How does aspect ratio affect the stalling angle of the wing?
Any finite span wing creates a wingtip vortex which causes an induced downwash profile that is maximum at the wingtips, and gets less as you move further inboard (towards the wing root). This is due to the vorticity causing a upwashing outboard of the wing tip, and a downwash inboard of the wing tip. If we took a theoretical wing where the aspect ratio is high enough that the span is 'near-infinite', there will be no tip-effect (vortex) induced downwash at the center (root).
Lower aspect ratio means that there is more induced downwash further inboard, and this local downwash actually helps to keep the airflow attached becuase the local flow is already flowing downwards along the later surface of the wing. You can also think of it as reducing the local incidence angle because the air is already flowing downwards.
Most wings are designed to stall first at the wing root, and for rectangular wings this is inherit due to the downwashing profile described above. For other wing shapes, designers will include &quot;wash-out&quot; to ensure that the root stalls first (Why does a rectangular wing stall first at the root?). Because the root is separated first, if we take away the section of the wing near the root (hence decreasing aspect ratio), the area that would have already separated is now removed, and the wing will be able to operate at a higher angle of attack.
Therefore the stall angle would increase if aspect ratio is decreased (ceteris paribus!). This is assuming that the wing is operating in the longitudinal plane only. Any asymmetrical movement (rolling and yawing), will mean that the wing could possibly stall at the tip.