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 datasets available for a catchment in the Scottish Highlands, an area where the impetus for regeneration of native pinewoods is growing. We then simulated two land cover change scenarios that incorporated forests at early (thicket) and late (old-open forest) stages of regeneration, respectively, and compared these to a present-day baseline simulation. Changes to forest structure (proportional vegetation cover, vegetation heights and leaf area index of pine trees) were modelled for each stage. Establishment of thicket forest had the greatest effect on water partitioning/ages and connectivity, with increased losses to interception evaporation driving reductions in below-canopy fluxes (soil evaporation, groundwater recharge and streamflow) and generally slower rates of water turnover. Effects on streamflow were most evident for low and moderate summer flows rather than winter high flows. Whilst full forest regeneration was limited to hillslopes, resultant changes to the spatial dynamics of flux partitioning could also cause drying out of the valley bottom. The more open nature of the older forest generally resulted in water fluxes, ages and connectivity characteristics returning towards baseline conditions. Our work implies that the ecohydrological consequences of natural forest regeneration on degraded land 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 management that effectively balances landscape restoration with demand for ecosystem services. Tracer-aided ecohydrological models were also shown to be useful tools for land cover change simulations and further potential of such models was highlighted.

Using isotopes to understand landscape‐scale connectivity in a groundwater‐dominated, lowland catchment under drought conditions

“…The use of soil water or groundwater as a transpiration source shows dependency on tree species, age, stand density and distribution (Song et al, 2016), with a tendency for deeper water sources in more arid climates (Evaristo & McDonnell, 2017). Potential impacts of pine plantations (i.e., uniform age distributions in stems) on the regional groundwater levels have been also highlighted by Nützmann et al (2011) and evidence‐based assessment of water footprints of different land use will be essential in studying and managing local subsurface and surface water resources (Neill et al, 2021).…”

Time variable effectiveness and cost-benefits of different nature-based solution types and design for drought and flood management

“…Although results suggested planting would reduce low flows and recharge, as found elsewhere [49,90,99], the impacts may have differed with alternative designs; species, age or distribution [100]. Model representations of tree planting are also often limited, specifically by temporal changes as a tree canopy develops e.g.…”

“…Simulation of NBS scenarios through contrasting hydro-climatic conditions helped identify how the different water fluxes changed for the different types of NBS. Whilst planting scenarios appeared to increase sub-surface storage and retention of event water, the short retention time meant this had little impact on low flows compared to planting-associated increases in ET and water use [49]. Planting scenarios therefore reduced streamflow and recharge below baseline, which could be problematic in catchments where water yield was of concern.…”

“…Models enable simulation of this, avoiding installation of multiple NBS and extensive monitoring campaigns [29]. Although modelling is associated with uncertainties, these can be reduced by calibrating models with local empirical data, and results can provide useful insights into direction of change [48,49]. Given NBS affect processes at different spatial scales, modelling also enables extraction of results at relevant scales for e.g., cost-benefit analyses, to address issues specific to local surroundings or stakeholders.…”