Sheet stacking apparatus, sheet processing apparatus and image forming apparatus

A sheet stacking apparatus comprising:

• an upper discharge port and a lower discharge port, located below the upper discharge port, each capable of discharging a sheet;
• a stacking portion having an upper tray and a lower tray, located below the upper tray, each capable of stacking sheets discharged from one of the upper and lower discharge ports and moving vertically;
• and
• a controller which controls the vertical motion of the upper tray and the lower tray, wherein
• the controller is capable of setting a first mode and a second mode in which a lower limit position of the upper tray stacked with sheets discharged from the upper discharge port is different,
• the lower limit position of the upper tray in the first mode is above the lower discharge port,
• the lower limit position of the upper tray in the second mode is a position in which the upper tray or sheets stacked on the upper tray blocks the lower discharge port, and
• the controller executes change-over between the first mode and the second mode depending on the total quantity of sheets scheduled to be discharged to the upper and the lower trays.