New Forests (2006) 32:71–86 DOI 10.1007/s11056-005-3872-2
Springer 2006
-1
Provenance variation in seed morphometric traits, germination, and seedling growth of Cordia africana Lam. ABRAHAM LOHA1,2, MULUALEM TIGABU1, DEMEL TEKETAY3, KENNETH LUNDKVIST4 and ANDERS FRIES1,* 1 Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, SE-901 83, Umea˚, Sweden; 2Permanent Address: Wondo Genet College of Forestry, P.O. Box 128, Shashemene, Ethiopia; 3Forest Stewardship Council, African Regional Office, P.O. Box RY615, Kumasi, Ghana; 4Department of Plant Biology and Forest Genetics, Swedish University of Agricultural Sciences, SE-750 07, Uppsala, Sweden; *Author for correspondence (e-mail:
[email protected]; phone: +46-90-786-8368; fax: +46-90-786-8165)
Received 11 November 2004; accepted in revised form 30 September 2005
Key words: Ethiopia, Genetic variation, Growth capacity, Seed weight, Tropical trees, Viability Abstract. Patterns of genetic variation in Cordia africana, a tropical timber species, were evaluated at the population level. Bulk seed samples were collected from six natural populations in Ethiopia and examined for variations in seed morphometric traits, seed germination, and seedling growth at nursery stage. Analysis of variance revealed significant differences among provenances in all studied attributes except root collar diameter after 4 months of growth. The provenance effect, as determined by broad sense heritability, was 71–98% for seed morphometric traits, 80% for germination capacity, 42% for germination energy, 57–58% for seedling height and 3–13% for root collar diameter. Seed weight showed a significant positive correlation with altitude and negative correlation with mean annual temperature of seed origin. Germination energy was significantly correlated with longitude and mean annual rainfall. Seedling parameters and geo-climatic variables of seed origin were fairly correlated. A significant intercharacter correlation was found between seed length and seed weight, between root collar diameter at the age of 4 months and seed length and weight, as well as between seedling height after 4 and 8 months of growth. It can be concluded that the observed patterns of variation will have implications for genetic resources conservation and tree improvement.
Introduction Cordia africana Lam. (synonym: Cordia abyssinica R.Br.) is a widely distributed tree species in tropical Africa from the Sudan and Ethiopia in the North to North-East Transvaal in the South and as far as tropical Arabia (Warfa 1988; Friis 1992). In Ethiopia, it is widespread in the broadleaved Afromontane rain forests, undifferentiated Afromontane forests and riverine forest of the northwest and southeast highlands, and very common in western lowlands at altitudes ranging from 550 to 2600 m (Friis 1992). Its
72 growth habit varies from a shrub (generally less than 9 m) to a large tree (growing to a height of 25 m), and it grows over a wide rainfall range, 700–2000 mm per annum. Cordia africana is one of the major timber species in Ethiopia that have been exploited commercially (Abebe 2003a). The moderately hard (basic density = 0.35–0.39 g/cm3) and durable wood makes it a valuable raw material for making high quality furniture and household materials (Ishengoma et al. 1999). It is also the most important multipurpose tree species widely occurring on croplands, grazing areas, homesteads and farm boundaries. For instance, it is a good source of firewood, food (fruit), medicine (bark, root), and bee forage (Bekele et al. 1993; Fichtl and Adi 1994; Negash 1995). Several other uses including ethno-pharmaceutical values are documented by Jansen (1981). In addition, it is one of the common shade trees of coffee in Ethiopia (Teketay and Tegineh 1991), and enriches soil fertility and productivity through litter fall that decomposes readily (Teklay and Malmer 2004). In Ethiopia, there is insufficient knowledge about provenance and genetic variability of important indigenous species in general and C. africana in particular. Since environmental