Cottonwood hybridization effects tannin and nitrogen content of leaf litter and alters decomposition

Cottonwoods are dominant riparian trees of the western United States and are known for their propensity to hybridize. We compared the decomposition of leaf litter from two species (Populus angustifolia and P. fremontii) and their hybrids. Three patterns were found. First, in one terrestrial and two aquatic experiments, decomposition varied twofold among tree types. Second, backcross hybrid leaves decomposed more slowly than those of either parent. Third, the variation in decomposition between F₁ and backcross hybrids was as great as the variation between species. These results show significant differences in decomposition in a low-diversity system, where >80% of the leaf litter comes from just two species and their hybrids. Mechanistically, high concentrations of condensed tannins in leaves appear to inhibit decomposition (r¹=0.63). The initial condensed tannin concentration was high in narrowleaf leaves, low or undetectable in Fremont leaves, and intermediate in F₁ hybrid leaves (additive inheritance). Backcross hybrids were high in condensed tannins and were not different from narrowleaf (dominant inheritance). Neither nitrogen (N) concentration nor the ratio of ash-free dry weight to N (a surrogate for carbon:nitrogen ratio) were significantly correlated with decomposition. The N content of leaf material at the end of each year's experiment was inversely correlated with rates of litter mass loss and varied 1.6- to 2.1-fold among tree classes. This result suggests that hybrids and their parental species are used differently by the microbial community.