OIKOS 90: 311–320. Copenhagen 2000
Investigating the relationship between neighbor root biomass and
belowground competition: field evidence for symmetric competition
belowground
James F. Cahill, Jr. and Brenda B. Casper
Cahill, J. F., Jr. and Casper, B. B. 2000. Investigating the relationship between
neighbor root biomass and belowground competition: field evidence for symmetric
competition belowground. – Oikos 90: 311 – 320.
Little is known about how small-scale variation in neighbor biomass can influence
the strength of root competition experienced by an individual plant. In this study,
modified root exclusion tubes were used to vary the accessibility of the soil space
surrounding Amaranthus retroflexus target plants to the neighboring plants. A
gradient of root accessibility was created by drilling varying numbers of holes into
standard root exclusion tubes, made of 15 cm diameter PVC pipe. Belowground
competitive intensity, defined as biomass reduction due to root interactions alone,
relative to growth in the absence of neighbors, was then measured along the resulting
gradient of neighbor root densities. At low neighbor root abundances the strength of
belowground competition was proportional to neighbor root biomass, consistent with
prior evidence that belowground competition is symmetric. If belowground competition were asymmetric, neighbor roots should have had little effect on target plants
when they are rare relative to those of the target plant. At higher neighbor root
abundances, belowground competitive intensity should increase rapidly. The strong
relationship found between neighbor root biomass and belowground competitive
intensity suggests relatively small variations in root biomass could lead to large
variations in belowground competition. Reduced belowground competition in areas
with low root biomass could have important implications for the establishment and
growth of poor belowground competitors, suggesting a...