Consistent pattern of higher lability of leaves from high latitudes for both native Phragmites australis and exotic Spartina alterniflora

Global variation in litter decomposition rates is driven by climate, decomposer taxa and litter quality. Most large‐scale studies of litter quality have made comparisons across species, rather than within a species sourced from different latitudes. Here, we aim to explore latitudinal variation in leaf litter decomposition rate and litter quality within a species. We investigated whether the typical interspecific pattern of increased litter lability at higher latitudes also holds within plant species, by comparing leaf litter decomposition rates and litter traits from stands of native Phragmites australis and exotic Spartina alterniflora in Chinese coastal wetlands spanning 20° of latitude using parallel greenhouse and field transplant experiments. We found that leaf litter from high‐latitude plants decomposed 46.6 ± 5.1% faster than that from low‐latitude conspecifics, with similar patterns in both plant species in both decomposition experiments. Higher lability of leaves from high latitudes was associated with lower levels of plant defensive compounds (phenolics and tannins) and higher levels of nutrients (Ca, Mg and Na). Litter from the exotic S . alterniflora decomposed faster than litter from the native P . australis , being consistent with lower defences and higher nutrient concentrations in S . alterniflora litter. Our results show that individual species follow the same pattern of increasing litter lability at higher latitudes previously reported in cross‐species geographical comparisons. Moreover, this pattern can develop rapidly (<4 decades) in an introduced species, raising the question of whether it is caused by phenotypic plasticity or adaptation. A free Plain Language Summary can be found within the Supporting Information of this article.