Lower temperatures in the early morning

Temperature is getting low in early morning.(4.27)
The lowest temperature of visitor center is -5℃.
During this season,trees are seems welcomed to coming spring.
It is difficult to know this picture so marked red oval.
They get over the severe cold and Birch is getting active while raising temperature.
Sound of falling drop while walking around in forest.
Thought it was rain but was sound of dropping sap and hit dead leaves.
Sap is flowing out from scar Dendrocopos leucotos picked at birch then dropped on a ground while go along stem.
Suck up nutrients and moisture from root for summer, passing through the stem and transmit to branch.
Finally flowers, animals and trees have started to act for spring.