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The ecology of planted Engelmann spruce (Picea engelmannii Parry) seedlings on subalpine forest cutovers

University of British Columbia

At high elevations in south-central British Columbia conifers are slow to re-establish after logging and the vegetation on many harvested sites is dominated by herbs and shrubs. At present, Engelmann spruce (Picea engelmannii Parry) is widely planted on subalpine cutovers to increase stocking levels, but the growth and survival of planted seedlings is often poor and is highly variable. The objectives of this research were to study: 1) variation in the environments within which Engelmann spruce establishes on subalpine cutovers; 2) the performance of planted seedlings in these environments both with and without interference from non-crop species; 3) the relationship between the growth of planted seedlings and naturally established Engelmann spruce and; 4) the responses of non-crop species to post-logging disturbances. Engelmann spruce seedlings were planted in 1987 into patches of herbs and shrubs and on skid trails on two subalpine cutovers that were winter-logged in 1983. Seedlings grew poorly beneath canopies of all non-crop species where low light levels were a dominant factor limiting growth despite differences between patch types in other environmental factors. Differences in light availability between patches were associated with a relatively greater impact of herbs on stem diameter and lateral growth and of shrubs on height growth. The removal of non-crop species increased light levels and soil temperatures and resulted in significantly greater growth and ratios of shoot:root biomass in open-grown seedlings. Increases in the size or number of most components in open-grown seedlings were strongly correlated. Needles, however, responded differently to treatments than other seedling components. Increases in shoot:root ratios were size-related and due to differences in the relative growth rates of roots and shoots. There was no evidence of shifts in carbon allocation within seedlings in response to variation in resource availability. There were significant differences between the characteristics of planted and naturally established Engelmann spruce seedlings. Open-grown planted seedlings were larger than naturals of the same age and had higher relative growth rates, but similar ratios of needle:stem biomass. Shaded planted seedlings were also larger than naturals but had lower relative growth rates and lower ratios of needle:stem biomass. There were also differences between naturals and planted seedlings in the morphology of root systems. After the removal of above-ground vegetation, dominant herbs re-established cover within one season, mainly from persistent below-ground structures. Dominant shrubs recovered more slowly but were not replaced by new species, even after the removal of both above- and below-ground biomass. Shifts to new dominants occurred after the removal of total biomass in herb patches and also in undisturbed herb patches. Species shifts in undisturbed herb patches as well as increases in total cover in both herb and shrub patches over the study period suggested that the plant communities on the study sites were not at equilibrium. The results of this research indicated that shifts in carbon allocation within seedlings are not part of Engelmann spruce's strategy for establishment in heterogeneous subalpine environments. It is suggested that patterns of growth are conservative rather than competitive and that increased levels of resources must be delivered directly to seedlings to improve early performance. The removal of non-crop vegetation is one way to do this but dominant non-crop species, particularly herbs, will re-establish rapidly relative to the rate of spruce establishment, suggesting limited benefits from vegetation management on subalpine cutovers. Any form of harvesting that causes mimimum understorey disturbance on these sites will result in levels of spruce regeneration that fail to meet current stocking standards. Planting large and vigorous seedlings in areas where resources are high and interference is low may alleviate this problem, but changing silvicultural expectations to better reflect the constraints on conifer regeneration in subalpine environments may be a more effective solution.

Text

2011-01-28

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http://hdl.handle.net/2429/30967

Forestry

University of British Columbia

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