Population pyramids provide a way to visualize the age and sex composition of a geographic region, such as a nation, state, or county. A standard population pyramid divides sex into two bar charts or histograms, one for the male population and one for the female population. The two charts mirror each other and are divide age into 5-year cohorts. The shape of a population pyramid provides insights into a region’s fertility, mortality, and migration patterns. When a region has high fertility and mortality, but low migration the visualization will look like a pyramid, with the youngest age cohort (0-4 years) representing the largest percent of the population and each older cohort representing a progressively smaller percent of the population.

In many regions fertility and mortality have decreased significantly since 1970, as people live longer and women have fewer children. With lower fertility and mortality, population pyramids are shaped more like a pillar.

While population pyramids can be made for any geographic region, when interpreting population pyramids for smaller areas (like counties) the most important force that shapes the pyramid is often in- and out-migration (Wang and vom Hofe, 2006, p. 65). For smaller regions, population pyramids can have unique shapes.

This data archive provides the resources needed to generate population pyramids for the United States, individual states, and any county within the United States. Population pyramids usually require significant data cleaning and graph making skills to generate one pyramid. With this data archive the data cleaning has been completed and the R script provides reusable code to quickly generate graphs. The final output is an image file with six graphs on one page. The final layout makes it easy to compare changes in population age and sex composition for any state and any county in the US for 1970, 1980, 1990, 2000, 2010, and 2017.

Quercus chungii, a rare and endangered endemic tree species, is found exclusively in subtropical regions of China. Understanding the population structure and temporal dynamics of Q. chungii is pivotal for effective conservation and restoration of its populations and associated ecosystems. However, large knowledge gaps remain about its population structure and temporal change, and its key demographic rates across size classes. In this study, we investigated the population structures of Q. chungii in 2013 and 2023 in a nature reserve specifically established to better conserve this species and its associated ecosystems. We found that Q. chungii increased in its overall abundance, and tree size in the past decade, suggesting active regeneration and a rapid growth rate for this species and the effectiveness of past conservation efforts. The age structure in 2023 showed a pyramid shape, with a sharp decline in the numbers of individuals from germinated seeds to seedlings and from seedlings to saplings. These led to the low numbers of seedlings and saplings and high age-specific death probabilities at the early developmental stages. These results indicated potential risks of future population decline. These risks may have already manifested over the past decade, as a high mortality rate during the seedling-to-sapling transition could be one of the primary reasons contributing to the decreased proportion of saplings in 2023 compared to 2013. We propose that future studies may benefit from in-depth studies on the regeneration processes of Q. chungii by considering seed predation and germination under changing climate. This study improves the prediction of population development of Q. chungii, thereby offering theoretical guidance essential for its conservation.