The ongoing fast climate change is currently affecting the physiology and distribution
of species. When projecting future species repartition, most prediction models don’t take
into account the evolutionary mechanisms behind species response to changing parameters.
The aim of the study was to characterise the adaptive capacities of populations by (i)
studying the extent to which the stomatal variations are under genetic control (adaptation)
or under environmental control (phenotypic plasticity) and (ii) identify the environmental
parameters which trigger a stomatal response.
The study focused on a Sessile oak (Quercus petraea (Matt.) Liebl), a broadleaved
species largely distributed throughout Europe. Leaves were collected from four common
gardens among Europe (France, Denmark, Poland, England) with each of them gathering
seeds originating from 10 European provenances. Investigations on Stomatal response
were conducted through the study of stomatal density SD and size SS. To disentangle local
adaptation and phenotypic plasticity in stomatal response, Linear Mixed Effect Models
were studied for each of the stomatal traits and used as explanatory variables the trial site,
seed provenance origin and an interaction term between provenance and trial.
For both stomatal traits, interaction with the environment (trial site) was the first predictor
in explaining the observed variability. Phenotypic plasticity appears as the main
reaction mechanism in oaks. For both models, there was a genetic variability in the phenotypic
plasticity displayed. Finally, if low evidence of local adaptation process were found,
an adaptative lag appears when testing the effect of Temperature of mean temperature of
warmest quarter ecodistance on Stomatal size.
A significant negative correlation was found between the stomatal size and the trial’s mean
temperature of the wettest quarter (1987-2019 period), precipitation of spring 2021, and
2020 annual precipitation. These results suggest that heat and dryer environments lead
to a leaf response through stomata reduction to avoid water depletion. Correlations with
the stomatal size and the seed’s provenance site precipitation regime (annual, seasonality,
in the driest quarter for the 1961-1990 period) suggest that trees originating from drier
climates tend to have smaller stomata. Stomatal density trait was less correlated and exhibited
inverse tendencies than stomatal size.
To conclude, global warming is a quick process which calls for a fast response of trees to
changing habitat. Hence, the high oak plasticity observed in our experiment is a good sign
as to the ability of the oaks to maintain themselves. This reinforces the statement of oak
as a resilient species. If our study didn’t highlight limits in the plastic response expressed
by oaks to changing conditions, phenotypic plasticity may not be sufficient to ensure the
adjustment of phenological traits to climate change. Thus, oaks could be forced to adapt
via migration.