Success with white pine: planting under an overstory
Although planting under an overstory sounds befuddled, it's true, best results happen when you plant white pine under an existing canopy of trees. Unlike red pine or jack pine, white pine can easily tolerate growing in the shade of other trees. Aspen, birch, oak and other hardwoods are good choices for the overstory. Look for and choose the sites: where aspen ( or birch) is mature and the stand is breaking up, where a hardwood stand just underwent thinning or where a stand was partially harvested. Heavy or continuous shade can be detrimental to white pine growth, so, as a goal, maintain approximately 40 to 60 percent shade. By the way, most unmanaged stands have a natural shading of 70 to 80 percent. You can tell if you're on the right track with the correct amount of shading if the white pine seedlings grow about two feet in height per year after being established for three years.
Over and above this, planting under an overstory has two major benefits, namely, reduced risk of attack by white pine weevil and protection from white pine blister rust infection.
White pine weevil larvae cause the wilting and death of the tree's leader (or terminal) which is a loss in height growth ( up to three years' growth) and may cause forking of the stem. The risk of attack is reduced by growing white pines in the shade of overstory trees. The cooler daytime temperatures and thinner diameter terminals grown in the shade discourage attacks by the weevil.
White pine blister rust causes branch death, topkill, stem cankers and can be fatal. To successfully infect a white pine, blister rust spores need 48 hours of temperatures less than 68 F and a film of water on the needles in late summer. White pines growing under an overstory are warmer and drier than trees growing in an open field or small opening in the forest canopy. The infection process can be halted as temperatures exceed 68 F and /or as the film of water dries up.
Night time temperatures in northern Minnesota are usually in the 60's and cool day time temperatures aren't rare. Pine foliage rapidly looses its heat as the sun goes down and the moisture laden air condenses on the foliage producing a film of water during cool nights. Cool, still nights followed by cool, cloudy days produce ideal conditions and enough time for infection. A simple analogy can be taken from a common experience, camping out (if you disregard the presence of mosquitos). Where would you rather set out your sleeping bag, in an open field or under the forest canopy? You'll be soaked by dew and cold if you sleep out in the open field, but fairly warm and dry under the forest canopy. The same holds true for pine foliage. So, the presence of a forest canopy protects white pine foliage from infection by maintaining warmer temperatures through the normally cool nights and by reducing moisture formation on the needles.
One of the worst places to attempt managing white pine is in a "small opening" in the forest canopy. Small opening means the diameter of the opening in the canopy is less than the height of the surrounding trees. White pine needles in small openings remains wet all day and all night, and cool air collects and remains there. And, as you now know, conditions like this are ideal for blister rust infections.