Wind burn in a Christmas tree plantation. Courtesy of Linda Haugen, USDA Forest Service, Bugwood.org (#1400013)
Air pollution has been shown to affect all conifers, with each species responding differently. In general, eastern white pine is the most susceptible, but spruce may be the most sensitive in industrial areas.
A tree’s response to airborne pollutants is affected by temperature, light intensity, water availability, and soil nutrients. Airborne pollutants may cause a decrease in photosynthesis, resulting in carbohydrate shortage in older needles. This leads to a decrease in new needle development and growth. Trees attempt to conserve energy when exposed to high levels of air pollution and may undergo early needle drop or senescence (Figures 1 and 2). Trees growing in shallow or poorly drained soils may already be near the limits of their environmental requirements and would be predisposed to stress from air pollution. Trees growing in better soils are more susceptible to damage in late summer due to the high rates of photosynthesis at that time. Decreased radial growth (diameter expansion) often results, but tree death may also occur. Some experiments have shown decreases in tree hardiness and frost resistance due to an increase in ozone levels caused by pollution. However, others have shown that high ozone levels did not affect frost resistance.
Figure 1. Air pollution damage on eastern white pine. Courtesy of USDA Forest Service Region 8 Archive, Bugwood.org (#1505030)
Figure 2. Needle discoloration and drop as a result of air pollution. Courtesy of USDA Forest Service Northeastern Area Archive, Bugwood.org (#1398007)
On the positive side, a few studies have shown that an increase in carbon dioxide (CO2) improves photosynthesis and root-to-shoot ratio in field-grown plants, provided they are of optimal nutrition. Some naturally occurring antioxidants have been identified in trees. These compounds may reverse some of the effects of air pollution. From available research, it has been shown that conifers have a higher tolerance to pollution during their dormant period than when actively growing.
Suggestions for limiting damage from air pollution include pruning dead shoots, culling dying trees to prevent other pests, avoiding planting near a source of pollution, and planting air-pollution-resistant species.
Drought damage is the result of trees needing more moisture than what is available in the soil. It affects all ages of trees, but new transplants are the most vulnerable.
Symptoms generally start at the top of the tree and continue downward and may include the loss of needles, fading or yellowing of needles, and overall slow growth. During extreme drought, tree mortality may occur (Figure 3). The effects of drought may be from one growing season of significantly decreased moisture or from several seasons of below-average rainfall. Drought also leads to other problems, especially insect pests and diseases.
Figure 3. A whole block of dead seedlings as a result of drought. Courtesy of PDA
Heat and Sunscald
Extreme heat from intense sunlight can damage all Christmas trees, especially during bud break. Heat damage generally occurs on the southern or southwestern exposure, where needles quickly turn reddish brown, causing the entire tree to appear burned (Figure 4).
Sunscald is caused when young, thin-barked trees are exposed to concentrated sunlight and the cambium under the bark dies. It may also coincide with drought conditions. The bark will take on a coppery brown color during summer and branches above the site may die. Eventually, the bark may slough off. Damage may be limited to a certain area or may occur over the entire tree but is generally restricted to the southern or southwestern exposure. Trees with few branches are more likely to experience sunscald. Douglas-fir is the most susceptible to sunscald, with true firs following closely behind (Figure 5). Basal pruning in fall or winter and commercial tree wraps may help reduce this problem.
Figure 4. Foliage burn on the southern side of the tree caused by intense sunlight. Courtesy of Cathy Thomas, PDA
Figure 5. Douglas-fir showing symptoms of sunscald. Courtesy of USDA Forest Service Ogden Archive, Bugwood.org (#1467233)
Frost injury occurs in early spring and is most common on Douglas-fir, true firs, and spruce. Pines are damaged only on rare occasions of extremely late season frost. Trees that break bud early or are in frost-prone areas on northern slopes are most susceptible to this type of damage. Within a few days of the frost event, the new growth wilts and turns pinkish brown. As the season progresses, damaged shoots turn dark brown and may remain on the tree into summer (Figure 6). Damaged growth can be removed during shearing, and the tree should recover within one year. Repeated frost damage will result in trees that are underdeveloped and bushy. Pines are best suited for planting in known frost pockets or on northern exposures.
Figure 6. New growth of Fraser fir damaged by frost. Courtesy of Ricky Bates, Penn State
Winter injury is more common with Fraser and Concolor fir, eastern white pine, and Norway spruce. Both winter burn and drying may occur, causing the needles to turn brown. Occasionally, an entire tree will be discolored above the snow line (Figures 7 and 8). Needle loss may occur, especially on the southern side. On eastern white pine, winter injury causes needles to droop downward. On Scotch pine, yellow spots are found on damaged needles.
Figure 7. Snow protecting foliage from extreme winter elements. Courtesy of PDA
Figure 8. Foliage that sat above the snow line exhibiting dieback. Courtesy of Rayanne D. Lehman, PDA
Wind damage often occurs on the northwest or west side of the trees. It is also more common around the perimeter of the tree block. Needles will show discoloration from the tips of the needles to the back (Figure 9). On closer inspection, needles may have a stippled effect.
Figure 9. Tree near the perimeter of a block exhibiting wind damage. Courtesy of Rayanne D. Lehman, PDA
Delayed Winter Dormancy
Delayed winter dormancy may occur when climate conditions prior to and at the time of harvesting Christmas trees are warmer and/or wetter than usual. Typically, growers in the East begin harvesting trees in mid- to late November. When November temperatures are warmer than usual, trees may not have the opportunity to become dormant before being cut. When this occurs, trees are more likely to dry out and prematurely drop needles. Delaying tree cutting until after temperatures have dropped may be a way to prevent early needle loss.