conditions vary extensively within the natural range of the species, it is reasonable to expect genetic differentiation among C. africana populations in a number of traits. In addition, the species is known to be insect pollinated (Negash 1995), which probably reduces gene flow and thus enhance the genetic differentiation between populations. In fact, our casual observations on variations in flower and fruiting phenology (although seed production is generally annual), vegetative growth and growth habit of the species in its natural habitat are indicators for the existence of genetic variation. Previous genetic studies on economically useful tropical plantation species such as Gmelina arborea Roxb., Tectona grandis L.f., and Hevea brasiliensis (Willd. ex Adr. de Juss) Muell.-Arg. have documented the existence of substantial genetic variation in natural populations for a variety of quantitative traits (Alika 1980; Jayasankar et al. 1999a; Hodge and Dvorak 2004; Lauridsen 2004). It is generally assumed that populations within the same regions of provenance are derived from the same random mating or base population (Stern and Roche 1974). The genetic component of this variation among populations from different regions can, therefore, be identified by provenance testing and exploited through selection of superior populations for seed collection. Conservation and sustainable use of genetic resources is dependent upon the knowledge of the extent and pattern of intra-specific variation. In a country like Ethiopia where no a priori information exists, knowledge of the pattern of genetic variation is of fundamental importance to identify seed sources, select plus trees and prevent risks associated with transfer of seeds into new environments. In this paper, we report results on variations in seed- and seedling-related parameters among six provenances of C. africana, which are
73 representative of the natural distribution range of the species. The populations were examined for differences in seed morphometric traits, germination and seedling growth in the nursery. Materials and methods Seed collection Seeds of C. africana were collected from six provenances, representing the natural distribution of the species in Ethiopia. Provenances were selected following the tree seed zoning system developed for the country (Aalbæk 1993). In this study, the term provenance denotes the original geographic area from which seeds were obtained (Sensu Zobel and Talbert 1984). The geographic locations and climatic conditions of the different provenances are given in Table 1 and Figure 1. The Zeghie Peninsula and Bedelle provenances represent the northwestern and southwestern ranges of distribution of the species, respectively. Alemaya, Degaga, Dilla and Wondo Genet represent the widest range of distribution of the species in the southeastern highlands (Friis 1992). Drupes were collected from 10 phenotypically superior trees in each provenance. Dominant or codominant trees with clear bole, well developed crown and with abundant drupe were selected for seed collection. To ensure maximum genetic variation within the population, the selected trees were kept at least 100 m apart from each other (FAO 1975). After collection, drupes were bulked by provenance, depulped manually and the seeds were air-dried to 6–8% moisture content.
Figure 1.
Location of the different provenances of Cordia africana represented in this study.
74 Table 1. Geographic locations (latitude, longitude and mean altitude) and climatic conditions (mean annual rainfall and mean annual temperature) of the different provenances of Cordia africana represented in this study. Provenance
Geographic location Lat.–Long.
Altitude (m a.s.l)
Rainfall (mm)
Temperature (C)
Dilla Wondo Genet Degaga Bedelle Alemaya Zeghie
625¢ N–3818¢ E 710¢ N–3835¢ E 735¢ N–3854¢ E 827¢ N–3623¢ E 926¢ N–4203¢ E 1136¢ N–3724¢ E
1670 1880 2550 2090 2125 1890
1253 1244 1333 1966 880 1521
20.1 18.6 12.6 18.0 17.1 18.3
Climatic data were obtained from the Ethiopian Meteorological Service Agency.
Seed morphometric characters To examine the variability in seed morphometric characters, seed length, width and 100-seed weight were determined. From each provenance, a total of 100 seeds were randomly selected (5 replicates of 20 seeds) and used for measuring each morphometric character. Seed length and width were measured on individual seeds using a digital micro caliper while seed weight was determined for each replicate (20 seeds) using sensitive balance and then 100-seed weights were computed. Germination test The germination test was conducted at the National Tree Seed Project laboratory of the Forestry Research Center in Addis Ababa (902¢ N and 3845¢ E), Ethiopia. Five replicates of 20 seeds from each provenance were sown on sand medium in Petri dishes, which was kept moist. The Petri dishes were placed on a germination table maintained at 20 ± 2 C with continuous illumination (Fluorescent lamp F 40 W/33 RS cool white light). The germination process was monitored every day for 60 days and germinated seeds were counted when the radicle reached 2 mm and had a normal appearance (International Seed Testing Association 1999). The following parameters were determined: germination capacity (GC) and germination energy (GE). GC is the proportion of total germinated seeds to that of sown seeds, expressed in percentage. GE, also expressed in percentage, is computed as the proportion of germinated seeds after 21 days to that of total germinated seeds after 60 days. GE is one of the commonly employed indices of speed of germination (International Seed Testing Association 1999). Seedling performance test For assessment of the variability in seedling growth parameters, a nursery experiment was performed at Wondo Genet College of Forestry nursery, which
75 is located 260 km south of the capital, Addis Ababa. For geo-climatic variables of Wondo Genet see Table 1. Initially, seeds from each provenance were sown on seedbeds and germinants were later transplanted into polythene plastic pots of size 8 · 15 cm when flat. The pots were filled with a mixture of humus-rich forest soil, local mineral soil and fine sand in the ratio of 5:3:1, respectively. Seedlings were grown under partial shade (50% full sunlight) using locally available grasses. To maintain an ideal condition of soil moisture, watering was done twice a day at the beginning, thereafter as often as needed. No fertilizer or mycorrhizal inoculation was used. The experimental design adopted was a completely randomized design comprising of 25 seedlings per provenance. After 4 and 8 months of growth, seedling height and root collar diameter were measured.
Statistical analysis The data were subjected to one-way analysis of variance (ANOVA). Prior to ANOVA, the percentage data sets (germination capacity and germination energy) were arcsine transformed to meet the normality assumption for the analysis of variance (Zar 1996). The generalized linear model (GLM) procedure of SPSS (SPSS 10 Copyright: SPSS Inc.) was employed for ANOVA along with the variance components procedure using the restricted maximum-likelihood method of estimation to calculate variances due to provenance and environment (error). Means that exhibited significant differences were compared using Bonferonni’s procedure at the 0.05% level. To compare the magnitude of variation due to provenance and environment, provenance coefficient of variation (PCV) and environmental coefficient of variation (ECV) were computed for each seed- and seedling-related character using the expected mean square of the provenance variance (r2pro), environmental variance (r2e) and the overall mean as given below: 1=2
PCV ¼
ðr2pro Þ 100 Mean
ð1Þ
ðr2e Þ1=2 100 Mean
ð2Þ
ECV ¼
To determine to what extent the provenance variation contributed to the total variation, broad sense heritability (H2) was calculated as a ratio of expected mean square of the provenance variance (r2pro) to the total (phenotypic) variance (r2pro + r2e) as follows: H2 ¼
ðr2pro Þ ðr2pro þ r2e Þ
ð3Þ
76 Partitioning of total variance into genetic and environmental components is described elsewhere (Falconer and Mackay 1996). Pearson product–moment correlations were calculated to examine relationships between geo-climatic data and seed- and seedling-related parameters, as well as between seed- and seedling-related parameters.
Results Seed morphology Significant differences among provenances were detected for all seed morphometric characters (Table 2). The average seed length, seed width and 100-seed weight are shown in Figure 2a. The mean seed length varied from 8.2 to 9.54 mm. Seeds collected from Degaga and Zeghie had the largest value for seed length followed by those from Alemaya and Dilla, and the lowest being for seeds collected from Bedelle and Wondo Genet. With regard to seed width, seeds collected from Degaga and Dilla had significantly higher values compared to other provenances. The mean seed width varied from 5.4 to 7 mm. Seed weight markedly differed among provenances except seeds collected from Wondo Genet and Dilla that did not show significant difference. The mean 100-seed weight varied from 27.4 to 41.3 g. Seeds collected from Degaga were the heaviest while those collected from Wondo Genet and Dilla were the lightest. For all morphometric characters, a clear contrast was discernible within the southeast provenances (Wondo Genet, Alemaya, Dilla and Degaga), as well as between southwest (Bedelle) and northwest (Zeghie) ranges of distribution. Estimates of provenance and environmental effects indicate that the PCV was large for seed weight compared to seed length and seed width (Table 3). In general, environmental factors appeared to play a minor role in shaping these characters, as 71–98% of the total variation was attributed to provenance variation as indicated by the broad sense heritability (Table 3). Of the seed morphometric characters, only seed weight showed a significant positive correlation with altitude and a significant negative correlation with mean annual temperature of seed origin (Table 4). Among morphometric characters, seed length and weight had a significant positive correlation (Table 5). Although not statistically significant, seed length and seed width had a fairly good correlation (Table 5).
