In addition to naturally grown riparian woody plants, agricultural woody structures can contribute to shading the watercourse and thus cooling it down. The fall of leaves in autumn can also serve as a food source for microorganisms living in the water body. Plants act against erosion and can reduce pollutant inputs from agriculture into the watercourse. Woody agricultural plantations along watercourses can also contribute to flood protection, as the trees have a retention effect due to their flow resistance and thus slow down flood waves.

So what exactly must these agricultural wood structures be like, i.e. which tree species should be used and how should they be planted so that they have a positive effect on the climate in and around the watercourse, on the soil and on biodiversity? The TU Dresden's science partners have launched a comprehensive measurement campaign at three sites in the Dresden region to answer this question.

Climate and woody plant parameters are the focus of the investigations

Another agricultural wood structure included in the investigations is located at the Wiesengrundbach in Klingenberg-Colmnitz near the Tharandt Forest. Here, the surveys are in the area of flowing waters, as well as at another site at a renaturalised watercourse section of the Mortelbach near Waldheim (LK Mittelsachsen). The Chair of Hydraulic Engineering collects information on the hydraulics of watercourses, the microclimate of watercourses and the macrozoobenthos (benthic invertebrate fauna) and investigates the correlation between these parameters.

In addition to the woody plant physiological, climatic and hydraulic measurements, the Chair of Biodiversity and Nature Conservation is supplementing the investigations with analyses of the woody plant structure and, the position of the trees and the flowing water in the space using high-resolution, terrestrial 3D laser scanning. Based on the laser scanning data, a digital terrain model of the study areas is created. Furthermore, the soil composition and the vegetation composition in the plantation itself and in the surrounding agricultural areas will be investigated and the diaspora bank analysed on the basis of soil samples.

The TU Dresden's measurement campaign will run this year and next. It is intended to provide information on the multifunctional and biodiverse implementation of an agricultural wood structure.

Staff members of the Chair of Biodiversity and Nature Conservation at the TU Dresden supervised a student project work within the OLGA project as part of the Master's degree course in Spatial Development and Natural Resource Management, which is carried out in cooperation between the TU Dresden and the Leibniz Institute for Ecological Spatial Development. As part of the work, the area potential for planting agricultural trees in Dresden and the surrounding districts was analysed. In addition to the energetic utilisation of the fast-growing wood, the ecological condition of flowing waters is to be improved in accordance with the EU Water Framework Directive. 
A catalogue of criteria regulating requirements, exclusion areas and legal framework conditions for potential areas of agricultural timber planting served as a starting point:

• Current land use: arable land, grassland, fallow land
• Distance to water bodies: 60 m surrounding
• Distance to settlement and traffic areas: 8 m
• Exclusion: Area natural monuments, core zones of national park and biosphere reserve, legally protected biotopes, floodplains
• Slope: max. 20 %
• Area size: 2-20 ha per agroaric wood plantation

Using data sources from the Saxon State Office for Environment, Agriculture and Geology and the Saxon State Office for Geographic Information and Surveying, the potential areas were determined with the help of the geographic information system ArcGIS Pro.  

After the analysis had been carried out, 2,480 areas were considered for the establishment of agroforests, which lie within a radius of 40 km around the city limits of Dresden and fall within the specified area size of 2-20 ha per agroforestry plantation. These have a total area size of 8,951 ha, which corresponds to a share of grassland, arable land and fallow land of 7 %. Most of the potential areas are rather small and reach the minimum size of 2 ha. The average size of all potential areas is 3.6 ha, with the largest area being 17.2 ha. Areas larger than 20 ha are also potentially suitable for planting several agroforestry plantations, the size of each not exceeding 20 ha. 

The results of the work provide an initial assessment of possible land uses with agricultural wood in the Dresden region.  The catalogue of criteria will be further specified in the further course of the project in order to finally determine the land potential and feasible areas for implementation.

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HERZIG, Marie, SCHWARZ, Lena (2021): Etablierung von Gehölzen zur Verbesserung des ökologischen Gewässerzustandes - Analyse zum Flächenpotenzial in der Stadt Dresden und umgebenden Landkreisen (GIS-Auswertung). Student work at the TU Dresden, Faculty of Environmental Sciences in the Master's programme in Spatial Development and Natural Resource Management, within the framework of the OLGA project. 

Floods are natural events. The starting point is long-lasting, large-scale continuous rainfall or short, heavy rainfall events. When the resulting water masses become too large, rivers and streams overflow their banks and flood the foreland. These floodplains act as natural retention areas that can absorb and hold back large amounts of water. In this case, the vegetation along the watercourses can have a braking effect on the water masses, delay the flood wave and reduce the runoff peaks.

