Is there a latitudinal gradient in seed production?

We compiled data from seed rain studies at 33 sites from around the world to determine whether the greater mean seed mass of tropical plants is associated with production of fewer seeds per square meter of ground. We found no significant linear relationship between latitude and annual seed rain density, but found some evidence for a mid-latitude peak in seed rain density (quadratic relationship, p=0.018; R²=0.23). Combining seed rain data with seed mass data suggests that vegetation at the equator produces between 19 and 128 times more total mass of seed per year than does vegetation at 60°. This gradient in seed production would far outweigh the doubling in net primary productivity (NPP) over the same range of latitudes. Thus, our (admittedly small) dataset suggests that tropical vegetation allocates a much greater proportion of NPP to reproduction. This raises two important questions for the future: 1) why might tropical vegetation commit more energy to seed production than vegetation further from the equator? 2) What aspect of plant growth might receive proportionally less energy in tropical ecosystems?