Genotype by genotype interactions and genetic diversity influence leaf litter decomposition in a large-scale riparian common garden

Background/Question/Methods

In riparian environments, leaf litter inputs provide crucial sources of organic matter to riparian soils and subsidies of carbon and nutrients to in-stream communities. The physical nature of these environments means leaf litter may decompose in place (under donor canopies) or be transported downstream to decompose under the canopies of other riparian tree populations. Using a large-scale common garden of Fremont cottonwood (Populus fremontii) that includes 16 genotypes collected from across the range of P. fremontii (from southern AZ to northern UT), we selected five genotypes from across this range and collected naturally abscising leaf litter. We compared leaf litter decomposition rates of these genotypes under three different riparian conditions: 1) at home (under a donor canopy of 16 individual clones of the same genotype), 2) away (under the canopy of another genotype, again a stand of 16 individual clones), and 3) under a diverse canopy of all 16 different genotypes growing together. Home and away pairs of genotypes varied in terms of genetic relatedness and geographic distance between origins, but all leaf litter decomposition took place within the 16-ha common garden.

Results/Conclusions

We found that the home versus away treatment influenced the five different genotypes in different ways. In some cases litter decomposed faster in away environments and in other cases litter decomposed slower. In addition, in some cases the single genotype environments resulted in faster litter decomposition than genetically diverse environments, in other cases decomposition was slower. The mechanisms for these differences are varied, but include aboveground productivity and shading of the decomposition environment, differences among microbial communities in stands dominated by different genotypes or diverse genotypes, and interactions among leaf litter chemicals. The variability among genotypes in both initial litter chemistry and their responses to the decomposition environment adds another layer to genes-to-ecosystems research and provides needed insight into genetic by environment and genotype by genotype interactions.