The impact of global change is mainly studied on species phenology and distribution. The impact on growth has been studied on tree species for decades. The recent development of digitized herbaria opens onto a wide range of new possibilities to study the impact of global change on plants over the last centuries. By using a novel approach based on the comparison of herbaria with contemporary specimens collected in temperate areas of France, we aimed to assess the impact of environmental changes on the size of temperate herbaceous species. We used forest species as model species to limit the direct impact of agricultural practices and detect growth changes linked to the fertilizing effects of the CO2 increase, global warming and nitrogen deposition. We compared plant height and leaf size of 503 specimens belonging to 12 species and 7 botanical families collected between 1704 and 1950 (average year: 1888) with the same traits measured on 1,090 specimens of these species collected after 2000 (average year: 2016). Specimens were collected in the same period of year, on the 131st day and the 133rd on average for historical and recent specimens, respectively. Recent specimens were also collected in the same forests as historical ones to highlight temporal changes and limit spatial bias.
Results/Conclusions
We observed an increase in plant height for 9 of the 12 species and an increase in leaf area for 11 of the 12 species. The average increase reached 12 and 26 % for plant height and leaf size, respectively. This increase in size remained significant when models were used to control for the period of the year, the phenological stage, the forest canopy cover or the field sampling method. Results also showed an increasing trend in size around the city of Paris, where many specimens were collected, as well as in the rest of France. The large size of the plant sample, its distribution both in the preindustrial period and in the 21st century, and the large study area highlight a likely footprint of global change on the evolution of the size of herbaceous species. The magnitude of the growth change was similar to that observed on tree species, showing that global change modifies the growth of plants at the ecosystem scale. The change in plant size could impact plant metabolism as well as community interactions and ecosystem functioning.