Patterns of genetic variation in mountain hemlock (Tsuga mertensiana (Bong.) Carr.) with respect to height growth and frost hardiness

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Abstract

Genetic structure (variance among and within populations) and geographic pattern of variations in growth and frost hardiness of mountain hemlock (Tsuga mertensiana (Bong.) Carr.) populations from coastal British Columbia (BC) were examined. Populations fell into two main groups — south coast (16 sources) and north coast (two sources). Height growth of greenhouse grown seedlings was measured during the first growing season on a weekly basis whereas frost hardiness and branch water content were evaluated during the fall at monthly intervals. Significant differences among the populations were found in maximum instantaneous growth rate, height at the end of the growing season and in October and November frost hardiness. There were no differences in date of maximum growth rate, branch water content and September frost hardiness. Most of the genetic variance was found within populations: 77% for height, maximum growth rate and November frost hardiness and 87% for October frost hardiness. Geographic trends were identified using multiple linear regression and canonical correlation analyses based on population means and latitude, longitude and elevation of the population origin. The uneven distribution of sources limited the extrapolation, but some general trends appeared. Maximum instantaneous growth rates increased with latitude, elevation and decreased with longitude (R2=0.64) and consequently were negatively correlated with the length of the growing season in the seed collection sites. Both populations from the north coast developed frost hardiness earlier than any population from the south coast. The relationship between climate coldness and growth rates has potential implications for seed transfer guidelines. Due to their higher growth rates, plants from higher altitudes may accomplish the same amount of growth within a shorter period compared to plants from lower altitudes when both are transferred north. Results of the frost hardiness tests indicate that seed transfer along the British Columbia coast of more than 3° northward will considerably increase chances of frost damage in plantations.

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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