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Summary1. This study was undertaken to address a curious ecological phenomenon that occurs over extensive areas of northern Vancouver Island, Canada. Slow tree growth and symptoms of nutrient deficiencies appear within 10 years of harvesting old-growth cedar-hemlock (CH) forests, but do not occur on adjacent hemlock-fir (HA) stands. We investigate the underlying causes of distinct differences in productivity and fertility of the two ecosystems. 2. We hypothesized that the differences in nutrient supply and tree growth arise from small, but ecologically significant, differences in soil moisture with attendant effects on redox potential (Eh), aeration, soil C stores and plant species distribution. Our field studies determined (i) whether neighbouring CH and HA forests differ in soil moisture and aeration, (ii) whether soil carbon stores differ among CH and HA forests, (iii) whether plant community composition in the two forest types is related to soil moisture and aeration and (iv) the impact of harvesting CH and HA forests on moisture and aeration conditions. 3. As hypothesized, soils in CH forests were wetter and less aerated, and had shallower aerated depth and higher frequency of anaerobic conditions compared with HA forests. About 43% of the samples taken from CH forests had average redox values <+300 mV, a threshold below which anaerobic conditions develop and root growth is impeded. Soil carbon stores and rates of decomposition of cellulose did not differ among the forest types. Soil aeration explained 25% of the variability in species composition of plant communities. 4. The two forest types responded differently to clear-cut harvesting, and there were indications that clear-cut harvesting could trigger reductions in soil aeration in some HA forest soils which could lead to paludification and a decline in site productivity of these ecosystems. 5. Synthesis. Our findings confirmed that these ecosystems bracket a critical biological threshold below which low soil oxygen availability caused by excessive moisture becomes limiting for biological processes. Recognition of this redox threshold and its ecological implications could contribute to improved ability to manage ecosystems which may be near this threshold or become so in a changing climate.

The ericaceous shrub, salal (Gaultheria shallon Pursch) is associated with forest regeneration problems on a variety of site types along the west coast of North America. On dry sites, salal is a serious competitor for water; on wet (cedar-salal) sites, salal is thought to limit nutrient availability to trees by competing, interfering with mycorrhizae and "short-circuiting" the nitrogen cycle through production of phenolic compounds. Short-term field experiments and laboratory and greenhouse studies provided some evidence of each of these effects. However, long-term results from a suite of silvicultural trials and associated ecological studies indicated that salal is not the fundamental problem for regenerating conifers on cedar-salal sites. Fertilization of cedar-salal sites caused a large and sustained response in tree growth regardless of the presence of salal, and growth responses to salal removal were much smaller than fertilization responses. Greenhouse experiments indicated that salal does not have allelopathic effects on germination or growth of conifer seedlings. In laboratory studies, salal did not demonstrate a greater capacity to take up organic N forms than the conifers, and molecular studies uncovered a vast diversity of mycorrhizal fungi associated with salal and hemlock roots. Together these findings indicate that the nutrient "short-circuiting" hypothesis, based on assumptions about ericoid versus non-ericoid mycorrhizal plants does not adequately describe the nutrition of plants in these ecosystems. We conclude that salal should be viewed as a symptom of the underlying problem of poor nutrient supply on cedar-salal sites, and that fertilization, rather than salal control, is the optimal solution for improving forest regeneration on these sites.Key words: ericaceous shrubs, fertilization, scarification, allelopathy, mycorrhizae, Gaultheria shallon, nitrogen RÉSUMÉ Un arbuste de la famille des Éricacées, la gaulthérie shallon (Gaultheria shallon Pursch) est associé aux problèmes de régénération forestière sur différents types de station de la côte ouest de l'Amérique du Nord. Sur les stations sèches, la gaulthérie shallon est un féroce compétiteur pour l'eau, sur les stations humides (thuya-gaulthérie), on considère que celle-ci perturbe la disponibilité en éléments nutritifs des arbres par compétition, et interfère au niveau des mycorhizes et « court-circuite » le cycle de l'azote par la production de composés phénoliques. Des essais à court terme sur le terrain ainsi que des études en laboratoire et sous serre ont permis de mettre en évidence chacun de ces effets. Toutefois, les résultats à long terme d'une série de travaux en sylviculture et d'études associées en écologie ont démontré que la gaulthérie shallon ne constitue pas un problème fondamental de régénération des conifères sur les stations de type thuyagaulthérie. La fertilisation des stations de type thuya-gaulthérie a entraîné une réaction importante et durable de la croissance des arbres sans égard à la présence de la gaulthér...

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