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

Abstract

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.

Holocene treeline and timberline changes in the South Carpathians (Romania): Climatic and anthropogenic drivers on the southern slopes of the Retezat Mountains

“…They also suggest that Picea abies reached its highest elevation between 3500 and 3000 cal. yr BP, probably because of more favourable soil forming conditions (Henne et al, 2011). To investigate the role of summer temperature changes and human activities on the timberline and treeline, we compare the reconstructed treeline and timberline changes (Figure 8) with the charcoal and human indicator pollen proxies ( Figure 7) and with the chironomid-inferred July temperature changes in Lake Brazi on the northern slope of the Retezat Mountains (Tóth et al, 2015).…”

Palaeoclimate records 60–8 ka in the Austrian and Swiss Alps and their forelands

“…Furthermore, it was decided to mainly focus on established proxies which allow quantitative, or at least semi-quantitative estimates of past variations in climate variables, since these reconstructions are directly comparable with other quantitative records of past climate change, climate model output data, and can be used as input variables to derive model based assessments of past ecosystem change (e.g. Heiri et al, 2006;Henne et al, 2011). However, an effort was made to cover a wide range of archives and palaeoclimatological methods, including palaeoecological, geochemical and geomorphological approaches.…”

Sensitivity Analysis of a Land-Use Change Model with and without Agents to Assess Land Abandonment and Long-Term Re-Forestation in a Swiss Mountain Region

“…LandClim was designed to examine the impact of climate change and forest management on forest development and structure [94]. The model has been tested in the Central Alps, North American Rocky Mountains, and Mediterranean forests, and has been used to simulate current, paleo-ecological [95][96][97] and future forest dynamics [55,94]. LandClim simulates forest growth in 25 m × 25 m cells using simplified versions of tree recruitment, growth and competition processes that are commonly included in forest gap models [98].…”