Original Research ArticleTrees and shrubs as components of the storage of coarse particulate organic matter and instream wood in Mediterranean intermittent streams
Introduction
Coarse particulate organic matter (CPOM; leaves, needles, twigs, and other organic particles larger than 1 mm in size) together with large instream wood (LW; dead trees or their fragments with ≥1 m length and ≥0.1 m diameter) belong to the elements transported by stream flows along with sediments and nutrients (Shumilova et al., 2019; Wohl et al., 2019). Both CPOM and LW are crucial and dynamic components of healthy freshwater ecosystems surrounded by tree riparian vegetation, which support biodiversity of invertebrates and fish species by providing habitats, shelters or food (Dolloff and Warren, 2003; Enefalk and Bergman, 2016; Harmon et al., 1986; Richardson, 1992; Shumilova et al., 2019). In particular, stable LW pieces and log jams increase flow resistance and act as flow obstacles, which in turn influences final channel morphology and its stability (Gurnell et al., 2002; Wilcox et al., 2011; Wohl and Scott, 2017). However, small wood pieces and CPOM can also have at least a temporaly effect on channel morphology and hydraulic processes, mainly by decreasing the porosity of log jams or boulder steps in mountain headwater channels (Haga et al., 2017; Přibyla et al., 2016). Despite the existence of a direct relationship between the transport rate of CPOM and discharge, CPOM can be mobilized even during ordinal flow conditions due to its relatively small size (Bunte et al., 2016; Iroumé et al., 2020; Richardson et al., 2009; Turowski et al., 2013). On the contrary, LW transport is usually strictly episodical in dependence on the dimensions of the channel, the size and shape of the LW and the flow magnitude (Kramer and Wohl, 2017; Ruiz-Villanueva et al., 2016a).
Storage and retention of CPOM have been frequently investigated during the past decades primarily by field experiments based on the release of material (typically leaves or small twigs) into stream channels. These experiments documented that, during low-flow conditions, in-channel macro-roughness elements such as boulders or individual LW pieces are preferential sites of CPOM storage in single-thread channels, while riparian vegetation (or adjacent floodplain surface in general) plays an important role in CPOM retention during overbank flows (Brookshire and Dwire, 2003; Lininger et al., 2021; Oelbermann and Gordon, 2001; Small et al., 2008; Speaker et al., 1988). For nonperennial reaches, also pool habitats were identified as sinks for temporary storage of CPOM and its later decomposition (Abril et al., 2016; Dieter et al., 2011). The retention of LW in fluvial systems is usually related to specific sites of high roughness or the presence of flow obstacles. Field observations together with physical and numerical modelling demonstrated preferential deposition of mobilized LW on irregular channel banks, elevated bar deposits, and floodplain surface or stacking of LW by trees living in riparian corridors (Gurnell et al., 2000; Haga et al., 2002; Ruiz-Villanueva et al., 2016b, Swanson et al., 1998). This suggests a direct relation between the stream transport capacity (or hydrological regime in general), geomorphic structure of the channel and adjacent floodplain, presence of living vegetation, and the final pattern of CPOM and LW storage at a reach scale.
However, such relationships have not been complexly investigated so far in the case of dynamic single- or multithread nonperennial channels despite the fact that these fluvial systems including ephemeral and intermittent watercourses occupy a large portion of the global fluvial network and act as highly vulnerable hotspots of biodiversity especially in (semi)arid landscapes (Bonada and Resh, 2013; Borg Galea et al., 2019; Datry et al., 2014; Skoulikidis et al., 2017; Tooth, 2000). Nonperennial streams are different from perennial ones by their (i) high annual flow variations from complete drying up to high-magnitude flash flooding and (ii) frequent succession of herbal and tree vegetation in the active channel due to local moisture availability in contrast to surrounding landscape under (semi)arid climatic conditions (Busch et al., 2020; Hooke, 2016; Merritt, 2013; Tichavský et al., 2020). In the context of possible biogeomorphic interactions, woody plants that occupy the active channel have the potential to act as sources of organic material and, at the same time, as flow obstacles that trap transported CPOM and LW during high flow events, contributing to overall channel heterogeneity and related biodiversity in nonperennial streams (Dunkerley, 2014; Galia et al., 2018; Pettit et al., 2006). The increasing spatial and temporal extent of regularly drying reaches is documented due to water abstraction by rising water demand for agricultural, industrial, urban and touristic development (Datry et al., 2014; Skoulikidis et al., 2017). Together with climate changes, these interventions lead to alterations in CPOM and LW transport, storage, and decomposition processes in nonperennial watercourses (Abril et al., 2016; Arroita et al., 2015) as well as extension of riparian vegetation at the expense of bare surfaces within the active channels (Garófano-Gómez et al., 2013; Martínez-Fernández et al., 2018). This underlines the importance of understanding complex interactions among the LW, CPOM, and living vegetation in nonperennial fluvial systems.
