Silva Fennica 35(1) research articles

Growth and Phenology of Hybrid Clones ( tremula L. × Michx.)

Qibin Yu, P. M. A. Tigerstedt and Matti Haapanen

Yu, Q., Tigerstedt, P.M.A. & Haapanen, M. 2001. Growth and phenology of hybrid aspen clones ( L. × Populus tremuloides Michx.). Silva Fennica 35(1): 15–25.

Height, basal diameter, diameter at breast height, bud burst, and development were recorded in a 5-year-old hybrid aspen clonal trial. The fi eld trial consisted of four aspen hybrid clones (Populus tremula × Populus tremuloides ) and one local P. tremula seedling source. Phenological traits were observed in the 3rd year. Growth patterns were recorded during the 3rd and 4th years. Phenological traits were explored in relation to hybrid vigor expressed as growth traits. Differences were observed for phenological and growth traits among hybrid clones and P. tremula. The growth period varied from 143–158 days for the four hybrid clones, and was 112 days for P. tremula. The correlation between growth period and yield was highly signifi cant. The annual growth rate of height for the hybrids was 4.2 cm per 7 days (2.4 for P. tremula) in the 3rd year and 6.4 cm per 7 days (2.9 for P. tremula) in the 4th year. After 5 years, mean estimated stem volume of the hybrids was 3.9 times that of P. tremula. Signifi cant clone by year interaction was observed for height, diameter, and volume growth. The hybrid vigor seems to be mainly attributable to a longer growth period.

Keywords hybrid aspen, growth pattern, growth rate, phenology Authors’ addresses Yu and Tigerstedt, Department of Biology, P.O. Box 27, FIN-00014 University of Helsinki, Finland; Haapanen, Finnish Forest Research Institute, P.O. Box 18, FIN-01301 Vantaa, Finland E-mail qibin.yu@helsinki.fi Fax +358 9 7085 8727 Received 24 March 2000 Accepted 8 January 2001

1 Introduction in high-quality papermaking. The Finnish forest industry has shown a fresh interest in utilising Populus tremula is a widely distributed species aspen for producing the short fi bres. Interest in in Finland, often growing in mixed stands with cultivating selected clones of hybrid aspen has pine, spruce and birch. Recent developments in increased. The hybrid between European aspen P. fi bre research have allowed short fi bres to tremula L. and North American trembling aspen be used in admixture with coniferous long fi bres P. tremuloides Michx. has shown superior growth

