Tree Blog

Quercus nigra | Stanford

A spot of mistletoe distinguishes the Water Oak at Stanford in the winter. Here the tree is just putting on its foliage in late March. In the Bay Area, mistletoe is an occasional feature, on deciduous trees only, and is typically seen only on exotic oaks. The fact that you do not find mistletoe on trees which hold green leaves in the winter, is a clue to the life cycle, and adaptation, of mistletoe. Deciduousness evolved because of cold temperatures. When water freezes, it expands. For broadleaf trees, this expansion is typically the end of the leaf, as it dies after the cell walls are burst. So most broadleaf trees evolved the ability to drop their leaves in the winter, and wait to put them back on until cold temperatures have typically passed in their native climate. Virtually all evergreen trees in cold winter climates are conifers, whose needles contain little or no water, and are well adapted to even subzero temperatures. Yes, there are a few broadleaf evergreen in cold climates, but they are extremely rare.

Dave Muffly