yes, a vent stack; it helps air flow. There are three two-inch (5cm) holes at the bottom end to admit air. An earlier iteration of this design had screens over the holes to keep critters out, but it turned out to not be an issue so we have not screened the holes on this unit.
We saw a design for a solar dryer popular with Peace Corps volunteers - a heat-collector box with a small dryer compartment on top and three little trays in a stack. But I thought, why not put the dryer trays right inside the heat collector? I just checked the temperature and with partly-cloudy skies, it is 50C in the box.