15 Silva Fennica 35(1) research articles in Finland (Beuker 1989). Genetic gain from Breeding and the University of Helsinki. The trial aspen breeding programs were demonstrated in is located in Viikki, area of Helsinki University the (Einspahr 1984) and Europe (lat. 60°14´ N, long. 25°05´ E, alt 10 m). The (Melchior 1985). Interspecifi c hybrids have trial was planted in May, 1994 with one-year-old grown faster than the progenies of intraspecifi c , and consists of four aspen hybrid clones crosses (Heimburger 1936, Einspahr and Benson (Populus tremula × P. tremuloides) and one P. 1964, Melchior and Seitz 1966, Heimburger tremula seedling source. Clone 1 was a hybrid 1968, Zsuffa 1969, 1973). There is much genetic between P. tremula (Tuusula, south Finland) × P. diversity in aspen, and it can be manipulated tremuloides (Lake Aleza, B.C. ). Clone 2 through selective breeding, interspecifi c hybridi- was a hybrid between P. tremula (Tuusula, Fin- zation and cloning (Li 1995). land) × P. tremuloides (Maple, Ontario, Canada). Hybrid vigor has been observed in poplar and Clones 3 and 4 were selected from a mixed plan- has been one of the driving forces in poplar breed- tation of hybrids, between P. tremula (Tuusula, ing (Muhle Larsen 1970). The phenomenon is also or Punkaharju, Finland) and P. tremuloides (Lake well documented for aspen hybridisation in the Aleza or Maple, Canada). Thus their exact ped- U.S. (Li and Wu 1996, 1997, Li et al. 1998). This igrees are unknown. The local aspen seedlings increase in vigor may have many components, were progenies of single open pollinated superior including carbon allocation patterns, water and in Karkkila (lat. 60°32´ N, long. 24°15´ E), nutrient use effi ciency, and shoot growth phenol- Finland. Thus they are predominantly half-sib. A ogy. Traits that affect Populus performance include randomized complete block design was used with phenology (Michael et al. 1990, Ceulemans 1992), 4 × 4 plants spaced at 2.5 m × 2.25 m per plot and leaf and stomatal morphology and photosynthetic fi ve blocks. capacity. Large clonal differences in phenology, Total height, basal diameter 15 cm from the leaf area development, and photosynthetic produc- ground and breast height diameter at height 1.3 tion have been found (Ceulemans 1987, Orlovic m were measured. Measuring positions were 1998, Thomas et al. 1997). Li et al. (1998) found marked on the stems to assure that repeated meas- that interspecifi c aspen hybrids grew faster than urements were taken at exactly the same position. intraspecifi c hybrids at the juvenile stage. The Stem volume was estimated as height × basal authors ascribe this to larger internode number and diameter2 (Causton 1985, Ceulemans 1992, Li length and leaf number. Heterosis of P. tremula × et al. 1998). P. tremuloides hybrids for stem volume was prob- Height, basal diameter and breast height diam- ably the result of delayed bud set, which gave a eter were measured in the autumns of the 2nd to longer duration of height growth. the 5th year. For observation of dynamic seasonal In this paper we compare hybrid aspen clones growth patterns, the fi ve tallest were sampled and local P. tremula for yield, growth pattern and for measurements of growth traits in each plot in phenology. The aim of the study was to obtain the spring of the 3rd year. Height, basal diameter better fundamental understanding of hybrid vigor and breast height diameter were measured at of aspen hybrids. We present results of studies on intervals of approx. 2 to 3 weeks over the course of yield components and their relationship in hybrid the growing season in the 3rd year (10 measure- aspen clones and local P. tremula. The growth ments) and the 4th year (8 measurements). The patterns were studied mainly from a phenological growth rates for height, basal diameter and breast perspective, in an attempt what has hitherto been diameter were expressed as increments (cm or loosely termed heterosis of species hybrids. mm) over a 7 day period. The annual growth rates were calculated by bringing total growth made over the total growth measuring period (days) to a 7 day basis. 2 Materials and Methods Phenological traits were recorded in the 3rd year. Growth initiation was classifi ed into three The material used in this study was a clonal trial stages counting from the fi rst of May: 1) the day established by the Foundation for Forest Tree when the bud emerged 5 mm beyond the bud

