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Abstract. We describe an in situ fossil flora of Late Cretaceous age (;73 Ma [megaannum or million years]) from Big Cedar Ridge in central Wyoming, USA, which we sampled using a modified line-intercept method to quantify the relative abundances of 122 taxa at 100 sites across 4 km of exposed sedimentary deposits. We also measured three physical variables at each site: paleotopographic level, grain size, and total organic content. Paleoenvironmental conditions and paleofloral composition at Big Cedar Ridge covary strongly and are highly heterogeneous on small spatial scales. The reconstructed vegetation has some similarities with extant topogenous fens, but also important differences. Non-monocot angiosperms were abundant only on wet, mineral substrates that had been disturbed shortly before preservation, consistent with the weedy life histories that are inferred for their Early Cretaceous ancestors. Many non-monocot angiosperms grew in small, dispersed populations, consistent with the hypothesis that they were biotically pollinated. Overall, non-monocot angiosperm abundance was low compared with many modern wetlands. A single species of coryphoid palm was the dominant on moist, stable, moderately organic-rich sites, a pattern seen in some subtropical to tropical wetlands in the present day. Fern thickets at Big Cedar Ridge occupied highly organic, possibly low-nutrient substrates, and were dominated by Dipteridaceae, Gleicheniaceae, Schizaeaceae, and Matoniaceae. The overall high diversity and abundance of pteridophytes is unusual in the context of modern vegetation, regardless of climate zone, and probably represents a late occurrence of pteridophyte-dominated vegetation that was common earlier in the Mesozoic. Plant distributions at Big Cedar Ridge combine aspects of pre-angiosperm and modern vegetation in a way that suggests both niche conservatism and niche evolution on geological time scales.

The hypothesis that cattle browsing on inflorescences of the soaptree Yucca elata reduces reproductive success was investigated by comparing recruitment and population structure in six populations protected from grazing, six grazed during the flowering season and five grazed outside the flowering season. Cattle consumed 98% of inflorescences, which were found to be highly nutritious. Reduced recruitment in flower-grazed populations could not be attributed to reduced flower survival, as recruitment in non-flower grazed populations was also reduced. Changes in population structure were due to cattle browsing small caudices, including both genets and ramets. An alternate hypothesis of limited germination in soils compacted by cattle was not supported. Cattle browsing of inflorescences reduced reproductive effort, which may be due to the inability of the plants to resorb nutrients after flowering. Browsing also increased branching, probably through lack of apical dominance, whereas physical trampling increased procumbency. Cattle browsing implies a lack of genetic recruitment, possible local extinction of the yucca moth Tegeticula yuccasella, the exclusive pollinator of Y. elata, and local reduction in insect and bird biodiversity.

A demographic study was conducted in the northern Australian Astrebla grasslands to determine the importance of habitat type in influencing invasion patterns of Acacia nilotica , an exotic leguminous tree from Africa and Asia. One of the repeated patterns observed for A. nilotica is that denser populations are often associated with riparian habitats. Data available on this species do not enable us to determine which of a number of processes has lead to the formation of this pattern. Several explanations were tested for patterns in tree abundance: (i) that more seedlings emerge in wetter habitats; (ii) that mortality is lower in wetter habitats; (iii) that growth rates are faster in wetter habitats; and (iv) that plants are more vigorous (as indicated by leaf cover, flowering intensity and predation rates) over longer periods in wetter habitats. The study was stratified across three habitat types, perennial and ephemeral riparian and non-riparian, which are characteristic of Astrebla grasslands and differentiated by the availability of water. In addition to testing for habitat-linked differentiation in demography, data were also used to test whether seedling emergence, mortality, growth and vigour varied between sites with cattle versus sheep. The data collected indicated that seedling emergence, determined primarily by livestock dispersal, was likely to be the dominant influence on patterns of A. nilotica invasion. Mortality and growth rates were similar in ephemeral riparian and non-riparian habitats, whereas perennial riparian habitats had more rapid growth rates, which may increase the rate of invasion in these areas. Plant vigour was also greater over longer periods in perennial riparian habitats with greater leaf cover, longer flowering season and fewer insect borer holes. Livestock species were found to have little influence on the demography of A. nilotica plants in this study. Very low growth rates and high mortality in A. nilotica populations are likely to lead to net decline in ephemeral riparian and non-riparian habitats in the long term. The importance of episodic recruitment in the maintenance of A. nilotica populations is discussed.

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