Shifts in seed size across experimental nitrogen enrichment and plant density gradients

We tested the hypotheses that increased soil resource availability selects for larger seeded plants by: (1) increasing community seed density and seedling competition, (2) increasing light competition, which favours larger seeded species if their additional seed resources are allocated to shoots. We also tested the hypothesis that plants respond to increased nutrient availability by producing larger seeds. In a controlled environment experiment, we grew monocultures of five species of co-occurring annual plant species at two N addition rates and measured seed size and the number of seeds produced. In a second experiment, we sowed seed of the five species together, manipulated nitrogen (N) addition rate and community seed density and measured species performance. In the first experiment, elevated N addition increased seed size, but only in larger seeded species. In the second experiment, high N addition increased community seed production. However, contrary to our hypothesis small seeded species were selected under high seed density, possibly because they germinated and grew more rapidly thus enabling them to pre-empt limiting resources. Larger seeded species were favoured by high N addition, as hypothesised. However, our data suggest that this was due to their seed reserves boosting survival below the denser canopies of fertile conditions rather than via competitive advantages. Our results point to a largely overlooked role of recruitment in determining community response to fertilisation. Although our results may not generalise to all plant communities they suggest that seed traits play a role in community response to both the direct effect of N addition and the stimulation of seed density caused by increased productivity. These results are also consistent with the view that the advantage of large seed size is not a competitive one, but greater survival in the face of environmental hazards.