The persistence of perennial herbaceous species is threatened by increasing aridity. However, summer dormancy is a strategy conferring superior survival to grasses adapted to hot and dry summers. The role of temperature on the induction of summer dormancy was investigated in the perennial grass Dactylis glomerata to analyse the potential expression of this strategy under warmer climates.

We tested seven populations of D. glomerata originating from Morocco to Norway across the same latitudinal gradient in a five-site experiment. One population of the highly summer-dormant grass Poa bulbosa was used as a reference. Plants were grown from autumn in pots under full irrigation for one year mostly under open-air shelters.  Heading date (ear emergence preceding flowering) was recorded and foliage senescence was assessed from end of spring until autumn. The maximum plant senescence under summer irrigation indicated the level of dormancy expression. Summer dormancy onset, release, expression and duration were modelled as a function of climatic variables.

From north to south, the duration of summer dormancy of the Mediterranean populations of D. glomerata and P. bulbosa ranged from 0 to 122 days, and 79 to 200 days respectively. P. bulbosa was always completely dormant, while dormancy expression of D. glomerata was positively correlated with the sum of temperatures from winter onset (R²=0.57) and with the mean of minimum temperatures in summer (R²=0.73). Dormancy onset, release and duration were also positively correlated with thermal time from winter onset, while the duration of summer dormancy was longer as maximum temperatures increased. Mapping the European regions with climates allowing the expression of summer dormancy in D. glomerata, showed that the potentially inductive areas for this strategy may expand in parallel with increasing summer aridity under a future climate warming scenario.

We tested 7 populations of cocksfoot and 1 population of Poa bulbosa in 5 sites from Morocco to Norway. Plants were grown for 1 year under irrigation. Summer maximum foliage senescence allowed to detect plants that were summer dormant (over 70% summer maximum foliage senescence under irrigation). Dates of onset and release of summer dormancy were calculated accordingly as well as dormancy duration.

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