Symbiotic nitrogen (N)-fixing trees are rare in temperate forests, accounting for 1% or less of tree basal area. This seems counterintuitive given that many temperate forests are N limited, which should favor N-fixing trees. One explanation for this pattern is that N-fixing trees are more susceptible to herbivory than are non-fixing trees because they often have high N content. On the contrary, if herbivory is a major selective agent, then N-fixing trees should be able to invest their N in defense against herbivores. Given that the mechanism underlying the need for defense relates to N content rather than N fixation itself, it would help be instructive to investigate plant defenses across a gradient of N availability. Here, we studied investment in a physical defense—thorns—on the most common N-fixing tree in the United States, Robinia pseudoacacia. We grew Robinia seedlings in a greenhouse and Robinia saplings in a field setting in Black Rock Forest, New York. In the greenhouse we inoculated half of the seedlings with their symbiotic bacteria, and added N fertilizer from 0.3 to 40 g N m-2 yr-1. In the field we grew saplings for four years at 0, 100, and 150 kg N ha-1 yr-1.
Results/Conclusions
In the greenhouse, Robinia seedlings were N limited in the low N treatments but not in the high N treatments. Their tissue N content reflected their limitation status: They had low tissue N content in the low N treatments and high tissue N content in the high N treatments. Investment in thorns rose with N fertilization and varied with inoculation. At our low levels of N addition (0.3, 0.9, 1.5, 3.3, 6.6, and 10 g m-2 yr-1), uninoculated Robinia seedlings invested an average of 0.15% of their tissues in thorns. At the higher levels of N addition (20, 30, and 40 g N m-2 yr-1), uninoculated Robinia seedlings invested 0.5% of their biomass in thorns. The inoculated seedlings responded much less to N fertilization, averaging 0.3% of their biomass without much change across fertilization levels. In the field, saplings in our low N treatment (no N added) invested 0.5% of their biomass in thorns, whereas those in the fertilization treatments invested 0.2-0.3% of their biomass in thorns. Overall, we found evidence that N supply, either through the soil or from fixation, enhanced investment in thorns, consistent with the idea that N-fixing trees use N to defend themselves against herbivory.