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  • 1. Sabater, Ana M.
    et al.
    Ward, Helen C.
    Hill, Timothy C.
    Gornall, Jemma L.
    Wade, Thomas J.
    Evans, Jonathan G.
    Prieto-Blanco, Ana
    Disney, Mathias
    Phoenix, Gareth K.
    Williams, Mathew
    Huntley, Brian
    Baxter, Robert
    Mencuccini, Maurizio
    Poyatos, Rafael
    Transpiration from subarctic deciduous woodlands: Environmental controls and contribution to ecosystem evapotranspiration2019In: Ecohydrology, ISSN 1936-0584, E-ISSN 1936-0592, article id e2190Article in journal (Refereed)
    Abstract [en]

    Potential land–climate feedbacks in subarctic regions, where rapid warming is driving forest expansion into the tundra, may be mediated by differences in transpiration of different plant functional types. Here, we assess the environmental controls of overstorey transpiration and its relevance for ecosystem evapotranspiration in subarctic deciduous woodlands. We measured overstorey transpiration of mountain birch canopies and ecosystem evapotranspiration in two locations in northern Fennoscandia, having dense (Abisko) and sparse (Kevo) overstories. For Kevo, we also upscale chamber‐measured understorey evapotranspiration from shrubs and lichen using a detailed land cover map. Subdaily evaporative fluxes were not affected by soil moisture and showed similar controls by vapour pressure deficit and radiation across sites. At the daily timescale, increases in evaporative demand led to proportionally higher contributions of overstorey transpiration to ecosystem evapotranspiration. For the entire growing season, the overstorey transpired 33% of ecosystem evapotranspiration in Abisko and only 16% in Kevo. At this latter site, the understorey had a higher leaf area index and contributed more to ecosystem evapotranspiration compared with the overstorey birch canopy. In Abisko, growing season evapotranspiration was 27% higher than precipitation, consistent with a gradual soil moisture depletion over the summer. Our results show that overstorey canopy transpiration in subarctic deciduous woodlands is not the dominant evaporative flux. However, given the observed environmental sensitivity of evapotranspiration components, the role of deciduous trees in driving ecosystem evapotranspiration may increase with the predicted increases in tree cover and evaporative demand across subarctic regions.

  • 2. Tang, Jing
    et al.
    Miller, Paul A.
    Crill, Patrick M.
    Olin, Stefan
    Pilesjö, Petter
    Investigating the influence of two different flow routing algorithms on soil–water–vegetation interactions using the dynamic ecosystem model LPJ-GUESS2015In: Ecohydrology, ISSN 1936-0584, E-ISSN 1936-0592, Vol. 8, no 4, p. 570-583Article in journal (Refereed)
    Abstract [en]

    This paper compares two flow routing algorithms’ influences on ecohydrological estimations in a northern peatland catchment, within the framework of an arctic-enabled version of the dynamic ecosystem model LPJ-GUESS. Accurate hydrological estimations are needed to fully capture vegetation dynamics and carbon fluxes in the subarctic peatland enviroment. A previously proposed distributed hydrological method based on the single flow (SF) algorithm extracted topographic indices has shown to improve runoff estimations in LPJ-GUESS. This paper investigates model performance differences caused by two flow routing algorithms, and importantly both permafrost processes and peatland hydrology are included in the model. The newly developed triangular form-based multiple flow (TFM) is selected due to its improved consideration of flow continuity and more realistic flow estimation over flat surfaces. A variety of measured data is included to assess both hydrological and ecological accuracy, and the results demonstrate that the choice of flow algorithm does matter for mesoscale ecohydrology applications. The allowance of flow convergence and consideration of flow partition differences from different terrain forms in the TFM algorithm yield better correspondence with the observed hydrological processes and also carbon fluxes. By directing flow to only one downslope cell together with its poorer depiction of flow over flat areas, the SF algorithm can result in too high runoff estimations for low-flat regions and overestimate carbon uptake and release in the peatland. The results of this study also highlight the need for care when selecting flow routing algorithms for biogeochemical estimations, especially within hydrologically and climatically sensitive environments. Copyright © 2014 John Wiley & Sons, Ltd.

  • 3. Tang, Jing
    et al.
    Pilesjo, Petter
    Miller, Paul A.
    Persson, Andreas
    Yang, Zhenlin
    Hanna, Edward
    Callaghan, Terry V.
    Incorporating topographic indices into dynamic ecosystem modelling using LPJ-GUESS2014In: Ecohydrology, ISSN 1936-0584, E-ISSN 1936-0592, Vol. 7, no 4, p. 1147-1162Article in journal (Refereed)
    Abstract [en]

    Northern high-latitude regions could feed back strongly on global warming because of large carbon pools and the fact that those regions are predicted to experience temperature increases greater than the global average. Furthermore, ecological functioning and carbon cycling are both strongly related to the prevailing hydrological conditions. In this study, we address these issues and present a newly developed model LPJ distributed hydrology (LPJ-DH) with distributed hydrology based on the dynamic global ecosystem and biogeochemistry model LPJ-GUESS. The new model is an enhanced version of LPJ-GUESS, introducing parametrizations of surface water routing and lateral water fluxes between grid cells. The newly introduced topographic variables in LPJ-DH are extracted from digital elevation models. LPJ-DH is tested at a 50-m resolution in the Stordalen catchment, northern Sweden. Modelled runoff is evaluated against the measured runoff from 2007 to 2009 at six outlet points. We demonstrate that the estimated monthly runoff from LPJ-DH agrees more closely with the measured data (adjusted R-2 = 0.8713) than did the standard LPJ-GUESS model (adjusted R-2 = 0.4277). However, there are still difficulties in predicting low-flow periods. The new model shows a possible advantage in representing the drainage network as well as topographic effects on water redistribution. The modelled birch tree line is in the range of the imagery observation, and the model captures the observed values of vegetation biomass in the region. Significant changes in biomass and carbon fluxes are also observed in the new model. Generally, the study justifies the feasibility and advantages of incorporating distributed topographic indices into the dynamic ecosystem model LPJ-GUESS. Copyright (C) 2013 John Wiley & Sons, Ltd.

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