Seed germination Provenances displayed significant differences with respect to germination capacity and germination energy (Table 2). The mean germination capacity
77
Figure 2. Mean seed length, seed width and 100-seed weight (panel a), and germination capacity and germination energy (panel b) of six provenances of C. africana. Bars with the same letter (s) are not significantly different at the 0.05 level.
varied between 4 and 91% (Figure 2b). The germination performance of seeds collected from Alemaya was superior to other provenances while the percentage germination of seeds collected from Zeghie was extremely low. The speed of germination, as determined by the germination energy, also showed considerable variation among provenances (10–89%). Seeds collected from Alemaya germinated rapidly, i.e., 89% of the seeds germinated with 21 days, compared to other provenances (Figure 2b). Most of the total variation in germination capacity was due to the provenance effect (Table 3). The environmental effect on the speed of germination was comparably higher than that of provenance effect, as shown by the relatively lower percentage contribution of provenance variation to the total variation (Table 3). Correlation analyses revealed that germination capacity had a fairly good correlation with longitude of seed origin, albeit not statistically significant (Table 4). Germination energy had a significant positive correlation with the longitude and negative correlation with mean annual rainfall of the
78 Table 2. Analysis of variance for seed morphometric traits (SL – seed length; SW – seed width; Swt – seed weight), germination parameters (GC – germination capacity; GE – germination energy) and seedling growth attributes (H4 – height at age 4 months; H8 – height at age 8 months; D4 – root collar diameter at age 4 months; and D8 – root collar diameter at age 8 months). Source of variation
Degrees of freedom Variance components
Morphometric traits Provenance (P) 5 Error (E) 24 Germination parameters Provenance (P) 5 Error (E) 24 Seedling growth parameters Provenance (P) 5 Error (E) 144
SL 0.29* 0.12 GC 534.9* 134.7 H4 62.9* 46.9
SW 0.30* 0.10 GE 454.9* 619.9 H8 151.3* 107.6
Expected mean squares
Swt 24.9* 0.48
r2e + 5r2Pro r2e r2e + 5r2Pro r2e
D4 0.03 0.88
D8 0.17* r2e + 25r2Pro 1.14 r2e
*Significant at p £ 0.05.
seed origin (Table 4). Both germination capacity and germination energy had very weak correlation with all seed morphometric characters (Table 5).
Seedling growth Provenances displayed significant differences in seedling height after 4 and 8 months of growth in the nursery (Table 2). Seedling height was considerably higher for Alemaya compared to other provenances both after 4 and 8 months (Figure 3a). The slowest height growth was observed for Dilla provenance. A marked contrast in height growth was discernible among the southeast range of
Table 3. Provenance and environment coefficient of variation, and broad sense heritability for seed- and seedling-related characters of C. africana. Characters
Seed parameters Length (mm) Width (mm) Weight (g) GC (%) GE (%) Seedling growth parameters Height, 4 months (cm) Height, 8 months (cm) Diameter, 4 months (cm) Diameter, 8 months (cm)
Overall mean
Coefficient of variation (%)
Heritability (%)
Provenance
Environment
8.86 6.24 32.2 45 46
6.1 8.8 15.5 51 46
3.9 5.1 2.2 26 54
71 75 98 80 42
20.6 32.5 3.1 5.5
39 38 5.6 7.5
33 21 30 19
57 58 3 13
79 Table 4. Correlations between geo-climatic variables (Latitude, longitude, mean altitude, mean annual rainfall, mean annual temperature) of seed origin and seed morphometric traits, germination and seedling growth traits of six C. africana provenances. Seed and seedling traits
Seed length Seed width Seed weight GC GE Height, 4 mo Height, 8 mo Diameter, 4 mo Diameter, 8 mo
Geo-climatic data Lat.
