  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.
