Can soil development limit upslope migration of tree species in a warming climate? Implications from paleoecology and dynamic modeling

Background/Question/Methods Paleoebotanical reconstructions from the Central European Alps demonstrate the sensitivity of high-elevation forests to Holocene temperature change. However, temperature alone can not account for changes in key ecological parameters such as treeline elevation, or tree species distributions. For example, a dramatic lowering of treeline after 4500 cal yr B.P. is inconsistent with temperature trends. Likewise, the arrival of spruce (Picea abies) to Central Europe lags the onset of favorable temperatures by about 5000 years. These inconsistencies must result from additional climatic forcings (e.g., changes in precipitation seasonality or abundance), ecological constraints (e.g., migrational lag, soil development), or a change in disturbance regimes (e.g., anthropogenic impacts). Dynamic forest landscape succession models afford the opportunity to test such competing hypotheses explaining Holocene forest dynamics. We coupled the LandClim model, with lake sediment paleobotanical reconstructions from the Swiss Central Alps. Holocene vegetational dynamics were modeled in a contiguous high-elevation (1100 – 2800 m asl) landscape located within a 3 km radius of Gouillé Rion, an alpine lake with well-dated macrofossil and palynological reconstructions. LandClim calculates soil moisture availability using a simple bucket model. We measured soil parameters over an elevational gradient near Gouillé Rion, and used an observed negative relationship between soil development and elevation to estimate bucket size across the landscape. Bucket size is <3 cm in poorly-developed, high-elevation soils, but can exceed 12 cm in deeper soils downslope. Climatic inputs were generated using a chironomid-based Holocene temperature reconstruction from the Northern Alps and 20^th-century temperature and precipitation normals.

Results/Conclusions Treeline dynamics simulated with current soil conditions and reconstructed Holocene temperatures are consistent with paleobotanical evidence during the early to mid-Holocene. However, modeled treeline elevation remains above reconstructed treeline elevation after 4500 cal yr B.P. Thus, model results support the hypothesis that early-human activity, not climate controlled late Holocene treeline elevation. Sensitivity tests using the temperature reconstruction but varying precipitation abundance suggest moisture availability may have limited spruce arrival during the Holocene. Spruce populations establish near treeline only on well-developed soils (i.e., bucket size >6 cm), and only when growing-season precipitation is abundant. These results indicate that soil development may have delayed spruce migration into the Central European Alps even under suitable temperatures. Our combined paleoecological and modeling studies imply that poorly-developed soils at high elevations may inhibit upslope migration by temperature and drought-sensitive species in a warming climate.