Relationships among soil moisture, aeration and plant communities in natural and harvested coniferous forests in coastal British Columbia, Canada

1. 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.