Heat stress in trees

We have had a pretty hot and dry summer throughout much of South Carolina and, although we are seeing some precipitation now, many trees are showing signs of stress.

Heat, especially combined with drought conditions, can damage trees in several ways. Most trees grow best in the 75°F to 85°F (24-30°C) temperature range. Above about 95°F (35°C) photosynthesis becomes less efficient, using more energy than it creates. Most plants cannot survive temperatures above 112°F (44°C). At these temperatures enzymes denature, nucleic acids (DNA and RNA) unravel, and cellular function ceases. Trees can cool down, most efficiently by transpiration, but this requires water, lots of water! When temperatures stay high for multiple days, trees feel it!

Trees respond differently to excessive heat and drought. Some species, like many pines, tulip poplar and sycamore, will drop older foliage to reduce transpiration loads and water loss. Others, like maples and many oaks, keep their foliage on but are thrifty with available water so that many leaves get a scorched appearance, usually around the edges of the leaves. Most of these trees will look poorly, but should recover once water is available and temperatures have cooled down.

Water is key to helping trees survive and is most effective BEFORE the trees are stressed. When weather is hot trees need more water. Watering in the heat of the day can be wasteful… much of the water is lost to evaporation so it is better to water in the evening. Avoid watering at the base of the tree; many of the roots that absorb water and nutrients are at or beyond the drip line, the ground with foliage directly overhead. Applying fertilizers can be counterproductive; stimulating new growth will increase the water requirements of the tree.

Trees are resilient. With a little extra care and the right weather, stressed trees should come out healthy next spring!

Maple leaves showing the scorched symptoms typical of draught or heat stress. Some fungal diseases cause similar symptoms, but usually when it is cooler and wetter.

These oaks are showing signs of stress from the long (more than one week) exposure to temperatures at or close to 100 degrees.

What city trees and their pests can tell us about climate change

Planting trees in cities is a great thing!  Trees regulate temperatures, filter air, and reduce runoff. And a tree-lined cityscape is beautiful!

But anyone who works as a tree health specialist knows that if you want to find a sick tree, go to the city. Many urban trees are plagued with a number of insults: reduced space for roots, pollution and other factors can reduce the growth and vigor of trees. What about higher temperatures?

Cities are known to be “heat islands” and can be several degrees warmer than surrounding rural environments, especially at night. This is a result of heat produced by dense motor traffic and the concrete and asphalt re-radiating the sun’s heat. If there is inadequate water heat can be devastating to trees. The stomates, minute holes in leaves that allow for gas exchange, stay open to evaporate water and cool the tree down, but this requires water. If there is no water, the plant begins to wilt.

It appears that heat can affect tree health in more subtle and indirect ways.

Recent work from the lab of Dr. Steven Frank at North Carolina State University has shown that scale pests of some urban trees are more abundant on trees in hotter parts of the city and that these insects have a negative impact on the infested tree’s health.

Red maples and willow oaks are popular trees in the urban landscape. Look closely at specimens of these trees in cities and you are likely to find scale insects: gloomy scale (Melanaspis tenebricosa) on maple, and oak lecanium (Parthenolecanium quercifex) on oaks. You would be hard-pressed to find these scale insects on their hosts during a walk in the woods. The leafless limbs on many maples and willow oaks planted in parking lots are often a result of these pests.

Oak lecanium scales on willow oak.

Gloomy scale on the bark of a maple.

 

Why are they more common on urban specimens? Stressed trees are often more susceptible to pests. They are too busy trying to survive and allocate fewer resources to defending themselves. But this is not true for sap-sucking insects like scales and aphids which are often more common on the healthiest plants. Greenhouse experiments showed that increased temperature, not lack of water or fertilizer, was the only measured variable accounting for the increase in scale insects.  

Like all insects, scales are cold-blooded and reproduce faster at higher temperatures (just a note: this is true within an optimum range of temperatures that varies according to the species; above a species’ maximum their survivorship rapidly declines).

But if it was simply a matter of faster reproduction as a result of warmer temperatures we would expect other insect pests to increase on trees in warmer areas, too, but that is not the case. Although willow oaks and red maples have a number of insect pests associated with them, including several other scale insect species, oak lecanium and gloomy scale stand out as pests of oaks and maples in warmer environments.

What does this tell us about the impacts climate change will have on our forests? It tells us that it’s complicated. There will be winners and there will be losers. There are many trees that do very well in urban environments. In fact, oaks and maples are great urban trees if planted in the right place. It tells us that some insects are predisposed to take advantage of warmer environments and these may become considerable pests in warmer environments.

