Climate change has been identified as a causal factor for diverse ecological changes worldwide. Warming trends over the last couple of decades have coincided with the collapse of long-term population cycles in a broad range of taxa, although causal mechanisms are not well-understood. Larch budmoth (LBM) population dynamics across the European Alps, a classic example of regular outbreaks, inexplicably changed sometime during the 1980s after 1,200 y of nearly uninterrupted periodic outbreak cycles. Herein, analysis of perhaps the most extensive spatiotemporal dataset of population dynamics and reconstructed Alpine-wide LBM defoliation records reveals elevational shifts in LBM outbreak epicenters that coincide with temperature fluctuations over two centuries. A population model supports the hypothesis that temperature-mediated shifting of the optimal elevation for LBM population growth is the mechanism for elevational epicenter changes. Increases in the optimal elevation for population growth over the warming period of the last century to near the distributional limit of host larch likely dampened population cycles, thereby causing the collapse of a millennium-long outbreak cycle. The threshold-like change in LBM outbreak pattern highlights how interacting species with differential response rates to climate change can result in dramatic ecological changes.

https://www.pnas.org/content/pnas/107/47/20576.full.pdf

Climatic warming disrupts recurrent Alpine insect outbreaks

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.

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

Fire, humans and landscape in the European Alpine region during the Holocene

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.

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

European larch (Larix decidua Mill.) forests in the Alps are cultural landscapes that have been shaped by humans for centuries through traditional management. Biological and historical data sources were employed, and a multi-scale approach was adopted to capture the influence of factors affecting the structure of these forests. Landscape and stand scale dynamics were analyzed in four watersheds (c. 13,000 ha) of the western and central Italian Alps that have experienced different land-use intensities. Observed landscape changes were generalized using path analyses developed from a common conceptual model. Stand structure and a range of environmental variables were sampled in 203 circular plots, and land use and anthropogenic variables were derived from thematic maps and aerial photographs. We used multivariate statistical analyses (ordination and SEM models) to relate forest structure, anthropogenic influences, land uses, and topography. The most commonly observed land cover transition was an expansion of forests at the expense of open areas. All studied watersheds were dominated by larch forests, but their structure and spatial pattern differed greatly. Anthropogenic variables were less important at Ventina, the least accessible site, but emerged as fundamental to explain stand structure in the other study sites. Complexity of topography and proximity to roads had influenced past human activities mainly in the most accessible sites. Regeneration density was greatest at lower elevations and closer to human settlements. Quantification of the role played by forest harvesting and cattle grazing in past centuries is critical for understanding how global change factors may influence future dynamics of mountain forests in the European Alps and similar cultural landscapes worldwide.

/pdf/land-use-history-and-topographic-gradients-as-driving-5anxd4gm39.pdf

G. Mischi

Papers

Land-use history and succession of Larix decidua in the Southern Alps of Italy–An essay based on a cultural history study of Roswitha Asche

Larjei - 1000 Jahre Bewirtschaftung der Lärche im Campilltal, Südtirol