16 Yu, Tigerstedt and Haapanen Growth and Phenology of Hybrid Aspen Clones...

scale, 2) the day unfolded, 3) the day the is the interaction of the i th clone and k th block leaves attained full size. Likewise, three stages of in the j th year and εijkl is random error. Clonal growth cessation were distinguished: 1) the day effects were considered to be fi xed, while the year when half of the leaves yellowed, 2) when all of and block were treated as random effects. the leaves yellowed, 3) when all leaves defoliated. “Bud emerged” and “half of the leaves yellowed” were taken as indicators of growth initiation and growth cessation. The length of the growth period 3 Results was estimated as the number of days from growth initiation to growth cessation. Each individual 3.1 Growth was observed from the same direction and from about the same relative position with respect to Highly signifi cant differences in growth traits the tree crown, every other day in the spring, and (height, basal and stem volume) were observed every third day in the autumn. among the blocks, clones, years and their inter- All calculations were based on plot means. actions (Table 1). All hybrid clones had out- The difference between the hybrid clones and performed the P. tremula by the end of the 5th the P. tremula were established by an analysis year. The best clones were clone 2 and 1, with of variance (ANOVA) using the PROC GLM stem production volumes of 17.82 dm3 and 17.57 procedure of the SAS statistical software pack- dm3, respectively (Fig. 1). Clonal ranking in the age (1989; SAS Institute, Cary, NC) with the 5th year remained nearly the same as in the 2nd type III estimation of sum of squares. Means of year. Clone 3 was shorter than clone 4 at the end phenological and growth traits were separated of the 5th year, but because of its larger diameter, by Tukey’s multiple range test at p ≤ 0.05 prob- it had a larger stem volume (Table 2). ability level of signifi cance. Pearson correlations Growth patterns of mean height, basal and were calculated to assess the linear relationships breast diameter of the hybrid clones and P. trem- between the studied traits. Data from all the four ula in the 3rd and the 4th year are shown in Fig. years were combined and the following statistical 2. The hybrids displayed superior growth for all model was used: traits during the entire growth period (Table 3). The growth rate of height, basal and breast Xijkl = µ + Ci + Yj + Bk(j) + CiYj + CiBk(j) + εijkl height diameter was highest from the beginning of June to the end of July in the 3rd year. The where Xijkl is an observation on the l th tree from same pattern for the basal and breast height diam- the i th clone in j th year and k th block, µ is the eter was found in the 4th year. However, for overall mean, Ci is the effect due to the i th clone, height growth, the highest growth rate occurred Yj is the effect due to the j th year, Bk(j) is the during mid August. The growth rate of hybrid effect due to the k th block in the j th year, CiYj is aspen clones had two peaks, the fi rst in mid-July, the interaction of the i th clone and j th year, CiBk(j) and the second in mid-August. For the local P.

Table 1. Analysis of variance for height (cm), basal diameter (cm) and stem volume (dm3).

Source Height a Basal diameter Stem volume df MS Pr > F MS Pr > F MS Pr > F

Clone 4 779.3 0.0006 76.79 0.0002 1543.6 0.0351 Year 3 5390.0 0.0001 529.63 0.0001 9574.3 0.0001 Block (Year) 16 81.7 0.0001 6.23 0.0001 251.0 0.0001 Clone × Year 12 71.5 0.0141 5.81 0.0107 418.4 0.0001 Clone × Block (Year) 64 30.4 0.0001 2.37 0.0001 90.1 0.0001 Error 1332 3.7 0.54 14.9

a Sums of squares are multiplied by 10–3

17 Silva Fennica 35(1) research articles

tremula, however, there was only one peak of growth in mid-June. (Fig. 2). The growth patterns were different for differ- ent clones. Even for the same clone, the growth patterns for basal and breast diameter differed from the pattern for height growth. Clone 3 had the highest growth rate for basal diameter in mid- July in the 3rd year, but ranked third for height and breast height diameter (Fig. 2). The growth patterns in the 3rd and the 4th year were different (Fig. 2). Compared with hybrid clones in the 3rd year, the annual growth rate of the P. tremula was 56% for height, 73% for basal diameter and 71% for breast height diameter. The respective values in the 4th year were 45%, 67% and 56%, respectively (Table 3a, 3b).

3.2 Phenology

Hybrids fl ushed earlier and grew longer than P. tremula (Table 4). The mean time for each stage of growth initiation of the hybrids was earlier than for P. tremula: 9 days for bud emerged, 7 days for leaves unfold, 7 days for full size leaves. The ranges of variation of growth ces- sation varied from clone 3 to the P. tremula : 4–41 days for half leaves yellowed, 11–45 days for all leaves yellowed, and 16–54 days for all leaves defoliated. For the hybrids, averaged half leaves yellowed, all leaves yellowed and all leaves defo- liated were all 32 days later than for P. tremula. Fig. 1. Mean total tree height, basal diameter, and total Hybrids had a growth period of between 158–143 stem volume at the end of each of the four growing days compared to 112 for P. tremula. The mean seasons for aspen hybrid clones and P. tremula. of the growth period of hybrids was 41 days

Table 2. Mean height, basal diameter, breast height diameter and stem volume of four hybrid aspen clones and one local P. tremula seedling source at the end of 2nd and 5th year of growth.