 

Fire blight

We are noticing a lot of Callery pears (some people call them Bradford pears) with brown and black wilted foliage and stems, an indication that they are infected with fire blight. That’s good news for those of you who don’t like Callery pears, but it could mean trouble for edible pears, quince and apples!

Wilted stems and foliage with scorched appearance characteristic of infection by fire blight.

 

Fire blight is caused by the bacteria Erwinia amylovora and is especially common in warm, wet springs. It attacks a variety of plants in the rose family, particularly pears and quince, though apples, hawthorns, cotoneasters and pyracantha can also be infected. Oddly enough, the Callery pear is supposed to be resistant to fire blight, but it isn’t this year!

The bacteria most often enters through blossoms, transmitted by pollinators, including honey bees, and splashing by rain. It can also enter through wounds. The infected blooms and stems wilt and turn brown, reducing fruit production. The bacteria can be transported through the vascular system. If the disease becomes systemic and reaches the roots the tree can die.

Infected branches can be pruned and destroyed to prevent the bacteria from reaching the vascular tissue in the trunk. However, pruning can stimulate growth of tender tissues which are more susceptible to fire blight. The addition of nitrogen fertilizers should be avoided for the same reason.

If you do decide to prune, cut at least one foot below where the symptoms are visible and make sure your pruning implement is cleaned before/after each pruning cut to avoid transmitting the bacteria to new branches. Pruned material should be removed from the area and destroyed to reduce the risk of re-infection.

To avoid loss of production fruit growers sometimes apply antibiotics, but this must be done PRIOR to infections. In some locations fire blight has shown resistance to commonly used antibiotics and they are no longer effective. Some work has shown that innocuous bacteria or yeasts can outcompete the fire blight bacteria and protect blossoms and stems if applied PRIOR to infection.

Pest Alert: Spotted lanternfly

Be on the lookout for a new exotic insect, the spotted lanternfly (Lycorma delicatula)! Native to Southeast Asia, the spotted lanternfly moved to the Korean peninsula in 2006 where it has become a pests of fruit crops, including grapes, apples and pears, and forest trees. In 2014 a population was discovered in Pennsylvania, but there is evidence that it had been there at least two years prior.

An adult specimen of the spotted lanternfly with its wings spread. Adults are about 1 inch long. Photo credit Lawrence Barringer, Pennsylvania Department of Agriculture, Bugwood.org.

An adult and nymph spotted lanternfly on a tree trunk. Photo credit Lawrence Barringer, Pennsylvania Department of Agriculture, Bugwood.org.

Late-stage nymph of the spotted lanternfly. Photo credit Lawrence Barringer, Pennsylvania Department of Agriculture, Bugwood.org.

Early-stage nymph of the spotted lanternfly. Photo credit Lawrence Barringer, Pennsylvania Department of Agriculture, Bugwood.org.

The spotted lanternfly is a phloem-sucking planthopper. Although it has wings as an adult and can fly it tends to be more of a hopper. Females lay their egg masses on a variety of smooth surfaces, including tree bark, dead wood, outdoor furniture, and vehicles. Currently limited to Pennsylvania, unwitting transport of eggmasses could rapidly expand the range of this insect.

The adults prefer to feed on tree-of-heaven, Ailanthus altissima, which will be good news to native plant lovers, but it also feeds on grape vines and poses a grave threat to grape growers. The nymphs feed on a much wider variety of hosts, including a number of stone and pome fruits, and forest trees including pines, maples, and oaks.

The adults and the nymphs tend to feed nocturnally in groups, so it is best to look for them at dusk or at night. Feeding often results in weeping wounds on the tree bark and the build-up of honeydew and sooty mold at the base of the tree.

The adults and nymphs are striking, which often indicates that an animal is unpalatable. It appears they sequester toxins from the tree-of-heaven that protect them from vertebrate predators, such as birds.

It is too early to say what impact this insect could have on South Carolina’s forests and agriculture if it arrived, but it is better to be prepared. In its native habitat natural enemies and diseases keep populations of the spotted lanternfly below damaging thresholds. In Korea and North America there are no natural enemies and so populations get larger than they normally would.

If you suspect that you have found spotted lanternfly please contact your local extension agent or the South Carolina Forestry Commission at (803) 896-8838 or djenkins@scfc.gov

Cankerworm

I received a call recently about using banding to prevent outbreaks of cankerworm.

A female fall cankerworm. Note that she is wingless and has to crawl up the trunk to lay her eggs. Photo credit: James B. Hanson, USDA Forest Service, Bugwood.org.

Egg mass of fall cankerworm. All of these eggs have hatched. Photo credit: Pennsylvania Department of Conservation and Natural Resources, Forestry Archives, Bugwood.org.

Larvae of fall cankerworm. Photo credit: E. Bradford Walker, Vermont Department of Forests, Parks and Recreation.

