Dominance Of C4 Grasses Research Articles

Mycorrhizal‐herbivore interactions and the competitive release of subdominant tallgrass prairie species

Abstract Plant‐microbial‐herbivore interactions play a crucial role in the structuring and maintenance of plant communities and biodiversity, yet these relationships are complex. In grassland ecosystems, herbivores have the potential to greatly influence the survival, growth and reproduction of plants. However, few studies examine interactions of above‐ and below‐ground grazing and arbuscular mycorrhizal (AM) mycorrhizal symbiosis on plant community structure. We established experimental mesocosms containing an assemblage of eight tallgrass prairie grass and forb species in native prairie soil, maintained under mycorrhizal and nonmycorrhizal conditions, with and without native herbivorous soil nematodes, and with and without grasshopper herbivory. Using factorial analysis of variance and principal component analysis, we examined: (a) the independent and interacting effects of above‐ and below‐ground herbivores on AM symbiosis in tallgrass prairie mesocosms, (b) independent and interacting effects of above‐ and below‐ground herbivores and mycorrhizal fungi on plant community structure and (c) potential influences of mycorrhizal responsiveness of host plants on herbivory tolerance and concomitant shifts in plant community composition. Treatment effects were characterized by interactions between AM fungi and both above‐ground and below‐ground herbivores, while herbivore effects were additive. The dominance of mycorrhizal‐dependent C4 grasses in the presence of AM symbiosis was increased (p < 0.0001) by grasshopper herbivory but reduced (p < 0.0001) by nematode herbivory. Cool‐season C3 grasses exhibited a competitive release in the absence of AM symbiosis but this effect was largely reversed in the presence of grasshopper herbivory. Forbs showed species‐specific responses to both AM fungal inoculation and the addition of herbivores. Biomass of the grazing‐avoidant, facultatively mycotrophic forb Brickellia eupatorioides increased (p < 0.0001) in the absence of AM symbiosis and with grasshopper herbivory, while AM‐related increases in the above‐ground biomass of mycorrhizal‐dependent forbs Rudbeckia hirta and Salvia azurea were eradicated (p < 0.0001) by grasshopper herbivory. In contrast, nematode herbivory enhanced (p = 0.001) the contribution of Salvia azurea to total biomass. Synthesis. Our research indicates that arbuscular mycorrhizal symbiosis is the key driver of dominance of C4 grasses in the tallgrass prairie, with foliar and root herbivory being two mechanisms for maintenance of plant diversity.

Orbital Influence on Precipitation, Fire, and Grass Community Composition From 1.87 to 1.38 Ma in the Turkana Basin, Kenya

The Turkana Basin in northern Kenya and southern Ethiopia has yielded hundreds of hominin fossils and is among the most important localities in the world for studying human origins. High resolution climate and vegetation reconstructions from this region can elucidate potential linkages between hominin evolution and environmental change. Microcharcoal and phytoliths were examined from a 216 m (1.87–1.38 Ma) drill core (WTK13), which targeted paleo-Lake Lorenyang sediments from the Nachukui Formation of the Turkana Basin. A total of 287 samples were analyzed at ∼32–96 cm intervals, providing millennial-scale temporal resolution. To better understand how basin sediments record fire and vegetation from the watershed, the paleorecord was compared with nine modern sediment samples collected from Lake Turkana along a transect of increasing distance from the 1978 to 1979 shoreline. This included vegetation surveys and phytolith production data for species from areas proximal to the basin. We found that phytolith and microcharcoal concentrations decreased predictably moving off shore. However, phytoliths from plants sourced in the Ethiopian Highlands increased moving off shore, likely the result of increased exposure to the Omo River sediment plume. In our down-core study, microcharcoal was well-preserved but phytolith preservation was poor below ∼60 m (∼1.50 Ma). Spectral analysis revealed that microcharcoal often varied at precessional (∼21 kyr) periodicities, and through a correlation with δDwax, linked orbitally forced peaks in precipitation with elevated fire on the landscape. Phytoliths revealed that alternating mesic C4 versus xeric C4 grass dominance likely varied at precessional periodicities as well, but that grass community composition was also mediated by basin geometry. Two high eccentricity intervals of particularly high amplitude and abrupt environmental change were centered at ∼1.72 and 1.50 Ma, with the intervening period experiencing high fire variability. With the switch from lacustrine to fluvial-deltaic deposition at the core site by 1.5 Ma, mesic C4 grasses dominated and fire activity was high. This upper interval correlated to the time interval from which Nariokotome Boy (Homo erectus/ergaster) was discovered 3 km east of our drill site. Phytoliths indicated a seasonally wet and open landscape dominated by xeric C4 grasses, sedges, and other herbaceous plants.

