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Factors limiting early conifer growth in salal-dominated cutovers on northern Vancouver Island, British Columbia

University of British Columbia

Nutritional stress has been reported in planted and naturally-regenerated conifers growing in association with an ericaceous species, salal (Gaultheria shallon Pursh), in cutovers previously occupied by old-growth western redcedar (Thuja plicata Donn) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) (CH sites) on northern Vancouver Island. No such stress was apparent in cutovers previously occupied by natural, second growth western hemlock and amabilis fir (Abies amabilis (Dougl.) Forbes) stands (HA sites) that developed following windthrow in 1907. The CH ecosystem type occupies as much as 100,000 ha in coastal British Columbia. In the spring of 1987, a series of field and pot experiments was initiated to investigate some of the ecological processes affecting the early growth of conifers on recently logged and burned 2- to 10-year-old CH and 2- to 4-year-old HA sites. The overall objective of the research was to quantify some of the possible factors limiting early conifer growth on northern Vancouver Island. The research encompassed studies of: (1) below- and above-ground non-crop vegetation recovery, forest floor nutrient availability and soil microenvironmental modification following clear-cutting and burning; (2) competition for nutrients by the non-crop vegetation; (3) interference by salal of the mycorrhizal development on conifer seedling roots; (4) conifer seedling growth under several different experimental conditions, and; (5) relationship between microsite factors and western redcedar seedling growth within clear-cut and burned CH sites. Salal was the main non-crop species found on the CH sites. It reestablished itself rapidly, both above- and below-ground, following clear-cutting and burning on this type of site. The total above-ground vegetation biomass quadrupled from 1372 kg ha ̄¹on the 2-year-old CH sites to 5574 kg ha ̄¹on the 8-year-old CH sites, whereas the total below-ground biomass increased six times from 1908 kg ha ̄¹ on the 2-year-old CH sites to 11415 kg ha ̄¹ on the 8-year-old CH sites. Similar amounts of total above-ground non-crop biomass were found on HA cutovers for the first 4 years, but the non-crop vegetation was composed of half salal half fireweed (Epilobium angustifolium L.). The regrowth of the non-crop vegetation immobilized annually 9 and 0.9 kg ha ̄¹of N and P, respectively, on the CH sites during the first eight years. This was estimated to represent potentially between 30 and 45% of the available N on these sites. A model of the development of live fine-root, leaf, stem and rhizome biomass of salal over a 60 year period is proposed based on the result of this study and of other studies. This model suggests that the net immobilization of nutrients in salal biomass will cease between 10 and 20 years after clear-cutting and burning as salal stops expanding its biomass. This model implies that the nutritional stress caused by salal should decline after 10 to 20 years at which time conifer nutrition and growth should slowly improve, especially as salal is eliminated by light competition as the conifer canopy begins to close. Several biotic and abiotic factors were studied in relation to their possible effects on the early growth of western hemlock, Sitka spruce and western redcedar. The growth of these three conifer species was compared between 2- to 4-year-old CH cutovers (2+B CH), 8- to 10-year-old CH cutovers (8+B CH), and 2- to 4-year-old HA cutovers (2+B HA) with and without the influence of the non-crop vegetation. No moisture deficit was measured in the field throughout the year on all sites. Both the growth of the conifers and the availability of N (+36%) and P (+25%) were increased by the removal of the non-crop vegetation; however, no difference in cellulose decomposition and soil moisture, and only very small difference in soil temperature was measured. The better conifer growth on 2+B HA > 2+B CH > 8+B CH without the influence of the non-crop vegetation was associated with an overall better forest floor nutrient status; however, no difference in soil moisture and pH, and only small difference in soil temperature were measured. Western redcedar and western hemlock were the best growing species on CH and HA sites, respectively. Both western hemlock and Sitka spruce (Picea sitchensis) were very responsive to the different site conditions (CH vs HA sites) and to planting treatments that increased or decreased conventional measures of nutrient availability caused by the different treatments, whereas western redcedar was not. The presence of salal was found to have no effect on the total percent mycorrhizae found on the roots of the three conifer species studied three years after planting. Both field and pot experiments yielded comparable results. Slow-release fertilizer, at the time of planting, increased growth only for the first two years after application. Western redcedar growth was significantly greater in depressions than on flats and mounds, but this difference was not related to any major differences in the forest floor variables measured between the three microtopographic positions. These results indicate that the nutritional stress and poor growth reported in conifers, especially in Sitka spruce, on CH cutovers on northern Vancouver Island can be explained by a combination of (1) inherently low forest floor fertility in cutovers originating from the old-growth CH forests, (2) salal competition for scarce nutrients and their immobilization in salal biomass, and (3) declining site fertility caused by the termination of the flush of nutrients that occurs in the immediate post-logging and burning period on CH sites.

Text

2011-02-08

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http://hdl.handle.net/2429/31130

Forestry

University of British Columbia

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