Lon.
Alt.
Rainfall
Temp.
0.40 0.30 0.21 0.17 0.36 0.42 0.38 0.22 0.18
0.23 0.12 0.09 0.65 0.92* 0.78 0.79 0.62 0.72
0.49 0.24 0.87* 0.46 0.15 0.59 0.64 0.73 0.06
0.29 0.40 0.03 0.33 0.86* 0.50 0.53 0.54 0.69
0.57 0.37 0.91* 0.34 0.09 0.49 0.56 0.74 0.12
Values are Pearson product–moment correlation coefficient, r. *Correlation is significant at the 0.05 level.
distribution (Alemaya, Degaga, Wondo Genet and Dilla). The diameter growth after 4 months was similar for all provenances (p = 0.52), but significant difference among provenances was found after 8 months of growth (Table 2). At 8 months of age, root collar diameter was bigger for Wondo Genet and Alemaya compared to other provenances (Figure 3b). More than 50% of the total variation in seedling height was attributed to provenance effect (Table 3). The environmental effect on root collar diameter was remarkably higher than the provenance effect, as shown by the low broad
Table 5. Intercharacter correlations of seed and seedling parameters of six C. africana provenances (n = 6). Parameters
SL SW Swt GC GE H4 H8 D4 D8
Seed and seedling parameters SL
SW
Swt
GC
GE
H4
H8
D4
D8
1.0
0.70 1.0
0.83* 0.53 1.0
0.18 0.34 0.09 1.0
0.05 0.10 0.10 0.72 1.0
0.33 0.18 0.40 0.73 0.56 1.0
0.39 0.09 0.47 0.72 0.58 0.99* 1.0
0.83* 0.52 0.82* 0.38 0.41 0.71 0.77 1.0
0.17 0.27 0.31 0.34 0.63 0.56 0.54 0.10 1.0
Values are Pearson product–moment correlation coefficient, r, and SL – seed length; SW – seed width; Swt – seed weight; GC – germination capacity; GE – germination energy; H4 – height at age 4 months; H8 – height at age 8 months; D4 – root collar diameter at age 4 months; and D8 – root collar diameter at age 8 months. *Correlation is significant at the 0.05 level.
80
Figure 3. Mean seedling height (panel a) and root collar diameter (panel b) of six provenances of C. africana after 4 and 8 months of growth in the nursery. Bars with the same letter (s) are not significantly different at the 0.05 level.
sense heritability values (Table 3). Although not statistically significant, seedling height had a reasonably good positive correlation with longitude and altitude of seed origin and negative correlation with rainfall and temperature (Table 4). Diameter at the age of 4 months showed a fairly good correlation with all geo-climatic variables except latitude of seed origin, while diameter at the age of 8 months fairly correlated with longitude and rainfall (Table 4). Among seed-related parameters, seed length and seed weight showed a positive significant correlation with root collar diameter at the age of 4 months (Table 5). Germination capacity and germination energy exhibited a fairly good correlation with seedling height, so also seedling height and root collar diameter (Table 5). While seedling height after 4 and 8 months of growth showed a 100% correlation, root collar diameters measured after 4 and 8 months showed a very poor correlation (Table 5).
