Ecophysiologically driven irrigation scheduling for safeguarding reforestation efforts in drought-prone calamity areas in Central Europe

Droughts, storms, and insect infestations have severely damaged European forests in the past decade, necessitating urgent reforestation. However, tree saplings lacking deep roots are highly vulnerable to drought, especially on exposed clear-cut sites with poor soil water retention. During droughts, irrigation could reduce sapling mortality and safeguard reforestation efforts. Until now, irrigation was not a critical topic in Central European forestry since severe droughts were rare, thus irrigation thresholds have not been established. In order to fill this gap, we performed a study in Northern Bavaria, Germany, with field and greenhouse experiments. Based on our results, we propose ecophysiologically driven irrigation thresholds for the three common tree species Fagus sylvatica, Quercus robur, and Pseudotsuga menziesii. Easily available dendrometer recordings were used for estimating tree water deficit (TWD) based irrigation thresholds. We defined a lower irrigation threshold reflecting the point where trees cannot refill their stem water reserves during nighttime, and an upper irrigation threshold corresponding to a 12 % xylem conductivity loss (Ψ₁₂) as the ultimate threshold preventing severe hydraulic damage. Irrigation demand was estimated as a percentage from 1 % (lower irrigation threshold) to 100 % (upper irrigation threshold). In a greenhouse experiment, soil water content significantly correlated with irrigation demand (ρ = -0.85) and explained 72 % of its variance overall. Field validation showed significant but weaker correlations between irrigation demand and drought conditions. Irrigation relieved drought stress and reduced the irrigation demand. In the face of climate change, our findings provide a drought mitigation strategy in forestry to improve reforestation success.