Managing Drought Stress in Landscape Trees

	Author Mike Kuhns is Extension Forestry Specialist in the College of Natural Resources. More information about drought stress and trees in general can be found at the Extension Forestry Web site.

	Prolonged drought stress on this linden has led to reduced growth, and a noticeably thin crown.

Water is the most critical factor limiting growth and survival of trees nearly everywhere in the world. As you travel across North America from east to west you see the effects of water on natural forests: dense eastern forests with occasional non-forested areas to scattered forests mostly at high elevations west of the Missouri River. This change in forest cover is mainly a function of decreasing precipitation. When natural precipitation drops below about 15 to 18 inches a year there is not enough water to sustain even the most drought-hardy trees.

So what is drought? Drought is a period of "prolonged dry weather." To give you some perspective on drought and what dry weather is, consider that a desert typically gets less then 10 inches of precipitation a year. Forests, on the other hand, need greater than 15 to 20 inches a year, and many of the tree species we grow in our cultivated landscapes need 25 to 40 inches.

HOW DOES DROUGH AFFECT TREES?

In dry Utah, lack of water exposes plants to the effects of drought. Drought affects plants through a sequence of events that looks like this:

Prolonged dry weather (drought) causes...

drying soil, which leads to...
- decreased plant water uptake, resulting in...
  - plant tissue dehydration, which ultimately affects the plant through...
  - reduced food production (photosynthesis) & storage
  
  - reduced shoot/root growth
  
  - reduced membrane integrity
  
  - reduced self defense ability
  
  - reduced survival

Reduction in some or all of these factors results in poor survival of young, newly transplanted trees, and weakens older, established trees. Drought can kill mature trees if it is severe enough and lasts long enough, but more often weakens them, opening the door to insects or diseases that come in and finish the job. Thankfully, many trees have mechanisms to tolerate lack of water. Trees tolerate drought either by keeping their tissues moist by maintaining water uptake and decreasing water loss, or they develop tolerance to tissue dehydration when tissue drying cannot be avoided.

WHAT CAN BE DONE ABOUT DROUGHT AND TREES?

Whether growing trees in a large-scale production nursery or taking care of a residential landscape, short-term and long-term droughts have to be dealt with to keep our trees healthy. With climate getting warmer and drier in some areas, and population growing, sources of irrigation water are becoming more limited. Here are some ways we can plan for and deal with drought in landscapes with trees:

1. Tree Selection

The best way to deal with drought is to favor trees and other plant material that can tolerate drought. Look for species native to drier regions of the U.S. and the rest of the world - they likely are adapted to dry conditions. Very drought tolerant tree species include most oaks, elms, hawthorns, locusts, some maples, many pines, and junipers. Species that are intermediate in drought tolerance include some oaks and pines, many maples, lindens, ashes, firs, and spruces. Examples of trees that are low in drought tolerance include willows, most cottonwoods, and birches. Regions that are home to many drought tolerant species include much of the western U.S., the Great Plains, the U.S. Southwest and Mexico, the Mediterranean, and drier portions of Asia. Be careful to avoid use of species that are pest-prone when drought stressed unless you can deliver them extra water. A prime example is blue spruce, that does fine when well-watered, but is bark beetle-prone when water stressed.

2. Irrigate to Promote Drought Tolerance

As with turf, deep, infrequent irrigation of trees promotes deep, extensive root systems that can access more water when the soil begins to dry. Also, gradually exposing many trees to some drying in early to mid-summer will actually encourage them to adjust the chemical makeup of their cell contents and membranes, and can help prevent the membrane leakage and chemical disruptions that happen when drought gets worse later in the summer.

3. Create Landscape Zones Based on Water Needs

Most landscapes contain plants with varied levels of drought tolerance. If you are in a position to design or redesign landscapes, group plants at least partly by their drought tolerance. This makes it much easier to irrigate according to specific plant needs.

We in Utah know what it is like to grow plants in landscapes that are exposed to drought. Understanding how it affects trees and other plants, and how they react to it, can make the task of coping with drought easier and more likely to be successful.

-Mike Kuhns, Extension Forestry Specialist