Information from this article comes from Dr. Peter Kolb (MSU Extension Forester). Dr. Kolb will be presenting on tree management in Fort Benton at the Memorial Ambulance building at 3:00 p.m. on Thursday, April 4th. The public is welcome to attend.
During dry and cold winters, evergreen trees growing in windbreaks or other exposed areas are afflicted with a bad case of needle discoloration that ranges from brown to purple followed by needle drop. Often this is most prevalent on the south side or windward side of the tree, and in some cases only last year’s new needles are impacted and in other cases most of the older needles are disproportionately afflicted. The reason for this type of variability is because multiple factors might be interacting on the tree, though the major cause is typically a winter where severe temperature shifts have occurred.
The general phenomenon is commonly referred to as “windburn” or “winterburn” and results from needles drying out. As mentioned, multiple factors can influence this condition. Green needles on evergreen trees are designed to function between 3-7 years on a tree, and thus
must remain alive throughout the winter. To prepare for extreme cold, the cell structure and contents of woody plants undergo changes in the fall to winterize or “harden off”. Among these are increases in cellular sugars and lipids that both decrease the freezing point of cell cytoplasm but also act to break apart cell water content into micro-sized droplets that can “supercool”, reaching temperatures as low as -38 Fahrenheit before they freeze. As water freezes it expands, and within living cells this can result in cell wall rupture. As a result, trees or other plants not adapted to cold temperatures suffer from direct cell ice formation that kills living tissue. Drought stressing trees and plants automatically decreases live tissue water content and can act to increase cold tolerance, which is why watering in the fall is often called into question as potentially raising the potential of freeze damage. However in the case of spring browning on needles on spruces and pines the mechanism may be a bit different.
Another form of cold damage is when water trapped in-between cells freezes. Although this causes minimal damage as no cells are directly damaged, ice has the tendency to attract liquid water from surrounding tissue. Thus when trees go through multiple and rapid freeze thaw cycles, the ice between cells has a greater potential to pull the super-cooled water droplets out from inside the surrounding cells, thereby causing severe dehydration and cell death. This is the same mechanism that causes “freezer-burn” in produce or meat stored in a chest freezer too long or that has thawed and refrozen several times. The outcome is almost always severe dehydration and disruption of the cell structure and a chemical change of many organic compounds that give freezer-burned foods their unique “taste”.
Although the results from tests vary greatly, a well hydrated tree typically survives the winter better than a dehydrated tree, even though dehydration in theory is supposed to make trees more cold tolerant. This is most likely because a well hydrated tree can photosynthesize better and has a surplus of sugar, which in turn helps cells lower their freezing point, super cool, and hold onto water better. Sugar is hygroscopic – which means it will hold onto water when intracellular ice crystals want to draw it out of cells. Some of a trees ability to withstand freeze thaw cycles is genetically controlled as well, which is why in a row of trees some will survive winter healthy looking than others, that display severe needle discoloration or loss. Aphid and mite feeding during the previous summer will reduce needle sugar content as well as create small perforations in needles, which both reduces freezing resistance and allows for quicker water loss out of needles. Finally, wind and direct sun will impact individual trees. Wind pulls water out of trees faster than calm air, and direct south facing sun will allow for faster and greater temperature changes over short timer periods. Often a snow patch in front of trees will allow reflected sun from the ground to “double dose” an exposed tree with sunlight.
Good tree management begins with raising a healthy tree. Do not fertilize with nitrogen any time other than spring and then apply sparingly as fall nitrogen prevents trees from hardening properly. Keep your evergreens hydrated. Check for aphids and mites and control when needed. Anti-transpirants with white residue reflect sunlight and can help reduce water loss. Anti-transpirants are applied in the Fall. Planting a row of deciduous trees such as green ash, Siberian elm, Alberta hybrid poplar or other cold and drought hardy deciduous trees on the sunny and windward side of spruces and pines will help protect them from warm/cold cycles in the winter. Deciduous trees do not have leaves that are effected in the winter, but their branches can provide some shade from direct and reflected sunlight as well as slow the wind. Make certain they are not planted too close so as to block sunlight in the summer as spruces and pines need direct sunlight to grow. Remember the sun has a much lower angle on the horizon in the winter, which deciduous trees can block.
Afflicted trees can recover. Do not prune until new branch buds have had a chance to sprout as buds are often much hardier than needles. Most pine trees can regrow needles, though a full complement may take three years. If no new needles have resprouted by mid-June then it is safe to prune off the branch if it has lost all its needles. Water and fertilize with a balanced fertilizer such as 10-10-10 in late May to help restore growth. Approximately one to two pounds for a fifteen foot tree should be plenty distributed evenly from the stem to 1 ½ times the distance to the edge of the canopy.
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