Patterns of genetic variation in mountain hemlock (Tsuga mertensiana (Bong.) Carr.) with respect to height growth and frost hardiness
Introduction
Although mountain hemlock (Tsuga mertensiana (Bong.) Carr.) is considered a ‘minor’ commercial species in British Columbia (BC), it is an important component of sub-alpine habitats in terms of watershed protection and creation of wildlife habitat. It occurs mainly in coastal areas between 1000 and 1500 m in southern BC at lower elevations farther north, and at higher elevations in disjunct populations in southeastern BC, northern Idaho, northwestern Montana, and in the Cascades south to California (Arno, 1977). As timber harvesting moves to higher elevations, particularly in coastal areas, there is increasing pressure on mountain hemlock populations. At present, there is no tree improvement program or gene conservation plan for this species (Edwards et al., 1993), and there is little information on its genetic variation (Meagher, 1976; von Rudloff and Lapp, 1989; Edwards and El-Kassaby, 1998; Benowicz and El-Kassaby, 1999). It is important that information be available on genetic diversity, regeneration potential and adaptive traits to ensure that natural and artificial regeneration of mountain hemlock will be successful. Knowledge of the extent of adaptive variation within and among populations in British Columbia should be useful in defining seed transfer rules with respect to elevation, latitude and longitude.
Among adaptive attributes, low temperature survival and growth performance are two characteristics of practical importance. Differences at the population level in both cold resistance and growth traits are often found for tree species that grow in a wide range of environmental conditions (see Morgenstern, 1996). Frost hardiness and growth are also frequently inter-related: plants from colder regions are often smaller and develop frost resistance earlier than plants evolved in warmer locations.
The objectives of this study were: (1) to estimate the variation within and among populations of mountain hemlock from coastal British Columbia in height growth and fall frost hardiness, (2) to relate the pattern of inter-population differentiation to the geographic variables of population origin, and (3) to utilize the information from the two previously stated objectives to develop preliminary recommendations for seed transfer guidelines.
Section snippets
Plant material
Seeds of 18 mountain hemlock populations were obtained from wild-stand seed collections at the Ministry of Forests Seed Centre in Surrey, BC. Geographic locations ranged from 48° to 56° latitude, 122° to 129° longitude, and 600 to 1280 m elevation (Fig. 1). The populations fell into two main groups: southern and northern comprised of 16 and 2 seed sources, respectively (Fig. 1). Seedlings were grown in a greenhouse in a commercial nursery located on the Saanich Peninsula of southern Vancouver
Frost hardiness
There were significant differences (P<0.05) among the populations in frost hardiness on October 4 and November 8 but not on September 7 (Table 1). In October, the populations formed two groups based on similar level of frost hardiness as northern populations (17 and 18) were significantly more frost hardy (P<0.05) than all southern populations, while no differences were apparent within both groups (Fig. 2). In November, both northern populations were more frost hardy than most southern
Frost hardiness
Most of the variation in fall frost hardiness resided within populations (above 77%). That is despite rather large latitudinal range of the sampled populations, spanning almost 8°. These results correspond to the relatively low level of inter-population differentiation found in mountain hemlock in other studies. Based on one inland population and 11 coastal populations, Benowicz and El-Kassaby (1999) found that between 5 and 15% of the genetic variance was attributed to the population effect in
Acknowledgements
This study was funded by a grant from the Forest Renewal British Columbia # HQ 96059-RE. Support of the British Columbia Ministry of Forests Research Branch and the technical assistance of C. Cook, J. Halusiak and H.A. Mehl are highly appreciated.
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