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Abstract

This study tests the predictions of two different hypotheses of trophic organization the 'bottom-up' and 'top-down' hypotheses respectively using plants in the boreal forest understory. The experiment manipulated plant resource levels by fertilization and consumer levels (vertebrate herbivory rate) using exclosures, and monitored the response of transplanted seedlings and the leaf area of the established vegetation. Survival and growth transplants was poorest at the highest fertilizer levels; a result not predicted by either 'bottomup' or 'top-down'. Herbivore exclosures had no significant effects on survival or growth at low or moderate herbivore densities; fertilizer addition did not increase leaf area. These results suggest that the resources added by fertilization and that the herbivores excluded were not limiting at these herbivore densities. At high herbivore densities transplant survival and growth was consistently greater inside exclosures which lends support to the 'top-down' hypothesis for seedling survival and performance, but leaf area did not significantly respond to either treatment but was greater inside exclosures especially when fertilized. A model of trophic relations in the boreal forest between understory plants, their resources and their consumers should only include herbivores as a limiting factor when their densities are abnormally high and even then the 'top-down' hypothesis is supported only from transplant data and not from existing vegetation.