A proper base layer is both moisture resistant and well ventilated. Keeping you dry when its cold will allow your body to more easily regulate its body temperature. Proper ventilation and air flow will allow your body to only keep the heat it needs and remain cool in excesses of that. A well-made base layer can easily accomplish both of these goals and be useful to a runner year round.
The fabric best suited for quality construction, moisture management, and temperature regulation is merino wool. This is the material we would recommend for your base layer, no matter the season.