Clone Year 2 Year 5 Height Basal Breast height Stem Height Basal Breast height Stem (cm) diameter diameter volume (cm) diameter diameter volume (cm) (cm) (dm3) (cm) (cm) (dm3)

Clone 1 187.8 a 2.3 a 1.2 ab 1.07a 512.3 a 5.6 a 4.3 a 17.57a Clone 2 194.2 a 2.1 ab 1.4 a 1.03ab 548.6 a 5.2 ab 4.3 a 17.82a Clone 3 176.5 a 2.0 b 1.0 bc 0.78b 435.4 b 4.9 bc 3.8 b 11.69b Clone 4 184.5 a 1.9 b 1.1 b 0.83ab 462.1 b 4.7 c 3.6 b 11.19b P. tremula 144.7 b 1.6 c 0.9 c 0.41c 324.9 c 3.3 d 2.3 c 4.09c

Note: Means in vertical sequence not followed by the same letter are signifi cantly different at P < 0.05 according to Tukey’ s test.

18 Yu, Tigerstedt and Haapanen Growth and Phenology of Hybrid Aspen Clones...

Fig. 2. Growth pattern of diameter at breast height (DBH), basal diameter (BD) and height (HT) in the 3rd (Figs. a,b,c) and 4th year (Figs. d,e,f) for the hybrid clones and P. tremula. For key to symbols, see Fig. 1.

longer than for P. tremula, the four hybrids all phenological and growth characters. The three fell within a 15 day range. stages of growth initiation were negatively cor- related with stages of growth cessation. Growth traits were positively correlated with growth ces- 3.3 Correlations sation and growth period, but negatively cor- related with growth initiation (Table 5). This Signifi cant correlations were observed among means that late starters generally stopped grow-

19 Silva Fennica 35(1) research articles g hybrid clones and 172.5 b 176.3 b 193.6 b 206.1 b 215.4 b 7.8 b 217.8 b 219.0 b 8.2 b 219.1 b 9.0 c 219.6 b 220.6 b 16.8 b 10.4 c 2.47 c 17.2 b 10.9 c 17.8b 11.2 c 19.8 b 11.4 c 21.0 b 11.5 b 21.0 b 11.5 c 21.4 b 11.7 c 21.6 b 0.24 a 22.0 b 22.2 b 0.31 c P. tremula P. tremula P. P. tremula P. in 3rd year (a) and 4th (b). Mean annual growth rates, increment of height, basal diameter and breast height diameter at different observational dates amon observational rates, increment of height, basal diameter and breast height at different Mean annual growth P. tremula P. a) CV% Breast height diameter (mm) Clone 1 Clone 2 14.5 13.8 12.6 12.8 14.2 14.8 15.1 15.4 15.7 29.2 CV% 13.0 ab 17.1 a Basal diameter (mm) 13.5 a Clone 1 17.3 a 15.7 ab 25.1 a 19.5 a 17.6 ab Clone 3 24.3 23.2 21.3 19.1 20.0 20.2 20.6 20.9 20.8 19.99 25.8 a Clone CV% 4 21.4 a 18.9 ab 11.6 ab 12.3 ab 27.9a 19.5 ab 12.1 ab 22.9 a 13.0 ab 19.7 ab 13.6 bc 30.2 a 23.2 a Clone 14.2 bc 2 20.1 a 15.1 bc Clone 3 31.6 a 13.4 24.0 a 15.7 abc 13.3 11.6 10.9 11.2 12.2 12.1 11.9 12.06 25.7 a Clone 16.1 abc 4 20.2 ab 16.7 bc 23.7 a 31.9 a 16.2 bc 24.4 a 26.3 a 20.2 ab 21.9 ab 16.9 abc 16.6 bc 23.8 a 32.5 a 0.39 a 22.8 a 17.2 abc 24.7 a 28.5a 17.3 ab 17.4 ab 24.7a 32.9 a 24.0a 25.1 a 30.6 a 17.3 bc 17.5 abc 27.9 a 17.7 bc 33.1 a 17.4 bc 26.4 a 0.35 a 32.8 a 0.32 a 28.6 a 0.29 a 33.5 a 27.2 ab 33.0 a 28.9 a 27.7 a 0.46 a 33.8 a 29.4 a 28.0 ab 34.3 a 28.4 ab 30.0 a 34.8 a 28.6 ab 30.4 a 29.3 a 35.2 a 30.7 a 0.40 abc 0.44 ab 0.35 bc Table 3. Table days) (/7 Height (cm) rate Clone 1 227.5 ab 236.4 a 253.0a Clone 2 May 24 277.4 a Clone 3 242.9 a Clone 298.2 a June 1 4 208.9 ab 251.0 a 304.3 a 223.7 ab 215.1 ab June 15 274.7 a 231.0 ab 229.6 ab 309.0 a June 29 250.0 ab 246.1 ab 295.3 a 311.3 a 268.9 ab 260.6 ab July 16 318.2 a 287.1 ab 262.4 ab 315.4 a 289.7 ab 264.9 ab July 27 324.8 a 319.6 a 291.4 ab 264.2 ab 331.6 a Aug. 9 5.10 a 291.6 ab 265.2 ab 292.9 ab 267.4 ab 335.4 a Aug. 24 293.8 ab 3.23 bc 340.4 a 3.89 bc Sept.13 342.4 a Oct. 5 4.65 a Annual growth

