are also observed in southern than northern crown positions (MESSER 1956). The frequency of full seeds is also evidently dependent on the abundance of pollen shed. During a poor flowering year, the percentage of full seeds is considerably lower (SARVAS 1968).

8.1.4. Flowering in seed orchards

The first male and/or female strobili appear on Picea abies grafts at the age 3 to 5 years (DORMLING1970; KOZUBOV et al. 1981; MELCHIOR 1987; EFIMOV 1993). However, abundant flowering in seed orchards (more than 20 cones per graft) can be expected 12 - 20 years after grafting (NILSSON and WIMAN1967; ERIKSSON et al. 1973, CHALUPKA 1988). It is possible to accelerate flowering by several years through grafting scions on the top of stocks about 0.5 m in height (DIETRICHSON and TUTTUREN 1978).

In general, the numbers of female and male flowers are well correlated, but in some years female flowering is much poorer than male flowering (ERIKSSON at al. 1973; SKR0PPA and Tutturen 1985; CHALUPKA 1988; NIK-KANEN and RUOTSALAINEN 2000). Abundant female flowering is not always well correlated with cone crop, as crop production is strongly affected by the abortion of strobili and cones owing to injury caused by late frosts at the time of receptivity (ERIKSSON et al. 1973). The percentage of aborted cones ranges from 1 or 2 to over 12 percent and is similar in both greenhouse and outdoor Picea abies seed orchards (JOHNSEN et al. 1994a).

Flowering and seed production in Picea abies seed orchards is strongly affected by inter- and intra-clonal variation (NILSSON and WIMAN 1967; ERIKSSON etal. 1973; Werner1980; SKR0PPA and TUTTUREN 1985; CHALUPKA 1988, CHALUPKA and ROZKOWSKI1995; NIKKANEN and RUOTSALAINEN 2000). Distinct variation in the flowering periodicity and abundance among grafts of the same clone could be associated with topophysis (DORMLING 1970; SARVAS 1970), height (REMROD 1972; ERIKSSON et al. 1973), and/or stock origin (MELCHIOR 1987).

Long-term studies of flowering on Picea abies seed orchards indicate marked periodicity of cone crops similar to that observed in mature stands (Werner 1980; DIETRICHSON 1989; NIKKANEN and RUOTSALAINEN 2000). The periodicity of Picea abies clones flowering in the same seed orchard can also be influenced by their geographic origin. Northern clones moved to southern latitudes produce cones and seeds more frequently and in larger amounts than in their place of origin (SKR0PPA and TUTTUREN 1985; MELCHIOR 1987; CHALUPKA 1988; EFIMOV 1993; CHALUPKA and ROZKOWSKI 1995). At first this finding seemed to support the idea of an accelerated and increased production of Picea abies seeds by moving clones from a cold to warmer climate, i.e. from north to south as well as from high to lower elevations, or growing them in greenhouses under a temperature regime higher than ambient. How ever, it was observed that such treatments cause unexpected aftereffects. The altered environment in which sexual reproduction occurs causes increased abortion at early stages of embryogenesis and significantly affects many qualitative traits of Picea abies seedlings derived from seeds developed in this manner (SKR0PPA and JOHNSEN 2000; OWENS et al. 2001; Saxe et al. 2001).

8.1.5. Natural protection against selfing

Norway spruce is partly able to protect itself against selfing. Metandry is the first barrier to self-pollination, but its effectiveness is diminished because of prolonged female strobili receptivity, which overlaps with pollen shed on the same tree. However, metandry is assisted by a limited capacity of the pollen chamber. Generally only the first pollen grains contacting the surface of nucellus have an opportunity to achieve fertilization. Thus, the limited capacity of the pollen chamber acts to prefer early pollinators, resulting in important genetic consequences (Sarvas 1968). Another barrier against selfing is embryo abortion caused by lethal genes during the different phases of proembryo development (from syngamy to the second mitosis). This results in the rejection of most self-fertilized embryos (Koski 1971). These natural protection mechanisms against selfing could be very ineffective in a year of insufficient pollen shed. The probability of self-pollination in such years is very high, and the genetic quality of the seed diminishes. Consequently, Norway spruce seeds should be collected only in years of abundant cone crops (Sarvas 1968).

8.1.6. Periodicity of flowering and seed crops

Some self-regulation of the periodicity of flowering and seed crops exists in Norway spruce stands. Abundant flowering will substantially reduce the number of vegetative buds in the crown. In addition, cones that are formed during the same growing season as the seeds utilize a considerable amount of assimilated carbon. Consequently, this diminishes the number of new shoots and buds in the year after flowering and markedly reduces the possibility of new strobili initiation (Tiren 1935, GORCHAKOVSKI1958; ZYKOV 1967; CHALUP-KA et al. 1975b). In general, abundant cone crops in Norway spruce are not observed in two consecutive years even though the environmental conditions are suitable for flower induction (CHALUPKA and GIERTYCH 1973; INNES 1994; Kantorowicz 2000).