Carbon isotopic composition of plant waxes, bulk organics and carbonates from soils of the Serengeti grasslands

The carbon isotopic composition of different carbon-bearing materials, organic and inorganic, is commonly used to reconstruct distributions of tropical C4 grasslands. However, no study of modern soils has yet combined carbon isotopic compositions (δ13C) of bulk soil organic matter (SOM), soil carbonate (SC), plant wax n-alkanes (alk) and n-alkanoic acids (acid) to allow for comparison between all four materials. Here, we studied carbonate-precipitating, grassland soils across the Serengeti ecosystem, Tanzania to directly compare δ13C values of these four materials using samples from 11 sites along a NW–SE transect and depth profiles at each location. Among the soil carbonates found at 9 sites, the δ13C results (δ13CSC mean = 0.5 ‰, 1σ = 0.8‰, n = 70 nodules collected), denote C4 grasses exclusively, but soil carbonates are absent from two of the three sites with trees. Organics were measured at all 11 sites including bulk (δ13CSOM −13.1 ± 1.8‰) and plant wax biomarkers (e.g., δ13C29alk −23.5 ± 3.1, δ13C31alk −22.2 ± 2.0, δ13C33alk −21.9 ± 1.8, δ13C28acid −19.1 ± 2.1‰, δ13C30acid −21.1 ± 2.6‰, and δ13C32acid −20.1 ± 1.7‰). δ13C values in organic materials record the dominance of C4 grasses at the sampled sites. Surface samples are more negative than those at depth at most sites, reflecting changes in atmospheric composition and surficial inputs in recent decades, and degradation at depth over centuries. We found significant correlations between each of the organic proxies, but the δ13C29alk is overly responsive to tree inputs (greater C29 concentration and 13C-depletion relative to other homologues), whereas the δ13C31alk, δ13C33alk and δ13C32acid are robust recorders of vegetation cover. Of all materials considered (plant wax, SOM and soil carbonates), plant wax n-alkanes and n-alkanoic acids have the broadest applicability, extending to non-carbonate bearing soils, and to lacustrine and marine sedimentary archives.

Landscape paleodynamics in siliciclastic domains with the use of phytoliths, sponge spicules and carbon isotopes: The case of southern Espinhaço Mountain Range, Minas Gerais, Brazil

Mountain ranges formed mainly of siliciclastic rocks in humid tropical regions, such as that of Espinhaço, Brazil, present particular landscape features related to processes not yet fully understood. These processes are mainly controlled by the response of the rocks to climate and vegetation characteristics occurring through time. The present study investigates the geomorphological dynamics leading to the formation of recurrent, relatively flat slopes, developed near quartzite outcrops, through the inference of climatic variations and insights into dominant relief processes. These analyses are based on a geomorphological approach with the support of data from phytoliths, sponge spicules and carbon isotopes. Results of field and laboratory analyses of twelve soil profiles sampled in three distinct areas (Guinda, Morrinhos and Chapadinha) show no major overall changes in the vegetation type over 14C dated time, though there are variations along the slope profiles. In Guinda and Morrinhos, phytolith stocks do not follow the pattern of decreasing with depth, varying instead according to the granulometry. In Chapadinha, the stocks decreased with depth, D/P (tree density) ratios were always low (0.06–0.29) and Bi (water stress) indices were very high (48–94%). In Guinda and Chapadinha, the phytoliths present a high degree of alteration, indicating intense erosive processes, while in Morrinhos the presence of well preserved phytoliths indicates greater geomorphological stability of the slopes. δ13C results show dominance of C4 grasses. The presence of woody species varies with slope stability but decreases with depth were observed in all areas enabling the inference that a less humid environment occurred at ca. 6000 yr cal BP and also that cerrado vegetation has predominated since then. Results showed that the applied analyses have a great potential for temporal reconstruction therefore helping the interpretation of geomorphological processes operating during paleoclimates.

