Tree mortality risks under climate change in Europe: assessment of silviculture practices and genetic conservation networks

General context: Climate change can positively or negatively affect abiotic and biotic drivers of tree mortality. Process-based models integrating these climatic effects are only seldom used at species distribution scale.

Objective: The main objective of this study was to investigate the multi-causal mortality risk of five major European forest tree species across their distribution range from an ecophysiological perspective, to quantify the impact of forest management practices on this risk and to identify threats on the genetic conservation network.

Methods: We used the process-based ecophysiological model CASTANEA to simulate the mortality risk of \textit{Fagus sylvatica}, \textit{Quercus petraea}, \textit{Pinus sylvestris}, \textit{Pinus pinaster} and \textit{Picea abies} under current and future climate conditions, while considering local silviculture practices. The mortality risk was assessed by a composite risk index \textit{(CRIM)} integrating the risks of carbon starvation, hydraulic failure and frost damage. We took into account extreme climatic events with the \textit{CRIM$_{max}$}, computed as the maximum annual value of the \textit{CRIM}.

Results: The physiological processes' contributions to \textit{CRIM} differed among species: it was mainly driven by hydraulic failure for \textit{P. sylvestris} and \textit{Q. petraea}, by frost damage for \textit{P. abies}, by carbon starvation for \textit{P. pinaster}, and by a combination of hydraulic failure and frost damage for \textit{F. sylvatica}. Under future climate, projection showed an increase of \textit{CRIM} for \textit{P. pinaster} but a decrease for \textit{P. abies}, \textit{Q. petraea} and \textit{F. sylvatica}, and little variation for \textit{P. sylvestris}. Under the harshest future climatic scenario, forest management decreased the mean \textit{CRIM} for \textit{P. sylvestris}, increased it for \textit{P. abies} and \textit{P. pinaster} and had no major impact for the two broadleaved species. By the year 2100, 38\% to 90\% of the conservation units are at extinction threat (\textit{CRIM$_{max}$}=1), depending on the species.

Conclusions: Using a process-based ecophysiological model allowed us to disentangle the multiple drivers of tree mortality under current and future climate. Taking into account the positive effect of increased CO$_2$ on fertilization and water use efficiency, the average risks may increase or decrease in the future depending on species and sites. However, considering extreme climatic events, future projections are as pessimistic than those obtained with bioclimatic niche models.

Abbreviation for column:

X                                    Longitude
Y                                    Latitude
LAImax                            Leaf area index max reach
Nha                                Density per hectar
Vha                                Volume per hectar
NEE                                Net ecosystem exchange
NPP                                net primary production
Reco                                Respiration ecosystem
GPP                                Gross primary production
Etveg                            Evapotranspiration canopy
Etsol                                Evapotranspiration sol
TR                                tree transpiration
ETP                                evapotranspiration potentiel
BiomassOfReserves        Biomass of reserve
rw                                    ring width
dbh                                diameter at breast heast
height                            height
BBday                            Budburst date
rFD                                risk of frost
CRIM_max                        Maximum combined risk index of mortality reach
rNSC                                risk of carbon starvation
rPLC                                risk of embolism
rPLC_max                        Maximum risk of embolism reach
CRIM                            combined risk index of mortality
Climate                            Climatic model
rNSC_max                        maximum risk of carbon starvation reach
rFD_max                        Maximum risk of frost  reach
Scenario_Sylvicol            null means no silvulcture simulated
species                            species
Country                            Country
alt_watch                        altitude of climate simulated
grid_watch                    number of the pixel point of WATCH
grid_eurocordex                number of the pixel point of Eurocordex
Pinus_sylvestris                0 abscence ; 1 presence
Fagus_sylvatica                0 abscence ; 1 presence
Quercus_petraea            0 abscence ; 1 presence
Picea_abies                    0 abscence ; 1 presence
Pinus_pinaster                0 abscence ; 1 presence