Riparian plant biodiversity and community composition of ditches and straightened streams : identifying key environmental drivers in a boreal forest setting

Abstract: Modified ecosystems are typically seen as degraded and having no ecological value. In many cases, the modification of the ecosystem was done more than 100 years ago and may have had time to recover and become a refugia for species that were once more common in the landscape. Ditches and straightened streams are one such ecosystem that are at risk of being destroyed through forest management practices like ditch cleaning and poor riparian buffer practices. The overall aim with this study was to get new insights in the poorly explored topic of the ecology of boreal forest ditches and straightened streams. With this study, I start filling a knowledge gap regarding the importance of these artificial waterways, by analysing riparian plant biodiversity (i.e., species richness and Hill-Shannon diversity) and community composition. New knowledge gained from this study can help when deciding on how to manage these systems in the future. In the Krycklan Catchment Study, close to Vindeln, Sweden, I surveyed ground cover vegetation in plots on 0cm, 20cm and 40cm elevations from the water surface along sixteen ditches and straightened streams. These were on till soil across a catchment area size gradient (0.5 – 50 ha), which allowed me to study what effect catchment area has on riparian plant biodiversity and community composition. I included six small ditches (< 5 ha) on peat soil to study differences in species richness and community composition between soil types. I hypothesised that the species richness, Hill-Shannon diversity, and community composition would all depend on catchment area, soil type, elevation above the water surface and if the waterway was a ditch or a straightened stream. The species richness of ditches and straightened streams was significantly related to catchment area size while the Hill-Shannon diversity was not. Species richness was significantly higher in plots on till soil on 40 cm elevation than plots on peat soil on 40 cm elevation. Furthermore, Hill-Shannon diversity was significantly lower in plots on 0 cm elevation on till soil than plots on 0 cm elevation on peat soil. Riparian plant community composition was significantly different between ditches and straightened streams and between soil types. Catchment area, carbon – nitrogen ratio, proportion of litter and large wood were the variables driving the composition. I suggest that the underlaying factors driving the differences in biodiversity and plant community composition in ditches and straightened streams are a combination of hydrological (related to catchment area) and edaphic. The variation in biodiversity and community composition in these systems across soil types and catchment areas needs to be acknowledged and carefully considered when planning for future management.