Plant Water Use and the Impact of Drought
All areas of Massachusetts experienced some level of drought in 2016. Although the timing and severity may have differed, many plant responses were similar. To understand why plants have certain responses to water stress and to predict longer term impacts of drought, it is important to consider some of the ways plants use water.
Plants use water in four main ways including transpiration, transport of mineral nutrients, metabolic processes, and for turgor pressure. Transpiration is the evaporation of water from plants to the atmosphere. It is the process by which plants are cooled and is dependent on humidity. Lower humidity and higher temperatures increase the rate of transpiration. Mineral nutrients from the soil are dissolved in water and move from the roots through the xylem to the leaves where they are utilized in processes such as photosynthesis. Water is also needed for photosynthesis, as well as other metabolic reactions. The production of carbohydrates, proteins, plant hormones, and secondary metabolites can all be disrupted by water stress. Turgor pressure provides structure and is needed for cell elongation. Turgor pressure is needed for non-woody plants to remain upright.
Most horticultural plants have mechanisms that are utilized or developed in order to retain or obtain water during drought. These mechanisms can be plant traits that are present before drought stress occurs such as deep root systems, growing in an ecological niche, or leaf features. Deep root systems allow plants to have access to water over a greater area of the soil profile. Some plants grow in areas where drought is unlikely or may be an ecotype that is more tolerant of drought. Leaf features that help with drought include thick, fleshy stems and leaves that can store water, waxy coated leaves that help to reduce transpiration, hairy leaves that help to reduce wind movement around leaves (reducing transpiration), light colored foliage (silver, gray, blue, white) which reflects light (reducing heating), and narrow leaves, which reduces the surface area for heating.
Other mechanisms are in response to drought. Plants close their stomata (tiny openings in the leaf undersides, through which gases and water vapor pass) in response to soil drying in order to decrease transpiration. Leaf movements, including rolling, wilting, and changing orientation, can reduce heating of leaf surfaces via light and/or limit transpiration. Rolling leaves reduces the leaf areas exposed to light (therefore heating), reduces area movement along the leaf surface, and creates a high humidity environment within the roll which reduces transpiration. In some cases, plants may even drop some or all of their leaves to reduce transpiration.
Short term drought symptoms can include marginal leaf scorch, rolled or folded leaves, leaf drop, early fall color, wilting, off-colored or yellow leaves, twig and branch dieback, and reduced growth. These symptoms can be the result of reduced water for transpiration, nutrient deficiencies due to reduced transport, and/or reduced photosynthesis. Reduced photosynthesis can be the result of fewer leaves, closed stomata, or changes in leaf orientation. Over long periods of time, this can lead to reduced growth and reduced carbohydrate production and storage.
Moving into 2017, the extent of continued damage will be variable depending on type of plant, extent of establishment, type of soil, the area the plant is located, and how long the drought continues. There is a greater likelihood of damage with new plants, plants with smaller root systems, plants with shallow root systems, plants with injuries, or plants with poor or damaged root systems.
Damage caused by water stress in 2016 may become evident in ways other than drought stress symptoms. During the winter, drought stressed plants will be more likely to have weakened branches break during storms. Drought stressed plants are also more susceptible to typical winter injuries including desiccation (especially broadleaved evergreens), sunscald, frost splitting, winter burn, or dieback. Stressed plants are also more likely to have increased injury from de-icing salts as high concentrations of de-icing salts can cause additional drought stress.
It can be challenging to identify symptoms of drought stress, as many of the symptoms can be similar to those of other stressors such as nutrient deficiencies or diseases. In general, injury from drought stress usually occurs from the top of plant down and the outside in. For evergreens, needle browning occurs from the tip downward. Other symptoms can include fewer and/or smaller leaves, shorter branches, fewer flowers and/or fruit, loss of branches, heavy seed loads, and dieback.
Root hairs and feeder roots which are generally located in the upper foot of soil are usually the first to die during drought. These are the roots that take up the greatest amounts of water and nutrients, so, even as the drought lessens, plants can still have reduced water and nutrient uptake, as it can take years for root systems to be repaired. This can lead to nutrient deficiencies even when nutrients are present in the soil.
Drought-weakened plants have increased susceptibility to insects and diseases. Disruption of metabolic processes also means that that the production of protective chemicals by the plant is reduced. Plants, especially plants with physical damage such as cracks in branches, should be monitored. Root rots, cankers, wood rots, spider mites, and wood boring insects are more likely to occur in response to drought stress.
Moving Forward….
In developing new landscapes or maintaining existing landscapes, it is important to consider water conservation principles in design, installation, and maintenance to help promote more sustainable landscapes.
Design with irrigation and plant water needs in mind.
- Group plants by water needs and to allow for reduced and efficient irrigation practices.
Improve or maintain soil structure to promote water conservation.
When irrigation is possible, make sure it's efficient.
- Use drip irrigation or soaker hoses.
- Maintain irrigation systems to avoid water loss and improve application efficiency and uniformity.
- Adjust applications according to environmental conditions and changing plant water needs.
- Plants need around 1” of water per week, applied slowly to a depth of 8-12”. Frequency and duration will depend on soil and weather.
- Remember plants need 2-3 years for establishment.
Mulch to reduce evaporation from the soil.
- Apply a 2-4” layer, kept away from the trunk/base of plant.
- Helps control fluctuating soil temperatures.
- Adds organic matter to the soil.
- Helps reduce weeds.
Choose the right plant for the right place (including turf)
- Group plants to help improve irrigation efficiency.
Appropriate maintenance
- Avoid over-fertilization, as it increases growth which increases the water demand of the plant.
- Control weeds to reduce water competition.
- Only prune damaged or dead branches to avoid increasing the amount of stress on the plant.
Remember that drought stress symptoms can be delayed as plants use up stored carbohydrates and that the effects of severe drought stress can take years for recovery. Help reduce other stresses as plants recover.
Sources:
Caldwell, Ainsley. Drought and Urban Trees. City of Atlanta Department of Planning & Community Development. 2017, 2/13.
Douglas, Sharon. 2002. Minimizing the Long-Term Effects of Drought on Trees and Shrubs. The Connecticut Agricultural Experiment Station.
Hopkins and Hüner. 2004. Introduction to Plant Physiology.3rd Edition. John Wiley & Sons Inc., Hoboken, NJ.
Kujawski, Ron. 2011. Long-term Drought Effects on Trees and Shrubs. UMass Extension.
Seymour, R. M. and G. L. Wade. Make Every Drop Count: Xeriscape- Seven Steps to a Water-Wise Landscape. University of Georgia Extension.
Mandy Bayer, Extension Assistant Professor of Sustainable Landscape Horticulture, University of Massachusetts Amherst
