Is Physical Space a Soil Resource?

We examined effects of differing levels of physical belowground space on the performance of several colonizing annual species in two experiments. Individual plants were grown in controlled greenhouse conditions under an experimental regime that independently varied nutrient addition rates and the physical space available for the deployment of roots while maintaining water supply at adequate rates. When equal amounts of nutrients were given, plants grown in greater volumes had greater vegetative growth and often higher reproductive output. Species differed in patterns or response to physical space in both vegetative and reproductive parameters. Abutilon theophrasti given larger soil volumes increased allocation to reproductive tissues relative to vegetative tissues. Setaria faberii, however, responded through a phenological shift: earlier flowering and greater reproductive output in smaller volumes. These results indicate that physical underground space, released in the formation of certain types of gaps, may influence the performance of colonizing annuals beyond providing access to other soil resources, such as nutrients and water.