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Understanding patterns of soil C and N accumulation in restored prairies is necessary to determine the sequestration potential of these systems. Estimates of C and N accumulation following prairie restoration are generally based on chronosequence studies. However, few studies have compared a chronosequence approach to direct measures of accumulation over time. Restored prairies in the Cowling Arboretum of Carleton College, Northfield, MN, were established annually from 1995 to 2007. In 2000 and 2010–2011, prairies were sampled for soil %C and %N and soil bulk density, allowing for a comparison of the effects of prairie age using a chronosequence approach to direct measures of C and N accumulation rates. Soil C and N content increased in all restorations, although there was no effect of field age across the chronosequence on C or N pools. However, rates of soil C and N accumulation decreased with restoration age, suggesting a rapid increase in the years immediately following agricultural conversion. This rapid accumulation was likely due to the turnover of annual and biennial species that dominated plant cover in the early in succession. This suggestion was supported by an observed increase in δ¹³C values with increasing prairie age. Thus, contributions from early successional species may result in rapid recovery of soil pools following restoration from agriculture. Direct measures of soil C and N accumulation may be necessary when initial variability among sites limit the ability of a chronosequence approach to detect significant changes in soil C and N content over short time periods.

The once-extensive tallgrass prairie community of North America has been reduced to small rem- nants, many of which are surrounded by intensive corn (Zea mays) agriculture. We investigated adult corn- rootworm beetles (Chrysomelidae: Diabrotica spp.), important pests of corn, on sunflowers (Asteraceae: He- lianthus spp.) in prairie remnants in southeast Minnesota. Large numbers of beetles invaded the prairie from surrounding corn fields in late summer. D. barberi and D. virgifera were captured on sticky traps in all loca- tions in the prairie, but abundance was much greater near the edge adjacent to corn. We observed D, barberi (but not D. virgifera) feeding extensively on sunflower pollen and occasionally on other flower parts, such as petak. Sunflowers located nearer corn fields sustained morefloral damage than those farther from om. To de- termine the effect of beetle &mage on seed set, we enclosed sunflower heads in bags with either zero, two, or four D. barberi adults. Seed set was reduced in heads enclosed with D. barberi Thus, this agriculturalpest may interfere with the successful reproduction of sunflowers and possibly otherprairie composites thatflower in late summer. Given the small size of mostprairie remnants and the abundance of thisflowerrfeeding beetle in land- scapes dominated by corn agriculture, D. barberi may affect the sustainability ofprairie plantpopulations.

Recent null models that place species ranges randomly within a bounded domain have produced controversial results. Many such geometric constraint models predict a peak in species richness in the centre of domains in the absence of underlying environmental gradients or interspecific interactions. We used two-dimensional simulation models to explore different ways that species ranges could interact with the domain boundary. In the rejection model, a randomly generated range that overlaps a domain boundary is removed from the simulation. In the reshaping model, a range that overlaps the domain boundary is reshaped so that the entire range is placed within the domain. The truncation model allows potential ranges to extend across the boundary, but only that portion of the range within the domain is included in the realized range. Both rejection and reshaping models produced a drop in species richness near domain boundaries, though the effect was less pronounced in the reshaping model. Our truncation model did not produce any spatial pattern in species richness. Thus the random placement of species ranges within a bounded domain does not necessarily lead to a mid-domain effect. Range truncation is consistent with bioclimate envelope models, which can successfully predict a species range in response to the availability of appropriate climate conditions. We argue that such flexible range sizes are more realistic than the assumption that range size is an unvarying characteristic of a species. Other range characteristics, including size and shape, can change near domain boundaries in the null models, including the truncation model. A broader consideration of range characteristics near domain boundaries could be productive.

