The various studies that have looked at this actually show conflicting results. It seems that a short jog in cold weather results in more saliva, while a marathon on a warm day actually reduces saliva production. Your body may initially be trying to offset the drying effect of the extra mouth breathing, but over longer periods dehydration sets in and your body reduces saliva production to conserve water.
All exercise, regardless of the intensity, also makes you secrete more of a protein called MUC5B. This makes your saliva more sticky and viscous, which contributes to that dry mouth feeling you can get after exercising.