Our previous work in California annual grasslands indicates that nitrogen released from seedling thinning accounts for 37-63% of annual internal N cycling, suggesting that seed densities could have profound effects on plant N supply. Restoration seeding rates are often orders of magnitude lower than natural densities, which could be detrimental to plant establishment if seedling thinning is a significant source of plant N. In order to test the impact of seedling thinning on stand biomass and N uptake, we grew California annual grasses at typical early-season densities (80,000 seeds/m², allowing for seedling thinning), and at end-of-season densities (8,000 seeds/m², typical densities after thinning has been completed). These density treatments were performed in monocultures of the two dominant grasses, Avena barbata and Bromus hordeaceus, and in mixtures of the two species.

Results/Conclusions

Regardless of species composition, higher seed densities doubled end-of-season stand biomass and N content. These patterns were seen at a whole-plant level, as well as individually for shoots, roots, and seeds. These patterns suggest that plant N limitation may be diminished by the N released from dying seedlings in high-density neighborhoods. Density treatments did not impact end-of-season live tissue C/N ratios, but significantly decreased litter C/N values in monocultures. Together, these results suggest that increased planting density not only increases the current season’s N availability and plant productivity, but will also likely enhance plant biomass and N content in subsequent growing seasons through two different pathways. Seedling thinning inputs are likely to be higher in the 2^nd growing season due to higher seed density and N content, and litter decomposition inputs should also increase due to both doubled litter N content, and higher litter quality. While increasing seed densities would substantially increase restoration costs, it would likely yield long-term benefits in plant establishment.