Abstract Themeda and Heteropogon are closely related grass genera frequently dominant in tropical C4 grasslands. Relationships between them are poorly resolved, impeding ecological study, especially of T. triandra with a broad distribution from Africa to East Asia, and H. contortus with a pantropical distribution. Our analyses of plastome and nuclear genomes with comprehensive sampling of Themeda and Heteropogon demonstrate that neither genus is monophyletic as currently circumscribed. Plastome and nuclear data place H. melanocarpus and H. ritchiei in Themeda. Nested in T. triandra are T. quadrivalvis and T. unica, demonstrating that this widespread species is more morphologically diverse than previously recognized. Heteropogon fischerianus is nested in H. contortus. The picture is more complex for H. triticeus that is sister to H. contortus in the nuclear analysis and to Cymbopogon in the plastome analysis. This incongruence between nuclear and plastid phylogenetic trees suggests hybridization between Cymbopogon-related genome donors and H. contortus. Plastome dating estimates the Themeda–Heteropogon crown age at c. 7.6 Myr, consistent with the Miocene C4 grassland expansion. Themeda triandra and H. contortus diversified 1–2 Mya in the Pleistocene. These results establish a foundation for studying the history of these ecologically significant widespread grasses and the ecosystems they form.
Abstract Vegetation succession depends on the availability of suitable propagules in the soils, and an understanding of soil seed banks is important for effective vegetation restoration of abandoned croplands. Aims of this study were to identify characteristics of soil seed bank, relationship with standing vegetation, and potential significance of soil seed bank for vegetation restoration in abandoned croplands on the hilly-gullied Loess Plateau. Results of field survey, germination, and correspondence analysis showed that density of soil seed bank ranged from almost 900 through almost 6470 seeds m−2 at 0–5 cm depth and almost 120 through almost 2470 seeds m−2 at 5–10 cm depth, with species richness of 7–14. Early successional species dominated soil seed bank, while later successional species occurred only at low densities. Similarity between soil seed bank and standing vegetation was low (with an average of 0.261). Most important variables contributing to the variation of the standing vegetaion included soil water, extractable P, soil seed bank density, and aspect. Soil seed bank alone explained 32% of vegetation community variation. Potential for vegetation restoration from soil seed banks is limited, and it is recommended to transplant some later successional species into abandoned croplands to accelerate succession. Keywords: community similaritycorrespondence analysisseed germination methodsoil erosion We thank the NSFC projects (40771126; 40571094; 40271074) for funding this research, the assistance of Ansai Ecological Experimental Station of Soil and Water Conservation, CAS. We are also grateful to X. H. Ma, Z. G. Zhang, L. R. Xu, H. Y. Zou, and J. M. Cheng for field survey and experiment suggestions. We would like to thank the three anonymous reviewers for constructive comments. Notes SW0-5, soil water content in 0–500 cm layers; SW2-5, soil water content in 200–500 cm layers; SOM, soil organic matter. CV, Coefficient of variation; SE, Standard Error. Frequency of seeds (%) in the soil seed bank is based on presence in 144 sampling points and in vegetation is based on 72 sampling points; Seed density is the range of the mean value (6 sub-samples) in the plots in which it occurred. CVA, Canonical Variate Analysis; PCVA, Partial Canonical Variate Analysis; ∗P < 0.05; ∗∗P < 0.01; the inertia of CVA is 3.000; "/", "partialling out" the following variable. Bs, species forming a persistent seed bank; V, possessing lateral vegetative spread; VBs, species characterized by Bs and V; W, producing numerous widely dispersed seeds.
Semi-natural grasslands on steep slopes often show high plant species diversity. These grasslands were traditionally maintained through mowing and/or grazing. The traditional management practices help to maintain species diversity, whereas land abandonment reduces diversity by increasing competition from dominant species and reducing seedling recruitment. The reintroduction of management can reverse species diversity declines, but suitable grassland restoration programs are scarce in Japan. To study the effect of short-term abandonment on seedling ecology, we monitored the vegetation of a Susogari grassland that had been abandoned for 3 years; the grassland occupies a steep slope (ca. 50°) on a hillside above paddy fields, and was traditionally mown. We monitored the vegetation before abandonment, in the 3rd year of abandonment, and in the 1st and 2nd years after restoration of mowing management. Emergence and survival of seedlings was monitored for 18 months after reintroduction of management. We monitored 1,183 seedlings of grassland species and non-target annuals in ten 1-m 2 plots. After mowing was reintroduced, most grassland species reappeared or increased in the first and second years. Few seedlings of perennial plants and no seedlings of annuals flowered. An exotic species, Solidago altissima , had a lower survival rate (10%) than grassland species (>30%), and all but two grassland species survived over the 18-month period. Although vegetation composition was not fully recovered, our findings suggest that a steep slope acts as a strong filter that inhibits the establishment of non-target species while enhancing persistence of target grassland species.
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