CHANGES IN NUTRIENT AVAILABILITY AND FOREST FLOOR CHARACTERISTICS IN RELATION TO STAND AGE AND FOREST COMPOSITION IN THE SOUTHERN PART OF THE BOREAL FOREST OF NORTHWESTERN QUEBEC

Changes in forest floor properties and nutrient availability along a boreal post-fire sere succession covering a time span of 231 years were assessed using soil analysis and exchange resins. A decline in pH (from 5.5 to 3.65), effective cation exchange capacity (CEC) (from 72.5 to 39.6 cmol(+) kg(-1)), exchangeable cation concentrations, as well as in Ca, Mg, K and PO4 concentrations on resin was observed. Contrary to what has been reported so far for boreal forests, NO3 and NH4 concentrations did not decrease in the course of succession. Ammonium concentrations remained constant while those of NO3 reached their highest values at age 27, decreased abruptly between ages 27 and 47, and started to increase in older stands. No correlation was found between NO3 availability and pH, P concentrations or C/N ratio. Interactive effects of stand age and stand composition on nutrient concentrations on resins were evaluated using path analysis. While it is not possible to fully disentangle the contribution of single factors, the results showed a positive effect of aspen on Ca concentrations. These results stressed the need to evaluate the long term impacts of successive rotations of softwood species coupled with the elimination of hardwood competition on the soil base status of plantations.