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.