Within the OLGA project, the Institute of Hydraulic Engineering and Technical Hydromechanics at TU Dresden is investigating the extent to which riparian vegetation (including agroforestry structures) can reduce the flood risk in downstream settlement areas. One study area in OLGA is the renaturalised section of the Wiesengrundbach stream in Klingenberg-Colmnitz in the district of Sächsische Schweiz-Osterzgebirge.

The simulation below shows that the agricultural wood plantation established to the left of the road running from the northwest to the south retains the water only minimally and thus does not really serve as a retention area in this case. The reasons for this are the damming of the water to the right of the road, because the Wiesengrundbach crosses underneath it through a pipe, and the deepening of the stream at the level of the plantation.

In order to evaluate our study areas at the Peickwitz millrace near Senftenberg (Bandenburg) and at the Wiesengrundbach near Dresden (Saxony) from a nature conservation perspective, vegetation surveys were carried out in the herb layer in early summer 2021. We investigate the influence of the agroforestry plantations on plant diversity and use adjacent reference areas (agricultural areas, marginal, flowering and riparian strips of water bodies) as a comparison. As there is forest in the vicinity of the agroforestry plantations, we extended the study design by five circular survey areas along the transect* agroforestry plantation - agroforestry area - forest and additionally five along the transect agroforestry plantation - agroforestry area/ riparian strip - railway line at the Peickwitz millrace. The aim of the transect analysis is to identify possible dispersal routes of plant species and to find out whether the agricultural wood plantations could be part of a biotope network in the respective landscape context. Railway lines are also relevant for the spread of neophytes.
In total, the species composition and cover was recorded on 55 plots, 25 plots are located at the Wiesengrundbach and 30 plots at the Peickwitz millrace.

In the agricultural woodland plantation at the Wiesengrundbach, a total of 22 species were logged, with common colewort (Geum urbanum) and meadow bluegrass (Poa pratensis) being particularly common and having high cover levels. Almost two thirds of the species are found both in the forest and in the open country. 

At the Peickwitz millrace, 19 species could be recorded. Catchweed (Galium aparine) was the most common species with the highest cover percentages. More than half of the species are purely open-land species that occur, for example, in herbaceous vegetation, margins or as field weeds. One third of the species can be found both in the forest and in the open landscape. On the reference areas, the total number of species ranged from 9 (Wiesengrundbach, reference area agricultural area) to 36 species (Peickwitz millrace, field edge).

Further analyses will be carried out in the coming months. We will report on their results in due course.

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*Transect: Along a transect, a marked straight line, data are recorded at fixed intervals. This method is used when you want to establish a spatial relationship to the object of investigation. This method is often used in ecology (botany and zoology), for field investigations (mapping) and in geology and geoecology.
(Source: https://www.pflanzenforschung.de/de/pflanzenwissen/lexikon-a-z/transekt-10152, accessed on 30.11.2021)

In OLGA, agroforestry systems along watercourses at the Peickwitz and Klingenberg-Colmnitz test sites are being studied with regard to their microclimatic effects on the watercourse itself, the soil and the immediate surroundings. In addition to climatic analyses, further analyses are carried out on woody plant physiology, vegetation diversity and biomass. Selected results of last years measurements are presented below.

Fig. 1: Study sites of the measurement campaigns in the OLGA project

Monitoring the microclimate inside and outside the agroforestry system
Initial information on radiant heat shows that the temperature outside the agrowood stand on the farmland can be up to 10°C higher on a warm summer day. The recording of temperature and humidity from June to October 2021 by permanently installed climate sensors on the test sites in Peickwitz and Klingenberg-Colmnitz shows that within the wooded strips there is an equalisation of the daily minima and maxima. This means that, on the one hand, the night temperatures decrease less and, on the other hand, the daily maximum temperatures increase less than outside the area. Fig. 2 illustrates the temperature course within the experimental stands from 28th of June to 4th of July 2021 and, in addition, the course of the temperature measured at the Klettwitz weather station of the German Weather Service located near Peickwitz. The graphs show temperature differences between the stand (Peickwitz agrowood structure) and the open area (DWD Klettwitz weather station).

