Soil Legacies of a Nitrogen-Fixing Invader in the Pacific Northwest.

It is often presumed that when an invasive plant species is removed, its effects are removed as well.  However, invasive species can leave behind soil legacies that affect native species regeneration.  We examined the legacy effect of a widespread nitrogen-fixing exotic shrub (Scotch broom) in the Pacific northwest whose invasion inhibits reforestation of Douglas-fir.  We have identified two primary mechanisms of this invader’s legacy: nitrogen enrichment as well as potential allelopathic (negative) effects.  We hypothesize that both these mechanisms can affect Douglas-fir regeneration directly as well as indirectly via the mycorrhizal mutualists of Douglas-fir.  We planted Doug-fir seedlings across a chronosequence of ten sites that had been clearcut and subsequently invaded by Scotch broom, representing 5 to 40 years of invasion. Within each site, we planted seedlings into two patch types: patches that had had Scotch broom removed (broom soils), and patches that had never been invaded (native soils).  We simultaneously inserted single-ion exchange resin bags into the soil under a subset of seedlings in both patch types; we retrieved and deployed new bags at three intervals over the growing season (from April to November 2015) across all sites. We found that irrespective of duration of invasion soil nitrogen availability was an order of magnitude higher in broom soils compared to native soils, suggesting a positive soil legacy. However, Douglas-fir seedlings had ~25% greater mortality when planted into broom soils versus native soils across all sites, again irrespective of invasion duration.  These results suggest that despite increased nitrogen availability, broom imparts negative effects that decrease Douglas-fir seedling survival – a finding that has implications for Douglas-fir regeneration efforts especially in poor soils.