The diversity of neotropical plant species is a central topic in ecological theory. While there is much attention on large-scale trends in species diversity, such as the increase with decreasing latitude, there is less documentation of changes within the tropics. Understanding tropical plant diversity is crucial for resolving debates about the regulation of species diversity. There is controversy over how to measure diversity, with some advocating for species richness and others emphasizing equitability. Whittaker (1977) argues that species richness is the most appropriate measure of diversity, which this chapter uses. Species richness depends on a standard sample area, and the relationship between species number and sample area is known as the species-area curve. This relationship, when log-log transformed, follows a power function equation: S = kA^z, where A is area, z is the slope, and k is the y-intercept. Preston (1948, 1962) popularized this equation. Most studies of the species-area relationship in plants have focused on islands or temperate vegetation. Few have studied tropical forests, and even fewer have included small plants. Some studies have compared species-area patterns in different tropical communities. Overall, tropical plant communities have more species than temperate ones, and those on poorer soils have fewer species than those on better soils. However, Huston (1979, 1980) challenged the positive correlation between plant diversity and soil fertility, suggesting that rich soils may lead to higher growth rates and competitive displacement, reducing diversity. His findings, based on Costa Rican data, show a negative correlation between tree species diversity and certain soil nutrients. However, the significance of these results is questionable, as the Costa Rican sites are on relatively rich soils.The diversity of neotropical plant species is a central topic in ecological theory. While there is much attention on large-scale trends in species diversity, such as the increase with decreasing latitude, there is less documentation of changes within the tropics. Understanding tropical plant diversity is crucial for resolving debates about the regulation of species diversity. There is controversy over how to measure diversity, with some advocating for species richness and others emphasizing equitability. Whittaker (1977) argues that species richness is the most appropriate measure of diversity, which this chapter uses. Species richness depends on a standard sample area, and the relationship between species number and sample area is known as the species-area curve. This relationship, when log-log transformed, follows a power function equation: S = kA^z, where A is area, z is the slope, and k is the y-intercept. Preston (1948, 1962) popularized this equation. Most studies of the species-area relationship in plants have focused on islands or temperate vegetation. Few have studied tropical forests, and even fewer have included small plants. Some studies have compared species-area patterns in different tropical communities. Overall, tropical plant communities have more species than temperate ones, and those on poorer soils have fewer species than those on better soils. However, Huston (1979, 1980) challenged the positive correlation between plant diversity and soil fertility, suggesting that rich soils may lead to higher growth rates and competitive displacement, reducing diversity. His findings, based on Costa Rican data, show a negative correlation between tree species diversity and certain soil nutrients. However, the significance of these results is questionable, as the Costa Rican sites are on relatively rich soils.