The growth habit of perennial plants indigenous to temperate zones is generally characterized by a relatively short period of active shoot elongation followed by a lengthy "dormancy." Although dormancy is an adaptation to permit plant survival during periods of stress, i.e., drought, frost, etc., a plant is not equally resistant to factors of the environment during the entire dormant period, nor are the stages of dormancy normally defined in relation to stress resistance. The material in this unit will discuss the current knowledge of plant physiology during the dormancy period, the effect cultural activities may have upon the endogenous annual growth cycle, and the significance of nursery management upon the survival potential of out-planted coniferous seedlings.
This section will provide a summary of the annual growth cycle of Douglas-fir seedlings and the significance of this cycle in the production of vigorous plants which have a high survival potential after out-planting. References listed at the end of this section present additional detailed material which further illustrates the major points.
1. By the end of this unit, you should be able to define the term "dormancy," discuss in detail the phases of dormancy with particular reference to the environments associated with each, and identify the parts of the seedling which become dormant.
2. You should be able to outline the effects of nursery irrigation schedules upon the endogenous dormancy cycle of Douglas-fir seedlings and to specify appropriate nursery management techniques for the production of seedlings for outplanting in: (a) brushfields of the Oregon coastal mountains; (b) mesic sites in Cascade Mountains, Oregon, and (c) extremely droughty areas in southwestern Oregon.
Primitive man lived in the tropic or subtropic environments where the needs for shelter and clothing were minimal. Only as he learned to protect his body against weather extremes was he able to colonize areas with more severe climates. Yearly migrations of birds and animals are a manifestation of another means whereby life is able to exist, if only in a transitory manner, in areas characterized by extreme cold or drought. But, perennial plants are fixed for life in the site where their seed germinate and must tolerate weather extremes with no exogenous protection. How do they accomplish this feat?
Plants have avoided injury primarily through the evolution of distinct annual cycles in which periods of active growth are interspersed with stress resistant resting stages wherein there is no evidence of active growth, but during which there may be preparation for the following period of rapid root and shoot expansion. It should be recognized that, while man might wish his crop species would make the most rapid growth possible, the major evolutionary pressure upon a species is that of survival rather than for growth per se. Therefore the annual growth cycle of a typical temperate zone plant will maximize the potential to avoid injury from climatic extremes but only rarely utilize the entire periods of low or nonstress weather to achieve maximum possible growth.
Douglas-fir is one of the major timber producing trees in North America with an extensive range which includes coastal British Columbia, Washington, Oregon, and California as well as the Rocky Mountains from central B.C. to Mexico. This range is characterized by winters of varying severity, but never as intense as those in boreal regions, and summers with drought periods varying from several months in northern California to several weeks in coastal British Columbia. The species has evolved in accord with this general climate and the resultant annual growth cycle is conservative in that the active growth period, at least in so far as shoot extension is concerned, is much shorter than the frost free period. The plant's adaptation to the climate is achieved through its response to several environmental signals: (1) shoot elongation begins in the spring after a minimum number of heat units have accumulated, (2) shoot extension ends in midsummer generally in response to moisture stress, (3) dormancy develops during late summer and early fall in response to a continuing moisture stress, and (4) cold hardiness is triggered during the fall primarily in response to a shortening photoperiod. In the following material we will examine the annual growth cycle of Douglas-fir in more detail and indicate how this cycle is affected by nursery and silvicultural operations.
The first evidence of active growth resumption in the spring is when the buds, which contain the apical meristems, swell and finally break. This growth is believed to occur in response to the gradually increasing temperatures which commonly occur during late winter and early spring. Most species apparently have a requirement for a definite minimum number of heat units in excess of 3°C to stimulate bud break. Presumably this heat requirement ensures that bud break will normally not occur until the danger of a late spring frost is past. It is probable that local races have developed such that trees indigenous to frost pockets initiate growth later in spring than trees in neighboring stands which are free of this stress. Conversely, trees growing in areas characterized by early summer drought frequently initiate shoot growth early.
Many temperate zone perennial plants, especially conifers and including Douglas-fir, are characterized as having a determinate growth habit. In other words, the entire growth flush in the spring and early summer consists of the enlargement of cells initiated during the previous late summer and fall. When this enlargement is complete, the tree commonly develops new buds (bed set), normally in early to mid-July. These new buds will contain initials which will be expanded during the year they are formed (lammas shoots) or the following year. Lammas shoots occur only when the drought period in summer is broken by sufficient rain to relieve the moisture stress in the tree, and only then when the tree has been dormant for a relatively short time. If the rains occur in late August or September, the annual growth cycle of the tree will have proceeded far enough that a growth response will not likely occur as a consequence of a favorable moisture supply.
We have mentioned "dormancy" in the above discussion without defining what is meant by the term. The classical definition of dormancy is "when an organ which is predisposed to elongate or grow in some other manner does not do so." This is really a poor definition in that it neither describes the organ from an anatomical or morphological standpoint, nor does it provide information with regard to the physiology involved. Nonetheless, this is the definition employed by botanists, foresters, and horticulturalists.
With many temperate zone plants, dormancy begins in mid to late summer and extends until bud break the following spring. But to complicate matters, it has been found that the growth response of a given plant during this "dormant" period differs during this time.
