Background: Plant richness world-records have been reported to occur either in tropical rainforests, or, at smaller spatial scales, in chronically disturbed grasslands. The tight relationship between scale and record richness suggests that there is an unyielding limit to diversity. Hypotheses: If such limit exists, current records should hardly be beaten. Chronic disturbance in grasslands allows richness to approach the limit. Studied species: All vascular plants at the study site. Study site and years of study: A natural, semiarid grassland at Concepción Buenavista, Oaxaca, Mexico, in 2012 and 2014. Methods: At 21 sites we recorded species richness in randomly sampled 0.1 × 0.1 m quadrats, and measured chronic disturbance caused by different agents. We estimated the maximum richness at each site through maximum likelihood. At one site we searched for high richness quadrats. Results: The study site shares the current world record of plant richness at the 0.1 × 0.1 m scale with 25 species. The estimated maximum diversity at each site decreased with land degradation, but had a maximum at intermediate-high levels of livestock activity. Conclusions: Our results support the idea of a hard limit to species richness, and thus theories of plant coexistence that envisage such limit. Our study site shares attributes with other record-holding grasslands, such as severe resource limitations and chronic disturbance by ungulates, suggesting that these conditions promote high species richness. We hope that, by reporting this diversity record, we also help to change the notion that Mexican grasslands are secondary communities having a negligible biological value. Keywords: Chronic anthropogenic disturbance, semiarid grassland, species richness, coexistence, intermediate disturbance hypothesis. Un récord mundial de diversidad en un pastizal de Oaxaca, México ResumenAntecedentes: Los récords mundiales de riqueza de especies de plantas se han reportado en selvas perennifolias o, en escalas pequeñas, en pastizales con disturbio crónico. La relación estrecha entre escala y riqueza récord sugiere la existencia de un límite inviolable para la diversidad. Hipótesis: Si tal límite existe, los récords actuales difícilmente serán rotos. El disturbio crónico permite que la riqueza de los pastizales se acerque al límite. Especies en estudio: Las plantas vasculares en el sitio de estudio. Sitio de estudio y fechas: Un pastizal natural semiárido, Concepción Buenavista, Oaxaca, 2012 y 2014. Métodos: En 21 sitios registramos la riqueza de especies en cuadros de 0.1 × 0.1 m y medimos el disturbio crónico causado por distintos agentes. Estimamos la diversidad máxima de cada sitio por verosimilitud máxima. En un sitio buscamos los cuadros con mayor riqueza. Resultados: Nuestro sitio comparte el récord de riqueza en la escala de 0.1 × 0.1 m con 25 especies. La riqueza máxima estimada se redujo con la degradación del suelo, pero alcanzó un máximo en un disturbio medio-alto por ganado. Conclusiones: Nuestros resultados apoyan la idea de que e...

Plant-soil feedback (PSF) occurs when plants change the biota and physicochemical properties of the soil, and these changes affect future survival or growth of plants. PSF depends on several factors such as plant functional attributes (e.g., life cycle or photosynthetic metabolism) and the environment. PSF often turn positive under dry conditions because soil biota confers drought tolerance. Conspecifics and close relatives share pathogens and consume similar resources, exerting negative PSF on each other. These ideas have mostly been tested under controlled conditions, while field studies remain scarce. To reevaluate these findings in nature, we analyzed plant-soil feedbacks over a drought-stress gradient in a phosphorus-limited semiarid grassland. We planted seedlings of 17 species in plots where community composition had been monitored for six years. To determine PSF intensity, we measured how seedling longevity was affected by previous occupancy of conspecifics and heterospecifics. The previous occupancy-survival relationship (OSR) was used as a proxy for PSF. Evidence for OSRs was found in one-third of the species pairs, with inconclusive evidence for the rest suggesting weak feedbacks. This is in line with the expectation that PSFs in the field are weaker than under controlled conditions. As expected, positive PSFs were more frequent as drought stress increased. The strongest OSRs were caused in dry plots by C 4 perennial grasses, which had very positive OSRs on several C 3 annual forbs, but negative effects on each other. Well-documented differences between these two functional groups may explain this result: C 3 plants are more sensitive to drought, and thus may be favored by tolerance-conferring microbiota; in contrast, water-efficient C 4 perennial grasses compete for phosphorus strongly, perhaps driving strong negative PSFs between them. Finally, close relatives had more negative OSRs on each other than on distant relatives as expected, although only in dry plots. This pattern was mostly due to the negative effects of closely related C 4 grasses under dry conditions, and their positive effects on distantly related dicots. Our results highlight the importance of plant traits and of the environmental context in determining the direction and strength of PSFs under field conditions.

Microsite limitation (ML) occurs when the number of suitable sites for development determines population size. Seed limitation (SL) arises when seeds cannot occupy all microsites. Both limitations have been measured on relative scales, but absolute values would allow analyzing the relationship between them and their contributions to population regulation. A tradeoff is expected because small‐seeded species are prolific and thus could have low SL, whereas large seeds perform better under different conditions, lowering ML. We assessed whether there is such tradeoff, and if it is mediated by seed size. We measured the absolute magnitudes of limitations through seed addition experiments, and tested whether they determine population density in 14 herbs with different seed sizes in a semiarid grassland. All species were limited by microsites and seeds. The tradeoff occurred only during germination, but was seemingly not mediated by seed size. Annual species were more limited by microsites than perennials, which suffered more SL. Population densities were smaller for species with greater ML measured over the life cycle, but marginally so when ML was measured during germination. SL only reduced annual‐plant density. These negative correlations indicate that our measurements correspond to actual limitations. Microsites limited populations depending on the process analyzed (germination, survival and whole life‐cycle) and the life history of species, suggesting that an integral approach to ML is required rather than focusing only on germination. Comparing annuals and perennials, we found that the strongest limitation is the main determinant of population size. The tradeoff was consistent with seed size and competition–colonization tradeoff, but our results were inconclusive. The SL–ML tradeoff seems to arise from still‐unidentified evolutionary constraints, and may contribute to the maintenance of species diversity, preventing any single species from becoming dominant and excluding its neighbors.

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