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

Abstract

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 m3/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.

Drivers of variability in large wood loads along the fluvial continuum of a Mediterranean intermittent river

“…Despite the frequent flood events of high magnitudes causing significant damages on infrastructure and fatalities (six floods in the period 1993−2016; Karaouzas et al, 2017), overall low in‐channel and floodplain LW volumes likely prevented the effective clustering of transported LW into jams. The same relation between low LW volumes and low frequency of jams was previously reported from non‐perennial steep channels in Crete (Galia et al, 2019) or semi‐arid glaciogenic basins of the Rocky Mountains (Scott & Wohl, 2018) with these observations suggesting low biomass productivity of riparian corridors in semi‐arid conditions in relation to LW recruitment.…”

“…Our analyses confirmed our first hypothesis on the impact of flow intermittency, together with characteristics of riparian stands, on LW loads in the fluvial corridor. At a global scale, previous reports have indicated a decrease in in‐channel LW volumes at increasing channel width and catchment area (Gurnell et al, 2002; Ruiz‐Villanueva et al, 2016; Wohl, 2017), with similar results reported in the Mediterranean region, (in steep non‐perennial streams of Crete, Greece; Galia et al, 2019). In the case of the studied intermittent river, we found no inverse correlation between in‐channel LW retention (represented by LW frequency per stream length and LW volume per channel area) and channel width (4.8–37.7 m) or catchment area (36–1775 km 2 ).…”

“…We used a simple methodology for determining the tree basal area of living and dead trees in the potential recruitment zone of LW by bank erosion or tree falls (following Galia et al, 2019). These parameters were obtained at three locations (upstream end, middle and downstream end of a reach) in quadrats covering an area 10 m along the bank top and 25 m back from the river edge across the bank top and thus aligned perpendicular to the channel axis (i.e.…”

“…Studies of perennial and non‐perennial headwater streams in Portugal, highlighted that wildfires are an important LW recruitment agent in Mediterranean landscapes, jointly with forest management practices of riparian corridors (Vaz et al, 2011, 2013). Direct links between LW retention and the tree composition of riparian corridors in relation to altitude as a proxy indicator of annual precipitation, were also documented in steep mountain streams of the island of Crete in Greece (Galia et al, 2019).…”

“…Slow decay of floodplain LW was also documented in semi‐arid regions, compared to more humid climate regions, which can play a positive role in floodplain LW retention in Mediterranean‐climate waterways (Andersen et al, 2016). However, low biomass productivity of riparian forests often prevails in these relatively dry regions, implying low in‐channel and floodplain LW loads (Vaz et al, 2013; Lininger et al, 2007; Wohl et al, 2018; Galia et al, 2019). To improve the management of fluvial corridors in Mediterranean‐climate regions in relation to the potential ecological LW benefits, but also to control the flood hazard related to intensive transport of LW, we need to fill the knowledge gaps on recruitment, mobility and retention of LW in non‐perennial fluvial systems.…”