Testing soil legacy effects of the invasive shrub Cytisus scoparius on Douglas fir seedling growth

Background/Question/Methods   The exotic shrub Cytisus scoparius (Scotch broom) has been implicated in failed reforestation efforts in the Pacific Northwest. While competition from C. scoparius may directly affect the establishment success of Douglas fir (Pseudotsuga menziesii), there is also the potential for indirect interactions through effects on soil. First, as a nitrogen-fixer, C. scoparius may change nitrogen availability, causing a fertilization effect that could have either direct positive or indirect negative effects on Douglas fir. Second, alkaloids in C. scoparius litter could affect Douglas fir growth directly or indirectly through mycorrhizal associations. We performed a greenhouse experiment to quantify Douglas fir growth in soils long invaded by C. scoparius relative to uninvaded forest soils. A factorial design was used to test the effects of adding C. scoparius mulch to soils, and to separate the mechanisms of nitrogen fertilization (with a sugar addition treatment) and allelopathy (with a charged-carbon treatment).

Results/Conclusions   We found that seedling growth was substantially impaired in the long-invaded soils relative to forest soils. However, adding C. scoparius mulch led to an increase rather than a decrease in growth, particularly in the invaded soils. We found evidence for a complex combination of positive and negative effects of the invader on soils, and thereby on Douglas fir seedling growth. These soil impacts could result in a long-term “legacy effect” of the invader on both managed and wild lands. Even after eradication of C. scoparius, impacts of the invasion on soil nutrient dynamics and microbial communities should be considered in forest restoration efforts in the Pacific Northwest.