Umeå universitets logga

Riparian buffers are commonly used to mitigate the negative effects of forestry operations near water, particularly sediment transport to streams. In Sweden, current practices typically involve 5-7 m wide riparian buffers along small streams. Historical forest management, which favored conifers up to the channel edge, has resulted in these narrow buffers having a simplified tree species composition and structure, making them prone to windthrow. While windthrow can contribute large wood (LW) to streams, windthrow also risks increasing sediment inputs if rootwads are exposed near stream edges. This disturbance affects sediment connectivity, or the movement of particles through the fluvial system, but the interaction between LW dynamics and sediment connectivity in small boreal streams is not well understood. We investigated sediment connectivity at the Trollberget Experimental Area in northern Sweden, where six 100 m stream reaches had either 5 m or 15 m wide riparian buffers. Pre-harvest and one-year post-harvest data on windthrow, hydrology, and sediment yields were collected. Forest harvesting increased sediment connectivity in the streams regardless of buffer width, indicating that buffers wider than 15 m are necessary to reduce sediment input impacts in small headwater streams. Windthrow affecting stream channels was more common in the 5 m buffers, leading to significantly higher deposition of very fine sediments (<250 μm) compared to the 15 m buffers. Coarse (>1 mm) and fine sediments (250 μm – 1 mm) were also higher in the 5 m buffers. We found that sediment connectivity in streams was closely linked to LW dynamics, negatively before harvest but positively after harvest. Before harvest, LW trapped sediment and prevented downstream transport, but after harvest, the increased sediment input overwhelmed this function. Our results highlight a trade-off between the recruitment of LW and minimizing sediment connectivity, two key objectives in riparian buffer management.