Grassland fire ecology has roots in the late Miocene

That fire facilitated the late Miocene C4 grassland expansion is widely suspected but poorly documented. Fire potentially tied global climate to this profound biosphere transition by serving as a regional-to-local driver of vegetation change. In modern environments, seasonal extremes in moisture amplify the occurrence of fire, disturbing forest ecosystems to create niche space for flammable grasses, which in turn provide fuel for frequent fires. On the Indian subcontinent, C4 expansion was accompanied by increased seasonal extremes in rainfall (evidenced by δ18Ocarbonate), which set the stage for fuel accumulation and fire-linked clearance during wet-to-dry seasonal transitions. Here, we test the role of fire directly by examining the abundance and distribution patterns of fire-derived polycyclic aromatic hydrocarbons (PAHs) and terrestrial vegetation signatures in n-alkane carbon isotopes from paleosol samples of the Siwalik Group (Pakistan). Two million years before the C4 grassland transition, fire-derived PAH concentrations increased as conifer vegetation declined, as indicated by a decrease in retene. This early increase in molecular fire signatures suggests a transition to more fire-prone vegetation such as a C3 grassland and/or dry deciduous woodland. Between 8.0 and 6.0 million years ago, fire, precipitation seasonality, and C4-grass dominance increased simultaneously (within resolution) as marked by sharp increases in fire-derived PAHs, δ18Ocarbonate, and 13C enrichment in n-alkanes diagnostic of C4 grasses. The strong association of evidence for fire occurrence, vegetation change, and landscape opening indicates that a dynamic fire-grassland feedback system was both a necessary precondition and a driver for grassland ecology during the first emergence of C4 grasslands.

The stable isotope ecology of mycalesine butterflies: implications for plant–insect co‐evolution

Summary One of the most dramatic examples of biome shifts in the geological record is the rapid replacement of C3 vegetation by C4 grasses in (sub‐) tropical regions during the Late Miocene–Pliocene. Climate‐driven biome shifts of this magnitude are expected to have a major impact on diversification and ecological speciation, especially in grazing taxa. Mycalesine butterflies are excellent candidates to explore the evolutionary impact of these C3/C4 shifts on insect grazer communities. Mycalesine butterflies feed on grasses as larvae, have radiated spectacularly and occur in almost all extant habitats across the Old World tropics. However, at present, we lack a comprehensive understanding of the larval ecology of these butterflies and this hampers investigations of co‐evolutionary patterns among the geographically parallel radiations of mycalesine butterflies and the remarkable evolutionary history of their host plants. By conducting several experiments under defined environmental conditions, we demonstrate that the feeding history of mycalesine larvae on C3 and C4 grasses can be traced by analysing δ13C in the organic material of the adult exoskeleton, while values of δ18O in the adult reflect atmospheric humidity during larval development. To show the power of these isotopic proxies for ecological studies, we analysed the isotopic composition of organic material obtained from adult butterflies sampled in two extensive longitudinal surveys. We observed strong associations among the larval ecology, habitat preferences of the adult butterflies and patterns of seasonality, such that mycalesine species that inhabit open environments are more opportunistic in their host plant choice but utilize C3 grasses more frequently during the dry season. Crucially, the ability to process the less palatable C4 grasses appears to be phylogenetically clustered within mycalesine species, suggesting that novel feeding adaptations may have evolved in response to the ecological dominance of C4 grasses in open savanna habitats. A lay summary is available for this article.