The gap between two consecutive abundant crop years varies depending on geographic location. In Byelorussia, as well as in the northeastern part of European Russia, large crops occur every 3-5 years (USKOV1962; MOLCHANOV 1967). On average, seed crop frequency in Poland is comparable to that of Sweden and Finland, where abundant seed crops occur every 11-12 years or 12-13 years, respectively (Sarvas 1957, 1968; HAGNER1965; CHALUPKA and

GIERTYCH 1973; KANTOROWICZ 2000). Good seed crops are most frequent in the central regions of the geographic range of Norway spruce, and their frequency declines as one moves towards the limits of the range (DOLGOSHOV 1958; CHALUPKA and GIERTYCH 1973).

8.1.7. Factors affecting cone and seed production Soil fertility, stand characteristics, and individual variation MOLCHANOV (1950) reports that seed crops in Picea abies differ depending on the forest site type with the highest crops observed on the richest sites. However, this relationship is apparent only in years of a poor cone crop, whereas in good seed years, Norway spruce stands yield seeds in quantities unrelated to site index (BARABIN 1969).

The growth characteristics of trees and their crown position within the stand also play a role in flowering and seed production. The number of male and female strobili increases substantially with an increase in the average dominant height of a Picea abies stand (SARVAS 1968; CHALUPKAetal. 1975a). The mean crop on trees of Picea abies in the 1st and 2nd KRAFT class was much higher than on trees from the lower classes, and trees from these two upper classes produced 86% of the total cone crop in the stand (MESSER 1956). A similar positive correlation exists between tree diameter at breast height and cone crop (HAGNER 1958; USKOV 1962; ELIASON and CARLSON 1968; CHALUPKA etal. 1975a). Climatic factors

The influence of climatic factors on reproductive development must be divided into two separate periods: (1) before strobili initiation, and (2) during flower and cone development. The period through strobili initiation appears more sensitive to the influence of weather than the second period. Many authors attach considerable importance to air temperature, particularly at the time of strobili initiation, e.g. a year before flowering. It has been observed in many studies, that the mean daily temperature in late June and early July or in July is significantly correlated with the cone crop abundance the following year (TIREN1935; EKLUND1957; BR0NDBO1970; CHALUPKA 1975a; LINDGREN et al. 1977; ILSTEDT and ERIKSSON 1982).

An important climatic factor operating together with temperature is insolation which is also well correlated with flowering abundance. Abundant cone crops in Picea abies were observed when the mean hours of sunshine in the previous June attained a minimum 9 hours per day (ZVIEDRE 1970; CHALUPKA 1975a). Similarly, low precipitation during the summer affects the initiation of flower buds and results in a good cone crop the following year (TIREN 1935; TYSZKIEWICZ1949), whereas excessive precipitation has an opposite ef fect (BR0NDBO 1970; BASTIDE, la and VREDENBURCH, van 1970). Based on the information reviewed above, it is possible to predict good seed years in advance with some degree of reliability (LEIKOLA etal. 1982; PUKKALAl987a).

8.1.8. Artificial stimulation of flowering Wound treatments

Among a number of methods, strangulation enhanced male strobili initiation on mature Picea abies seed-trees 2-3 years after treatment (STEFANSSON 1948), whereas ringing significantly stimulated female strobili initiation in 13-years-old Norway spruce trees (CHALUPKA1997). Top and branch pruning as well as root pruning did not influence flowering (KOZUBOV at al. 1981) and severe pruning resulted in a large reduction of cone production in a seed orchard (NILSSON and WIMAN 1967; SAMUELSON 1979). Mineral fertilizers

Using different fertilizers to stimulate cone and seed production in Picea abies trees can be effective when nutrients are carefully chosen for the local soil conditions (MÀLKONEN 1971). Cone production has been significantly increased in Picea abies stands when nitrogen, phosphorus, and potassium were applied to the soil a year prior to flowering (SKOKLEFALD 1970; MÀLKONEN 1971; ENESCU et al. 1973; CHALUPKA 1976). In a series of foliar nutrition experiments, VOGL (1960) noted a significant relationship between an increased accumulation of phosphorus in the buds of Picea abies and the floral induction process. Modification of climatic factors

Covering Picea abies grafts with polyethylene in late June and early July significantly enhanced male (BR0NDBO 1969; CHALUPKA and GIERTYCH 1977; CHALUPKA 1981) and female flowering next year (REMROD 1972). However, covering individual trees with polyethylene or growing them in polyethylene greenhouses may modify many microclimatic conditions and it is difficult to decide which factor is the most important one. The intensity of sunlight transmitted by polyethylene declines by 25% when the cover is dry, and by 60% when the cover is covered with moisture. However, in both cases the transmission of far-red light (above 670 T|m) increases and the temperature under the cover is elevated by several degrees (PUKACKI1981; CHALUPKA 1985).

