Disentangling the formation of contrasting tree-line physiognomies combining model selection and Bayesian...

Area of Science:

• Ecology
• Computational Ecology
• Biogeography

Background:

• Alpine tree-line ecotones exhibit significant spatial variation and diverse physiognomies.
• Understanding the minimal processes driving tree-line formation is crucial for predicting ecosystem responses.

Purpose of the Study:

• To identify the essential ecological processes required for the formation of alpine tree lines.
• To test a set of individual-based models against empirical data from Pinus uncinata ecotones.

Main Methods:

• Utilized a Bayesian approach with Markov chain Monte Carlo (MCMC) methods for parameter estimation.
• Employed model selection procedures to determine the minimal subset of processes explaining tree-line formation.
• Compared model outputs with data from four contrasting Pinus uncinata ecotones in the Spanish Pyrenees.

Main Results:

• Tree-line formation was primarily explained by nonlinear individual growth and mortality responses to the altitudinal gradient.
• Variations in tree-line physiognomy (e.g., diffuse vs. abrupt, krummholz presence) were driven by the relative importance of these nonlinear responses.
• Dispersal limitation and facilitation played secondary roles in shaping tree-line ecotones.

Conclusions:

• Nonlinear responses of individual organisms are key drivers of alpine tree-line ecotone structure.
• The findings provide insights into differential responses of tree line types to climate change.
• The presented methodological approach enhances transparency and efficiency in evaluating individual-based models.