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This study examined growth and seed yield of three perennial species, Desmanthus illinoensis (Illinois bundleflower, a legume), Leymus racemosus (mammoth wildrye, a C 3 grass), and Tripsacum dactyloides (eastern gamagrass, a C 4 grass), in monoculture, biculture, and triculture treatments, on two soils differing in initial fertility, and over 5 yrs. There were significant effects of site, treatment, and year on both aboveground biomass and seed yield. On average, bundleflower monoculture produced the greatest aboveground biomass, although the three-species mixture produced the peak biomass (814 g/m 2 ) in any given year. Among treatments, Illinois bundleflower monoculture yielded the most seed (maximum=122 g/m 2 ). Overall, biomass and seed yield were higher at the more fertile Site 1, but species differed in their dependence on soil fertility. Among species, bundleflower performed fairly independently of soil fertility, wildrye grew poorly on the less fertile soil, and gamagrass persisted at both sites although it grew less well at Site 2. In most cases, mixtures produced as well as the best-yielding monoculture. In 26 of 30 instances, biomass relative yield totals (RYTs) were statistically >1.0 and, in 19 of 21 cases, RYT for seed yield was statistically >1.0. In general, the overyielding effect appeared stronger at Site 2 than at Site 1, with seed yield RYT appearing to increase with time at Site 2. The results show that the seed yield of perennials can be high, and that some species can persist in mixture for several years. The data for 5 yrs point to the need to follow long-term patterns of yield and interspecific interactions within perennial grain polycultures in order to maintain species diversity and to make reasonable predictions. |
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