Soil physical characteristics predict sapling performance in recent afforestation projects in Flanders (northern Belgium) subjected to drought

The aim of this study was to investigate the importance of soil characteristics on tree performance in recent afforestation projects in Flanders (northern Belgium) subjected to periods of severe drought. Using a robust sampling design, covering three tree species (oak, hornbeam, and linden), and spanning large gradients in soil physical and chemical characteristics, we found that the vitality of young saplings planted one to three years ago was mainly determined by the plant available water capacity (PAWC) of the soil, with increasing soil PAWC leading to higher fractions of vital trees. These effects were consistent across the three studied species, but were most pronounced for oak, a species for which typically larger stocking material is used than for hornbeam and linden. PAWC in turn was mostly determined by soil texture, with a sharp decline in PAWC when the % sand exceeded 70 %. Remarkably, higher soil organic matter concentrations did not increase the PAWC in the subset of most sandy soils. Of the studied soil chemical variables, only soil pH affected tree performance with height growth being limited when soil pH (H2O) exceeded a value of ∼6.5. We conclude that maintaining a favourable soil water status, by keeping soils covered via mulching or by creating a more sheltered microclimate for the saplings by appropriate ground cover management, are important measures to maintain tree vitality, especially in the most sandy soils. Our results also hint at the fact that, where possible, using small planting stock with a balanced root:shoot ratio may be as important as species’ ecological characteristics in the first years after planting to maintain sapling vitality under drought.