Ecologists have generally assumed that there is a gradual transition in the form and function of plants and animals along the latitudinal gradient, rather than an abrupt change at the edge of the tropics. We used a dataset spanning 37 011 species-site combinations worldwide to quantify the shape of latitudinal gradients in plant height, seed mass, leaf area, specific leaf area, leaf Nmass, and leaf Pmass.
Results/Conclusions
Latitudinal gradients in all six plant traits displayed significant step changes at the edge of the tropics. Many of these steps were substantial, especially at the southern edge of the tropics where there was a 30-fold drop in leaf area, a 10-fold drop in seed mass, and a 7-fold drop in plant height. The step at the edge of the tropics remained significant in all six plant traits after accounting for mean annual temperature, minimum temperature, mean annual precipitation, within year temperature variability, and within year precipitation variability. We discuss the possibility that this major discontinuity in plant traits at the edge of the tropics might reflect the existence of alternative stable states.