Changes in the plant community and ecosystem properties that follow the conversion of agriculture to restored tallgrass prairies are poorly understood. Beginning in 1995, we established a species-rich, restored prairie chronosequence where ∼3 ha of agricultural land have been converted to tallgrass prairie each year. Our goals were to examine differences in ecosystem properties between these restored prairies and adjacent agricultural fields and to determine changes in, and potential interactions between, the plant community and ecosystem properties that occur over time in the restored prairies. During the summers of 2000–2002, we examined species cover, soil C and N, potential net C and N mineralization, litter mass, soil texture, and bulk density across the 6- to 8-year-old prairie chronosequence and adjacent agricultural fields in southern Minnesota. We also established experimentally fertilized, watered, and control plots in the prairie chronosequence to examine the degree of nitrogen limitation on aboveground and belowground net primary production (ANPP and BNPP). Large shifts in functional diversity occurred within three growing seasons. First-year prairies were dominated by annuals and biennials. By the second growing season, perennial native composites had become dominant, followed by a significant shift to warm-season C4 grasses in prairies ≥3 yr old. Ecosystem properties that changed with the rise of C4 grasses included increased BNPP, litter mass, and C mineralization rates and decreased N mineralization rates. ANPP increased significantly with N fertilization but did not vary between young and old prairies with dramatically different plant community composition. Total soil C and N were not significantly different between prairie and agricultural soils in the depths examined (0–10, 10–20, 20–35, 35–50, 50–65 cm). We compared the results from our species-rich prairie restoration to published data on ecosystem function in other restored grasslands, such as Conservation Reserve Program (CRP) and old-field successional sites. Results suggest that rapid changes in functional diversity can have large impacts on ecosystem-level properties, causing community- and system-level dynamics in species-rich prairie restorations to converge with those from low-diversity managed grasslands.

In some leaf-cutting ant species, minim workers ride on the fragments of leaves as they are carried back to the nest from the cutting site. There is convincing evidence that these ‘‘hitchhikers’’ can protect the leaf carriers from attack by phorid (Diptera: Phoridae) parasitoids, but we consider the possibility of other functions for the hitchhiking behavior. It has been hypothesized that the hitchhikers (1) feed on leaf sap from the edges of the cut leaves; (2) ride back to the nest to save energy; (3) get caught on the fragments as they are cut, and hitchhike because they cannot (or will not) get off; and (4) begin the process of preparing the leaf to enter the fungal gardens in the nest, perhaps by removing microbial contaminants. We observed hitchhikers of Atta cephalotes in 14 nests at the La Selva Biological Station in Costa Rica. There was no difference in the proportion of leaf carriers with hitchhikers between day and night. Because the nests we observed were largely nocturnal, more than 90 percent of the hitchhiking occurred at night. The phorid parasitoids are usually considered to be diurnal, so the preponderance of nocturnal hitchhiking suggests other functions in addition to parasitoid defense. Hitchhikers spent more time in the defensive head-up posture during the day, but spent more time in the head-down posture at night. The head-down posture may indicate cleaning or other leaf preparation. The hitchhikers were never observed feeding on sap. Hitchhikers frequently got onto and off of the fragments, and so they were not ‘‘marooned.’’ Few hitchhikers rode all the way back to the nest and were often moving on the leaf fragment; these observations make the energy conservation hypothesis less likely, although we cannot reject it. We conclude that parasitoid defense is an important function of hitchhiking but also that there are probably other functions when parasitoids are absent. Based on available data, the most likely possibility is preparation of the leaf fragment before it enters the nest.

Global surface temperatures are expected to increase by several degrees in the next century, with potentially large but poorly understood impacts on ecological interactions. Here we propose potential effects of increased temperatures on ecologically dominant New Zealand grasses (Chionochloa spp.) that mass flower and mast seed. Twenty-two years’ data from five masting Chionochloa species in New Zealand showed that the cue for heavy flowering was unusually high temperature in the summer of the year before flowering. Attack by predispersal insect seed predators was much reduced in mast years, apparently because predator populations were satiated. Increased temperatures would greatly decrease interannual variation in Chionochloa flowering, allowing seed predator populations to increase and potentially to devastate the seed crop annually. Similar responses are likely in masting species worldwide. This previously unrecognized effect of global warming could have widespread impacts on temperate ecosystems.

