Some me with the problem at certain times, the stomata on the tree leaves close. what environmental stimulus might cause this response?

Stomata close when the vacuoles in the guard cells fill with water and the cells become turgid.  The environmental stimulus that causes this response is the change from light to dark.  During the day, heat from the sun causes evaporation in the leaves, which leads to water loss from the leaves, by way of the stomata, in a process known as transpiration.  At night, the absence of the sun's heat slows the rate of evaporation, causing the vacuoles in the guard cells to fill with water by way of osmosis, causing the guard cells to become turgid and seal off the stomata.