I've spent some time the past few winters admiring and learning about the causes of various whitish markings found on a small stand of White Pines, Pinus strobus located at Westwood Hills Nature Center. Below are brief descriptions of the four main causes of these different spots I've found in this stand of pines.
Sap oozes from the tree when it is injured, such as when a branch breaks off, or the bark is cut into. It helps protect the tree from infection. It is often sticky and aromatic and will have a dripping to splotch-like appearance. The color can range from yellowish to white to clear and the texture is usually somewhere between blobish to crusty.
Appearing to be a part of the bark, these greenish white spots are crustose lichens. If you look very closely, you'll notice small dots inside the lichens. These are spore producing structures. Crustose lichens are difficult to identify. After looking a bit with a microscope and a some research I wonder if these lichens might be in the genus Porina.
White Pine Adelgids
These white spots are caused by White Pine Adelgids, Pineus strobi. When these minute aphids feed on the inner bark of the tree, they secrete a waxy, thread-like substance from their abdomen for protection. From a distance these secretions appear as white dots covering the tree. They feed in large groups so a tree can become covered in their secretions, which persist throughout the winter. They prefer to feed where the outer bark is smooth, so on older trees, it might only be the upper truck that is covered. There's a nice article that describes their life cycle at Northern Woodlands magazine
Also known as mute. In this stand of white pines, there are often Great Horned Owls or Barred Owls roosting in the winter. If one of the owls uses the same tree repeatedly to roost, their white droppings will become noticeable on and under the tree. A good way to find an owl is to look for this clue. Owl droppings can look a lot like pine sap dripping down the tree, but the droppings are always white and most resemble paint dripping down the tree.
Admiring the various spots, blemishes, and splotches on trees is a good winter time activity. For me, it focuses me and helps slow me down and keep me outside in the cold. And winter is an excellent time to research the often obscure organisms that cause these spots, and to contemplate the many things that live in plain sight, but are largely unnoticed.