Stomata sunken to allow gas exchange, minimize water loss. In north, shorter growing season. Leaves already in place to be more active in spring. Don't have to make all new leaves like deciduous tress. Stomata open during day for photosynthesis, close usually at night.
Water moves into guard cell from surrounding cells.
Water leaves guard cells, cell walls limp, stomata close. Low CO2 makes stomata open even at night. Photosynthesis uses up CO2. In drought water stress, stomata will stay closed in day. Circadian rhythm aka internal biological clock correlates opening and closing with what should be light and dark.