Fig. 2: Temperature profile at the Peickwitz and Colmnitz test sites and from the Klettwitz weather station from 28th of June to 4th of July 2021

Is there a connection between the spatial structure of the agrowood system and the growth of the trees?
The part of the agrowood plot in Peickwitz planted with the Fritzi Pauley poplar variety was established in 2015 and has a size of approx. 0.43 ha. When selecting the study plot, we ensured that it was as homogeneous as possible and had a low plant failure rate. In addition to phenological characterisation, the trunk diameter and the annual growth of the trees were recorded using a dendrometer. The aim of the study is to determine to what extent the structure of the narrow agricultural wood strip at the Peickwitzer Mühlgraben influences the growth of the trees. The average diameter at breast height (BHD) at the end of the 2021 growing season was 12.92 cm. It is noticeable that the individuals in the second row both at the edge of the field and directly at the watercourse have the highest BHD values. This is probably due to the weaker light competition compared to the inner area of the plot. Fig. 3 shows a typical temperature and dendrometer curve for the Peickwitz test site in the period 17th to 21st of June 2021. As the daily temperature increases, the circumference of the trunk decreases in each case due to the increased transpiration rate. Over the period, an increase in girth is recorded.

Fig. 3: Section of a typical temperature (red) and dendrometer curve for the Peickwitz test site

Determining the biomass growth per hectare with terrestrial laser scanning
For our test site at Peickwitz Mühlgraben, we were interested in how much biomass the poplar variety Fritzi Pauley has produced since 2015 and what the potential biomass production per hectare is for this site. Since no poplars need to be harvested for this estimate, a 3D point cloud of the trial plot with about 70 poplars was created with the terrestrial laser scanner, from which the individual woody plants were cut out using software (Fig. 4). Using the calculated 3D models, the biomass per poplar could be determined and then extrapolated to 1 ha. The advantage of this methodology is that a very accurate estimate of the woody plant volumes can be made even without harvesting. The results show that the biomass is distributed very evenly between the stem (53-41 % of the total volume) and crown segments (47-59 % of the total volume). The tallest poplar in the experimental plot is just over 16 m high, while the smallest is only about 6.50 metres. The latter is in the row with the lowest failures, which shows that competition between the poplars is probably particularly high.
Since spring 2022, new measurements are being taken with the terrestrial laser scanner. In the course of the year, all calculations will be repeated and the results compared. This will provide us with precise statements on the growth of the poplars within the 2021 vegetation period. From the analyses of the biomass growth, it can already be summed up that the agroforestry structure at this site is economically very profitable.

Fig. 4: Point cloud of a segmented poplar hybrid on the Peickwitz experimental plot (left), the three-dimensional quantitative structure model calculated for it (right). 

Findings on plant physiology and water balance of selected poplar varieties
So far, the poplar cultivars Max3 (Populus nigra x P. maximowiczii) and Fritzi Pauley (P. trichocarpa) have been compared on the test sites Peickwitz and Klingenberg-Colmnitz. Data were collected on above-ground biomass formation (including leaf and shoot mass, leaf area, shoot length, root-shoot ratio) and root mass and morphology. The comparison of varieties with three irrigation variants each (normal, water level 1/3 and 2/3 pot height) shows in summary that the variety Max3 produces an average of 8.22 g leaf dry mass across all irrigation variants. This is significantly more than the plants of the Fritzi Pauley variety with an average of 5.31 g. 
The dry matter between the treatment variants varies less with Fritzi Pauley than with Max3. The latter shows a clear reduction in leaf mass in the irrigation variant 2/3, which indicates less plasticity with regard to the environmental factor "prolonged high groundwater level". The development of the root system as well as the root-shoot ratio indicate how well the plant can penetrate the soil to reach groundwater or react to an oversupply or undersupply of water. The comparison of varieties shows that Max3 forms the longest roots across all diameter classes in the normal variant. Correspondingly, the shortest roots are formed in the variant with groundwater at 2/3 pot height (Fig. 5). Fritzi Pauley forms the longest roots in the 1/3 irrigation variant, reacts to the 2/3 flooding variant similarly to the normally irrigated approach. This result also indicates a variety-specific root-growth reaction and that a water requirement adapted to the individual genotype produces optimal growth performance.

Fig. 5: Variety comparison with regard to total root length, separated according to diameter classes (diameter class superfine=<0.5mm, fine=0.5-2.0mm, coarse=>2.0mm)

Apart from the economic efficiency of the agroforestry system, this result indicates that agrowood structures should be established with a mixture of varieties whose composition is adapted to the distance from the watercourse. In order to be able to make comparisons, the extensive measurement campaign from 2021 will be continued this year.