Dormancy for plants with a determinate growth habit actually begins when the expansion of the previous year's initials are complete. This initial stage of dormancy is called quiescence and is defined as dormancy which is controlled by conditions external* to the plant, i.e., in the case of Douglas-fir it is commonly a moisture stress; in the case of a boreal species such as white spruce, it is generally a shortening photoperiod. If, after a Douglas-fir seedling has developed some new initials and it is not subject to an environmental stress such as drought, the newly formed bud will elongate and a second flush of shoot growth ( normally much less than the initial flush) will develop to use up the newly formed initials. As noted above, this second flush is commonly referred to as "lammas growth," if the growth occurs from the terminal bud of a tree. If the growth occurs on branches, it is called "prolepsis". However, if the environmental stress persists, the plant will gradually develop a "deeper" dormancy until it is unable to respond with shoot elongation, regardless of the environment stimuli which may be present.
This second stage of dormancy is commonly called "rest" (Table V includes a range of terms which have been used to indicate the different stages of dormancy) and is that period during dormancy when the control of the potential growth response is seated in the bud itself rather than in some other part of the plant or in the environment. Although we do not understand the details, there is a definite change in plant physiology which occurs as the seedling moves from "quiescence" to "rest." A "quiescent" seedling if well irrigated or moved to a greenhouse where a favorable growth regime is maintained will break its bud and resume active shoot elongation. In contrast, a seedling in "rest" will not grow regardless of how favorable the environment is. We mentioned earlier that a plant's annual growth cycle was cued by the environment, i.e., initiation of dormancy is stimulated by either a shortening photoperiod or by moisture stress. The environment which is favorable for the transition from "quiescence" to "rest" includes a shortening photoperiod and mild temperatures together with a continuing moderate moisture stress. This, of course, is generally the weather which may be expected over much of the western part of North America in August and early September.
Table V. Nomenclature of Dormancy. Note the relationship among quiescence, correlated inhibition, and rest.
Once the seedling has entered the "rest" phase of dormancy, it requires a definite exposure to low temperatures, ~ 5°C, to permit it to resume growth the following spring. The length of this exposure is very definitely species specific. For example, Douglas-fir requires about three months of such low temperatures during which time the physiology of the seedling is gradually changed, until, when the chilling period is complete, the plant is able to resume growth rapidly if given a favorable environment. In contrast to Douglas-fir, white spruce, which you might expect to have a greater "chilling requirement" since it is native to a much colder climate, in actuality has a significantly shorter "chilling requirement," about 4 to 6 weeks. The difference in "chilling requirement" between species may be more understandable when we consider the probable reason for this period in the annual growth cycle. In Oregon and over much of the range of Douglas-fir, it is possible for warm periods to occur in mid-winter which would stimulate active shoot elongation if the species did not have a long chilling requirement. White spruce, on the other hand, is native to a climate with harsh, unbroken winters, and, therefore there is much less danger that a premature resumption of growth will be damaged by subsequent cold.
The weather favorable to the completion of the rest period includes relatively cool temperatures (~ 5°C) and short photoperiod. Again, the type of weather pattern we can expect during much of the winter. Periods of warm temperature are much less effective in breaking rest than are temperatures near 5°C. Indeed, temperatures above 10°C have little value in satisfying the "chilling requirements" of temperate zone plants and many such plants, if deprived of exposure to low temperatures by spending the winter in a greenhouse situation will respond with a greatly delayed, weak shoot growth the following spring.
Here is an interesting real life example to consider. A timber company in the United States wished to site a Douglas-fir seed orchard where the incidence of outside pollen would be minimal. They selected a site near Monterey in California where there were no natural Douglas-fir trees growing and planted the grafted seedlings which were intended to form the orchard. The first spring after planting, both the scion and the seedling materials produced some shoot growth but demonstrated considerable planting shock. The second spring, the growth was greatly reduced, and the third spring, little shoot growth occurred and the whole plantation was in very poor condition. The temperature mean for the three coldest months in this area is about 11-12°C, too warm to satisfy the chilling requirements of Douglas-fir. So it is probable that the lack of natural Douglas-fir and the poor growth of the plantation both reflect winters too warm for this species. It is interesting that the mean temperature of the three coldest months in the most productive forest regions of western Oregon and Washington, i.e., below 300m. is currently about 7-8°C. Climatologists predict that the "greenhouse" effect will probably increase mean annual global temperatures by ca 3°C in the next fifty years and that the majority of this effect will occur in the winter period. If their predictions are accurate, it is possible that much of the low forest areas in western Washington and Oregon and, possibly even on the lower British Columbia mainland and southern Vancouver Island will no longer be favorable for Douglas-fir.
After the seedling has completed the "rest" period, it is again said to be "quiescent", although the growth is prevented by low temperature rather than by moisture stress. The photoperiod is still short at this time (February-early March) but studies have shown that a long photoperiod is not essential for growth resumption of temperate zone trees which have received sufficient chilling to end rest. Finally, as we noted at the beginning of this discussion, sufficient heat units will accumulate to permit resumption of active shoot growth and the growth cycle will begin again. Table VI summarizes the annual dormancy sequence in Douglas-fir.
TABLE VI. Annual Dormancy Sequence in Douglas-fir.