Temperature is the major driver of distribution patterns for C4 and C3 BEP grasses along tropical elevation gradients in Hawai‘i, and comparison with worldwide patterns

The distribution patterns of C4 and C3 grasses in relation to climate have attracted much attention, but few studies have examined grass distributions along tropical elevation gradients. Previous studies identified either temperature, precipitation, or both variables as the major climatic factor(s) driving these distributions. Here we investigated relative dominance of C4 grasses in relation to climate along five elevation gradients in Hawai‘i. The transition temperature between C4 and C3 BEP (Bambusoideae, Ehrhartoideae, and Pooideae) grasses (where their relative dominance is equal) was determined; in our study, the subfamily Bambusoideae was not included. A worldwide synthesis of previous studies testing climatic factors and transition temperatures associated with C4 and C3 grass distributions was also carried out. Mean July maximum temperature was significantly correlated with C4 dominance along all elevation transects in Hawai‘i, while precipitation was only correlated along three transects when precipitation was correlated with temperature. A spatially explicit multiple regression model indicated that C4 relative cover was best explained by temperature. Temperature appears to be the major climatic factor shaping distribution patterns of C4 and C3 BEP grasses in Hawai‘i. According to the worldwide analysis, temperature primarily influenced grass distribution patterns more often in temperate studies (70%) than in tropical studies (45%). Degree of correlation or covariance between temperature and precipitation was rarely reported in previous studies, although this can strongly affect conclusions. C4-C3 BEP transition temperatures (mean July maximum) ranged from 18 to 21 °C in Hawai‘i; these transition temperatures are lower than those reported in temperate localities (26–31 °C), but similar to transition temperatures for other localities at tropical latitudes (21–22 °C). A warming climate is likely to shift C4 grass dominance upward in elevation, threatening higher elevation native communities by perpetuating a grass–fire cycle.

Insolation and glacial–interglacial control on southwestern African hydroclimate over the past 140 000 years

The past climate evolution of southwestern Africa is poorly understood and interpretations of past hydrological changes are sometimes contradictory. Here we present a record of leaf-wax δD and δC13 taken from a marine sediment core at 23°S off the coast of Namibia to reconstruct the hydrology and C3 versus C4 vegetation of southwestern Africa over the last 140 000 years (140 ka). We find lower leaf-wax δD and higher δC13 (more C4 grasses), which we interpret to indicate wetter Southern Hemisphere (SH) summer conditions and increased seasonality, during SH insolation maxima relative to minima and during the last glacial period relative to the Holocene and the last interglacial period. Nonetheless, the dominance of C4 grasses throughout the record indicates that the wet season remained brief and that this region has remained semi-arid. Our data suggest that past precipitation increases were derived from the tropics rather than from the winter westerlies. Comparison with a record from the Congo Basin indicates that hydroclimate in southwestern Africa has evolved in antiphase with that of central Africa over the last 140 ka.

Differential soil organic carbon storage at forb- and grass-dominated plant communities, 33 years after tallgrass prairie restoration

Dominance of C4 grasses has been proposed as a means of increasing soil organic carbon (SOC) sequestration in restored tallgrass prairies. However, this hypothesis has not been tested on long time scales and under realistic (e.g. N-limited) environmental conditions. We sampled a restoration in southern Illinois 33 years after establishment to determine the effects of varying plant communities on SOC sequestration in the top 50 cm of soil. SOC, total nitrogen (TN), and the stable isotopic composition of SOC (δ13C) were used to calculate SOC sequestration rates, N storage, and the relative contributions of C3 vs. C4 plant communities as a function of soil depth. While both a forb-dominated and a mixed forb-grass plant community showed positive sequestration rates (0.56 ± 0.13 and 0.27 ± 0.10 Mg C ha−1 yr−1, respectively), a C4 grass-dominated community showed SOC losses after 33 years of restoration (−0.31 ± 0.08 Mg C ha−1 yr−1). Soil δ13C values were significantly more negative for forb-dominated plant communities, increasing the confidence that plant communities were stable over time and an important contributor to differences in SOC stocks among transects. These results suggest that functional diversity may be necessary to sustain sequestration rates on the scale of decades, and that dominance of C4 grasses, favored by frequent burning, may lead to SOC losses over time.

