A test of simultaneous resource and pollen limitation in Stylidium armeria.

This study tests the Haig-Westoby model, which predicts that seed output will be limited simultaneously by pollen and resources when plants optimally distribute their reproductive investment. The test was conducted over 2 yr using Stylidium armeria in a factorial design that fully crossed three pollination levels (small stigmatic loads, open pollination, and supplementation of natural loads) with three levels of resource availability (reduction through partial defoliation, unmanipulated resource conditions, and supplementation through nitrogen, phosphorus and potassium (NPK) addition). There was no evidence of pollen limitation from supplemental pollination; however, pollen reductions (to about half the normal mean stigmatic loads) sharply reduced seed output. There was no evidence of resource limitation, in that NPK addition did not, by itself, significantly elevate seed output in either year of the study, while resource reduction by defoliation lowered seed output in the second year. Simultaneous addition of both pollen and resources strongly and significantly increased seed production. These results match the direction of effects predicted by the Haig-Westoby model, and suggest that S. armeria plants at our site are at or near an equilibrium of joint limitation of seed production by pollen capture and resource availability.