Using forest product residuals to overcome abiotic thresholds to riparian forest restoration of a gravel mine in North Cascades National Park, Washington

Transporting soil amendments to remote ecological restoration sites can be prohibitively expensive.  Alternatively, local “wastes” or organic residuals can be utilized to promote revegetation on sites with poor edaphic conditions at significantly lower cost.  The objective of this experiment was to test the efficacy of locally sourced organic amendments for the ecological restoration of a low elevation riparian forest to an abandoned gravel mine in North Cascades National Park along Goodell Creek, a salmon bearing tributary of the Skagit River.  In the fall of 2001, organic residuals produced from the local forest industry and mulches processed from nearby plant residues were applied to the gravel mine, then seeded with an understory native species mix and planted with seedlings of three locally dominant riparian tree species; black cottonwood (Populus balsamifera), red alder (Alnus rubra), and Douglas fir (Pseudotsuga menzeisii).  Initial results showed slight differences between mulch treatments in terms of understory plant establishment, non-native plant invasion and edaphic conditions however after four years these differences were undetectable.  After four years the amendment treatments resulted in highly significant differences in both tree growth and soil conditions.   The rate of tree growth in the amended plots was as much as three times that of the unamended sites.  Soil carbon in the amended plots was six times greater at the surface depths (0-15cm) and almost double at deeper depths (15-30cm) than unamended plots.  Vegetation and soil results from the first year of the study will be compared with those after four years, and the implications for long term ecological restoration discussed.