Influence of grazing and fire frequency on small‐scale plant community structure and resource variability in native tallgrass prairie

In grasslands worldwide, grazing by ungulates and periodic fires are important forces affecting resource availability and plant community structure. It is not clear, however, whether changes in community structure are the direct effects of the disturbance (i.e. fire and grazing) or are mediated indirectly through changes in resource abundance and availability. In North American tallgrass prairies, fire and grazing often have disparate effects on plant resources and plant diversity, yet, little is known about the individual and interactive effects of fire and grazing on resource variability and how that variability relates to heterogeneity in plant community structure, particularly at small scales. We conducted a field study to determine the interactive effects of different long‐term fire regimes (annual vs four‐year fire frequency) and grazing by native ungulates (Bos bison) on small‐scale plant community structure and resource variability (N and light) in native tallgrass prairie. Grazing enhanced light and nitrogen availability, but did not affect small‐scale resource variability. In addition, grazing reduced the dominance of C4grasses which enhanced species richness, diversity and community heterogeneity. In contrast, annual fire increased community dominance and reduced species richness and diversity, particularly in the absence of grazing, but had no effect on community heterogeneity, resource availability and resource variability. Variability in the abundance of resources showed no relationship with community heterogeneity at the scale measured in this study, however we found a relationship between community dominance and heterogeneity. Therefore, we conclude that grazing generated small‐scale community heterogeneity in this mesic grassland by directly affecting plant community dominance, rather than indirectly through changes in resource variability.

Diminishing Spatial Heterogeneity in Soil Organic Matter across a Prairie Restoration Chronosequence

Abstract Habitat restoration resulting in changes in plant community composition or species dominance can affect the spatial pattern and variability of soil nutrients. Questions about how these changes in soil spatial heterogeneity develop over time at restoration sites, however, remain unaddressed. In this study, a geostatistical approach was used to quantify changes over time in the spatial heterogeneity of soil organic matter (SOM) across a 26‐year chronosequence of tallgrass prairie restoration sites at FermiLab, outside of Chicago, Illinois. We used total soil N and C as an index of the quantity of SOM. We also examined changes in C:N ratio, which can influence the turnover of SOM. Specifically, the spatial structure of total N, total C, and C:N ratio in the top 10 cm of soil was quantified at a macroscale (minimum spacing of 1.5 m) and a microscale (minimum spacing of 0.2 m). The magnitude of spatial heterogeneity (MSH) was characterized as the proportion of total sample variation explained by spatially structured variation. At the macroscale, the MSH for total N decreased with time since restoration (r2= 0.99, p < 0.001). The decrease in spatial heterogeneity over time corresponded with a significant increase in the dominance of the C4 grasses. At the microscale, there was significant spatial structure for total N at the 4‐year‐old, 16‐year‐old, and 26‐year‐old sites, and significant spatial structure for total C at the 16‐year‐old and 26‐year‐old sites. These results suggest that an increase in dominance of C4 grasses across the chronosequence is homogenizing organic matter variability at the field scale while creating fine‐scale patterns associated with the spacing of vegetation. Areas of higher soil moisture were associated with higher soil N and C at the two oldest restoration sites and at the native prairie site, potentially suggesting patches of increased belowground productivity in areas of higher soil moisture. This study is one of the first to report significant changes over time in the spatial structure of organic matter in response to successional changes initiated by restoration.

Aboveground Biomass Removal by Burning and Raking Increases Diversity in a Reconstructed Prairie

Abstract Prescribed spring burning often contributes to a predominance of C4 grasses and low forb abundance and is impractical at many sites, especially near development. We tested raking after mowing as an alternative to prescribed burning in a reconstructed Minnesota prairie. We also tested mowing without raking as a possible means of maintaining prairie communities. Frequency, flowering stem abundance, and cover were measured for all plant species and native functional groups (C4 grasses, C3 graminoids, forbs, legumes, and annual or biennial forbs). Mowing alone did not differ from the control in its effect on any functional groups of plants. Round‐headed bush clover (Lespedeza capitata), a legume, and Black‐eyed Susan (Rudbeckia hirta), a biennial, increased in frequency with treatments that removed biomass (i.e., fire or raking), but they did not have significantly more flowering stems. Thus, new plants established well from seed, whereas the vitality of mature plants did not change. Raking had similar effects to burning on most functional groups, although flowering stems of C4 grasses were significantly more abundant after fire than after raking. Burning reduced some C3 forbs and grasses and favored the dominance of C4 grasses. Therefore, raking after mowing in the spring provides an alternative to prescribed burning that has many of the same positive aspects as fire but does not promote aggressive C4 grasses to the same extent.

