Nutrient availability in natural gaps in old-growth forests in coastal British Columbia
Ahrens, O.. 2005.
Abstract
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
