Riparian Vegetation Patterns in Relation to Fluvial Landforms and Channel Evolution Along Selected Rivers of Tuscany (Central Italy)

Abstract

Riparian vegetation distribution patterns and diversity relative to various fluvial geomorphic channel patterns, landforms, and processes are described and interpreted for selected rivers of Tuscany, Central Italy; with emphasis on channel evolution following human impacts. Field surveys were conducted along thirteen gauged reaches for species presence, fluvial landforms, and the type and amount of channel/riparian zone change. Inundation frequency of different geomorphic surfaces was determined, and vegetation data were analyzed using BDA (binary discriminate analysis) and DCA (detrended correspondence analysis) and related to hydrogeomorphology. Multivariate analyses revealed distinct quantitative vegetation patterns relative to six major fluvial geomorphic surfaces. DCA of the vegetation data also showed distinct associations of plants to processes of adjustment that are related to stage of channel evolution, and clearly separated plants along disturbance/landform/soil moisture gradients. Species richness increases from the channel bed to the terrace and on heterogeneous riparian areas, whereas species richness decreases from moderate to intense incision and from low to intense narrowing.

Key Words:

• channel incision
• floodplains
• fluvial landforms
• riparian zone
• vegetation

Acknowledgments

Fieldwork was funded by a CNR short-term fellowship (1995/96). We thank Andrea Corini and Daniel Kroes for assistance in the field and the laboratory. We offer our gratitude to Bruna Gumiero, Waite Osterkamp, and Sammy King for constructive reviews of the manuscript and to Hervé Piégay, Angela Gurnell, and the section editor, Basil Gomez, for postsubmission critical review.

Notes

Notes: Locations of the gauging stations are shown in Figure 1.

A=drainage area; L=river length; H=average basin elevation; ΔH=difference between higher and lower basin elevation; R = average annual runoff; q_mean=average of mean-daily discharges; Q₂=peak discharge with two-year return period.

Notes: Locations of river reaches are shown in Figure 1.

A=drainage area; S=channel slope; D₅₀=median diameter of bed sediments; W=bankfull width; D=bankfull mean depth.

Vertical adjustments: I-L=absent or limited incision, corresponding to a bed lowering less than 0.5 m; I-M=moderate incision, corresponding to a bed lowering less than 2 m; I-I=intense incision, corresponding to a bed lowering more than 2 m.

Width adjustments: N-L=limited narrowing and/or uncertain changes; N-M=moderate narrowing, up to 50 percent of the initial (1954) width; N-I=intense narrowing, above 50 percent of the initial width.

Type of adjustments: The cases from A to H refer to the classification scheme of Figure 2.

Notes: Locations are shown in Figure 1.

For inundations given in percentage, the duration equaled or exceeded that obtained by the flow duration curve of mean daily discharge. For inundations not given in percentage, the number is the return period obtained by flow frequency analysis of annual peak discharges.

Notes: CB=Channel bed; B=active bar; HB=high bar; Be=bench; FP=floodplain; T=terrace.

^aThe genus Salix includes the species shown above and S. purpurea L., which occurred occasionally but was not distinguished separately in the field. Amorpha fruticosa L. was common along several reaches, but due to an error in abbreviations in the field its inclusion in this analysis was not considered reliable.

Note: DCA=detrended correspondence analysis.