Do impacts of an invasive nitrogen‐fixing shrub on Douglas‐fir and its ectomycorrhizal mutualism change over time following invasion?

Abstract

Summary Impacts of invasive species may change in magnitude and even direction with invasion age. Impacts could increase as the population increases, individuals grow in size and ecological changes accumulate. We used a chronosequence approach to characterize the development of soil impacts over time following the invasion of Cytisus scoparius, a widespread nitrogen‐fixing shrub thought to limit reforestation success. In a greenhouse experiment, we evaluated how abundance of ectomycorrhizal fungi, Douglas‐fir performance and leaf nitrogen changed across a 3‐ to 31‐year chronosequence of invasion. Each of the chronosequence sites were clearcuts where reforestation efforts were unsuccessful and where C. scoparius invaded. To estimate the contributions of the invasion separately from contributions of the accompanying disturbance, i.e. deforestation, we included soils from both invaded and uninvaded patches in each site of the chronosequence. In a complementary soil conditioning experiment, we examined the separate effects of host absence and invader presence on the mycorrhizal mutualism, leaf nitrogen and seedling growth. Ectomycorrhizal colonization was lower in invaded soil, but this effect did not intensify with time. Despite the suppression of the mutualism, Douglas‐fir grew larger in invaded soils. This positive response is likely due to nitrogen fertilization, a conclusion supported by higher concentrations of leaf nitrogen of Douglas‐fir grown in invaded soils. While leaf N concentration increased with invasion duration, Douglas‐fir survival and growth did not. Synthesis. Our findings suggest that soil impacts of an invader can develop rapidly and can be surprisingly stable over time. In such systems, recently invaded areas may be as difficult to restore as long‐invaded areas, especially where ectomycorrhizal fungi are important drivers of reforestation success. More chronosequence studies or long time series are needed to evaluate whether this is a general pattern.