Discussion The occurrence of C. africana over a wide geographical range, encompassing a great diversity in edapho-climatic conditions of its habitat (Friis 1992), is
81 expected to be reflected in the genetic constitution of its diverse populations. In the present study, a considerable variation in seed morphometric traits was observed among provenances (Figure 2a), and most of the total variation was a provenance effect (Table 2). In addition, morphometric traits especially seed weight, showed a strong correlation with altitude (75%) and mean annual temperature of seed origin (83%) (Table 4). This indicates that seed size is not only heritable but also strongly affected by environmental variations. While it is a well-known fact that altitude and temperature has an inverse relationship, we found a negative correlation between seed weight vs. altitude and mean annual temperature in this study, which is inconsistent with established fact. Studies have shown that it is the low temperature during seed development and maturation that increases seed mass rather than the overall annual temperature (Wulff 1995). Genetic control of seed morphometric traits has been indicated for several tree species (Ibrahim 1996; Jayasankar et al. 1999b; Gera et al. 2000; Sivakumar et al. 2002; Mkonda et al. 2003). There was also a strong intercharacter correlation, particularly between seed weight and seed length (Table 5). Correlated quantitative traits are of a major interest in an improvement program, as the improvement of one character may cause simultaneous changes in the other character. Similar results have been reported for Faidherbia albida (Del.) A. Chev. (Ibrahim 1996) and Tectona grandis L.f. (Jayasankar et al. 1999b). Provenances have also displayed significant differences in germination parameters, and the magnitude of variation due to provenance effect was much higher for germination capacity than germination energy (Table 2; Figure 2b). In most plant species, seeds vary in their degree of germinability between and within populations and between and within individuals (Benowicz et al. 2000, 2001; Gera et al. 2000; Sivakumar et al. 2002; Thomsen and Kjær 2002; Mkonda et al. 2003). Some of this variation can be of genetic origin, but much of it is known to be phenotypic, i.e. caused by the local conditions under which the seed matured. As reviewed by Gutterman (2000), the germinability of seeds can be markedly influenced by maternal factors, such as position of the seed in the fruit/tree, the age of the mother plant during seed maturation, as well as environmental factors such as day length, temperature, light quality, water availability and altitude. Although geo-climatic variables of seed origin and germination capacity showed a relatively low correlation, handling during collection and processing might cause erratic germination responses. Despite the fact that 80% of the total variation in germination capacity is attributed to provenance effect, the effect of other factors cannot be ruled out. The provenance effect on the speed of germination, as determined by the germination energy, was comparably low (42%). It is well-known that the loss of seed vigor precedes the loss of viability, and seed deterioration usually commences at physiological maturity and continues during harvest, processing and storage, which in turn is governed by the genetic constitution, environmental factors during seed development and storage conditions (McDonald 1999). Apparently, factors other than genetics considerably affect the speed of
82 germination. In the present study, more than 80 and 70% of the variation in germination energy is attributed to longitude (r = 0.92) and mean annual rainfall of seed origin (r = 0.86), respectively (Table 4). The strong negative correlation between germination energy and rainfall indicates an adaptation for ecological conditions of germination and seedling establishment. Under natural condition, rapid germination and seedling emergence could be a survival strategy that permits rapid exploitation of conditions favorable for germination, which is a typical characteristic of many pioneer species like C. africana (Larcher 1995; Abebe 2003a, b). None of the seed morphometric traits has shown good correlation with germination capacity or germination energy (Table 5). This indicates that seed morphometric traits have little importance in predicting the germinability of seeds of C. africana, and seed morphometric traits may be adapted for other purposes, such as dispersal. Evidence indicates that large seed size with fleshy edible fruits, such as that of C. africana, is one of the syndromes for endozoochorous modes of dispersal (Izhaki 2002; Jansen et al. 2002). Large differences in seedling height were found among provenances after 4 and 8 months of growth in the nursery (Table 2; Figure 3a), and as much as 58% of the total variation was due to provenance effect (Table 3). This indicates that there is adequate genetic variability for seedling height growth in the present material. Similar results have been reported for several broadleaved and coniferous species (Jayasankar et al. 1999a; Benowicz et al. 2001; Mantovan 2002; Shimizu et al. 2002; Cundall et al. 2003). Interestingly, a significant strong age–age correlation was found for mean height after 4 and 8 months (Table 5). This indicates that seedling height could be employed as a proxy indicator for early selection of provenances for further testing; however, a definitive recommendation to rely on such early selection must wait until the field test data is at least 1/3 of the rotation length and necessary analyses are completed. It is often advisable that the age of early selection should be sufficiently