Data from: Does one model fit all? patterns of beech mortality in natural forests of three European regions

Large uncertainties characterize forest development under global climate
change. Although recent studies have found widespread increased tree
mortality, the patterns and processes associated with tree death remain
poorly understood, thus restricting accurate mortality predictions. Yet,
projections of future forest dynamics depend critically on robust
mortality models, preferably based on empirical data rather than
theoretical, not well-constrained assumptions. We developed parsimonious
mortality models for individual beech (Fagus sylvatica) trees and
evaluated their potential for incorporation in Dynamic Vegetation Models
(DVMs). We used inventory data from nearly 19′000 trees from unmanaged
forests in Switzerland, Germany and Ukraine, representing the largest
dataset used to date for calibrating such models. Tree death was modelled
as a function of size and growth, i.e., stem diameter (dbh) and relative
basal area increment (relBAI), using generalized logistic regression
accounting for unequal re-measurement intervals. To explain the spatial
and temporal variability in mortality patterns, we considered a large set
of environmental and stand characteristics. Validation with independent
datasets was performed to assess model generality. Our results demonstrate
strong variability in beech mortality that was independent of
environmental or stand characteristics. Mortality patterns in Swiss and
German strict forest reserves were dominated by competition processes as
indicated by J-shaped mortality over tree size and growth. The Ukrainian
primeval beech forest was additionally characterized by windthrow and a
U-shaped size-mortality function. Unlike the mortality model based on
Ukrainian data, the Swiss and German models achieved good discrimination
and acceptable transferability when validated against each other. We thus
recommend these two models to be incorporated and examined in DVMs. Their
mortality predictions respond to climate change via tree growth, which is
sufficient to capture the adverse effects of water availability and
competition on the mortality probability of beech under current
conditions.