When you have successfully completed this section you will:
1. Understand the importance of dormancy and cold hardiness to the survival of long-lived, temperate woody plants.
2. Know the primary environmental signals involved in dormancy induction and bud break and how these factors can be manipulated by nursery managers to manipulate dormancy.
3. Be able to explain the two basic mechanisms woody plants use to survive low winter temperatures.
Since temperate woody plants live for many decades or even centuries they must have mechanisms in place which allow them to survive harsh winter periods. Dormancy is a phase in development which allows woody plants to survive these unfavorable conditions. In forest tree species, dormancy and cold hardiness are closely linked. Trees are generally most cold hardy when they are in deep dormancy. However, overall cold hardiness will vary even in dormant plants.
Without deep dormancy woody plants could never survive harsh winter conditions.
TYPES OF DORMANCY
Dormancy in shoots refers to a period of ceased growth and a resting bud which is typically enclosed in scales. True dormancy typically can only be broken from a period of sustained chilling. Other environmental factors (e.g. light, temperature) can substitute for some but not all of this cold requirement. A shoot in a true dormant state even if brought into warm, favorable conditions will not grow if adequate chilling has not occurred. Quiescence is a term used to describe a resting state in response to adverse environmental conditions. In a quiescent state growth will resume when the environmental conditions become favorable again. Roots typically are never truly dormant but during the winter are in a quiescent state. Even when only a portion of the soil is warmed, roots in the warmed region will grow. This can occur even when air temperatures are well below freezing.
The development of shoot dormancy typically occurs in phases. The first phase is termed pre-dormancy. This early phase is reversible in that if the plant is returned to favorable growing conditions it will resume growth. As pre-dormancy develops the range of environmental conditions that allow growth to resume narrows. Following pre-dormancy the plant enters true-dormancy. In true-dormancy growth will not resume even if the plant is returned to optimal growing conditions. The plant is often defoliated at this point (if it is a deciduous tree!), and a period of prolonged chilling is required before growth resumes. The final stage of dormancy is post-dormancy. This stage is typical of later winter and early spring. In post-dormancy the bud is capable of growing, but it is still suppressed by adverse environmental conditions (e.g. low temperatures).
TRIGGERS - DAYLENGTH
The main environmental signal which triggers the onset of dormancy is daylength. For most temperate woody plants, long days promote vegetative growth and short days trigger dormancy. As days begin to get shorter in later summer growth slows, and eventually a dormant bud develops. You may recall from an earlier biology class that it is actually the length of the night that is critical, not the length of the day. Short nights stimulate growth, long, uninterrupted nights stimulate dormancy. Daylength, of course, is a very reliable environmental signal since it is perfectly stable from year to year and trees will not be tricked into growing longer because of an abnormally warm fall. Daylength then is the primary trigger that results in the changes in growth regulator production which in turn results in dormancy development. The growth regulator abscisic acid (ABA) apparently plays a role in dormancy development and has been found to build up to high levels in the fall.
ENVIRONMENTAL TRIGGERS - INFLUENCE OF TEMPERATURE
Decreasing temperatures also play a role in dormancy development. Short days cause the plant to enter pre-dormancy (and maybe even true-dormancy). It is believed by some researchers that cool temperatures are needed for the plant to enter true-dormancy. Whatever the specific case, dormancy in many temperate woody plants develops more quickly when short days occur in combination with cool temperatures.
ENVIRONMENTAL TRIGGERS - INFLUENCE OF WATER AND NUTRITION
Both water supply and mineral nutrition also interact with dormancy induction. Water stress will deepen dormancy and if severe enough will result in a resting bud and defoliation in some trees. High mineral nutrition can result in delaying dormancy. This is particularly true with the mineral nitrogen. High levels of nitrogen should never be given to plants in late summer or early fall since they may actually flush and resume growth. For the deepest dormancy, nurserymen will reduce day length, reduce temperatures, cut back on fertilization and mildly water stress plants.
Temperate woody plants once in true-dormancy require chilling to enter post-dormancy. Temperatures above freezing and in the range of 2° to 4° C are considered best. The amount of accumulated hours at these temperatures varies but is typically between 500 and 2,000 hours. Often species or seed sources from more northern climates require more hours but this is not an absolute rule. Fruit growers and nursery managers will often keep track of these so called chilling-hours so they know when their trees have entered post-dormancy.
RELEASE FROM DORMANCY
Some researchers believe that during short days in the fall ABA builds up to high levels and induces dormancy. Chilling may be responsible for the breaking down of ABA. Until enough hours have accumulated to remove the inhibitory effect of ABA the plant will not break bud. When the soil begins to warm, promoters of growth such as gibberellin and cytokinins build up, signaling the bud to resume growth..
RELEASE FROM DORMANCY
Once adequate chilling has occurred and the plant is in a post-dormant condition, warm temperatures and increasing day lengths are required for normal shoot expansion. Warm temperatures are probably the most critical environmental factor at this point. Trees of the same species growing in the north will break bud later than the ones growing in the south. However, research has also shown that plants kept under warm temperatures but short daylengths broke bud later than plants kept in warm temperatures and long daylengths.
This black walnut twig is just beginning to break bud.
