Geographic Variation of Litter Chemistry and Palatability in an Invasive Plant Versus Its Native Competitor

Aim: Latitudinal variation in biotic interactions is recognized as a driver underlying variation in plant invasion success and therefore an important issue in conservation biogeography. However, previous studies have mainly focused on interactions between living plants and herbivores, whereas litter traits and detritivory have been hardly studied along latitude or compared between native and invasive plants. Our aim was to compare latitudinal variation in leaf litter chemistry and palatability to detritivores between invasive and native plants, and investigate which chemical traits determine detritivory and whether they are climate-driven. Location: China. Taxa: Spartina alterniflora, Phragmites australis, Porcellio laevis, Chiromantes dehaani. Methods: We combined field surveys with laboratory experiments to compare latitudinal variation in litter chemistry between the widespread invasive Spartina alterniflora and its native competitor Phragmites australis across their co-occurring range (20.9-40.7 degrees N, similar to 2200 km). For both species, we examined litter palatability to two common detritivores (Porcellio laevis and Chiromantes dehaani) along the same latitude. We also analysed relationships among climate, litter traits and detritivory. Results: In five of nine litter traits, we found latitudinal clines, with little difference between the two plant species in how they responded across the gradient. Litter palatability decreased with increasing latitude, but was generally higher in Spartina than Phragmites. Two key litter traits (C:P ratio and flavonoid content) were significantly associated with temperature of origin and with detritivory. Main Conclusions: There were geographic clines in litter traits and palatability, with strong links between climate, litter chemistry and detritivory, in both Spartina and Phragmites. Spartina litter, however, was more rapidly decomposed by detritivores, which could create positive feedbacks, and contributes to the successful Spartina invasion along China's coast. Future ecological restoration projects should therefore dispose Spartina plant tissue or litter off-site, to reduce the competitiveness of Spartina and support the conservation of native Phragmites.