Effectiveness of two mechanical shrub removal treatments for restoring sub-alpine grasslands colonized by re-sprouting woody vegetation.

The extent of European sub-alpine grasslands and their associated ecosystem services are decreasing due to woody plant encroachment. Commonly used methods of woody vegetation suppression like prescribed burning or clearcutting usually cause little damage to belowground bud-banks, offering poor results against re-sprouting shrubs. In this study, we assessed the effects on vegetation and soil properties of two mechanical shrub removal methods for restoring sub-alpine grasslands colonized by the re-sprouting shrub Rosa sp. in the Central Spanish Pyrenees: a commonly used method based on clearcutting (Clearcutting); and a non-previously assessed method based on pulling shrubs off the soil to remove both the aerial and belowground bud-banks (Uprooting). We set a parallel experiment to test whether or not clustering Rosa sp. debris generated in Uprooting (which held many mature fruits) at certain grassland locations may promote colonization of new grassland spots by Rosa sp. seedlings. By the end of the study period, vegetation composition and structure was more similar to the reference grassland in Uprooting than in Clearcutting. Indeed, woody vegetation cover was 71 % smaller in Uprooting than in Clearcutting three years after shrub removal. Nevertheless, by the end of the study period, chemical and microbiological soil properties were slightly more similar to the reference grassland in Clearcutting than in Uprooting. Additionally, the results of our study showed that clustering unusually high number of mature fruits of Rosa sp. at certain grassland locations increased shrub seedling colonization in comparison with other areas of the reference grassland, indicating that operational planning needs to take into account shrub phenology. In conclusion, our work showed that Uprooting may be a useful tool for land managers aiming to restore sub-alpine grasslands colonized by re-sprouting shrubs, though it is advisable using it for scatter shrub patches to prevent significant medium to long-term soil disturbance at landscape scale.