It has been proposed that some wind-pollinated plants have the necessary conditions for an optimal sex allocation that is male biased, though there are few data that address this prediction. We determined that two prairie grass species ( Andropogon gerardii and Sorghastrum nutans) had reproductive characteristics that theoretically would result in a male-biased allocation: both species were self-incompatible and neither species had increased seed set after supplemental hand pollination. The relative allocation to pollen and seed production was measured in terms of biomass, energy, nitrogen, phosphorus, potassium, magnesium, and calcium. Sex allocation in A. gerardii was significantly male biased (from 60 to 89% male) when measured in currencies of biomass, energy, potassium, and calcium; there was no significant bias in the sex allocation (from 49 to 57% male) when measured in currencies of nitrogen, phosphorus, and magnesium. Sex allocation in S. nutans was significantly male biased (from 69 to 81% male) for all currencies except phosphorus (61% male). This is the first evidence for male-biased sex allocation in any plant or animal hermaphrodite. Though the necessary conditions may be uncommon, male-biased allocation may be found in other species with similar reproductive biology.

SUMMARY Intraspecific variation in pollen production and pollen size were studied within a population of Chionochloa pallens, an alpine snow-tussock grass of New Zealand. I sampled plants from two sites at the altitudinal extremes (1620 and 1070 m) of the distribution of C. pallens on Mt. Hutt, South Island. There was a highly skewed distribution of pollen production among plants; the largest 10% of plants produced over a third of the pollen. In contrast to some other species, pollen production in C. pallens can be accurately estimated by simple measures such as tussock area and number of flowering shoots. Other components of yield (number of spikelets per shoot, number of florets per spikelet, and average anther length) varied significantly among individuals, but these characteristics had a small impact on total pollen yield. There was continuous variation in pollen size among individuals; between the plants with the largest and the smallest pollen grains, there was a 21 % difference in pollen diameter and a 76% difference in pollen volume. Interplant differences in pollen size have been found in a number of plants, although the biological significance of this variation is unknown. There were few differences in pollen characteristics between sites. Plants at the low-altitude site had slightly greater pollen yield because total plant size was greater.

Five species of Bromus were tested for self-fertility when bagged. Bromus inermis was selfincompatible. Bromus tectorum, B. kalmEi, B. ciliatus, and B. Iatiglumis were capable of selffertilization. Further evidence from flowering behavior suggested that B. tectorum almost always self-pollinated and that the remaining self-fertile species sometimes outcrossed. Florets in different positions on a spikelet varied in the proportion that set seed. Open-pollinated B. inermis plants had lower seed set than the self-fertile species, in agreement with the pattern in a variety of hermaphroditic plants. THE RATE of self-pollination can have a profound effiect on the reproductive and population biology of hermaphroditic plants. In comparison with closely related outcrossing taxa plants that have high rates of self-fertilization can be expected to have lower rates of gene flow via pollen, smaller neighborhood size more homozygosity, and greater genetic differentiation and ecotypic specialization among populations (Stebbins, 1957; Allard, Jain and Workman, 1968; Schoen, 1982; Layton and Ganders,1984). Ornduff(1969) has listed many other correlates of breeding system. In addition, Sutherland and Delph (1984) have recently shown a relationship between the breeding system and the rate of seed set in many difEerent plant species. To test the generality of these patterns and as part of a study of the reproductive biology of several species of bromegrass (Bromus L.) I investigated the breeding system and pattern of seed set in five congeneric species that show wide variation in breeding system. Like most grasses, species of the genus Bromus are wind pollinated. The flower is much reduced and, with the two subtending bracts is referred to as a floret; florets are grouped together on the plant into spikelets. Each floret

