Jack Pine (Pinus banksiana)
Among all Michigan tree species, the jack pine is uniquely adapted to exist and reproduce on the hottest and driest sites in Michigan. It thrives on dune sand and on the sandy glacial plains, where it often occurs in dense stands. It is called a "fire species" because wildfires sweep through jack pine stands, killing the trees and preparing the ground for a new stand, thus releasing seeds from its cones.
Jack pine was once thought to poison the ground on which it stood because no other trees and few ground plants would grow near it. In truth, many species grow with jack pine.
Most jack pine cones are sealed with a special resin. This resin prevents the cones from drying out and releasing their seeds except under certain conditions. The resin melts at 112 degrees F, a temperature easily reached during a forest fire, but which also may be achieved on bare ground in open sunlight on a bright summer day. After the resin is melted, the cones open, releasing the seeds from within. These resinous cones are called serotinous cones.
Not all jack pine cones are serotinous. Almost every jack pine has a few cones each year which are non serotinous. These cones open in the fall and provide a source of seeds in case fires do not occur.
Given proper conditions, the seeds will germinate almost immediately after falling, but if moisture is lacking when they fall, some seeds go into dormancy which may last up to three years or more. Studies have shown jack pine seedlings will continue to sprout up to three years after a forest fire. This adaptation guards against the loss of all of the young seedlings due to an unusually dry summer or because of a hard frost.
Jack pines in Michigan are often crooked and short generally not a favorite tree of most people. However, in some places in the Upper Peninsula and farther north into Ontario, they grow much straighter and taller. Much of the difference is due to insects and other problems that plague the jack pine in lower Michigan, which is the southern limit of the tree's natural range.
The long wood fibers of jack pine make it ideal for the manufacture of strong papers. Harvest is usually by clearcut to mimic the effects of a wildfire. The soil is then prepared for seeding or planting, and a new stand is established.
Jack pine pollen is shed in late May or early June and sometimes forms large clouds resembling smoke rising from the tops of the trees. It settles on the ground and forms a yellow border around puddles and a thin, yellow film on everything within miles.