Evaluation of Environmental Impacts of Short Rotation Coppice with Regard to the Amount and Quality of Groundwater Recharge
von Paul Schmidt-Walter
Datum der mündl. Prüfung:2019-09-23
Erschienen:2020-09-02
Betreuer:Prof. Dr. Norbert Lamersdorf
Gutachter:Prof. Dr. Norbert Lamersdorf
Gutachter:Prof. Dr. Alexander Knohl
Gutachter:Prof. Dr. Holger Kreft
Dateien
Name:DissPSW_revise_final_eDiss.pdf
Size:24.6Mb
Format:PDF
Zusammenfassung
Englisch
While there is broad agreement on the positive environmental impacts of short rotation plantations (SRC), possible negative impacts on groundwater recharge due to potentially high water consumption of trees on arabale land are a major issue. The water use of SRCs usually exceeds the water use of arable crops, but can also far exceed the water use of deciduous forests. This leads to a decrease in groundwater recharge, the extent of which is, however, subject to considerable uncertainty and disagreement. In order to expand the knowledge base on SRC water use at the field scale for developing adaptive, sustainable management strategies for woody biomass production systems, field studies on evapotranspiration, groundwater recharge and nitrate leaching were carried out in several SRCs, which differed greatly in their pedo-climatic site conditions, canopy closure, leaf area index and stand age. The first field study was carried out in the drinking water abstraction area Fuhrberger Feld in order to assess the effects of SRC cultivation on the amount and quality of groundwater recharge. For this purpose, the water balance of a willow SRC and a set-aside arable land was determined with the help of a process-based simulation model, which was validated against observations of soil water tension and stand precipitation. In addition, nitrate concentrations were measured in the seepage water of these field plots and other SRCs of different ages. A second study was carried out to characterise the transpiration and water use strategy (isohydric or anisohydric) of two poplar SRC of contrasting canopy closure and leaf area, in order to evaluate factors potentially influencing SRC water use that can be controlled by management. A third study provided additional information on water use and groundwater recharge of a poplar SRC at near optimum water supply, derived from an inverse modelling approach using the newly developed process-based simulation model LWFBrook90R, which was trained on observations of bulk soil water storage. Overall, the water consumption of the investigated SRCs varied greatly due to the different site conditions with regard to water supply and evaporation requirements, but in no case exceeded the evapotranspiration of deciduous forests. Nevertheless, the results from the third study showed that the water demand of SRCs can be very high, and transpiration rates greater 500 mm y 1 can be observed when water supply is ample. The high water demand of SRCs can therefore lead to a considerable decrease in groundwater recharge compared to conventional arable crops, which particularly affects sites with a high plant available soil water storage capacity. For regions with lower soil water availability, the results from the willow SRC in the Fuhrberger Feld (transpiration < 300 mm y-1) indicate that the high water demand of SRCs is not covered for the majority of sites potentially available for SRC cultivation (i.e., marginal arable land). On the one hand a low soil water availability leads to lower biomass yields, but on the other hand also limits a potential reduction in groundwater recharge. Considering also nitrate concentrations in seepage water of SRCs in the Fuhrbeger Feld, it can be concluded that the environmental impacts of SRC cultivation do not conflict with the protection requirements respecting the amount and quality of groundwater recharge in the Fuhrberger Feld water abstraction area. The results of the second study, which investigates the water use patterns of two poplar plantations, indicated that there is a certain potential for manipulating SRC water use through informed management. The differences in total evapotranspiration between the two SRCs of contrasting canopy closure and leaf area index were small, and strategies to limit leaf area or canopy closure appeared to be of secondary importance for the total evapotranspiration of SRC. However, a promising option for actually influencing transpiration through management decisions seems to be the water use strategy of the plant material. While isohydric poplar hybrids efficiently control transpiration with increasing evaporative demand, anisohydric poplar hybrids maintain high stomatal conductance even when evaporative demand is high, and soil water availability is low. This can lead to very large differences in the transpiration rate of poplar clones of different provenence. Since the water use strategy also influences biomass yields and site suitability of individual poplar hybrids, information on the water use behaviour of individual poplar clones could be used to make an informed selection of plant material that is optimally adapted to the ecological and economic requirements of a production site. For this purpose, a data base should be created, with regard to increasing land requirements for the production of woody biomass, but also with regard to the effects of climate change.
Keywords: short rotation coppice; evapotranspiration; water balance; water use strategy; isohydric