What is the white at the bottom of these trees? Well as I understand it, the "bobby-socks" on these lodgepole pines is a mineral deposit. These trees are in the hydrothermal area of Yellowstone National Park, not far from the famous Old Faithful geyser. Along with geysers the hydrothermal area has such things as hot springs, fumaroles, and mud pots.
The water you see on the ground is full of minerals, and these dead pines soak up the mineral-rich water. As the water evaporates, the minerals left behind turn the lower part of the trees white, which gives them the appearance of having "bobby-socks."