No abstract

Restoration of tallgrass prairie on former agricultural land is often impeded by failure to establish a diverse native species assemblage and by difficulties with nonprairie, exotic species. High levels of available soil nitrogen (N) on such sites may favor fast-growing exotics at the expense of more slowly growing prairie species characteristic of low-N soils. We tested whether reducing N availability through soil carbon (C) amendments could be a useful tool in facilitating successful tallgrass prairie restoration. We added 6 kg/m 2 hardwood sawdust to experimental plots on an abandoned agricultural field in the Sandusky Plains of central Ohio, United States, increasing soil C by 67% in the upper 15 cm. This C amendment caused a 94% reduction in annual net N mineralization and a 27% increase in soil moisture but had no effect on total N or pH. Overall, plant mass after one growing season was reduced by 64% on amended compared with unamended soil, but this effect was less for prairie forbs (À34%) than for prairie grasses (À67%) or exotics (À62%). After the second growing season, only exotics responded significantly to the soil C amendment, with a 40% reduction in mass. The N concentration of green-leaf tissue and of senescent leaf litter was also reduced by the soil C treatment in most cases. We conclude that soil C amendment imparts several immediate benefits for tallgrass prairie restoration--notably reduced N availability, slower plant growth, and lower competition from exotic species.

Delayed autonomous self‐pollination allows outcrossing to occur while also ensuring that seeds are produced in the absence of pollen vectors. We investigated variation in the efficacy of this pollination mechanism in populations of Hibiscus laevis. Recurvature of stylar branches occurred after 1 d of anthesis, and in plants from Ohio, Illinois, Arkansas, and Oklahoma this behavior resulted in autonomous selfing (surprisingly, stylar movement was facultative in that it did not take place when the stigmas were already pollinated). In contrast to these more northern populations, the distance between anthers and stigmas was too great to allow autonomous selfing in plants from Texas, Mississippi, and Louisiana. Greenhouse studies of plants from Ohio demonstrated that autonomous selfing resulted in an average of 29.5 seeds per flower, as compared to 59.9 seeds per flower from hand‐pollination of stigmas with self pollen. In an assessment of the possible significance of this selfing mode, emasculated flowers did not set significantly fewer seeds in a natural stand in Ohio, suggesting that few seeds resulted from autonomous selfing at that site. Modest inbreeding depression was detected at this population. Our results suggest that delayed autonomous selfing is more common in northern populations, where it may facilitate population establishment and persistence at times when pollinators are scarce.

Differences in pollen tube growth rates (certation) between heterospecific (foreign) and conspecific pollen may strongly influence whether hybrid offspring are produced after mixed pollen loads are delivered to a stigma. For both members of a sympatric species pair, Hibiscus moscheutos and H. laevis, pollination by pure loads of foreign pollen resulted in fruit set that was not significantly different from conspecific pollination, indicating that pure loads of foreign pollen could readily result in hybrid offspring. However, the number of seeds per fruit from pure foreign pollinations was significantly less than that of pure conspecific pollination. Simultaneous mixed pollination resulted in a proportion of hybrid seeds (detected by an electrophoretic marker enzyme) that was significantly lower than expected based upon the capacity of foreign pollen to effect fertilization when applied in pure pollinations. After these 50/50% pollen mixtures were applied to stigmas, 8.0 and 7.4% hybrids were produced when H. moscheutos and H. laevis were the ovule parents, respectively. For these Hibiscus species, pollen competition appears to function as a barrier to hybridization that is of moderate intensity compared with similar barriers occurring between other recently studied sympatric species pairs.

Hibiscus dasycalyx is a rare eastern Texas endemic that shares key morphologic traits with two widespread sympatric congeners, H. laevis and H. moscheutos. Working with an initial hypothesis that these taxa were possible ancestors of the endemic through diploid hybridization speciation, a ten‐enzyme electrophoretic screening was conducted to determine whether H. dasycalyx exhibits a hybrid genetic profile. The three taxa share predominant alleles for all enzyme systems except ADH, GPI, and PGM, for which H. dasycalyx and H. laevis display generally identical banding patterns that differ from those of H. moscheutos. An analysis of diagnostic leaf shape and calyx pubescence features of eastern Texas H. laevis revealed substantial variation among populations, including forms intermediate between typical H. laevis and H. dasycalyx. In greenhouse hand‐pollinations, H. dasycalyx plants serving as ovule parents were freely compatible with H. laevis. Due to the possession of identical isozymes, the occurrence of morphologically intermediate populations where the species cooccur and exhibit an apparent lack of reproductive isolation, the endemic H. dasycalyx may be best regarded as a subspecies or variety of H. laevis, the taxon it most resembles morphologically. Conservation efforts should consider the potential effects of contaminating gene flow with typical H. laevis.