Evapotranspiration of bush encroachments on a temperate mire meadow – A nonlinear function of landscape composition and groundwater flow Mateusz Grygoruk a, *, Okke Batelaan b, c , Dorota Miroslaw-  Swia ˛tek a , Jan Szatylowicz d , Tomasz Okruszko a a Warsaw University of Life Sciences – SGGW, Department of Hydraulic Engineering, ul. Nowoursynowska 159, 02-776 Warsaw, Poland b Flinders University, National Centre for Groundwater Research and Training, School of the Environment, GPO Box 2100, Adelaide SA 5001, Australia c Vrije Universiteit Brussel, Department of Hydrology and Hydraulic Engineering, Pleinlaan 2, 1050 Brussels, Belgium d Warsaw University of Life Sciences – SGGW, Department of Environmental Improvement, ul. Nowoursynowska 159, 02-776 Warsaw, Poland A R T I C L E I N F O Article history: Received 3 June 2014 Received in revised form 1 September 2014 Accepted 17 September 2014 Available online xxx Keywords: Evapotranspiration Wetlands Mires Land cover Bush encroachment Biebrza A B S T R A C T As widely recognized, bush encroachment on mire meadows induces species and habitat biodiversity. However, it is unclear if expansion of either a coherent, continuous forest or randomly distributed groups of trees, so-called tree isles, in formerly open landscapes of mire meadows influences differently the water balance of the system due to changing vegetation structure, landscape, and thus evapotranspiration. In this paper we use a quasi-3D unsaturated–saturated groundwater flow model to reveal the feedback between the actual evapotranspiration and diurnal phreatic groundwater level dynamics. A schematized numerical experiment was setup to elucidate the ecohydrological functioning of mire meadows in the Biebrza Valley (NE Poland), which have been influenced by bush encroachment due to abandonment of mowing. Model calibration and validation was based on field collected data on groundwater dynamics showing the feedback between the evapotranspiration and diurnal groundwater level fluctuations. The model was applied for different landscape compositions of encroachments as well as different stages of shrubby vegetation expansion in a formerly open meadow. It is concluded that for coupled unsaturated– saturated flow models of mires, analysis of diurnal groundwater level fluctuations combined with evapotranspiration quantification is an efficient yet simple method for model calibration and validation. Results of the modelling experiment indicated that the tree-isle-type of bush encroachment entails higher losses of water from the system due to evapotranspiration than coherent forest expansion. For a hypothetical summer drought and an encroachment cover of 50% the total evapotranspiration is shown to be 13% higher for the tree-isle-type encroachment than for expansion of a coherent forest. Consequently, it is concluded that conservation of mires requires continuous control of encroachment not only because of potential loss of biodiversity, but more importantly to limit significant loss of water due to increased evapotranspiration. ã 2014 Published by Elsevier B.V. 1. Introduction 1.1. Research context Vegetation and land cover of agriculturally maintained mire meadows evolve under the impacts of economic and natural changes (Galatowitsch et al., 2000). The open aspect of landscapes of temperate miremeadows is controlled by mowingorpasture grazing (Opdekamp et al., 2012). Hence, once agricultural activities cease due to e.g. high water level during the haymaking period, which makes mowing impossible or economically inefficient, the secondary succession of forest spreads within open areas of meadow and bush encroachment starts. The process of encroachment begins with the spreading of single shrubs and young self-seeding trees (Resler and Stine, 2009; Valk and Warner, 2009). The most intensive bush encroachment can normally be observed along the boundaries of forests. However, the abandonment of agricultural activities allows scattered single groups of trees, or so-called tree isles, to develop in the middle of formerly maintained meadows. Tree isles, which normally consist of a few trees, shrubs and remaining sedges, * Corresponding author. Tel.: +48 225935309; fax: +48 225935320. E-mail address: m.grygoruk@levis.sggw.pl (M. Grygoruk). http://dx.doi.org/10.1016/j.ecoleng.2014.09.041 0925-8574/ ã 2014 Published by Elsevier B.V. Ecological Engineering 73 (2014) 598–609 Contents lists available at ScienceDirect Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng