Species effects on resource supply rates: do they influence competitive interactions?

We examined two ways in which species effects on nitrogen supply rates could influence species competitive interactions and lead to coexistence between species with very different patterns of resource use. First, through differential effects on the rates of resource cycling, species may modify resources in divergent directions to better tolerate neighbors under self-modified conditions (tolerance modification). Second, plant-induced shifts in resource supply could enhance the degree to which species can suppress individuals (suppression modification). We addressed these hypothesized mechanisms in the non-successional moist meadow alpine tundra, which is codominated by Acomastylis rossii, a slow-growing roseaceous forb with high nutrient retention that is associated with slow rates of nitrogen supply, and Deschampsia caespitosa, a graminoid with rapid growth potential that is associated with fast rates of nitrogen (N) supply. Neighbors inhibited Acomastylis transplants (the species with high N retention) less than Deschampsia transplants (the species with rapid potential growth) in both neighborhood types, and hence neighborhood type did not influence species tolerance to neighbors. Likewise, Acomastylis neighborhoods inhibited transplant growth more than Deschampsia neighborhoods regardless of transplant species identity. When N supply rates were enhanced through repeated N additions in the two neighborhood types or in the presence of each species’ litter, the inhibitory effects associated with Acomastylis were offset to a greater degree than those associated with Deschampsia, as predicted by the suppression modification. These effects appeared to be density or size-dependent, with few effects observed at low density. Our results suggest that species effects on supply rates may influence competitive interactions, particularly if these effects are complemented by other sources of temporal or spatial variation such as pulses in resource availability. This revised version was published online in June 2006 with corrections to the Cover Date.