Even if temperature is a major factor influencing the process of flower primordia initiation in covered spruce grafts (TOMPSETT and FLETCHER 1977, 1979; OLSEN1978), light still remains a primary factor and can directly affects the process of flower induction. This supposition has been confirmed by detailed studies of light transmission through the bud scales of forest trees using fiber optics (CHALUPKA and Giertych 1979; PUKACKI and CHALUPKA 1982; Pukacki and Giertych 1982). Inserting optic fibers into the buds bypassed the protective role of bud scales and enhanced the transmission of full, direct sunlight inside the bud, stimulating the initiation of female strobili in mature Picea abies trees (Kosinski and Giertych 1982). Growth regulators

Unsuccessful trials with growth retardants first indicated a possible role of plant hormones in flower induction in Picea abies (Dunberg 1974). In later experiments, a significant interaction between CCC (chlorine chloride) and GA3 (gibberellic acid) in the stimulation of female and male flowering was achieved in P. abies grafts (BLEYMÜLLER1976; CHALUPKA 1979). In addition, GA3 applied alone a year before flowering stimulated both the number of flowering grafts and mean number of strobili per graft (CHALUPKA 1981).

The results of several studies on the metabolism of endogenous gibberel-lins indicated their likely involvement in floral induction in P. abies (Dunberg 1979; CHALUPKA etal. 1982; Dunberg etal. 1983). A more pronounced stimulation of P. abies flowering was obtained when less polar gibberellins were applied at the time of flower induction. Supplying a GA4/7 mixture by spraying or by stem injection significantly increased the number of flowers initiated on P abies grafts (Dunberg 1980; SCHACHLER and Matschke 1991; JOHNSEN et al. 1994b; HöGBERG and ERIKSSON 1994), whereas for P. abies seedlings a carry-over effect was observed in the second year following GA4/7 treatment (Bonnet-Masimbert 1987, 1989; CHALUPKA 1997). The effect of the GA4/7 mixture was enhanced by the addition of GA9 (Dunberg 1980), when combined with higher temperatures in a greenhouse (LUUKANEN 1979, JOHNSEN et al. 1994b) and ringing (Bonnet-Masimbert 1987, 1989; SCHACHLER and Matschke 1991). A stimulation effect of hormonal treatments on the flowering of P. abies grafts was also observed in the case of kinetin and maleic acid hydrazide (BLEYMÜLLER 1978; Ronis 1983) and no interaction was observed between the effects of G4/7 and root pruning (HöGBERG and ERIKSSON 1994) or NAA (naphtyl acetic acid) treatment (Dunberg 1980).

8.1.9. Cone and seed crop in relation to vegetative growth

Flowering, cone development, and seed maturation in Picea abies take place in one growing season. Consequently, it is essential to examine both flowering and seed crop production jointly when evaluating their effects on growth. By contrast, the joint effect on growth is both larger and easier to estimate than in the case of Pinus sylvestris, where flowering, cone development, and seed maturation extend over two growing seasons and may result in overlap with seed yield of other years (Fober 1976).

■0.40 L-,-,-,-1-,-1-,-,-1-1-1-,-1-,-1-,-,-,-,-,-,-,-,—

1950 1955 1960 1965 1970


Figure 8.2. A comparison of cone crop data and growth increment deviations from the fitted line (according to Chalupka 1975a)

■0.40 L-,-,-,-1-,-1-,-,-1-1-1-,-1-,-1-,-,-,-,-,-,-,-,—

1950 1955 1960 1965 1970


Figure 8.2. A comparison of cone crop data and growth increment deviations from the fitted line (according to Chalupka 1975a)

A significant negative correlation between cone crop and radial growth increment is readily observable (Fig. 8.2). The width of the annual ring in the years of abundant cone and seed crop production in Picea abies may be reduced by 18-42% compared with the growth increment of non-flowering years, and a growth reduction is also detectable in the year following seed fall (MIKOLA1950; DANILOV 1953; EKLUND 1954; HOLMSGAARD 1955; JONSSON 1969; BUYAK 1975). In years of an abundant cone crop, the percentage of P abies late wood as well as the specific gravity of wood declines (CHALUPKA et al. 1977; PUKKALA 1987b). Estimates in a mature stand of P. abies revealed that the production of 1 kg of dry mass of reproductive structures (cones, seeds, and male flower residues) is associated with a loss of about 3 kg of wood dry mass (CHALUPKA et al. 1975b).

Wiadystaw Chatupka, Polish Academy of Sciences, Institute of Dendrology, Kdmik.

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