliage damps the transmission of high notes. The leaves of rushes scrape and vibrate like the reeds of wind instruments; their susurration and sibilation gave rise to the Greek legend about their whispering: "Midas has asses' ears!"