Dominance not richness determines invasibility of tallgrass prairie

Many recent studies suggest that more diverse communities are more resistant to invasion. Community characteristics that most strongly influence invasion are uncertain, however, due to covariation of diversity with competition and crowding. We examined separately the effects of species richness and dominance on invasion by an exotic legume,Melilotus officinalis, in intact, native Kansas grassland. We manipulated dominance of C4grasses by reducing their abundance (i.e. ramet densities) by ∼25 and 50%. In addition, richness was reduced by removing species that were mainly rare and uncommon as might be expected with environmental changes such as drought and fragmentation. In both years of the study (2001–2002), invasibility, measured as peak establishment ofMelilotus, was not affected by a 3‐fold reduction in species richness, nor was there an interaction between loss of species and reduced dominance on invasion. In contrast, reductions in abundance of the dominants significantly reduced invasibility of the grassland plots in both years. Because the abundance of dominants was highly correlated with measures of competition (i.e. ratio of dominant biomass to total biomass) and crowding (total stem densities), this pattern was opposite to that expected if competition were indeed limiting invasion. Rather, invasion appeared to be facilitated by the dominant species, most likely because reduced dominance increased environmental stress. Our results suggest that dominance is the key community characteristic determining invasibility, because highly competitive and space‐filling species can either enhance or reduce susceptibility to invasion depending on whether dominants create a more competitive environment or alleviate stressful conditions.

Effects of land use on 15N natural abundance of soils in Ethiopian highlands

We used the natural abundance of 15N in soils in forests, pastures and cultivated lands in the Menagesha and Wendo-Genet areas of Ethiopia to make inferences about the N cycles in these ecosystems. Since we have described the history of these sites based on variations in 13C natural abundance, patterns of δ15N and δ13C values were compared to determine if shifts of 15N correlate with shifts of vegetation. At Menagesha, a > 500-yr-old planted forest, we found δ15N values from −8.8 to +3.5‰ in litter, from −3.5 to +4.5‰ in 0–10 cm soil layer, and from −1.5 to +6.8‰ at >20 cm soil depth. The low δ15N in litter and surface mineral soils suggests that a closed N cycle has operated for a long time. At this site, the low δ13C of the surface horizon and the high δ13C of the lower soil horizons is clear evidence of a long phase of C4 grass dominance or cultivation of C4 crops before the establishment of the forest >500 years ago. In contrast, at Wendo-Genet, high δ13C of soils reveals that most of the land has been uncovered by forests until recently. Soil δ15N was high throughout (3.4–9.8‰), and there were no major differences between forested, cultivated and pasture soils in δ15N values of surface mineral soils. The high δ15N values suggest that open N cycles operate in the Wendo-Genet area. From the points of view of soil fertility management, it is interesting that tall forest ecosystems with relatively closed N cycling could be established on the fairly steep slopes at Menagesha after a long period of grass vegetation cover or cultivation.

Isotopic Biogeochemistry ( 13 C, 18 O) of Mammalian Enamel from African Pleistocene Hominid Sites

The carbon and oxygen isotope composition of carbonate in enamel hydroxylapatite can provide information on photosynthetic pathways of plants at the base of food webs, and on hydrological conditions. Retrieval ofpalaeoenvironmental information from isotopic composition of vertebrate fossils is complicated by potential diagenetic overprinting. In this study, alteration has been assessed by examining the extent to which expected biological carbon and oxygen isotope patterns are disrupted in fossils of species whose diets can be independently predicted by other criteria. The biological patterns used are 1) the differences in carbon isotope composition between grazers and browsers, and 2) the differences in oxygen isotope composition between hippopotamus and terrestrial herbivores. Results obtained on enamel samples from Tighenif (Algeria, =700,000 yr), Melka-Kunture (Ethiopia, 0.7-1.5 myr), and Anabo Koma (Djibouti, -1.6 myr) suggest that in vivo carbon and oxygen isotope compositions are preserved in most cases. Moreover, in all three regions, modern patterns of C3 versus C4 grass dominance were present within the Pleistocene.