Variation in pollen production was measured within five hermaphrodite species of bromegrass (Bromus). Anther length is an excellent predictor of pollen production in this genus (R2 = 0.97). Anther length varied considerably within each of the species, both among and within individual plants. Within plants, most of the variation occurred among florets within spikelets; florets in upper spikelet positions were smaller and produced less pollen. In B. inermis, pollen production was decreased by defoliation and increased in shoots that grew on thatching ant (Formica obscuripes) mounds. Whole-shoot pollen yield was determined by spikelet number, number of florets per spikelet, and pollen production per floret. All of these yield components must be considered in attempts to estimate pollen production accurately. IN CONTRAST to the voluminous literature on intraspecific variation in fruit and seed yield, relatively little is known about variation in pollen production (Bertin, 1988). Yet pollen yield can have as much impact on fitness as seed yield, and recent empirical and theoretical studies have emphasized the importance of paternal fitness in the evolution of attractive structures in angiosperms (e.g., Bell, 1985; Charnov and Bull, 1986; Stanton, Snow, and Handel, 1986). As part of a study of the reproductive biology of bromegrass (Poaceae: Bromus L.), I measured intraspecific variation in pollen production in five Bromus species. Lloyd (1980) suggested that a plant's maternal allocation is affected by a series of adjustments through the flowering season; total seed production could be modified by changes in flower number, proportion of pistillate flowers, number of ovules per flower, fruit and seed set, and seed weight. Similarly, serial adjustment of a plant's pollen Received for publication 28 December 1987; revision accepted 16 June 1988. E. J. Cushing, L. F. Delph, D. G. Lloyd, P. A. Morrow, and C. J. Webb made helpful comments on various versions of the manuscript. Financial assistance was provided by several bodies at the University of Minnesota, including the Dayton-Wilkie Fund, the Field Biology Program, the Graduate School (which awarded a Doctoral Dissertation Fellowship and a Doctoral Dissertation Special Grant), and the Department of Ecology and Behavioral Biology. This work was also partially supported by NSF Grant DEB8114302-A02 to D. Tilman and J. Tester for Long-Term Ecological Research at Cedar Creek Natural History Area. Partial support while preparing the manuscript was provided by NSF Grant INT-8603569 and by a grant from the Miss E. L. Hellaby Indigenous Grasslands Research Trust of Dunedin, New Zealand. 2 Current address: Department of Biology, Carleton College, Northfield, MN 55057. production is possible by means of changes in flower number, proportion of staminate flowers, number of stamens per flower, and number of pollen grains per stamen. Bromus flowers are hermaphrodite and always contain three stamens; thus, the remaining options for adjustment of pollen yield are flower number and pollen grains per stamen. I measured these parameters in field populations of five species, and calculated their relative importance in determining pollen yield. MATERIALS AND METHODS-Bromus tectorum L. plants were collected in 1982 from a disturbed site near the Botany Greenhouse of the University of Minnesota, Minneapolis. Bromus inermis Leyss., B. kalmii Gray, and B. ciliatus L. were collected in 1983 from populations in the Cedar Creek Natural History Area, about 50 km north of Minneapolis. The B. latiglumis Shear (Hitchc.) population was sampled in 1982 in Fort Snelling State Park, St. Paul. Voucher specimens of the five species have been placed in the University of Minnesota Herbarium, St. Paul. The five Bromus species differ in a number of reproductive characteristics, including average anther length, pollen size, and sex allocation (McKone, 1985, 1987). The species span a broad range of outcrossing rates (McKone, 1985), from obligate outcrossing in B. inermis, through partial selfing in B. kalmii, B. ciliatus, and B. latiglumis, to nearly complete selfing in B. tectorum. Bromus tectorum is an annual; the other four species are perennial. Pollen production was estimated for individual flowering shoots (culms). I will refer to shoots as plants; except for B. inermis, the species studied are not rhizomatous, and only