Soil saturation limits early oak establishment in upland pastures for restoration of Atlantic oak woodlands

With woodland expansion providing a nature-based solution to help tackle anthropogenic climate change, the persistence and regeneration of fragmented Atlantic oak woodland is a critical feature of the wilding of uplands and conservation of Temperate Rainforests in Western Europe. Predicted shifts in precipitation with climate change are likely to be a major determinant of tree recruitment here, yet our understanding of the threshold effects of high soil moisture on oak establishment is limited, particularly at early-life-history stages when trees are most vulnerable. We address this gap in knowledge by investigating the effect of soil saturation states on; 1) the capacity for oak seedling development from acorns in peaty soils; and 2) the survival and performance of juvenile (one-year-old) saplings planted in upland pasture. In container experiments, we quantified Quercus robur establishment from acorns in four soil saturation states. There was complete recruitment failure in ‘flooded’ soils (water level 20 mm above acorn), and reduction (43% survival) at ‘high’ saturation (water level 81 mm below acorn) compared to ‘medium’ (77% survival - water level 155 mm below acorn) and ‘low’ (83% survival - water level 220 mm below acorn) treatments. Surviving seedlings exhibited reduced root:shoot ratio, leaf photosynthesis, and lower likelihood of late season shoot growth in soils of high saturation. In the field experiment, juvenile oak (Q. robur, Q. petraea) saplings were planted in UK upland freely draining, seasonally waterlogged, and waterlogged pastures soils and subject to livestock browsing treatments. Q. robur had greater shoot growth and leaf photosynthesis response to soil saturation than closely related Q. petraea. Results highlight the need for better understanding of soil-dependent influence on tree browsing. We conclude that climate-resilient restoration of Atlantic oak woodland must utilise both European oak species to support seedling and sapling recruitment events in the face of rapid but uncertain changes in precipitation and soil conditions. Such an approach might better allow future natural colonisation events across a gradient of projected soil scenarios and landscape trajectories. Further research should look to better characterise the role of soil moisture on the early-life-history stages of trees. Furthermore, understanding the soil factors that limit seedling and sapling establishment, especially when these factors are themselves perturbed by climate change, is central to both our wider understanding of plant community response to climate change and nature-based solutions to mitigate its effects.