Structural changes to forests during regeneration affect water flux partitioning, water ages and hydrological connectivity: Insights from tracer-aided ecohydrological modelling

Abstract

Abstract. Increasing rates of biodiversity loss are adding momentum to efforts seeking to restore or rewild degraded landscapes. Here, we investigated the effects of natural forest regeneration on water flux partitioning, water ages and hydrological connectivity, using the tracer-aided ecohydrological model EcH2O-iso. The model was calibrated using ∼ 3.5 years of diverse ecohydrological and isotope data available for a catchment in the Scottish Highlands, an area where impetus for native pinewood regeneration is growing. We then simulated two land cover change scenarios that incorporated forests at early (dense thicket) and late (old open forest) stages of regeneration, respectively. Changes to forest structure (proportional vegetation cover, vegetation heights and leaf area index of pine trees) were modelled for each stage. The scenarios were then compared to a present-day baseline simulation. Establishment of thicket forest had substantial ecohydrological consequences for the catchment. Specifically, increased losses to transpiration and, in particular, interception evaporation drove reductions in below-canopy fluxes (soil evaporation, groundwater (GW) recharge and streamflow) and generally slower rates of water turnover. The greatest reductions in streamflow and connectivity were simulated for summer baseflows and small to moderate events during summer and the autumn/winter rewetting period. This resulted from the effect of local changes to flux partitioning in regenerating areas on the hillslopes extending to the wider catchment by reducing downslope GW subsidies that help sustain summer baseflows and saturation in the valley bottom. Meanwhile, higher flows were relatively less affected, especially in winter. Despite the generally drier state of the catchment, simulated water ages suggested that the increased transpiration demands of the thicket forest could be satisfied by moisture carried over from previous seasons. The more open nature of the old forest generally resulted in water fluxes, water ages and connectivity returning towards baseline conditions. Our work implies that the ecohydrological consequences of natural forest regeneration depend on the structural characteristics of the forest at different stages of development. Consequently, future land cover change investigations need to move beyond consideration of simple forest vs. non-forest scenarios to inform sustainable landscape restoration efforts.

Using stable water isotopes to understand ecohydrological partitioning under contrasting land uses in a drought‐sensitive rural, lowland catchment

“…At such locations, evaporative effects on grassland may become more influential than in a shaded forest with the deeper rooting systems and more distributed root water uptake across depth. Of course, the combination of many factors like topography, soil properties, and hydroclimatic conditions dictate soil water content VWC (see Neill et al, 2021; Zhu & Lin, 2011; Zucco et al, 2014). Nevertheless, in our study, the time dependent effects of dominant vegetation showed a clear influence on the patterns of ecohydrological partitioning of soil moisture variability, for example, forest soils tended to be drier compared to those under grassland.…”

Leveraging sap flow data in a catchment-scale hybrid model to improve soil moisture and transpiration estimates

“…However, beyond revealing absolute errors in the seasonal cycle, the stomatal conductance model based on sap flow also demonstrates that the Jarvis-Stewart model is not able to reproduce diurnal hydraulic feedbacks along the soil-plant-atmosphere continuum reflected in the dips in canopy conductance during the midday water stress period. Mechanistic understanding of these stress responses in plant-water flow is still limited and representing them using existing ecophysiological models is challenging, especially beyond the individual tree (e.g., Grossiord et al, 2020;Kannenberg et al, 2022;Novick et al, 2019). On the other hand, these dynamics are embedded in the sap flow data and were adequately reproduced by the RNN for the purpose of hydrological modeling.…”

“…One caveat R. Loritz et al: Leveraging sap flow data in a catchment-scale hybrid model to improve soil moisture in using these measurements is that they represent an effective flux integrating evaporation from the canopy interception store and the soil with plant transpiration. An accurate partitioning of this integral flux into its components is, however, of key importance for improving transpiration modeling under changing conditions (Stoy et al, 2019), including effects of land-use changes such as deforestation (e.g., Hrachowitz et al, 2021) and forest regeneration (e.g., Neill et al, 2021). This is a key reason why sap flow is used as independent measurement technique to characterize transpiration dynamics in forest (e.g., Granier and Loustau, 1994) and agricultural ecosystems (e.g., Dugas et al, 1994).…”

Integrated ecohydrological hydrometric and stable water isotope data of a drought-sensitive mixed land use lowland catchment

“…Such experimental "outdoor laboratories" are invaluable scientific resources given the complexity of increasing pressures on water supplies (e.g. Cosgrove and Loucks, 2015), land use change (Neill et al, 2021) and the uncertain effects and non-stationarity of projected climate change (Milly et al, 2015). Ecohydrology adopts an interdisciplinary approach to investigating interlinkages between the structure and function of ecological systems and the partitioning, flux and storage of fresh water (Guswa et al, 2020).…”