This study aims to investigate the possible impact of living trees and shrubs in the active channel on the storage of CPOM and LW under semiarid climatic conditions in the European Mediterranean. Because of the frequent occurrence of woody plants in the active channel of nonperennial streams, we hypothesise that trees and shrubs will play an important role in organic matter retention. This retention pattern may differ from typical perennial streams, where woody species usually occupy riparian stands (i.e., out-of-channel units) or eventually stable in-channel landforms as islands in the case of relatively wide river channels. To fulfil this task, detailed field inventories were performed in three reaches of intermittent streams at different positions of the fluvial continuum, namely in a steep confined reach with signs of debris-flow processes, in a relatively wide but still confined reach upstream of a landslide-related knickpoint, and in a lowland reach close to the estuary.
Section snippets
Study area
Three studied reaches (Krathis_up Krathis_dw and Kerinitis) were located in the intermittent Krathis and Kerinitis streams, which drain the Helmos Mountains (up to ∼2500 m above sea level) occupying the northern part of the Peloponnese peninsula, Greece (Fig. 1, Fig. 2, Table 1). This area is characterised by steep slopes and deep V-shaped antecedent valleys with frequent occurrence of landslides, when this natural hazard is predisposed by local tectonics, lithology (presence of non-cohesive
Reach characteristics of CPOM deposits, LW and woody plants
We found variations in the storage of CPOM, LW and the measured stem basal areas of living trees and shrubs in the studied reaches. In total, we measured 231 accumulations of CPOM (50 in Krathis_up, 142 in Krathis_dw and 39 in Kerinitis) and 183 LW pieces included in 125 LW deposits (13 in Krathis_up, 60 in Krathis_dw and 52 in Kerinitis) in the surveyed 10-m wide bands covering six cross-sections per reach. The largest volume of CPOM (15.65 m3/ha) and LW (13.35 m3/ha) together with the highest
Discussion
This study investigated the role of living woody plants in the storage of CPOM and LW in the active channels of intermittent streams that drain semi-arid Mediterranean mountain landscape. We confirmed the hypothesis that trees and shrubs living in the active channel are important as CPOM storage elements in all three studied reach settings, but LW was effectively trapped exclusively in the reach with a notably higher frequency of living trees with diameters greater than 10 cm.
The observed
. Conclusions
Nonperennial streams belong to the principal surface water bodies in (semi-)arid regions, and one of their typical features is the occurrence of living woody plants in their active channels owing to moisture availability. These trees and shrubs cannot be perceived exclusively as CPOM and LW recruitment sources, but they also act as important retention elements for floating matter, which is transported in the wet season during high flow pulses. Based on our detailed field inventory of
CRediT authorship contribution statement
Tomáš Galia: Conceptualization, Methodology, Formal analysis, Investigation, Writing – original draft, Visualization. Václav Škarpich: Investigation, Writing – review & editing, Visualization. Ondřej Vala: Data curation, Visualization.
Declaration of Competing Interest
None.
Acknowledgements
We would like to express thanks to two anonymous reviewers for their constructive comments, which significantly improved the manuscript. The study was supported by an internal grant of the University of Ostrava (SGS04/PřF/2022) and Rector's Award of the University of Ostrava (Tomáš Galia).
Data availability statement
Data are available from the authors upon reasonable request.
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