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.
Highlights
Increasing rates of biodiversity loss and ecosystem degradation have highlighted the urgent need for landscape conservation and restoration (Rands et al, 2010)
A central requirement is that the process of natural forest regeneration be restarted through a reduction in grazing pressures and the establishment of new seed sources via targeted tree planting; this will lead to the proliferation and maintenance of self-sustaining native pinewoods (Thomas et al, 2015; zu Ermgassen et al, 2018)
At most sites, modelled volumetric water content in layer 1 (L1) and L2 was bracketed by the range observed across the soil profile, simulations were sometimes less dynamic than observations, and root mean squared errors could be large
Summary
Increasing rates of biodiversity loss and ecosystem degradation have highlighted the urgent need for landscape conservation and restoration (Rands et al, 2010). The dominant vegetation over much of the Highlands was open forests dominated by Scots pine (Pinus sylvestris) and birch (Betula spp.; Steven and Carlisle, 1959). Today, such native pinewoods cover only ∼ 1 % of their Holocene maximum extent (Mason et al, 2004). This is largely due to industrial exploitation in the 17th–19th centuries and interruption of natural regeneration due to intensification of sheep grazing and management of Highland estates for deer and grouse shooting since the mid-19th century (Steven and Carlisle, 1959; Wilson, 2015). A central requirement is that the process of natural forest regeneration be restarted through a reduction in grazing pressures and the establishment of new seed sources via targeted tree planting; this will lead to the proliferation and maintenance of self-sustaining native pinewoods (Thomas et al, 2015; zu Ermgassen et al, 2018)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
