Shrub encroachment modifies soil properties through plant resource economic traits

Background and Aims Shrub encroachment alters ecosystem functions. Yet, changes in plant community traits and soil properties along succession from grassland to shrubland in European mountains are poorly understood. Methods We investigated the cascading effects of shrubs on community weighted means (CWM) of plant traits and onto soil properties along a gradient of encroachment in subalpine grasslands in two sites in the Alps using a trait-based approach. We hypothesized that increasing shrub density shifts plant communities towards more conservative traits, which non-linearly increases carbon sequestration and impacts nutrient cycling. We tested our hypothesized model of cascading effects using structural equation models. This model accounted for biomass allocation to leaves and stems in CWM calculations. Results Consistent with expectations, CWM dry matter content (DMC) increased and CWM of nitrogen (N) and phosphorus (P) decreased with increasing shrub biomass. Increasing CWM DMC resulted in increasing soil C:N ratio and soil organic matter (SOM) concentration, and decreasing pH. Decreasing CWM P was coupled with decreasing soil available P, but changes in CWM N had no effect on available N. There was however no indication of nonlinear changes. Conclusion This study demonstrates that with shrub encroachment plant communities gradually become more conservative with tougher and nutrient-poor tissues, which leads to soil acidification, SOM accumulation and lower P availability. We also demonstrate that DMC, an easy measurable trait, is a sufficient indicator for plant tissue quality in shrub encroached subalpine grasslands and could be used in future trait-based models, allowing projections under climate change scenarios.