WW-R-3: Riparian vegetation of small, medium-sized water bodies-case study

The picture shows a narrow stream flowing through an agricultural area. At the edge of the stream there are individual large trees and some bushes that cast shadows on the water surface. Click to enlarge
Shading by riparian vegetation can help to prevent excessive increases in water temperatures.
Source: Photograph: © XtravaganT / stock.adobe.com

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents

 

WW-R-3: Riparian vegetation on the banks of small and medium-sized watercourses – case study

The associated shading effect of riparian woodlands help to counteract increases in water temperature. Watercourses with riparian woodland on one or both banks are now to be found only in a third or scarcely half the sections of small and medium-sized watercourses in examples from the Länder examined for the purpose of this analysis.

The stacked column graph shows the percentage of the riparian vegetation categories "forest or gallery on both sides", "forest or gallery on one side", "no or little woody vegetation on both sides", "special cases" and "no data" in the length of small watercourses for the states of Brandenburg, Rhineland-Palatinate and Saxony. The share of the category "forest or gallery on both sides" is highest in Rhineland-Palatinate (slightly more than 20 percent) and lowest in Brandenburg (just under 14 percent).
WW-R-3: Riparian vegetation on the banks of small and medium-sized watercourses – case study

The stacked column graph shows the percentage of the riparian vegetation categories "forest or gallery on both sides", "forest or gallery on one side", "no or little woody vegetation on both sides", "special cases" and "no data" in the length of small watercourses for the states of Brandenburg, Rhineland-Palatinate and Saxony. The share of the category "forest or gallery on both sides" is highest in Rhineland-Palatinate (slightly more than 20 percent) and lowest in Brandenburg (just under 14 percent). In Brandenburg, more than 61 percent of the length of the watercourse has "no or little woodland on both sides".

Source: LfU Brandenburg; LfU Rheinland-Pfalz; LfULG Sachsen (Watercourse structure mapping)
 

More shading – better cooling of water structures

In principle, natural and near-natural water structures provide more favourable living conditions for aquatic organisms than water courses whose structure has been interfered with. The richer the structure of water courses, the greater the variety of different micro-habitats which enable aquatic creatures to take evasive action when living conditions change. In view of climate change and associated increases in water temperature, riparian vegetation plays an important role.

Especially in the case of small or medium-sized, hence narrower watercourses, shading by means of riparian woodland vegetation can help to prevent water from warming excessively. Herbaceous vegetation on the banks of smaller watercourses is, however, of minor importance for shading. It is true to say that in respect of larger rivers, the associated riparian vegetation has less influence on shading the entire cross section of the watercourse and also on cooling, but even in this case it produces diversification and creates cooler, shaded riparian zones which in warmer summer months serve as sanctuaries for sensitive species thus helping to improve their habitat conditions.

Under natural circumstances, the banks of nearly all small and medium-sized watercourses would be wooded with alder, willow and ash trees. However, property development close to and in watercourses, intensification of land use extending well into zones of water margins, as well as the use of hydropower resulting in numerous watercourses losing their natural riparian vegetation. These incisive changes are perpetuated by means of regular maintenance of watercourses in order to counteract natural succession. The restoration of riparian woodlands in particular is a form of adaptation to climate change, which can actively counteract further increases in water temperatures.

As part of mapping watercourse structures, the Länder of the Federal Republic carry out surveys of different water structure parameters in order to assess to what degree their condition can be described as near-natural. To this end, the Länder apply, for recording and evaluating woodland vegetation, a homogeneous methodology developed by the Federal/Länder Working Group Water. The mapping outcomes are fed into the assessment of the ecological condition of waters in accordance with the implementation of the EU Water Framework Directive (WFD).

The latest outcomes from the mapping of the structure of water courses carried out by the environment authorities are illustrated in the indicator for Brandenburg, Rhineland- Palatinate und Saxony. The indicator is focussed on small and large streams as well as small rivers with an average water level width up to 20 metres, because in the case of this type of flowing waters, associated woodlands clearly influence the water temperature. The following percentages were found for riparian sections wooded on one or both banks: in Brandenburg 35.1 %, in Rhineland-Palatinate 44.6 % and in Saxony just under 32.9 % of the entire riparian section mapped. The reason for the distinctly higher percentage in Rhineland Palatinate is essentially that here 40.6 % of the terrain is forested compared to just under 35 % in Brandenburg and just 22.3 % in Saxony. Consequently, a greater proportion of streams and rivers flows through forested terrain.

In the case of woodland vegetation extending on only one bank of the watercourse, the orientation of this vegetation is crucial for the shading effect. Consequently, the existence of this type of vegetation does not necessarily constitute shading, either temporally or spatially. Several differentiated analyses would be required to clarify this. On the other hand, lack of riparian vegetation is not necessarily a clue for the absence of shading, for shade can also be cast by built structures on the margins of watercourses. Nevertheless, it can be assumed for such sections that the watercourse in question has been generally altered by anthropogenic activities.

Furthermore, it is debatable whether for a near-natural protective section of a watercourse it would be preferable to have gappy riparian vegetation alternating naturally between shaded and sunlit sections rather than having total shading (Grünverrohrung, roughly translated as a solid shield of dense green vegetation), in view of the fact that gappy vegetation is more consistent with the natural alternation of such structures. The mapping outcomes currently available do not, unfortunately, allow such differentiated evaluation.

The data obtained so far do not yet permit the illustration of time series which might show the progress achieved in rehabilitating watercourses by restoring the formerly associated near-natural riparian woodlands. For this purpose, regular consultation of water structure mapping databases would be required in future. Furthermore it will be necessary to enhance the indicator by involving and incorporating other Länder, and to collate the data, where appropriate, ordered in categories of physiographic baseline conditions thus allowing the identification of any specific actions required.

It is to be expected that further renaturation efforts for the restoration of near-natural water structures will have positive impacts on water temperatures.

 

Objectives

Achieving good condition of surface waters (WFD, Article 4 (1))

Increasing the barrier-free condition and structural diversity of watercourses; restoring and redynamising alluvial meadows (DAS, ch. 3.2.3)

Prioritising actions under WFD which maintain or enhance the natural adaptability of watercourses as well as the biotope or habitat diversity of our watercourses (DAS ch. 3.2.2)