long to achieve a reasonable level of accuracy in selection (Kumar and Singh 2001; Cundall et al. 2003). Similar result has been reported for Gmelina arborea Roxb. in which total height at 9 months of age was the best predictor of performance at 72 months (Padua 2004). As our result is based on nursery performance for a short time (8 months), further progeny tests in the field should be carried out for at least a third of the rotation period to come up with a definitive recommendations for early selection. Correlation was found between mean seedling height and longitude of seed origin (r = 0.78 and 0.79 for height after 4 and 8 months, respectively, Table 4). Such correlation is expected in a material which is autochthonous; i.e., from a population of trees which is known to have naturally regenerated without human interference since primary colonization. The stands from which seeds were collected for the present study were remaining relict natural forests. Similar results have been documented in provenance trials of other species where early vigor has been correlated with the latitude and longitude of the provenances under test (e.g. Kleinschmit et al. 1996). Seed morphometric traits
83 had poor correlation with seedling height while germination parameters showed a fairly good correlation (r = 0.73 for germination capacity and 0.58 for germination energy, Table 5). This suggests that any single trait could not be employed for selecting provenances for tree improvement programs. Root collar diameter at the early stage of growth (4 months) did not vary significantly among provenances, but as growth advanced (after 8 months) a significant difference in diameter was found among provenances (Table 2; Figure 3b). Provenance variation in seedling diameter at the nursery stage has been observed for F. albida after 3 months (Ibrahim 1996), for Dalbergia sissoo Roxb. after 6 months (Sagta and Nautiyal 2001) and for Tectona grandis after 8 months (Jayasankar et al. 1999a). These findings suggest that expression of genetic potential for seedling diameter growth is species-specific. In addition, crowding takes place among seedlings in nursery beds just as it does among trees in the forest. Variability in survival, hence seedling density in the beds, can have a large effect on the diameter of the surviving seedlings. Also the speed of germination affects initial size of the seedlings, and hence diameter. For trees, stem diameter appears to be more sensitive to environmental effects than height. As a result, heritability is usually higher for height than for diameter. As C. africana is a light-demanding species (Yirdaw and Leinonen 2002; Abebe 2003b), height growth at the early stage might be more important than diameter growth. The magnitude of variation due to provenance effect was much lower for root collar diameter compared to seedling height (Table 3), and the age–age correlation in diameter was rather poor (Table 5). This indicates that diameter is a relatively poor parameter to detect genetic variation at the early stage, and a longer growing time is needed for a better expression of the genetic potential for this trait. For example, one study suggests selection based on diameter could be done after age 1–3 years for Populus deltoids clones (Randall 1977) while another study on the same species indicated selection at age 1 may not be judicious (Kumar and Singh 2001). It has been also suggested that height and diameter growth could be under rather different genetic control, and diameter is considered to be more sensitive to environmental conditions than height (Costa and Durel 1996). Although seed width showed weak correlation with root collar diameter, diameter at the early stage of growth (4 months) had a significant correlation with seed length and weight (Table 5). This is, in fact, expected as the emerging seedling depends on the seed reserve for its initial growth until it becomes autotrophic. This is further evidenced from the poor correlation between seed weight and diameter at the later stage of growth (8 months).
Conclusions Results from the present study provide evidence that seed- and seedling-related parameters vary considerably among provenances of C. africana. For bulk seed collection, either for ex situ conservation or seedling production, collection
84 should be made from various sources, especially along the east-west range of distribution of the species, as longitudinal variation has been reflected on germination performances and height growth. The fact that none of the provenances displayed consistent variation for all seed- and seedling-related traits suggests the need for multiple criteria for selection of provenances for improvement program. As this study is the first systematic attempt in the country, the findings can serve as base-line information for detailed genetic studies in the future. Further research is also in progress to quantify the genetic variability on individual family level and to identify highly correlated traits for selection of materials for improvement program of C. africana. Finally, selecting and analyzing additional provenances in future studies could be considered in order to get more precise correlation with geographic data.
Acknowledgements The study was financially supported by the Swedish International Development Agency (Sida). Wondo Genet College of Forestry, Ethiopia is highly acknowledged for logistics support during the field work. Staff members of the National Tree Seed Center, Mr Yonas at Bedelle site, Dr Zerfu Hailu and many more are gratefully acknowledged for their splendid support during seed collection, characterization and germination tests.
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