Effects of multiple disturbances on large wood recruitment and distribution in mid-mountain headwater streams

Abstract

Large wood (LW) is an important component of headwater streams that drain forested mountain regions, and a variety of disturbances act as LW recruitment and transport agents, dictating the final distribution of LW in this part of the fluvial system. We focused on the patterns of LW recruitment and distribution in three small mid-mountain catchments in relation to their disturbance histories. In addition to a detailed LW inventory used to assess the distributional patterns, dendroecological dating was used to obtain the ages of tree death and the growth increments of the trees during the last 30 years of their lives. The studied reach that was impacted only by blowdowns indicated the highest LW volume (193 m3/ha), and the calculated concavity pointed to a uniform longitudinal distribution of LW. The studied reach that was treated by a high-magnitude cascade recruitment-transport process (blowdown and debris flow) had the lowest LW volume (82.2 m3/ha) and a highly irregular longitudinal LW distribution; a quite similar irregularity was also obtained for the control reach, in which neither high-magnitude recruitment nor transport disturbances had occurred. Dendroecological dating revealed that 52.1% of sampled LW died during the period of 2005–2011, including deaths due to a blowdown in September 2007 and a debris flow in July 2011. This is particularly evident in the reach impacted by both disturbances; in this reach, all sampled logs died during this period. Overall, the analysis of LW growth patterns pointed to harsh conditions in 56.3% of the sampled LW, reflected by the presence of growth disturbances and abrupt or gradual declines prior to death. In contrast, a slight predominance of LW with balanced or improved growth increments (53.9%) was identified in the reach impacted by both blowdown and debris flow, indicating a higher proportion of trees that would normally stay alive.