20 Yu, Tigerstedt and Haapanen Growth and Phenology of Hybrid Aspen Clones... s test. ’ cantly different at P < 0.05 according to Tukey at P < 0.05 according to cantly different fi 228.8 b 241.9 c 254.0 b 262.2 b 278.6 b 283.6 b 284.3 c 285.6 c 2.92 c 12.2 c 14.5 c 16.8 b 18.3b 22.0 c 19.4 b 25.5 b 19.6 b 27.9 b 19.7 b 30.0 b 19.8 c 31.0 b 0.37 b 31.0 b 31.0 b 31.2 b 0.45 b P. tremula P. P. tremula P. tremula P. rate (/7 days) b) (/7 Height (cm) rate Clone 1 327.4 a 353.5 ab 363.4 a Clone 2 May 31 398.4 a Clone 3 Clone 353.4 a 4 422.2 a June 20 278.6 ab 385.5 a 457.3 a July 6 295.1 ab 294.8 abc 320.6 ab 408.3 a 324.8 bc 464.0 ab 339.0 ab July 20 338.4 ab 468.7 ab 427.6 a 355.6 ab 348.4 ab 6.97 ab 380.7 ab Aug. 8 452.4 a 366.8 ab 386.8 bc 390.8 ab 492.1 a Aug. 19 393.1 bc 396.2 abc by the same letter are signi sequence not followed Note: Means in vertical 399.3 abc 5.64 ab 504.7 a Sept. 3 5.14 bc 512.4 a Oct. 19 7.83 a Annual growth CV% Breast height diameter (mm) Clone 1 Clone 2 15.0 14.7 13.9 15.2 14.4 14.1 20.6 CV% 21.7 a 26.5 ab Basal diameter (mm) Clone 30.5 a 25.6 ab 1 29.7 a 32.5 a 34.7 ab Clone 3 32.2 a 39.1 a 18.2 18.5 19.4 20.1 16.8 16.3 16.1 16.0 15.9 Clone 35.3 a 4 CV% 19.4 abc 34.4 a 42.5 a 18.2 bc 37.8 a 22.7 abc 26.7 a 21.4 bc 35.9 a 45.6 a 38.8 a Clone 2 24.4 ab 28.8 a 35.9 a Clone 47.8 a 3 39.6 a 26.7 ab 11.7 12.1 11.9 11.3 14.3 17.0 Clone 36.1 a 4 3.00 a 36.3 ab 49.0 a 30.9 ab 40.0 a 28. ab 39.5 a 28.4 bc 0.72 a 32.1 a 35.0 a 49.2 a 0.67 a 29.5 ab 33.0 ab 43.4 a 32.6 a 38.5 a 49.2 a 29.9 a 36.0 ab 45.5 a 33.1 ab 40.7 a 38.2 ab 0.72 a 29.8 b 47.5 a 0.68 a 39.4 ab 43.0 a 0.57 a 40.5 ab 49.5 a 44.5 a 40.6 ab 49.2 a 44.5 a 40.6 ab 49.5 a 0.60 ab 44.8 a 0.68 a 0.69 a