 

There are two species of cankerworm; the fall cankerworm (Alsophila pometaria) and the spring cankerworm (Paleacrita vernata). Both of these moths are in the geometer or inch-worm family, Geometridae. Both are very polyphagous, that is, they have wide host ranges, including oak, elm, apple, and beech, among many others. Both species are native to North America. In both species the female is completely wingless and must climb up the tree to lay their eggs. The fall cankerworm does so in November and December and the spring cankerworm does so in spring. The eggs of both species hatch at budbreak and begin to feed on the tender young foliage, leaving just the veins of the leaves.

Defoliation caused by cankerworm. Photo credit: USDA-Forest Service, Ogden ARchives, Bigwood.org.

 

Because the females have to crawl up the trunk to lay their eggs applying a band of sticky substance, such as Tanglefoot™, can completely control infestations on that tree. Banding, as this practice is called, is used by many homeowners and by some cities to keep populations at low levels.

Bands applied to trees to keep female cankerworms from climbing up the trunk to lay their eggs. Photo credit: Keith Douce, University of Georgia, Bugwood.org.

 

Charlotte/Mecklenburg County in North Carolina has had increasing populations of cankerworms, despite aerial pesticide applications. Because the defoliation occurs almost every year many urban trees are weakened and made susceptible to other pests and diseases. These dying urban trees often drop limbs and can cause property damage or injury in urban environments. The caterpillars can become so numerous that they cause allergic reactions in some people. As such, Charlotte has implemented a banding program. The city identifies especially susceptible trees, particularly older, weaker willow oaks, and applies bands to these trees in November and December to keep the female cankerworms from crawling up the tree.

Trees are more resilient than we often give them credit for. Most trees can withstand complete defoliation if they are healthy and if the defoliation is not repeated soon. In these cases preventative action does not need to be taken.

Trees that have been exposed to drought or other stress will likely decline after a defoliation episode and so these need to be protected. If you noticed a large outbreak last spring now (November and December) is the time to act. You can band the tree by wrapping a cotton cloth around the trunk of the tree. This cotton cloth should be at least three feet above the ground and below any limbs. The cloth can be attached with electrical tape. Place a strip of roofer’s felt over cloth, attached with a staple gun on larger trees or electrical tape on smaller trees. Using a rubber glove or a spatula apply a thick layer of Tanglefoot™ to the roofer’s felt. Apply it as if you were icing a cake.

Take care of your trees and they will take care of you!

Bark splitting

Many trees have noticeable splits in the bark of their trunks or limbs. There are a number of potential causes.

The outer layer of bark is a dead protective layer. As the trunk grows outward adding a new bark layer the old layer may split or slough off. This is especially noticeable in some trees, like river birches, white oaks and shagbark hickories.

The trunk of a shagbark hickory, Carya ovata.

Tree species with thin bark, including, ornamental cherries, maples, and fruit trees, can be damaged by sunscald or frost cracks. Damage by both sunscald and frost cracks will always be on the side of the tree receiving the most sun… the south or southwest. The temperature of bark receiving sun can change drastically in a short period of time. This causes tissue to shrink and expand and could rupture the bark. Frost cracks are likely to form in previous injuries.

Normal bark splits in a red maple, Acer rubrum.

Usually these splits are harmless and the tree heals itself. However, it can allow decaying fungi or wood-boring insects to enter, especially if it hasn’t healed in the spring and summer when these organisms are active.

Sunscald on the trunk of a peach tree, Prunus persica.

Some activities make trees more prone to bark splitting. Over-pruning limbs can suddenly expose more bark to the sun without giving the tree time to adapt. It is better to gradually prune any unwanted limbs, if you have to prune at all. Fertilizing, particularly with nitrogen, at the end of the growing season (late summer, early fall) can induce growth, making the tree more susceptible to bark splitting and frost damage. Trees that do not get adequate water during the growing season may be more prone to bark splitting damage.

Frost crack in the trunk of a black cherry, Prunus serotina.

A frost crack that has developed in a red maple, Acer rubrum. Although ugly, the tree is still able to efficiently translocate sugars and nutrients. However, this opening has allowed fungal rots to decay the heartwood and carpenter ants are nesting in the rotting wood.

Fruit growers prevent bark splitting by painting the trunks of their fruit trees with a white reflective paint. This will buffer the trunk bark from excessive temperatures during the day.

The mystery of fall colors

We all enjoy the vibrant colors of fall. But why do some trees develop such brilliant colors in the fall?