RELEASE FROM DORMANCY
In the spring of 2007 a very early warm period encouraged buds to break early. This was followed by sub- freezing temperatures which killed the new flush on many trees. Here is a green ash showing the dead terminal flush with a new flush appearing above it. Typically freezes such as this do not kill trees, but it does cause them to use up vital carbon reserves to produce new foliage.
GENETIC INFLUENCES - DORMANCY AND PROVENANCE
Provenances, seed sources and species of trees species can vary greatly in the environental requirements for bud formation and bud break (release from dormance).
For example seed sources can vary greatly in the length of day required for bud formation. Recall that northern latitudes have longer daylengths in the summer months and much shorter ones during the winter months. Typically seed sources from northern latitudes require longer photoperiods for a maintenance of active shoot extension. This is not a problem since the daylengths are longer there. However when seed sources are moved far outside their native ranges, problems can occur.
The black oak in the middle of the picture is far behind in leaf development in comparison to the northern red oaks on either side.
INFLUENCES - DORMANCY AND PROVENANCE
Lets look at a hypothetical example. A source of red maple from the north, lets say Maine, requires 14 hours of light to maintain active growth. Trees from this Maine source are then planted in Virginia where we will notice that they set bud at an earlier date than they do in Maine. This is because daylength falls below 14 hours at an earlier date in Virginia. The calendar date is not the critical factor for the tree, it is the length of the day. If seed sources are moved very long distances, differences in growth can be extreme. Sources of black alder from Norway were found to set bud and stop growth in July when planted in Pennsylvania. Normally these trees would continue growth well into August.
The black spruce seedling on the right is from a Yukon seed source. When grown in southern Ontario it set bud very early as compared to an Ontario seed source shown on the left which is still actively growing.
GENETIC INFLUENCES - DORMANCY AND PROVENANCE
The opposite problem occurs when seed sources from the south are moved north. They will often fail to set bud and only stop growth in the fall when injured by cold. Seed sources from southern latitudes set bud when exposed to shorter daylengths as compared to their more northern counterparts. Again a hypothetical example might help to clarify this. A source of red maple from southern Georgia sets bud when daylengths fall below 12 hours. If this seed source is moved far enough north, a 12 hour daylength will not occur until late into the fall and the trees will be injured by frost or cold. For example, Iranian sources of black alder planted in central Pennsylvania continued growth well into October where they were eventually killed back by a frost.
GENETIC INFLUENCES - IMPORTANCE OF CHILLING
Chilling hours required for the breaking of dormancy are a concern to fruit growers. Peach trees cannot be grown in southern Florida or California because the trees do not receive adequate chilling to break winter dormancy. When peaches are grown in the north, however, they often flush rapidly in the spring and become injured by frosts. Growers try to match peach varieties by the amount of chilling hours needed to break dormancy and regions of the country where these hours can be met safely.
Once temperate trees have become dormant, they must also develop deep cold hardiness to survive the below freezing temperatures which occur throughout the winter. As mentioned above, cold hardiness generally develops along with dormancy. However, a dormant plant does not necessarily indicate a plant is cold hardy enough to survive mid-winter temperatures. Levels of cold hardiness must be able to deepen in response to cold temperatures.
In cold climates, trunks of trees will often freeze solid resulting in what is called frost cracks. Frost cracks occur when the pressure which develops from the expanding ice exceeds the strength of the wood, often resulting in loud popping as the wood ruptures. This is not fatal since the xylem is dead tissue. However, plants must avoid freezing within their living tissues. If freezing occurs the damage done by the expanding ice crystals will result in cell death due to ruptured membranes. To prevent this, temperate woody plants utilize one of two mechanisms for cold hardiness:
1) deep supercooling
2) intracellular dehydration.
This large wound in a sweet cherry is the result of a frost crack that has repeatedly opened and healed over the years.
MECHANISMS - SUPERCOOLING
In the absence of any nucleating points, water will remain liquid down to -38.1° C. This temperature is called the Homogeneous Nucleation Point. Once an ice crystal forms it can continue to grow up to 0° C or less. Deep supercooling refers to water in the cells that is maintained in a liquid state below 0° C but above the homogeneous nucleation point. These plants avoid cold damage by not allowing nucleating points. The presence of dissolved solutes will lower the nucleating point a few degrees further so supercooling plants can survive temperature around -40° to -41° C. If temperatures fall much below the homogeneous nucleation point these plants will be damaged or killed. However for a large part of North America this is more than adequate to survive the winter and, in fact, most temperate woody plants utilize this mechanism.
MECHANISMS - INTRACELLULAR DEHYDRATION
For some plants cold tolerance down to -40° C is not good enough. Plants found growing in parts of the world where temperatures fall below -40° C utilize a different mechanism. Water in these plants actually does not deep super cool in fact it freezes quite readily. These plants avoid injury by preventing intracellular (within the cell) ice formation. Water freezes in the extracellular spaces creating a very high vapor deficit which pulls liquid water out of the living cells. The cells actually become quite dehydrated as water moves out and freezes in the extracellular spaces. Pores in the cell walls are too small for ice to move through, but large enough for liquid water to move out. These plants will often be injured by dehydration but not freezing. Plants which utilize intracellular dehydration can survive temperatures well below -40° C.