21 Silva Fennica 35(1) research articles

Table 4. Mean values and coeffi cient of variation of the traits among the hybrid clones and P. tremula seedling source.

Clone Growth initiation (days) Growth cessation (days) Growth period (days) Bud Leaf Full size Half leaves All leaves All leaves emerged unfolded leaves yellowed yellowed defoliated

Clone 1 5 d 7 d 13 c 29 c 43 b 45 b 148 b Clone 2 6 c 10 c 13 c 37 ab 42 b 46 b 154 a Clone 3 6 c 10c 13 c 41 a 45 a 54 b 158 a Clone 4 13 b 16 b 18 b 35 b 42 b 45 a 143 b P. tremula 16 a 18a 21 a 4 d 11 c 16 c 110 c CV (%) 6.9 6.4 12.7 8.8 3.8 4.47 1.8

Means followed by the same letters are not signifi cantly different at P < 0.05 (Tukey’s HSD test).

Table 5. Correlation coeffi cients (r) among the phenological and growth traits in the 3rd year.

Growth traits Growth initiation Growth cessation

Height Basal Breast Bud Leaf Full size Half leaves All leaves All leaves (cm) diameter height emerged unfolded leaves yellowed yellowed defoliated (cm) diameter (days) (days) (days) (days) (days) (days) (cm)

Basal diameter 0.91*** Breast diameter 0.95*** 0.98*** Bud emerged –0.66*** –0.76*** –0.72*** Leaf unfolded –0.68*** –0.77*** –0.73*** 0.99*** Full size leaves –0.67*** –0.76*** –0.73*** 0.96*** 0.95*** Half leaves yellowed 0.58** 0.60** 0.62*** –0.71*** –0.65*** –0.74*** All leaves yellowed 0.65*** 0.69*** 0.69*** –0.78*** –0.73*** –0.80*** 0.95*** All leaves defoliated 0.57** 0.66*** 0.65*** –0.79*** –0.79*** –0.80*** 0.95*** 0.98*** Growth period (day) 0.60*** 0.91*** 0.67*** –0.83*** –0.79*** –0.85*** 0.97*** 0.95*** 0.95***

ing early, and thus had a short growth period. periods. However, growth rate of height showed This must be ascribed to the different parental a strong cross over during the summer in 3rd and origins of the F1 hybrids. 4th year (Fig. 2), which indicates clonal geno- type × environment interaction. In the 3rd year (1997), the growth rate of height for P. tremula 4 Discussion had one peak in the mid-June. It then decreased constantly through the rest of the growth period. In our study, the superiority of the hybrid clones We conclude that the fast growth of hybrids can was best illustrated by stem volume. Hybrid be partly explained by the second growth peak clones had a signifi cantly greater wood produc- in mid-August. However, the hybrid clones nor- tion than P. tremula. In spite of obvious superior- mally had another growth peak in mid-August, ity in hybrid yield, hybrids varied considerable especially the height growth. In the 4th year for stem volume and annual growth rate. The (1998), P. tremula shows a slight tendency to hybrid clones showed signifi cantly higher growth form a second peak, but the hybrids behave far rates and yields than P. tremula at most observa- more erratically, responding strongly to a period tion dates over the entire growth season in the of warm weather in late summer. This indicates 3rd and 4th years (Table 3). Generally, the higher that the hybrid genomes are adapted to two differ- the yield, the higher the growth rates over all ent parental environments. The late fl ushing and

