It has been known for more than 200 years that the species richness of different groups of organisms is highest near the equator and decreases toward the poles. Dozens of hypotheses have been formulated to explain this phenomenon, but due to the lack of reliable data on a global scale, few of them have been rigorously tested. In a recent paper, “Co-limitation toward lower latitudes shapes global forest diversity gradients” which appeared in Nature Ecology and Evolution (https://doi.org/10.1038/s41559-022-01831-x), we show that tree species richness on Earth depends on many factors, the most important of which is mean annual temperature. Analyses of species composition and climate and soil data collected from some 1,300,000 study plots confirmed that the highest tree species richness (an average of 98 species per hectare of forest) occurs near the equator. In the northern hemisphere, in the belt from the equator to 10o latitude, species richness decreases at an average rate of 6 species per 1o. In this zone, species richness depends on many interacting factors: landforms, human activity, soil fertility and climate. Between 10o and 50o north latitude, tree species richness decreases at a much slower rate and depends mainly on temperature and precipitation, with the higher the average annual temperature, the more important precipitation is, and the drier it is, the more important temperature is. The average species richness of trees stabilizes at 50o northern latitudes (the latitude of Krakow) at an average of 4 species/ha. In the southern hemisphere, trends are similar, although the boundaries of the different zones are arranged at different latitudes.
Full text of article: Liang et al. Co-limitation toward lower latitudes shapes global forest diversity gradients. Nat Ecol Evol (2022). https://doi.org/10.1038/s41559-022-01831-x