Did soil development limit spruce (Picea abies) expansion in the Central Alps during the Holocene? Testing a palaeobotanical hypothesis with a dynamic landscape model

Aim Forest communities in the European Central Alps are highly sensitive to climatic change. Palaeobotanical studies have demonstrated that forests rapidly expanded upslope during Holocene warm intervals and contracted when temperatures fell. However, temperature alone cannot account for important changes in tree species abundance. For example, population expansion by Norway spruce (Picea abies), a dominant subalpine species, lagged suitable temperatures by about 3000 years in eastern and by 6000 years in western Switzerland. We hypothesize that spruce expansion was delayed by limited water availability in weakly developed soils and/or by drier-than-present climatic conditions. Location We examine the impact of reduced moisture availability on forest dynamics using a combined dynamic modelling/palaeoecological approach at two high-elevational lakes in the Swiss Central Alps. Methods We simulate Holocene vegetation dynamics with the LandClim model in landscapes surrounding the two lakes and validate the model output by comparison with palaeobotanical reconstructions from the same sites. We evaluate the impact of shallow soils on vegetation dynamics at these sites by varying soil water-holding capacity (i.e. bucket size) and precipitation abundance in model scenarios. Results Simulations with modern soil conditions and precipitation abundance matched reconstructed vegetation dynamics near the tree line, where temperature limits plant growth, but simulated abundant spruce during the entire Holocene. Spruce was absent only in simulations with a maximum bucket size of less than 7 cm, or when precipitation was reduced by at least 60%. In exploratory simulations of future conditions with average temperatures raised by 4 °C, the low water-holding capacity of shallow alpine soils, not temperature, determined the upper elevational limit of spruce. Main conclusions Spruce expanded in the Central Alps only after soils developed sufficient water-holding capacity and precipitation neared its modern abundance. Soil development will probably constrain the future response of tree species to warmer conditions (e.g. upslope migrations), as it did in the past.