PHOTOSYNTHETIC RATES AND VEGETATIVE PRODUCTION OF SORGHASTRUM-NUTANS IN RESPONSE TO COMPETITION AT 2 STRIP MINES AND A RAILROAD PRAIRIE

The effect of differing environmental conditions on competition for re sources was investigated by a comparison of net photosynthetic rate (P -N) and vegetative production of Indiangrass [Sorghastrum nutans (L.) Nash.] at two strip mine sites with differing reclamation histories, a nd a railroad prairie site where this species occurs naturally. The tr eatment for a competition experiment consisted of tying back all speci es of neighboring plants around a target plant, and measuring its P-N and vegetative performance during the growing season. Environmental va riables at each site were also measured during the growing season. Soi l bulk density and pH were higher at the two mine sites than at the pr airie site, and soil texture, nutrients, and water potential were diff erent at each of the three sites. P-N of target plants compared closel y among the three sites, and were lowest for plants at the railroad pr airie. The competition experiment indicated that lower canopy leaves w ere most affected by competition for photosynthetically active radiati on (PAR) at all sites. Significant differences in P-N of upper canopy leaves were found between treatment and control plants at one of the m ine sites. This site had higher soil water potentials and higher soil levels of P and K than the other mine site or the railroad prairie. Ta rget plants at the other mine site experienced a low competition for P AR, likely due to lower soil moisture availability and therefore lower aboveground productivity. The largest differences in P-N and irradian ces between upper and lower canopy leaves occurred in target plants wi th neighbors at the railroad prairie, likely due to inter-specific com petition. Vegetative production of the target plants also reflected th e environment at each site, but did not reflect P-N differences betwee n treatments. S. nutans is well adapted to the varying environment at these three sites, and aboveground competition for radiant energy was probably not as limiting for this C4 grass as belowground competition.