Foreseeable global warming will differentially affect Chara vulgaris populations from different altitudes

• A common garden experiment was performed on charophytes.
• Four Chara vulgaris populations from different altitudes grew differently under 20–24 °C.
• Low-altitude C. vulgaris populations produced more biomass than those from high altitude at 24 °C.
• A more expansive architecture occurred in low-altitude populations when T increased.
• A substitution of charophyte ecotypes is foreseen given climatic change.

A 2–4 °C increase in temperature is foreseen for the Mediterranean region by the end of the 21st century due to climate change. This warming is expected to occur as heat waves, and will be particularly important for shallow water bodies which are the main freshwater ecosystems in the Mediterranean. We subjected four populations of Chara vulgaris to a common garden experiment. Each population came from locations at different altitudes, hence living at different mean temperatures. Individual charophyte cultures were initiated with specimens from four origins and acclimatized at 20 °C in a controlled environment for 3 weeks. After this, all the populations were incubated under 3 water temperature treatments: 20 °C (control), 22 °C and 24 °C. A two-way ANOVA (temperature × population origin) showed that the temperature treatment significantly, and positively, affected the growth calculated on normalized dry weight, the length of the main axis, the number of branches per node, the horizontal biomass distribution and the respiration rate. Moreover, a significant effect of the temperature × population-origin interaction was detected for biomass production, total length and branch production. These results suggest that lower-altitude populations have a greater tolerance to temperature increase, and they would be favored by climate change. These different intraspecific responses to warming could impact the ecotype distribution and diversity.