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Nutrient availability in natural gaps in old-growth forests in coastal British Columbia

Ahrens, O.. 2005.


In the last decade forest management shifted in the coastal forest region of British Columbia from clearcutting regardless of ecosystem type to harvest systems such as variable retention harvesting which imitate the natural disturbance regime. However, the selection of a silvicultural system that emulates natural disturbance requires an understanding of the influence of natural disturbance on post-disturbance nutrient supply rates. The effect of gap formation on the availability of 14 nutrients was investigated at two sites in coastal old-growth forests dominated by western redcedar (Thuja plicata Donn ex D. Don) and western hemlock (Tsuga heterophylla Raf.) in southern British Columbia. In summer and winter nutrient availability was measured using ion-exchange resins (Plant Root Simulator (PRS)%u2122-probe) incubated in the forest floor for a burial period of 4 to 8 weeks in replicated plots in natural gaps and close-canopy forest. In addition, above-ground biotic and abiotic factors as vegetation coverage and light levels were examined for their influence on nutrient supply rates. Neither the canopy openness nor rooting substrate coverage was significantly different between forest and gaps. No significant changes in nutrient supply rates between gaps and forest were measured. Supply rates for all analysed nutrients had high microscale variability in each forest phase. Increased availabilities of inorganic NO3 , Mg2 , and Al3 in gaps were inconsistently compared to the other evaluated nutrients within and between sites, and sample seasons. It is concluded that the general breakup of the canopy of the studied old-growth forests create pre-disturbance conditions of forest climate, understorey vegetation, and microbes which are similar to the post-disturbance conditions. On the basis of the high variability measured, it is recommends using several indices of nutrient availability simultaneously to assess differences between gaps and forest in nutrients supply rates.

Key Words

natural disturbance; temperate coastal old- growth forest