Modelling tree survival for investigating climate change effects

TL;DR

This study models tree survival using advanced statistical methods to identify key environmental and defoliation factors influencing mortality, revealing species-specific drivers and potential early warning thresholds for forest health under climate change.

Contribution

It introduces a novel smooth additive Cox model with random effects and a new model selection scheme for analyzing large ecological datasets.

Findings

Environmental drivers vary by species but are consistently important.

Defoliation has a strong, monotonic effect on mortality across species.

Potential defoliation thresholds for early warning identified for some species.

Abstract

Using German forest health monitoring data we investigate the main drivers leading to tree mortality and the association between defoliation and mortality; in particular (a) whether defoliation is a proxy for other covariates (climate, soil, water budget); (b) whether defoliation is a tree response that mitigates the effects of climate change and (c) whether there is a threshold of defoliation which could be used as an early warning sign for irreversible damage. Results show that environmental drivers leading to tree mortality differ by species, but some are always required in the model. The defoliation effect on mortality differs by species but it is always strong and monotonic. There is some evidence that a defoliation threshold exists for spruce, fir and beech. We model tree survival with a smooth additive Cox model allowing for random effects taking care of dependence between neighbouring trees and non-linear functions of spatial time varying and functional predictors on defoliation, climate, soil and hydrology characteristics. Due to the large sample size and large number of parameters, we use parallel computing combined with marginal discretization of covariates. We propose a 'boost forward penalise backward' model selection scheme based on combining component-wise gradient boosting with integrated backward selection.