Restoration rocks: The long-term impact of rock dust application on soil, tree foliar nutrition, tree radial growth, and understory biodiversity in Norway spruce forest stands

Abstract

Norway spruce (Picea abies L.) forests are experiencing severe dieback due to drought and related bark beetle infestations. These disturbances are amplified by poor tree vitality, which is linked to acidification in soils and exacerbated by atmospheric deposition. However, the large-scale deployment of rock dust as a soil-restoring amendment is hampered by uncertainties about its benefits in the long term. This study reassessed an experiment that was set up in 1987 in Vorarlberg, Austria, and sampled again in 2021. It comprises three 3 ha-large twin plots (topsoil pHCaCl(2) = 3.0, eCEC = 5-10 cmol(c)/kg) consisting of Norway spruce-dominated selection system stands. The soil amendments consisted of a mixture of basalt and diabase rock dust, complemented with bentonite and lime at a total dose of 4.7 Mg/ha, the equivalent acid neutralising capacity is about 2.5-3.3 Mg CaCO3/ha. The topsoil pH increased in 1991 after application by about 0.3 units but became untraceable in 2021. In contrast, the 2021 samples confirmed treatment effects on a significant and large rise in forest floor eCEC (+9 cmol(c)/kg) and in topsoil base saturation (+15 %-point). Tree vitality and growth were evaluated for Norway spruce (63 trees) and silver fir (Abies alba) (17 trees) through foliar nutrient concentrations, defoliation, radial growth and Laser Ablation - Inductively Coupled Plasma - Mass Spectrometry (LA-ICP-MS). Overall, foliar and tree ring concentrations of P, Ca and Mg increased in plots amended with rock dust. The tree radial growth responded to the treatment from 1989 onwards, with differences in annual Basal Area Increments (BAI) peaking about 20 years after applications. Growth improvement was markedly larger for trees with an age below 150 years. Remarkably, the amended spruce trees with limited N-deficiency (needle N > 10 mg/g) had a factor 1.3 larger BAI in 2010 than control trees. In contrast, N-deficient trees, mostly located at higher altitudes with lower current N deposition (needle N < 10 mg/g), did only marginally respond to the rock dust. Herb layer plant species richness increased in 2021 with the addition of rock dust, with 13 additional species compared to the 24 found in the control. Biodiversity indices showed that a marginal change occurred both in richness and evenness. The Ellenberg R increased slightly in the most acidic site but without loss of the typical species for oligotrophic spruce forests. In conclusion, it is possible to increase vitality and growth via rock dust amendments, provided that N-deficiency or tree age are not growth-limiting factors.