Can carbon-13 in large herbivores reflect the canopy effect in temperate and boreal ecosystems? Evidence from modern and ancient ungulates

Local environmental conditions under dense canopy are known to result in depletion in 13C abundance in plants compared to an open land context. This canopy effect has been observed in tropical as well as in mid-latitude forest ecosystems. However, the impact of the canopy effect on tissue 13C abundance of temperate and boreal forest-dwelling herbivores has not been thoroughly explored. Nevertheless, the canopy effect has been suggested to explain a decrease of about 3‰ in collagen δ13C values in ancient large herbivores from western Europe during the forest expansion of the Late-Glacial–Early Holocene period (ca. 15,000–6000 cal BP). Some papers have considered the 13C decrease in large herbivore as the main result of global change in atmospheric CO2 content. A detailed review of δ13C values of large herbivores (reindeer, red deer, roe deer, and bison) from open and closed environments from high and mid-latitudes confirm that the canopy effect observed in plants is passed on to their consumers. In the Paris Basin, the decline in δ13C values of large herbivores at the Late-Glacial/Early Holocene transition around 10,000 years BP appears to be different according to the considered species, namely red deer, roe deer, and large bovines (bison and aurochs). Moreover, differences in the pattern of decrease in δ13C values are observed in red deer between French northern Alps and French Jura. These differences among species in their isotopic response through time for a given geographical location, and within species from different locations, suggest variance in ecological responses of species that are associated with the relative use of forested habitat. As a result, 13C abundances in collagen can be considered as a direct tracker of the degree of closure of the habitat of ancient herbivores.