Drivers of Low Instream Large Wood Retention and Imprints of Wood Mobility in Mountain Nonperennial Streams of a Mediterranean Semiarid Environment

The majority of research on instream large wood (LW) focuses on perennial streams in temperate zones, and much less is known about wood distribution and its driving factors in other environments, such as semiarid bioclimatic regions. In this study, we sought to predict LW retention by assessing reach-scale variables to infer the influence of external factors on the presence of LW in 37 independent reaches of ephemeral and intermittent streams draining a mountain range in a Mediterranean semiarid environment (Lefka Ori Mountains, Crete, Greece). In addition, we focused on LW pieces that had been mobilized by fluvial transport to find links between wood mobility and the external reach-scale characteristics of this environment. Markedly low average LW frequency (6.5 LW/100 m) and volume (9.06 m(3)/ha) were observed in the studied reaches in comparison with streams draining other bioclimatic regions. Only one complex general linear model was found to predict LW volume (including valley confinement and basal area of coniferous trees), and five explanatory variables (elevation as a proxy of annual precipitation, valley type, valley confinement index, basal area of coniferous trees, and basal area of all trees) were significant in separated models predicting LW volume. Only two variables (valley floor width and the basal area of broadleaf trees) were significant in separated models for prediction of LW frequency. The mobilized LW in the studied streams tends to be stored as individual, dispersed pieces rather than in jams, likely due to overall low LW abundance which effectively prevents clustering of wood.