Pigments are not just pretty; they often perform many important functions. In flowers and fruits bright anthocyanins and other flavonoid pigments attract the attention of pollinators or animals that will eat the fruit and disperse their seeds. Chlorophyll, probably the most abundant pigment on earth, captures the sun’s light to be used in photosynthesis. Carotenoids give us the oranges and yellows that characterize the fall colors of hickories, grapes and sassafras. Carotenoids are found in leaves throughout the year, but their colors only show through as the chlorophyll breaks down and is reabsorbed by the tree (trees are great recyclers). Xanthophylls are specialized carotenoids that absorbing excess radiation that can damage the chloroplasts.

Outside of their production in fruit and flowers, anthocyanins are only produced as the chloroplasts begin to break down in the fall, and that is a mystery… why would a tree begin to manufacture a relatively expensive compound in foliage that is about to fall from the tree? Anthocyanins show up in the foliage of a minority of tree species, but they are responsible for the bright reds and purples in tupelo, dogwood, maples, sourwood, sweetgum, sumacs and oaks. Several hypotheses have been put forward to explain the red colors.

Red poison ivy. Poison ivy can also be yellow in the fall.

Red maples and sourwoods showing their reds above the yellow foliage of witch hazel.

The distinctive oranges of sassafras.

 

Red leaves warn insect herbivores: The most popular right now is that the bright red or purple warns insect pests that the foliage is not edible. On face value this is a particularly weak hypothesis. The costs of herbivores feeding on autumn leaves would seem to be very low, after all this is tissue that the tree is about to be jettisoned to the forest floor.

Proponents of this hypothesis, expounded by the great D.W. Hamilton just prior to his death, note that anthocyanins are costly; indeed, anthocyanins are hydrocarbons, the energy currency of life on earth. But how do the trees benefit?

Proponents of Hamilton’s theory argue that aphids exert a substantial negative effect on tree fitness, transmitting debilitating viruses and removing vital nutrients. Aphid biologists know that aphids are attracted to green and yellow, but not to red. To “test” their claims, proponents of the “warning” hypothesis compared aphid production on red foliaged apple varieties to non-reddening varieties. Aphids fared better on green and yellow apple foliage than red. So we know that aphids don’t do as well on red foliage and we know that aphids are not attracted to red foliage. But is it worth the investment in red warning pigments to ward off populations of aphids?

What proponents of the “warning” hypothesis have not demonstrated is the benefit of the warning foliage to the trees producing them. They take it for granted that aphids are a strong selective force for trees.

There is no doubt that aphids removing sugars and nutrients from the phloem is a net negative. However, trees are much more resilient than we often give them credit for. Most trees can support enormous populations of aphids without a detectable decline in health.

We can’t completely discount aphids as a selective force for trees. The hemlock wooly adelgid and the grape phylloxera, insects that are related to aphids and feed in an almost identical manner, have had enormous impacts on hemlock and grape populations, respectively.

The most damaging aspect of aphids is their ability to transmit viruses. Although annual vegetables are often devastated by virus infections, little is known about viral diseases affecting trees. Professionals that work in tree health are familiar with fungi, bacteria and insects that attack trees, but viruses affecting tree health are not discussed much. Perhaps we don’t know enough about viruses affecting forest trees. But the argument that trees with red foliage are avoiding viruses transmitted by aphids is pretty weak. 

Many aphids have a special association with trees in the fall. After feeding on herbaceous plants all summer some aphids seek out trees in the fall, not to feed but to lay their eggs near the buds of these trees. When the young tender foliage bursts forth in the spring, the aphid eggs will hatch and begin to multiply. There are several reasons aphids arriving in fall would be very inefficient at transmitting viruses. These aphids are not likely to feed and if they did, the leaves they are infecting will very likely fall to the forest floor before a virus could be moved into the tree..

However, aphid populations feeding on young foliage may be particularly costly for the tree. The tree is still relying on energy reserves in the early spring and anything sucking out large quantities of nutrients can weaken the tree. By dissuading aphids from laying eggs in the fall, trees with bright colored leaves may avoid the stress of supporting aphid populations in spring.

The sunscreen hypothesis: Anthocyanins can function as a sunscreen, protecting tissues from radiation. Anthocyanins absorb blue-green and ultraviolet wavelengths, protecting cells from high-light stress. In fact, it is noted that brilliant fall colors are associated with brighter, sunnier days and cooler temperatures. But why protect cells in leaves that are about to fall to the forest floor?

The competition hypothesis: Studies of maple foliage with high anthocyanin content have revealed that these leaves can inhibit germination of seeds of other plant species. This is an attractive hypothesis. The trees are investing in a selective herbicide to keep competitors at bay. However, this effect is weak in maples and has not been reported for other species with anthocyanin-rich fall foliage.

None of these hypotheses have very persuasive evidence to support them and so we will have to wait for a satisfactory explanation. Until then, we can enjoy the fall colors!