22 Yu, Tigerstedt and Haapanen Growth and Phenology of Hybrid Aspen Clones...

early cessation of the local aspen is considered period and yield growth, which indicates that the to be an adaptation to the short summers in the fast growth for the hybrid can be largely explained far north. by the expanding growth period. However, when The signifi cant clone-by-block interaction for P. tremula was removed from the data, there was growth performance illustrated how sensitive no correlation between growth period and yield aspen clones are to microsite differences (Ceule- growth in the hybrid material (r = –0.002). Yield is mans et.al 1992). By the end of the 5th year, a complex character, and may well be determined block 5 in the trial produced only 48% of what by other factors than the growth period. Li et al block 1 produced. This must have been due to a (1998) noted that hybrid vigor in aspen hybrids considerable variation in soil conditions across (P. tremula × P. tremuloides) was associated with the trial. Li and Wu (1997) found signifi cant delayed bud set resulting in longer duration of genotype × environment interaction for growth growth in fi rst year. However, by the second year, traits of inter and intra-specifi c aspen crosses in no relationship was found between bud set and two contrasting environments. stem growth in either the intra- or inter-specifi c The ranking of clones for stem volume was not crosses. consistent from the 2nd to the 5th year. Signifi cant Strong selection for growth may increase the clone by year interaction in growth performance growth period (Wang and Tigerstedt 1996). Late probably indicates differences in hybrid reaction active growth may render the hybrids liable to to the climate (Table 2). Zsuffa et al. (1993) frost damage in fall and winter (Rehfeldt 1979, found that the ranking of poplar clones (P. × and Li and Adams 1993, Li et al. 1998). Li (1998) euramericana) by height changed considerably proposes the use of local aspen as female parents during the fi rst 6 years of growth. Our study in intraspecifi c crosses, in order to improve frost showed similar results. Stem volume of clones 3 hardiness of the hybrids. and 4 were the same in the 2nd year, 0.8 dm3. The fast overall growth of the aspen hybrids Because basal diameter of clone 3 increased faster is largely explained by their longer vegetative than clone 4, the stem volume of clone 3 was period. This response is not true heterosis as larger than that of clone 4 in the 5th year. In has been commonly assumed in hybrid aspen. It the 2nd year, the stem volume of clones 1 and must be borne in mind that an extension of the 2 were 1.1 dm3 and 1.0 dm3, respectively, but vegetative period of about 40 days in the hybrids after the 5th year, the stem volume were 17.6 and may drastically increase the risk of frost damage 17.8 dm3, respectively. This suggests that, for at both ends of the growth period. proper clonal selection several cumulative growth The considerably extended growth period may periods must be observed, perhaps over as long also upset a “natural” balance between the aspen as 10 years. and its pests and diseases. Thus, the increased The range in growth initiation was usually yield is likely to involve an increased risk. This smaller than it was for growth cessation. The bud situation should be carefully assessed before emerged of the hybrids started 8.85 days earlier large-scale cultivation of cloned hybrids can take than for P. tremula in the spring, whereas half place. leaves yellowed of the hybrids occurred 31.63 Our analyses were based on a small number days later in the autumn. Compared to growth of clones measured over only fi ve years. The initiation, the environmental control of growth results of the study need to be followed up by cessation is more site-specifi c. This may be due to larger and longer experiments. The superiority the interaction of edaphic and weather conditions of hybrids over their non-hybrid counterparts, in causing rust damage. The leaf decoloration and the underlying cause for such superiority may have been indirectly affected by cold and are unclear (Stettler et al. 1996). Valid estimates wet weather in the 4th year. P. tremula and clone require a suffi cient number of parents and their 1 had suffered from a strong infection of leaf rust, progenies to be tested in suffi cient numbers of which may have affected the timing of defoliation trials, replicated in space and time (Stettler et and growth cessation. al. 1996). It remains to be found out, if on top There was a strong correlation between growth of the extended vegetation period, there is in

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24 Yu, Tigerstedt and Haapanen Growth and Phenology of Hybrid Aspen Clones...

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